MICROPILE 

US Department 

of Transportation DESIGN AND CONSTRUCTION 

Federal Highway 

Administration GUIDELINES 

Priority Technologies 

Program 

NEW BRIDGE STRUCTURE 

E ABUTMENT 

SECTION A-A 

NEW MlCROPlLE 

FOUNDATION SUPPORT 

FOUNDATION SUPPORT SEISMIC RETROFIT OF BRIDGES 

NON RETICULATED 

MICROPILE STRUCTURE 

SLOPE STABILIZATION 

SURCHARGE 

ORIGINAL CONCRETECAP i”‘:‘y;]:,‘:rrr 

\ t*....*AG&&. ,I 

WALL FACING .-• 

EARTH RETENTION 

IMPLEMENTATION MANUAL 

PUBLICATION NO. FHWA - SA - 97 - 070 June 2000 

-- 
NOTICE 

The contents of this report reflect the views of the authors, who are responsible for the facts and the 

accuracy of the data presented herein. The contents do not necessarily reflect the policy of the 

Department of Transportation. This report does not constitute a standard, specifications, or regulation. 

The United States Government does not endorse products or manufacturers. Trade or manufacturer’s 

names appear herein only because they are considered essential to the objective of this document. 
Technical Report Documentation Pat 

1. Report No. 2. Government Accession No. 3. Recipient’s Catalog No. 

FHWA-SA-97-070 

4. Title and Subtitle 5. Report Date 

Micropile Design and Construction Guidelines Implementation Manual June, 2000 

Priority Technologies Program (PTP) Project 6. Performing Organization Code: 

7. Author(s) 

Tom Armour, P.E., Paul Groneck, P.E., James Keelev. P.E. and 

Sunil Sharma, P.E., PhD. 

<- I 

8. Performing Organization Report No. 

! 

9. Performing Organization Name and Address 10. Work Unit No.(TRAIS) 

DBM Contractors Inc. 

1220 S. 356th 

Federal Way, WA 98063 

11. Contract or Grant No. 

Coop Agreement No. 95-A-17-0046 

12. Sponsoring Agency Name and Address 13 Type of Report and Period Covered 

FHWA Office of Technology Applications Technical Manual 1995-l 997 

FHWA Federal Lands Highway Division Offices 

FHWA Region 10 

14. Sponsoring Agency Code 

15. Supplementary Note 

FHWA COTR: Alan P. Kilian, P.E. Western Federal Lands Highway Division, Vancouver, WA 

Technical Review by Joint FHWA / STATE DOT / INDUSTRY Technical Working Group 

16. Abstract 

The use of micropiles has grown significantly since their conception in the 195Os, and in particular since the mid-1980s. 

Micropiles have been used mainly as elements for foundation support to resist static and seismic loading conditions and less 

frequently as in-situ reinforcements for slope and excavation stability. Many of these applications are suitable for 

transportation structures. 

Implementation of micropile technology on U.S. transportation projects has been hindered by lack of practical design and 

construction guidelines. In response to this need, the FHWA sponsored the development of this Micropile Design and 

Construction Guidelines Implementation Manual. Funding and development of the manual has been a cooperative effort 

between FHWA, several U.S. micropile specialty contractors, and several State DOT’s This manual is intended as a 

“practitioner-oriented” document containing sufficient information on micropile design, construction specifications, 

inspection and testing procedures, cost data, and contracting methods to facilitate and speed the implementation and cost 

effective use of micropiles on United States transportation projects. 

Chapter 1 provides a general definition and historic framework of micropiles. Chapter 2 describes the newly developed 

classifications of micropile type and application. Chapter 3 illustrates the use of micropiles for transportation applications. 

Chapter 4 discusses construction techniques and materials. Chapter 5 presents design methodologies for structural foundation 

support for both Service Load Design (SLD) and Load Factor Design (LFD). Chapter 6, which was supposed to present a 

design methodology for slope stabilization, is not included in this version. Chapter 7 describes micropile load testing. Chapter 

8 reviews construction inspection and quality control procedures. Chapter 9 discusses contracting methods for micropile 

applications. Chapter 10 presents feasibility and cost data. Appendix A presents sample plans and specifications for Owner 

Controlled Design with Contractor Design Build of the micropiles, and/or micropiles and footings. 

17. Key Words 18. Distribution Statement 

micropile, structure foundation, slope No restrictions. This document is available to the public 

stabilization, underpinning, seismic retrofitting, from the National Technical Information Service, 

specifications, load testing, insitu reinforcement Springfield, Virginia 22161. 

19. Security Classif. (of this report) 

Unclassified 

20. Security Classif. (of this page) 2 1. No. of Pages 22. Price 

Unclassified 376 

Form DOT F 1700.7 (8-72) Reproduction of completed page authorized 

PREFACE 

The long-term performance of micropiles has been proven after 25+ years of use in Europe and 

North America. The purpose of this “practitioner-oriented” manual is to facilitate the 

implementation of micropile technology into American transportation design and construction 

practice and to provide guidance for selecting, designing and specifying micropiles for those 

applications to which it is technically suited and economically attractive. A comprehensive 

review of current design and construction methods has been made and results compiled into a 

guideline procedure. The intent of presenting the guideline procedure is to help ensure that 

agencies adopting use of micropile technology follow a safe, rational procedure from site 

investigation through construction. 

Chapter 1 provides a general definition and historic framework of micropiles. Chapter 2 

describes the newly developed classifications of micropile type and application. Chapter 3 

illustrates the use of micropiles for transportation applications. Chapter 4 discusses 

construction techniques and materials. Chapter 5 details the design methodologies for 

structural foundation support and includes worked design examples. Chapter 6 was intended to 

cover slope stabilization details, but due to a lack of consensus on design methods, this chapter 

is not included and is still under preparation. When finished, Chapter 6 will be made available 

as a supplement to this manual. Chapter 7 describes pile load testing. Chapter 8 reviews 

construction inspection and quality control procedures. Chapter 9 discusses contracting 

methods for micropile applications. Chapter 10 presents feasibility and cost data. Appendix A 

presents guideline plans and specifications for Owner Controlled Design with Contractor 

Design Build of the micropiles, and/or micropiles and footings. 

MANUAL LIMITATIONS 

This document is disseminated under the sponsorship of the Department of Transportation in the interest of 

information exchange. The United States Government assumes no liability for its contents or use thereof. The 

contents of this report reflect the views of the authors, who are responsible for the accuracy of the data presented 

herein. The contents do not necessarily reflect the official policy of the Department of Transportation. This report 

does not constitute a standard, specification, or regulation. 

The United States Government does not endorse products or manufacturers. Trade or manufacturers’ names appear 

herein only because they are considered essential to the objective of this document. 

FHWA-SA-97-070 (~00-06) i 
[BLANK PAGE] 

ii FHWA-SA-97-070 (~00-06) 
ACKNOWLEDGMENTS 

The preparation of this manual has been funded through a cooperative effort between FHWA 

and several U.S. micropile specialty contractors and manufacturers, including: 

FHWA Region 10 and Federal Lands Division Offices (using Priority Technology 

Program Funds provided by the FHWA Office of Technology Applications.) 

DBM Contractors, Inc. 

Dywidag Systems International, Inc. 

Hayward Baker, Inc. 

Condon-Johnson and Associates, Inc. 

Malcolm Drilling Company, Inc. 

Nicholson Construction Company 

Schnabel Foundation Company 

Special thanks are due particularly to Mr. James Keeley (FHWA, Central Federal Lands 

Division), for co-authoring Chapter 5 on “Structural Foundations; Mr. Al Kilian (FHWA, 

Western Federal Lands Division) and Mr. Ron Chassie (FHWA, Region lo), for providing the 

constant assistance, encouragement and funding support that made this manual possible and to 

Dr. Sunil Sharma (University of Idaho) for technical assistance and for assembling and 

publishing the final version. An interactive, CD-version of this manual is currently being 

developed by Dr. Sharma for release in Fall 2000. 

Special acknowledgment is made to the following volunteer Technical Working Group 

members (listed in alphabetical order) for significant contributions to the manual and for 

providing the QA/QC to help assure that the design and construction guidelines and guide 

specifications presented in the manual are sound, rational, and practitioner oriented: 

Tom Armour DBM Contractors, Inc. (Lead Coordinator) 

Don Chadboume Washington State Department of Transportation 

FHWA-SA-97-070 (~00-06) 

. . . 

111 
Ron Chassie FHWA-Region 10 

Kerry Cook FHWA-WFLHD 

Paul Groneck DBM Contractors, Inc. 

Francis Gularte Hayward Baker, Inc. 

Sam Holder FHWA-CFLHD 

Fahmi Ismail FHWA-WFLHD 

James Johnson 

Gary Kast 

Jim Keeley 

Al Kilian 

Jim Klamerus 

Heimich Majewski 

Jim Moese 

Sharon Naramore 

Condon-Johnson and Associates, Inc. 

Dwyidag Systems, Canada, Ltd. 

FHWA-CFLHD 

FHWA-WFLHD 

FHWA-CFLHD 

Malcolm Drilling Company, Inc. 

California Department of Transportation 

California Department of Transportation 

Seth Pearlman 

Thomas Richards 

Nicholson Construction 

Nicholson Construction 

Jan Six Oregon Department of Transportation 

David Weatherby Schnabel Foundation Company 

John Wolosick Hayward Baker, Inc. 

Special recognition is given to Dr. Fernando Lizzi of Italy, the “father” of micropiles and to Dr. 

Donald Bruce, formerly Technical Director with Nicholson Construction Company and 

currently a Principal with EC0 Geosystems, for preparing the FHWA State-of-Practice Review 

Reports from which some of the information in this manual has been excerpted, and for his 

tireless efforts as a “champion” of Micropile Technology in North America. 

iv FHWA-SA-97-070 (~00-06) 
Thanks are also due to the members of the Deep Foundation Institute (DFI) Micropile 

Technical Committee for their thorough technical review of the final draft manual. 

The many figures in this manual were originally prepared by DBM Contractors, Inc., and were 

subsequently enhanced and colorized by Dr. Sharma and Rob Barnes of the University of 

Idaho. Lastly, and most importantly, heartfelt thanks are given to Ms. Mary Ellen Bruce, who 

developed Chapters l-3, Mr. Don Olson (FHWA, retired), who provided technical assistance 

with Chapter 5 and Ms. Patty Roman0 of DBM Contractors, Inc., who typed the original 

version of the manual. 

FHWA-SA-97-070 (~00-06) V 
[BLANK PAGE] 

FHWA-SA-97-070 (~00-06) 
ENGLISH TO METRIC (SI) CONVERSION FACTORS 

The primary metric (SI) units used in civil and strnctural engineering are: 

. meter (m) 

. kilogram (kg) 

. second (s) 

. Newton (N) 

. Pascal (Pa = N/m2 ) 

The following are the conversion factors for units presented in this manual: 

Quantity 

Mass 

Force 

Force/unit length 

Pressure, stress, 

modulus of 

elasticity 

Length 

Area 

Volume 

From English 

Units 

lb 

lb 

kiP 

Plf 

klf 

Psf 

ksf 

psi 

ksi 

inch 

foot 

foot 

square inch 

square foot 

square yard 

cubic inch 

cubic foot 

cubic yard 

To Metric (SI) Multiply 

Units by 

kg 0.453592 

N 4.44822 

kN 4.44822 

N/m 14.5939 

kNlm 14.5939 

Pa 47.8803 

kPa 47.8803 

kPa 6.89476 

MPa 6.89476 

mm 25.4 

m 0.3048 

mm 304.8 

mm2 645.16 

ni2 0.09290304 

m2 0.83612736 

mm3 16386.064 

m3 0.0283 168 

m3 0.764555 

For Aid to Quick 

Mental Calculations 

1 lb(force) = 4.5N 

1 plf = 14.5N/m 

1 klf = 14.5kN/m 

1 psf = 48 Pa 

1 ksf = 48 kPa 

1 psi = 6.9 kPa 

1 ksi = 6.9 MPa 

1 in=25mm 

1 ft = 0.3 m 

1 ft=300mm 

1 sq ft = 0.09 m2 

1 sa vd = 0.84 m2 

A few points to remember: 

1. In a “soft” conversion, an English measurement is mathematically converted to its exact 

metric equivalent. 

2. In a “hard” conversion, a new rounded, metric number is created that is convenient to work 

with and remember. 

3. Use only the meter and millimeter for length (avoid centimeter). 

4. The Pascal (Pa) is the unit for pressure and stress (Pa and N/m’). 

5. Structural calculations should be shown in MPa or kPa. 

6. A few basic comparisons worth remembering to help visualize metric dimensions are: 

. One mm is about l/25 inch or slightly less than the thickness of a dime. 

. One m is the length of a yardstick plus about 3 inches. 

. One inch is just a fraction (l/64 inch) longer than 25 mm (1 inch = 25.4 mm). 

. Four inches are about l/16 inch longer than 100 mm (4 inches = 101.6 mm). 

. One foot is about 3/16 inch longer than 300 mm (12 inches = 304.8 mm). 

FHWA-SA-97-070 (~00-06) vii 
[BLANK PAGE] 

. . . 

Vlll FHWA-SA-97-070 (~00-06) 
TABLE OF CONTENTS 

CHAPTER 1 

INTRODUCTION.........................................................l- 1 

l.A PURPOSEANDSCOPEOFMANUAL.. ................................ l-l 

1 .B MICROPILE DEFINITION AND DESCRIPTION 

.......................... 1 - 2 

1 .C HISTORICAL BACKGROUND 

........................................ 1 - 5 . 

CHAPTER 2 

MICROPILE CLASSIFICATION SYSTEM 

.................................. .2 - 1 

2.A MICROPILETYPESINCURRENTUSE 

............................... .2- 1 

2.A.l Design Application Classification 

............................... .2 - 1 

2.A.2 Construction Type Classification 

................................ .2 - 6 

CHAPTER 3 

MICROPILE APPLICATIONS IN TRANSPORTATION PROJECTS 

.............. .3 - 1 

3.A INTRODUCTION 

................................................. ..3- 1 

3.B STRUCTURAL SUPPORT 

........................................... .3 - 4 

3.B.l New Foundations 

............................................ .3 - 5 

3.B.2 Underpinning of Existing Foundations 

............................ .3 - 6 

3.B.3 Seismic Retrofit 

............................................. .3 - 9 

3.C IN-SITU REINFORCEMENT (SLOPE STABILIZATION & 

EARTHRETENTION) 

............................................ ..3-10 

FHWA-SA-97-070 (~00-06) ix 
3 .D FACTORS INFLUENCING THE CHOICE OF MICROPILES .............. 3 - 15 

3 .D. 1 Physical Considerations ....................................... 3 - 17 

3.D.2 Subsurface Conditions ........................................ 3 - 18 

3.D.3 Environmental Conditions ..................................... 3 - 19 

3.D.4 Existing Structure Adaptation .................................. 3 - 20 

3.D.5 Micropile Limitations ........................................ 3 - 21 

3 .D.6 Economics of Micropiles ...................................... 3 - 22 

CHAPTER 4 

CONSTRUCTION TECHNIQUES AND MATERIALS .......................... 4 - 1 

4.A INTRODUCTION ................................................... 4-l 

4.B DRTLLING ......................................................... 4-2 

4.B.l DrillRigs ................................................... 4-4 

4.B.2 Drilling Techniques ........................................... 4 - 7 

4.B.3 Overburden Drilling Techniques ................................. 4 - 8 

4.B.4 Open-Hole Drilling Techniques ................................. 4 - 13 

4.C GROUTING ...................................................... 4-14 

4.C.l Grout Equipment ............................................ 4 - 17 

4.C.2 Grout Mixing ............................................... 4 - 18 

4.C.3 Grout Placement Techniques ................................... 4 - 20 

4.c.3.1 Gravity Fill Techniques (Type A Micropiles) ............. 4 - 20 

4.C.3.2 Pressure Grouting Through the Casing (Type B Micropiles) . 4 - 20 

4.c.3.3 Postgrouting (Type C and D Micropiles) ................. 4 - 22 

4.C.4 Top-Off (Secondary) Grouting ................................. 4 - 26 

4.D REINFORCING STEEL ............................................. 4 - 26 

4.D.l Placement of Reinforcement ................................... 4 - 26 

4.D.2 Reinforcement Types ......................................... 4 - 27 

4.D.3 Reinforcement Corrosion Protection ............................. 4 - 36 

X FHWA-SA-97-070 (~00-06) 

___-~~. - ----. -- -. 
CHAPTER 5 

DESIGN OF MICROPILES FOR STRUCTURE FOUNDATIONS (CASE 1 PILES). ...... 1 

5.A INTRODUCTION ................................................. ..5- 1 

5.B COMMENTONTHEUSEOFTHISMANUALFORDESIGN .............. .5-4 

5.C EXPLANATION OF SLD AND LFD DESIGN METHODS ................. .5 - 5 

5.D GEOTECHNICAL DESIGN ......................................... .5 - 12 

5.D. 1 Geotechnical Investigation Requirements ......................... .5 - 12 

5.D.2 Geotechnical Bond Capacity ................................... .5 - 13 

5.D.3 SUIIIIIICQJJ of Grout-to-Ground a non,, Nominal sWength Values ............... 5 - 15 

5.D.3.1 Geotechnical Bond Length Tension And Compression 

AllowableAxialLoad-SLD ........................ .5- 17 

5.D.3.2 Geotechnical Bond Length Tension And Compression 

DesignAxial Strength-LFD ......................... .5 - 17 

5.D.4 Geotechnical End Bearing Capacity ............................. .5 - 17 

5 .D.5 Group Effect For Axially Loaded Micropiles ...................... .5 - 17 

5.E MICROPILE STRUCTURAL DESIGN ................................ .5 - 19 

5.E.l Notation ................................................... .5 -20 

5.E.2 Pile Cased Length Structural Capacity ........................... .5 - 21 

5.E.2.1 Pile Cased Length (Service Load Design) ............... .5 - 2 1 

5.E.2.2 Pile Cased Length (Load Factor Design) ................ .5 - 22 

5.E.3 Pile Uncased Length ......................................... .5 -23 

5.E.3.1 Pile Uncased Length (Service Load Design) ............. .5 - 23 

5.E.3.2 Pile Uncased Length (Load Factor Design) .............. .5 - 24 

5.E.4 Recommended Safety Factors and Test Loads for Field 

Verification and Proof Tests ................................... .5 - 25 

5.E.4.1 PileCasedLength ................................. ..5-2 9 

5.E.4.2 PileUncasedLength ............................... ..5-3 0 

5.E.5 GrouttoSteelBondCapacity .................................... 5-30 

5.E.6 DesignofPlungeLength.. .................................... .5-31 

5.E.7 Strain Compatibility Between Structural Components ............... .5 - 33 

FHWA-SA-97-070 (~00-06) xi 
5.E.8 Reinforcing Bar and Casing Connections ......................... 5 - 34 

5.E.9 Pile to Footing Connection .................................... 5 - 35 

5.F ADDITIONAL GEOTECHNICAL / STRUCTURAL CONSIDERATIONS .... 5 - 41 

5.F.l Prediction of Anticipated Structural Axial Displacements ............ 5 - 41 

5.F.2 Long Term Ground Creep Displacement .......................... 5 - 44 

5.F.3 Settlement of Pile Groups ..................................... 5 - 45 

5.F.4 Lateral Load Capacity ........................................ 5 - 46 

5.F.5 Lateral Stability (Buckling) .................................... 5 - 50 

5.F.6 Downdrag and Uplift Considerations ............................ 5 - 54 

5.G SAMPLE PROBLEM NO. 1 - BRIDGE ABUTMENT 

FOUNDATION SUPPORT .......................................... 5 - 55 

5.G. 1 Problem Statement ........................................... 5 - 55 

5.G.2 Step 1 -AbutmentDesignLoads ............................... 5-61 

5.G.2.1 ActiveEarthPressure-P, ............................ 5-61 

5.G.2.2 Earth Pressure Due to Live Load Surcharge - HL ........... 5 - 62 

5.G.2.3 Seismic Earth Pressure - P,, .......................... 5 - 63 

5.G.2.4 Seismic Inertia Forces ............................... 5 - 64 

5.G.3 Service Load Design (SLD) Method ............................. 5 - 65 

5.G.3.1 Step 2 (SLD) - Determine Pile Design Loads Per 

MeterLength ......................................... 5-65 

5.G.3.2 Step 3 (SLD) - Determine Allowable Structural and 

Geotechnical Pile Loads .............................. 5 - 67 

5.G.3.2.1 Step 3 (SLD) - Pile Cased Length Allowable Load . . 5 - 68 

5.G.3.2.2 Step 3 (SLD) -Pile Uncased Length Allowable Load 5 - 70 

5.G.3.2.3 Step 3 (SLD) -Allowable Geotechnical Bond Load . . 5 - 71 

5.G.3.2.4 Step 3 (SLD) - Verify Allowable Axial Loads ...... 5 - 72 

5.G.3.2.5 Step 3 (SLD) - Verify Allowable Lateral Loads ..... 5 - 72 

5 .G.3.2.6 Step 3 (SLD) - Compute Field Test Loads ......... 5 - 73 

5.G.3.2.7 Step 3 (SLD) -Pile Structural Design for Field 

TestLoads .................................. 5-74 

5.G.3.3 Step 4 (SLD) - Anticipated Axial Displacement ........... 5 - 77 

xii FHWA-SA-97-070 (~00-06) 
5.G.3.4 Step5(SLD)-PileConnectionDesign . . . . . . . . . . . . . . . . . . 5-79 

5.G.4 Load Factor Design (LFD) Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5 - 85 

5.G.4.1 Step 2 (LFD) - Determine Pile Required Strengths Per 

MeterLength. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5-85 

5.G.4.2 Step 3 (LFD) - Determine Structural and Geotechnical 

Pile Design Strengths . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5 - 88 

5.G.4.2.1 Step 3 (LFD) - Pile Cased Length Design Strength . . . 5 - 89 

5.G.4.2.2 Step 3 (LFD) - Pile Uncased Length Design Strength . 5 - 91 

5.G.4.2.3 Step 3 (LFD) - Geotechnical Bond Design Strength . .5 - 92 

5.G.4.2.4 Step 3 (LFD) - Verify Axial Design Strengths . . . . . . .5 - 94 

5.G.4.2.5 Step 3 (LFD) - Verify Lateral Design Strength . . . . . . 5 - 95 

5.G.4.2.6 Step 3 (LFD) - Compute Field Test Load. . . . . . . . . . . 5 - 96 

5.G.4.2.7 Step 3 (LFD) -Pile Structural Design for Field 

TestLoads...................................5-97 

5.G.4.2.8 Step 4 (LFD) -Anticipated Axial Displacement . . . . . 5 - 97 

5.G.4.2.9 Step 5 (LFD) - Pile Connection Design . . . . . . . . . . . . 5 - 97 

5.G.5 Step 6 (SLD & LFD) - Complete Detail Drawings . . . . . . . . . . . . . . . . . 5 - 102 

CHAPTER 6 

DESIGN OF MICROPILES FOR SLOPE STABILIZATION AND EARTH RETENTION 

6.A SPECIALNOTE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...6-1 

CHAPTER 7 

PILELOADTESTING.....................................................7- 1 

7.A INTRODUCTION ................................................. ..7- 1 

7.B TYPESANDPURPOSEOFLOADTEST ............................. .7-2 

7.B.l UltimateTest.................................................7- 3 

7.B.2 Verification Test ............................................. .7 - 4 

7.B.3 ProofTest ................................................. ..7- 4 

7.B.4 CreepTest...................................................7- 5 

FHWA-SA-97-070 (~00-06) 

. . . 

XIII 
7.C DETERMINATION OF PROJECT LOAD TESTING REQUIREMENTS ....... 7 - 5 

7.C.l NumberofLoadTests ......................................... 7-6 

7.C.2 Micropile Load-Testing Guidelines ............................... 7 - 7 

7.C.2.1 Micropile Load-Testing Guidelines for New Users 

(Owners/Contractors) ................................. 7 - 7 

7.C.2.2 Micropile Load-Testing Guidelines for Experienced Users 

(Owners/Contractors) ................................. 7 - 9 

7.C.3 Sample Problem No. 1 (Chapter 5) - Pile Load Testing Requirements . . 7 - 10 

7.C.4 TestLoadMagnitude ......................................... 7- 11 

7.D MICROPILE LOAD TESTING METHODS AND PROCEDURES ........... 7 - 12 

7.D.l Method of Load Application ................................... 7 - 12 

7.D.2 Load-Hold Duration .......................................... 7 - 13 

7.D.3 Load Test Acceptance Criteria .................................. 7 - 13 

7.E LOAD-TEST SETUP AND INSTRUMENTATION ....................... 7 - 14 

7.F DATA RECORDING AND PRESENTATION ........................... 7 - 22 

7.G PILE LOAD-TEST REPORT ......................................... 7 - 33 

7.H TESTPILEFALLURE............................................... 7-34 

CHAPTER 8 

CONSTRUCTION INSPECTION/QUALITY CONTROL ........................ 8 - 1 

8.A INTRODUCTION.. ................................................. 8-l 

8.B QUALITY CONTROL INSPECTION ................................... 8 - 2 

8.B.l MaterialHandlingandStorage .................................. 8-2 

8.B.2 Construction Monitoring ....................................... 8 - 6 

8.C QUALITY CONTROL DOCUMENTATION ............................ 8 - 13 

8.C.l. PileLoadTesting.. .......................................... 8-13 

8.C.2 ProductionPiles.. ........................................... 8-14 

xiv FHWA-SA-97-070 (~00-06) 
CHAPTER 9 

CONTRACTINGMETHODS 

............................................. ..9- 1 

9.A INTRODUCTION 

................................................. ..9- 1 

9.B SPECLFICATIONS...................................................9- 2 

9.B. 1 Owner-Controlled Design Methods 

.............................. .9 - 4 

9.B.2. Contractor Design/Build Methods 

............................... .9 - 8 

9.B.3 OtherMethods 

............................................. .9- 11 

9.C. CONTRACTPLANS................................................9-1 1 

CHAPTER 10 

FEASIBILITY AND COST DATA 

.......................................... 10 - 1 

10.A FEASIBILITY 

..................................................... lo- 1 

10.B MICROPILE COST DATA 

........................................... 10 - 2 

1O.C SAMPLE PROBLEMS-COST ESTIMATES ............................. 10 - 4 

10.C. 1 Sample Problem No. 1 (Chapter 5-Bridge Abutment Support) ......... 10 - 4 

lO.C.2 Sample Problem No. 2 (Seismic Retrofit) ......................... 10 - 5 

GLOSSARY OF TERMS 

REFERENCES 

APPENDIX A-l 

MICROPILE GUIDE CONSTRUCTION SPECIFICATION AND SAMPLE 

PLANS - CONTRACTOR DESIGN/BUILD OF MICROPILES 

1.0 DESCRIPTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Al-2 

1.1 Micropile Contractor’s Experience Requirements And Submittal. . . . . . Al - 3 

1.2 Pre-approved List . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Al - 4 

1.3 Related Specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Al - 5 

1.4 Definitions................................................. Al-5 

FHWA-SA-97-070 (~00-06) xv 
1.5 

1.6 

1.7 

1.8 

1.9 

1.10 

Referenced Codes and Standards. ............................... Al - 8 

1.5.1 American Society for Testing and Materials (ASTM) .......... Al - 8 

1.5.2 American Welding Society (AWS) ........................ Al - 9 

1.5.3 American Petroleum Institute (API) ....................... Al - 9 

Available Information. ........................................ Al - 9 

Construction Site Survey ..................................... Al - 10 

Micropile Design Requirements. ............................... Al - 11 

1.8.1 Micropile Design Submittals. ........................... Al - 12 

1.8.2 Design Calculations. .................................. Al - 12 

1.8.3 Working Drawings. ................................... Al - 13 

Construction Submittals. 

..................................... Al - 16 

Pre-construction Meeting. 

.................................... Al - 18 

2.0 MATERIALS. 

.................................................. ..Al-19 

3.0 CONSTRUCTION REQUIREMENTS ................................ Al - 22 

3.1 

3.2 

3.3 

3.4 

Site Drainage Control. ...................................... Al - 22 

Excavation .............................................. ..Al-2 3 

Micropile Allowable Construction Tolerances .................... Al - 23 

Micropile Installation ........................................ Al - 23 

3.4.1 Drilling ............................................ Al -24 

3.4.2 Ground Heave or Subsidence. ........................... Al - 24 

3.4.3 Pipe Casing and Reinforcing Bars Placement and Splicing. .... Al - 25 

3.4.4 Grouting. ........................................... Al -26 

3.4.5 Grout Testing ........................................ Al - 27 

Micropile Installation Records. ................................ Al - 27 

PileLoadTests ........................................... ..Al-2 8 

3.6.1 VerificationLoadTests ................................ Al -29 

3.6.2 Testing Equipment and Data Recording. ................... Al - 30 

3.6.3 Verification Test Loading Schedule. ...................... Al - 3 1 

3.6.4 Verification Test Pile Rejection .......................... Al - 33 

3.6.5 ProofLoadTests ................................... ..Al-3 4 

3.5 

3.6 

xvi FHWA-SA-97-070 (~00-06) 
3.6.6 Proof Test Loading Schedule . . . . . . . . . . . . . . . . . . . . . . . . . . . Al - 34 

3.6.7 Proof Test Pile Rejection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Al - 35 

4.0 METHOD OF MEASUREMENT. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Al - 36 

5.0 BASISOFPAYMENT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Al -36 

APPENDIX A-2 

MICROPILE GUIDE CONSTRUCTION SPECIFICATION AND SAMPLE PLANS - 

CONTRACTOR DESIGN/BUILD OF FOUNDATION (MICROPILES AND FOOTINGS) 

1.0 DESCRIPTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . AZ-2 

1.1 

1.2 

1.3 

1.4 

1.5 

Micropile Contractor’s Experience Requirements And Submittal. ..... A2 - 3 

Pre-approved List ........................................... A2 - 4 

Related Specifications ........................................ A2 - 5 

Definitions ................................................. A2-5 

Referenced Codes and Standards. ............................... A2 - 8 

1.5.1 American Society for Testing and Materials (ASTM) ......... A2 - 8 

1.5.2 American Welding Society (AWS) ........................ A2 - 9 

1.5.3 American Petroleum Institute (API) ....................... A2 - 9 

Available Information. ....................................... A2 - 9 

Construction Site Survey .................................... A2 - 10 

Micropile Design Requirements. .............................. A2 - 11 

1.8.1 Micropile Design Submittals. ........................... A2 - 12 

1.8.2 Design Calculations. .................................. A2 - 12 

1.8.3 Working Drawings. ................................... A2 - 13 

Construction Submittals. ..................................... A2 - 16 

Pre-construction Meeting. .................................... A2 - 18 

1.6 

1.7 

1.8 

1.9 

1.10 

2.0 MATERIALS. ................................................... A2-19 

3.0 CONSTRUCTION REQUIREMENTS ................................ A2 - 22 

3.1 Site Drainage Control. ...................................... A2 - 22 

3.2 Excavation ................................................ A2-23 

FHWA-SA-97-070 (~00-06) xvii 
3.3 Micropile Allowable Construction Tolerances .................... A2 - 23 

3.4 Micropile Installation ........................................ A2 - 23 

3.4.1 Drilling ........................................... .A2-24 

3.4.2 Ground Heave or Subsidence. ........................... A2 - 24 

3.4.3 Pipe Casing and Reinforcing Bars Placement and Splicing. .... A2 - 25 

3.4.4 Grouting. ........................................... A2 -26 

3.4.5 Grout Testing ........................................ A2 - 27 

3.5 Micropile Installation Records. ................................ A2 - 27 

3.6 PileLoadTests.............................................A2-2 8 

3.6.1 VerificationLoadTests .............................. ..A2-2 9 

3.6.2 Testing Equipment and Data Recording. ................... A2 - 30 

3.6.3 Verification Test Loading Schedule. ...................... A2 - 31 

3.6.4 Verification Test Pile Rejection .......................... A2 - 33 

3.6.5 ProofLoadTests .................................... .A2-34 

3.6.6 Proof Test Loading Schedule ............................ A2 - 34 

3.6.7 Proof Test Pile Rejection ............................... A2 - 35 

4.0 METHOD OF MEASUREMENT. .................................... A2 - 36 

5.0 BASIS OFPAYMENT ............................................. A2-36 

APPENDIX B 

Washington State DOT Guide for Preparation of a “Summary of 

Geotechnical Conditions” . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B - 1 

Example “Summary of Geotechnical Conditions” for Micropile 

Manual-SampleProblem 1 . . . . . . . . . . . . . . . . . ._ . . ., . . _. . , . . . . . . . . . . . . B-3 

. . . 

xv111 FHWA-SA-97-070 (~00-06) 
LIST OF TABLES 

Table 2-l. 

Table 3-l. 

Table 4-l. 

Table 4-2. 

Table 4-3. 

Table 4-4. 

Table 4-5. 

Table 4-6. 

Table 4-7. 

Table 5-l. 

Table 5-2. 

Table 5-3. 

Table 5-4. 

Table 7-l. 

Table 7-2. 

Table 7-3. 

Table 7-4. 

Details of Micropile Classification Based on Type of Grouting . . . . . . . . 2 - 9 

Relationship Between Micropile Application, Design Behavior, 

andConstructionType . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-3 

Overburden Drilling Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 - 9 

Dimensions, Yield, and Ultimate Strengths for Standard 

Reinforcing Bars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 - 28 

Dywidag Threadbars - Technical Data.(Courtesy of DSI) . . . . . . . . . . . 4 - 30 

Hollow Injection Bars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 - 32 

Dimensions and Yield Strength of Common Pipe Types and Sizes . . . . 4 - 34 

Corrosion Critical Values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 - 36 

Minimum Dimensions (mm) of Shell Thickness as Corrosion 

Protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-39 

Calibrated LFD (ho Factor for Various Ratios of Q n, Q L and Q E . . . . . 5 - 11 

Summary of Typical abond ,,ominal strensh Values (Grout-to-Ground Bond) 

for Preliminary Micropile Design that have been used in Practice. . . . . . 5 - 16 

P = Lateral Load for 6.35 mm (l/4 inch) Lateral Displacement 

in kN for Coarse Grained Soil (Pinned Pile Head Condition) . . . . . . . . 5 - 49 

Sample Problem No. l- Summary of Abutment Loads Per 

MeterLength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-62 

Suggested Micropile Load Testing Guidelines ..................... 7 - 8 

Amplification Factors for Micropile Verification Load Testing ...... 7 - 9 

Sample Problem No. 1 - Pile Testing Requirements ............... 7 - 11 

Example Tension Cyclic Load Test Schedule (Verification Test) 

fora322kNSeismicServiceDesignLoad ...................... 7-23 

FHWA-SA-97-070 (~00-06) xix 
Table 7-5. 

Table 7-6. 

Table 8-l. 

Table 8-2. 

Table 9-l. Contractor Design/Build Options ............................... 9 - 3 

Table 10-l. 

Table 10-2. 

Table 10-3. 

Table 10-4. 

Example Compression Cyclic Load Test Schedule (Verification Test) 

for the 721 kN Non-seismic Service Design Load .................. 7 - 25 

Example Ultimate Compression Load Test Schedule (Verification Test) 

for the 721 kN, Non-seismic Service Design Load ................. 7 - 28 

Micropile Installation Log .................................... 8 - 15 

Completed Micropile Installation Log .......................... 8 - 16 

Micropile Cost Influence Analysis ............................. 10 - 3 

Sample Problem No. 1 - Cost Analysis (Chapter 5 - Bridge 

AbutmentSupport) .......................................... 10-4 

Sample Problem No. 2 (Seismic Retrofit) ........................ 10 - 6 

Micropile Measurement and Payment Units ...................... 10 - 7 

xx FHWA-SA-97-070 (~00-06) 
LIST OF FIGURES 

Figure 1 - 1. Micropile Construction Sequence using Casing .................... 1 - 4 

Figure 1 - 2. Classical Arrangement of Root Piles for Underpinning .............. 1 - 5 

Figure 1 - 3. Typical Network of Reticulated Micropiles ........................ 1 - 7 

Figure 2 - 1. 

Figure 2 - 2. 

Figure 2 - 3. 

Figure 2 - 4. 

Figure 2 - 5. 

Figure 3 - 1. 

Figure 3 - 2. 

Figure 3 - 3. 

Figure 3 - 4. 

Figure 3 - 5. 

Figure 3 - 6. 

Figure 3 - 7. 

Figure 3 - 8. 

Figure 3 - 9. 

CASE 1 Micropiles (Directly Loaded) .......................... .2 - 2 

CASE 2 Micropiles - Reticulated Pile Network with Reinforced 

SoilMassLoadedorEngaged ................................. .2-3 

CASE 1 Micropile Arrangements .............................. .2 - 4 

CASE 2 Micropile Arrangements .............................. .2 - 5 

Micropile Classification Based on Type of Grouting. 

(RefertoTable2-1 fordetails) ................................ .2-7 

Classification of Micropile Applications ......................... .3 - 2 

Restoration Scheme for the Leaning Al Hadba Minaret (Lizzi, 1982) . . -3 - 4 

Micropiles for Foundation Support of Transportation Applications .... 3 - 7 

Underpinning Arrangement for Pocomoke River Bridge, Maryland 

(Bruceetal., 1990) .......................................... .3-9 

Typical Confignrations for Inclined Micropile Walls ............... 3 - 12 

State Road 4023 Micropile Slope Stabilization, Armstrong County, 

Pennsylvania ............................................ ..3-13 

Wall 600 Permanent Earth Retention, Portland, Oregon ............ .3 - 14 

CASE 2 Slope Stabilization, FH-7, Mendocino National Forest, 

California ................................................ .3 - 16 

Protection of an Existing Diaphragm Wall with a Secant Micropile 

Screen using Anti-acid mortar (Bachy, 1992.) .................... .3 - 20 

FHWA-SA-97-070 (~00-06) xxi 
Figure 4 - 1. 

Figure 4 - 2. 

Figure 4 - 3. 

Figure 4 - 4. 

Figure 4 - 5. 

Figure 4 - 6. 

Figure 4 - 7. 

Figure 4 - 8. 

Figure 4 - 9. 

Figure 4 - 10. 

Figure 4 - 11. 

Figure 5 - 1. 

Figure 5 - 2. 

Figure 5 - 3. 

Figure 5 - 4. 

Figure 5 - 5. 

Figure 5 - 6. 

Figure 5 - 7. 

Figure 5 - 8. 

Figure 5 - 9. 

Figure 5 - 10. 

Figure 5 - 11. 

Figure 5 - 12. 

Typical Micropile Construction Sequence Using Casing ............. 4 - 3 

Overburden Drilling Methods (Bruce, 1988) ..................... 4 - 10 

Effect of Water Content on Grout Compressive Strength and Flow 

Properties (Barley and Woodward, 1992) ........................ 4 - 15 

Various Types of Colloidal Mixers ............................. 4 - 18 

Various Types of Paddle Mixers ............................... 4 - 19 

Principle of the Tube a Manchette Method of Postgrouting Injection . . 4 - 24 

Use of Reinforcement Tube as a Tube a Manchette 

Postgrouting System ....................................... 4 - 25 

Multiple Bar Reinforcement with Bar Centralizer/Spacer ........... 4 - 28 

Details of Continuously Threaded Dywidag Bar (DSI, 1993) ........ 4 - 29 

Details of Composite High-capacity Type 1B Micropiles, 

Vandenberg Air Force Base, California ......................... 4 - 35 

GEWI Piles with (a) Grout protection only, and (b) Double 

corrosion protection (Courtesy DSI) ............................ 4 - 38 

SLD and LFD Description and Relationships ...................... 5 - 6 

Detail of a Composite Reinforced Micropile ..................... 5 - 19 

Comparison of Maximum Test loads for Micropiles, Soil Nails, 

andGroundAnchors ........................................ 5-27 

Detail of Load Transfer through the Casing Plunge Length. ......... 5 - 3 1 

Change in Load Transfer through Casing Plunge Length as Load 

Increases (Bruce and Gemme, 1992). ........................... 5 - 32 

Pile to Footing Connection Detail. ............................. 5 - 36 

Pile to Footing Connection Detail. ............................. 5 - 37 

Pile to Footing Connection Detail. ............................. 5 - 38 

Pile to Footing Connection Detail. ............................. 5 - 39 

Pile to New Footing Connection Detail - Simple Compression, 

ModerateLoad ............................................. 5-40 

Pile to New Footing Connection Detail - Simple Compression, 

HighLoad. ............................................... 5-40 

Pile to Existing Footing Connection Detail - Compression Only. ..... 5 - 41 

xxii FHWA-SA-97-070 (~00-06) 
Figure 5 - 13. 

Figure 5 - 14. 

Figure 5 - 15. 

Figure 5 - 16. 

Figure 5 - 17. 

Figure 5 - 18. 

Figure 5 - 19. 

Figure 5 - 20. 

Figure 5 - 21. 

Figure 7 - 1. Compression Load Test Arrangement .......................... .7 - 15 

Figure 7 - 2. Tension Load Test Arrangement .............................. .7 - 15 

Figure 7 - 3. Lateral Load Test Arrangement ............................... .7 - 16 

Figure 7 - 4. Typical Jack Calibration Curve. ............................... .7 - 18 

Figure 7 - 5. Piletopdisplacementvs.pileloadcurve.. ..................... ..7-2 9 

Figure 7 - 6. Pile Top Elastic/permanent Displacement Vs. Pile Load Curve ....... 7 - 30 

Figure 7 - 7. Displacement Creep Vs. Time Curve .......................... .7 - 31 

Figure 7 - 8. ElasticLengthvsPileLoadCurve ............................ .7-32 

Sample Problem No. 1 - Abutment Section Detail. ................ .5 - 56 

SampleProblemNo. 1 -PileDetails .......................... ..5-5 7 

Sample Problem No. 1 - Summary of Abutment Loads. ............ .5 - 58 

SampleProblemNo. 1 -SoilBoringLog ....................... .5-59 

Pile Top to Abutment Footing Connection Detail. ................ .5 - 80 

Comparison of SLD and LFD for Geotechnical Bond Length Design. .5 - 95 

Pile Top to Abutment Footing Connection Detail. ................. 5 - 98 

Page 1 of Sample Problem No. 1 - Working Drawing Submittal 

(SLD&LFDDesigns) ................................... ..5-10 3 

Page 2 of Sample Problem No. 1 - Working Drawing Submittal 

(SLD&LFDDesigns) .................................... .5-104 

FHWA-SA-97-070 (~00-06) . . . 

xx111 
[BLANK PAGE] 

xxiv FHWA-SA-97-070 (~00-06) 
LIST OF PHOTOGRAPHS 

Photograph 3 - 1. 

Photograph 3 - 2. 

Photograph 3 - 3. 

Photograph 3 - 4. 

Photograph 3 - 5. 

Photograph 4 - 1. 

Photograph 4 - 2. 

Photograph 4 - 3. 

Photograph 7 - 1. 

Photograph 7 - 2. 

Photograph 7 - 3. 

Photograph 7 - 4. 

Photograph 7 - 5. 

Photograph 7 - 6. 

Photograph 8 - 1. 

Photograph 8 - 2. 

Photograph 8 - 3. 

Photograph 8 - 4. 

Photograph 8 - 5. 

Photograph 8 - 6. 

Photograph 8 - 7. 

175mm Micropiles used under new abutments for bridge 

over Mahoning Creek, Armstrong County, PA 

(Pearlman and Wolosick, 1992). ........................... -3 - 6 

Underpinning of West Emerson Street Viaduct, 

Seattle, Washington .................................... -3 - 8 

Seismic Retrofit of I-l 10, North Connector, Los Angeles, 

California ........................................... .3- 11 

FH-7, Mendocino National Forest, California 

(Slope Stabilization) ................................... .3 - 17 

Low Headroom Micropile Installation ..................... .3 - 18 

Large Track-mounted Rotary Hydraulic Drill Rig ............. .4 - 5 

Small Track-mounted Rotary Hydraulic Drill Rig ............. .4 - 5 

Small Frame-mounted Rotary Hydraulic Drill Rig ............. .4 - 6 

Compression Load Test Setup ........................... -7 - 16 

TensionLoadTestSetup.. .............................. .7-17 

TypicalLoadTestJack ................................ ..7-18 

Micropile Load Test Instrumentation ...................... .7 - 20 

Micropile Load Test Instrumentation ...................... .7 - 2 1 

Micropile Load Test Instrumentation ...................... .7 - 2 1 

Cement Storage ........................................ .8 - 3 

Storage of Reinforcing Steel .............................. .8 - 4 

Double Corrosion Protected (Encapsulated) Reinforcing Steel .... 8 - 5 

DrillRigPressureGauge ............................... .8- 11 

Reinforcement Centralizers ............................. .8 - 13 

Grout Cubes for Compressive Strength Testing .............. .8 - 17 

BaroidMudBalanceTest ............................... .8- 18 

FHWA-SA-97-070 (~00-06) xxv 
[BLANK PAGE] 

xxvi FHWA-SA-97-070 (~00-06) 
CHAPTER 1 

INTRODUCTION 

l.A PURPOSE AND SCOPE OF MANUAL 

The use of micropiles has grown significantly since their conception in the 195Os, and in 

p~articular since the mid-1980s. Micropiles have been used mainly as elements for foundation 

support to resist static and seismic loading conditions, and as in-situ reinforcements for slope 

and excavation stability. Many of these applications are for transportation structures. 

In 1993, the Federal Highway Administration (FHWA) sponsored a desk study of the state-ofthe-practice of micropiles. The research group for this study consisted of contractors, 

consultants, academics, and clients. The document produced from this study, entitled Drilled 

and Grouted Micropiles - State-of-the-Practice Review (Federal Highway Administration, 

1997) provides a comprehensive international review and detailed analysis of available 

research and development results, laboratory and field testing data, design methods, 

construction methodologies, site observations, and monitored case studies. As part of this 

study, the limitations and uncertainties in the current state-of-the-practice were evaluated, and 

further research needs were assessed. One of the highlighted needs, voiced mainly by 

representatives of State Departments of Transportation, was a manual of design and 

construction guidelines intended for use by practicing, highway agency, geotechnical and 

structural engineers. 

Iu response to this need, the FHWA sponsored the development of this Micropile Design and 

Construction Guidelines, Implementation Manual. Funding and development of the manual 

was a cooperative effort between FHWA, several U.S. micropile specialty contractors, and 

several state DOT’s (See Acknowledgement Section). This manual is intended to be a 

“practitioner-oriented” document containing sufficient information on micropile design, 

construction specifications, inspection and testing procedures, cost data, and contracting 

FHWA-SA-97-070 (~00-06) l-l 
methods to facilitate and speed the implementation and cost-effective use of micropiles on 

United States transportation projects. 

Chapter 1 provides a general definition and historic framework of micropiles. Chapter 2 

describes the newly developed classifications of micropile type and application. Chapter 3 

illustrates the use of micropiles for transportation applications. Chapter 4 discusses 

construction techniques and materials. Chapter 5 details the design methodologies for 

structural foundation support and includes worked design examples. Chapter 6 was intended 

to cover slope stabilization details, but due to a lack of consensus on design procedures, 

this chapter is not included and is still under preparation. Chapter 7 describes pile load 

testing. Chapter 8 reviews construction inspection and quality control procedures. Chapter 9 

discusses contracting methods for micropile applications. Chapter 10 presents feasibility and 

cost data. Appendix A presents sample plans and specifications for owner-controlled design 

with contractor design-build of the micropiles. 

A basic introduction to micropiles can also be found in the December 1995 issue of Civil 

Engineering magazine, published by the American Society of Civil Engineers (Bruce et al., 

1995), in an article entitled “A Primer on Micropiles.” This article, authored by representatives 

of the FHWA state-of-the-practice research group, includes basic characteristics and 

definitions of micropiles, classifications of applications, and a discussion of the micropiling 

market. 

I .B MICROPILE DEFINITION AND DESCRIPTION 

Piles are divided into two general types: displacement piles and replacement piles (Fleming et 

al, 1985). Displacement piles are members that are driven or vibrated into the ground, thereby 

displacing the surrounding soil laterally during installation. Replacement piles are placed or 

constructed within a previously drilled borehole, thus replacing the excavated ground. 

A micropile is a small-diameter (typically less than 300 mm), drilled and grouted replacement 

pile that is typically reinforced. A micropile is constructed by drilling a borehole, placing 

l-2 FHWA-SA-97-070 (~00-06) 
reinforcement, and grouting the hole as illustrated in Figure 1- 1. Micropiles can withstand 

axial and/or lateral loads, and may be considered a substitute for conventional piles or as one 

component in a composite soil/pile mass, depending upon the design concept employed. 

Micropiles are installed by methods that cause minimal disturbance to adjacent structures, soil, 

and the environment. They can be installed in access- restrictive environments and in all soil 

types and ground conditions. Micropiles can be installed at any angle below the horizontal 

using the same type of equipment used for ground anchor and grouting projects. 

Since the installation procedure causes minimal vibration and noise and can be used in 

conditions of low headroom, micropiles are often used to underpin existing structures. 

Specialized drilling equipment is often required to install the micropiles from within existing 

basement facilities. 

Most of the applied load on conventional cast-in-place replacement piles is structurally resisted 

by the reinforced concrete; increased structural capacity is achieved by increased crosssectional and surface areas. Micropile structural capacities, by comparison, rely on highcapacity steel elements to resist most or all of the applied load. These steel elements have been 

reported to occupy as much as one-half of the hole volume. The special drilling and grouting 

methods used in micropile installation allow for high grout/ground bond values along the 

grout&round interface. The grout transfers the load through friction from the reinforcement to 

the ground in the micropile bond zone in a manner similar to that of ground anchors. Due to 

the small pile diameter, any end-bearing contribution in micropiles is generally neglected. The 

grout/ground bond strength achieved is influenced primarily by the ground type and grouting 

method used, i.e., pressure grouting or gravity feed. The role of the drilling method is also 

influential, although less well quantified. 

FHWA-SA-97-070 (~00-06) l-3 

1 .C HISTORICAL BACKGROUND 

Micropiles were conceived in Italy in the early 195Os, in response to the demand for innovative 

techniques for underpinning historic buildings and monuments that had sustained damage with 

time, and especially during World War II. A reliable underpinning system was required to 

support structural loads with minimal movement and for installation in access-restrictive 

environments with minimal disturbance to the existing structure. An Italian specialty 

contractor called Fondedile, for whom Dr. Fernando Lizzi was the technical director, developed 

the palo radice, or root pile, for underpinning applications. The palo rudice is a smalldiameter, drilled, cast-in-place, lightly reinforced, grouted pile. The classic arrangement ofpaZi 

radice for underpinning is shown in Figure l-2. 

Figure 1 - 2. Classical Arrangement of Root Piles for Underpinning 

FHWA-SA-97-070 (~00-06) l-5 

MICROPILE (TYP) 

VERTICAL HORIZONTAL 

CROSS- SECTION CROSS- SECTION A-A 
Although steel was in short supply in postwar Europe, labor was inexpensive, abundant, and 

often of high mechanical ability. Such conditions encouraged the development of these lightly 

reinforced, cast-in-place root pile elements, largely designed and installed by specialty 

contractors on a design-build basis. Load testing on these new root piles measured capacities 

in excess of 400 kN, although the nominal design capacity-based on contemporary 

conventional bored pile design methodologies- suggested loads of less than 100 kN. Direct 

full-scale load tests were performed at relatively little cost, fostering the acquisition and 

publication of a wealth of testing information. No grout/ground bond failures were recorded 

during these early tests. 

The use of root piles grew in Italy throughout the 1950s. Fondedile introduced the technology 

in the United Kingdom in 1962 for the underpinning of several historic structures, and by 1965, 

it was being used in Germany on underground urban transportation schemes. For proprietary 

reasons, the term “micropile” replaced “root pile” at that time. 

Initially, the majority of micropile applications were structural underpinning in urban 

environments. Starting in 1957, additional engineering demands resulted in the introduction of 

systems of reticoli dip&i radice (reticulated root piles). Such systems comprise multiple 

vertical and inclined micropiles interlocked in a three-dimensional network, creating a laterally 

confined soil/pile composite structure (Figure l-3). Reticulated micropile networks were 

applied for slope stabilization, reinforcement of quay walls, protection of buried structures, and 

other soil and structure support and ground reinforcement applications. 

Other proprietary micropiles were developed in Switzerland and Germany, and the 

technologies were quickly exported overseas by branches or licensees of the originating 

contractors. The Far East soon became a major market. 

Fondedile introduced the use of micropiles in North America in 1973 through a number of 

underpinning applications in the New York and Boston areas. The micropile technology did 

not grow rapidly in the United States, however, until the mid-1980s, after which time an 

abundance of successful published case histories, consistent influence by specialty contractors, 

l-6 FHWA-SA-97-070 (~00-06) 
TYPICAL COLUMN 

PLAN VIEW 

‘“.. 

,A 

Figure 1 - 3. Typical Network of Reticulated Micropiles 

and the growing needs of consultants and owners working in old urban environments 

overcame the skepticism and concerns of the traditional East Coast piling market (Bruce, 

1988.). The abundance of relatively cheap labor, the shortage of steel, and the need for 

reconstruction programs in urban environments had all promoted the growth and use of 

micropiles in Europe. Conversely, the slower and later growth of micropile usage in North 

America is reflective of the abundance of cheap steel, relatively high labor costs, and the need 

for capital works projects typically outside of the cities. These circumstances fostered the 

growth of the comparatively low-technology, driven-pile techniques governed by prescriptive 

specifications. Today, construction costs and technical demands are similar throughout the 

world and so continue to foster the growth of micropile demand, largely through geotechnical 

contractors with design-build capabilities. 

FHWA-SA-97-070 (~00-06) l-7 
[BLANK PAGE] 

l-8 FHWA-SA-97-070 (~00-06) 
CHAPTER 2 

MICROPILE CLASSIFICATION SYSTEM 

2.A MICROPILE TYPES IN CURRENT USE 

One of the most fundamental achievements of the research team that developed the Micropile 

State-of-the-Practice Report (Federal Highway Administration, 1997) was the new 

classification criteria for micropiles. This is important because it resolved disagreement within 

the industry about fundamental differences in design and behavior of elements that are visually 

similar and constructed with common equipment, materials and techniques. 

The State-of-the-Practice Report includes a micropile classification system based on two 

criteria: 1) philosophy of behavior (design) and 2) method of grouting (construction). The 

philosophy of behavior dictates the method employed in designing the micropile. The method 

of grouting defines the grout/ground bond capacity, which is generally the major constructional 

control over pile capacity. The classification system consists of a two-part designation: a 

number, which denotes the micropile behavior (design), and a Zetter, which designates the 

method of grouting (construction). 

2.A.l Design Application Classification 

The design of an individual or group of micropiles differs greatly from that of a network of 

closely spaced reticulated micropiles. This led to the definition of CASE 1 micropile elements, 

which are loaded directly and where the pile reinforcement resists the majority of the applied 

load (Figure 2-l). CASE 2 micropile elements circumscribes and internally reinforces the soil 

to make a reinforced soil composite that resists the applied load (Figure 2-2). This is referred 

to as a reticulated pile network. 

FHWA-SA-97-070 (~00-06) 2-l 
VERT’CALDRILLED 

MICROPILES 1 

LATERAL-+DRlLLED 

SHAFT MICROPILES SHAFT 

COMPRES 

STRATUM 

BEARING 

STRATUM 

TO SUSTAIN AXIAL LOADS EXTENT OF 

SOIL-PILE TO SUSTAIN LATERAL 

INTERACTION 

LOADS 

SOIL-PILE 

INTERACTION 

Figure 2 - 1. CASE 1 Micropiles (Directly Loaded) 

2-2 FHWA-SA-97-070 (~00-06) 
RESULTANT CENl 

GRAVITY OF SOIL-STRUCTURE 

UNIT TO IMPROVE STABILITY 

TO SUPPORT VERTICAL LOAD 

AND LATERAL LOAnn “F NFFnFn’ 

CAP BEAM 7. _“, 

I RADICE (ROOT PILE) 

RETICULATED STRUCTURE 

Figure 2 - 2. CASE 2 Micropiles - Reticulated Pile Network with Reinforced Soil Mass 

Loaded or Engaged 

-ER 

FHWA-SA-97-070 (~00-06) 2-3 
CASE 1 micropiles can be used as substitutes for more conventional types of piles to transfer 

structural loads to a deeper, more competent or stable stratum. Such directly loaded piles, 

whetherfor axial or lateral loading conditions, are referred to as CASE 1 elements. The load 

is primarily resisted structurally by the steel reinforcement and geotechnically by the 

grout/ground bond zone of the individual piles. At least 90 percent of all international 

applications to date, and virtually all of the projects in North America, have involved CASE 1 

micropiles. Such piles are designed to act individually, although, they may be installed in 

groups. Typical arrangements of CASE 1 micropiles are illustrated in Figure 2-3. 

NEW BRIDGE STRUCTURE 

BRIDGE ABUTMENT 

NEW MICROPILE 

FOUNDATIONS 

SUPPORT (TYP) 

FOUNDATION SUPPORT 

REINFORCED 

CONCRETE CA 

INISHED G 

APPROXIMATE 

FAILURE PLANE 

CASE 1 NON RETICULATED 

MICROPILE STRUCTURE (TYP) 

SLOPE STABILIZATION 

Figure 2 - 3. CASE 1 Micropile Arrangements 

2-4 FHWA-SA-97-070 (~00-06) 
The remaining applications involve networks of reticulated micropiles as components of a 

reinforced soil mass, which is used for stabilization and support. These micropiles are referred 

to as CASE 2 elements. The structural loads are applied to the entire reinforced soil mass, as 

opposed to individual piles. CASE 2 micropiles are lightly reinforced because they are not 

individually loaded as CASE 1 elements. They serve to circumscribe and then to internally 

strengthen the reinforced soil composite. A typical network of reticulated micropiles is 

illustrated in Figure 2-4. 

SURCHARGE 

ORIGINAL GRADE C(pPDCTE 

m------m--wE 

WALL FACINGFINAL GRADE \ 

EARTH RETENTION 

Figure 2 - 4. CASE 2 Micropile Arrangements 

FHWA-SA-97-070(vOO-06) 2-5 
There are combination design philosophies between CASE 1 and CASE 2 micropiles. An 

example is the case of a row of micropiles installed throughout a failure plane to achieve slope 

stabilization. Recent research (Pearlman, et al, 1992) suggests that pile/ground interaction only 

occurs near the slide plane. In this situation, the pile acts as a CASE 1 element because it 

directly resists the load. Above the failure plane, the pile group does add a certain degree of 

continuity to the reinforced soil composite structure. This behavior is of CASE 2 type. 

Therefore, this example is between CASE 1 and CASE 2. 

This philosophy of behavior (design) of an individual CASE 1 micropile is the same as that of 

a group of CASE 1 micropiles. A group of CASE 1 elements is defined as a closely spaced 

(typically parallel) arrangement of micropiles, each of which will be loaded directly. The 

behavior and design approach of a group of CASE 1 elements should not be confused with 

those of a reticulated network, although their geometries may appear to be similar. Group 

effects on micropile capacities are discussed in Chapter 5. Design methodologies for 

individual CASE 1 elements and groups of CASE 1 elements are discussed in Chapters 5 

and 6. Design methodologies for networks of CASE 2 elements are beyond the scope of 

this manual. 

2.A.2 Construction Type Classification 

The method of grouting is generally the most sensitive construction control over grout/ground 

bond capacity. Grout/ground bond capacity varies directly with the grouting method. The 

second part of the micropile classification consists of a letter designation (A through D) based 

primarily on the method of placement and pressure under which grouting is used during 

construction. The use of drill casing and reinforcement define sub-classifications. The 

classification is shown schematically in Figure 2-5. 

2-6 FHWA-SA-97-070 (~00-06) 
Type A: The type A classification indicates that grout is placed under gravity 

head only. Sand-cement mortars, as well as neat cement grouts, can be used 

because the grout column is not pressurized. The pile hole may be underreamed to 

increase tensile capacity, although this technique is not common or used with any 

other pile type. 

NPE A 

(GRAVITY) 

NPE B NPE C TYPED 

(PRESSLJ~~~;~OlJGH (SINGLE GLOBAL (MULTIPLE REPEATABLE 

POSTGROUT) POSTGROUT) 

PRESSURE GAGE 

Figure 2 - 5. Micropile Classification Based on Type of Grouting.(Refer to 

Table 2-1 for details) 

FHWA-SA-97-070 (~00-06) 2-7 
Type B: Type B indicates that neat cement grout is placed into the hole under 

pressure as the temporary steel drill casing is withdrawn. Injection pressures 

typically range from 0.5 to 1 MPa, and are limited to avoid hydrofiacturing the 

surrounding ground or causing excessive grout takes, and to maintain a seal 

around the casing during its withdrawal, where possible. 

Type C: Type C indicates a two-step process of grouting: 1) neat cement grout is 

placed under gravity head as with Type A, and 2) prior to hardening of the 

primary grout (after approximately 15 to 25 minutes), similar grout is injected one 

time via a sleeved grout pipe without the use of a packer (at the bond zone 

interface) at a pressure of at least 1 MPa. This pile type appears to be used only in 

France, and is referred to as IGU (Injection Globale et Unitaire). 

Type D: Type D indicates a two-step process of grouting similar to Type C with 

modifications to Step 2. Neat cement grout is placed under gravity head as with 

Types A and C and may be pressurized as in Type B. After hardening of the 

initially placed grout, additional grout is injected via a sleeved grout pipe at a 

pressure of 2 to 8 MPa. A packer may be used inside the sleeved pipe so that 

specific horizons can be treated several times, if required. This pile type is used 

commonly worldwide, and is referred to in France as the IRS (Injection RepCtitive 

et Selective). 

Table 2- 1 describes in more detail the micropile classification based on method of grouting 

(construction). Subclassifications (Numbers 1,2, and 3) are included in the table to describe 

the use of drill casing and reinforcement for each method of grouting. These subclassifications also represent the type of reinforcement required by design (e.g. rebar, casing, 

none). It is emphasized that Table 2-l is intended to present a classification system based on 

the type of micropile construction. It is not intended to be used in contract specifications. 

2-8 FHWA-SA-97-070 (~00-06) 
Table Z-l. Details of Micropile Classification Based on Type of Grouting 

Micropile Type 

and Gl;otiting 

Method 

Subtype 

Al 

Reinforcement 

None, monobar, cage, tube 

or structural section 

Drill Casing Grout 

Sand/cement mortar or 

neat cement grout, 

tremied to base of hole 

(or casing), no excess 

pressure applied 

Temporary or 

unlined (open 

hole or auger) 

Permanent, full 

length 

Type A Drill casing itself 

Gravity grout only 

A2 

Drill casing in upper shaft, 

bar(s) or tube in lower shaft 

(may extend full length) 

Permanent, upper 

A3 shaft only 

Type B 

Pressure - grouted 

through the casing 

or auger during 

withdrawal 

Temporary or 

unlined (open 

hole or auger) 

Neat cement grout is first 

tremied into drill 

casing/auger. Excess 

pressure (up to 1 MPa 

typically) is applied to 

additional grout injected 

during withdrawal of 

casing/auger 

Monobar or tube (cages 

rare due to lower structural 

capacity) 

Drill casing itself 

Bl 

Permanent, 

partial length B2 

B3 

Permanent, upper 

shaft 

Drill casing in upper shaft, 

bar(s) or tube in lower shaft 

(may extend full length) 

Monobar or tube (cages 

rare due to lower structural 

capacity) 

Neat cement grout is first 

tremied into hole (or 

casing/auger). Between 

15 to 25 minutes later, 

similar grout injected 

through tube (or 

reinforcing pipe) from 

head, once pressure is 

greater than 1 MPa 

Temporary or 

unlined (open 

hole or auger) 

Type C 

Primary grout 

placed under gravity 

head, then one 

phase of secondary 

“global” pressure 

grouting 

Cl 

Not - conducted 

c2 

Not conducted 

c3 

Neat cement grout is first 

tremied (Type A) and/or 

pressurized (Type B) into 

hole or casing/auger. 

Some hours later, similar 

grout injected through 

sleeved pipe (or sleeved 

reinforcement) via 

packers, as many times as 

necessary to achieve bond 

Temporary or 

unlined (open 

hole or auger) 

Monobar or tube (cages 

rare due to lower structural 

capacity) 

Dl Type D 

Primary grout 

placed under gravity 

head (Type A) or 

under pressure 

(Type B). Then one 

or more phases of 

secondary “global” 

pressure grouting 

Possible only if Drill casing itself 

regrout tube 

placed fulllength outside 

casing 

Permanent, upper 

shaft only 

D2 

Drill casing in upper shaft, 

bar(s) or tube in lower shaft 

(may extend full length) 

D3 

Source: after Pearlman and Wolosick (1992) 

FHWA-SA-97-070 (VOO-06) 2-9 
[BLANK PAGE] 

2- 10 FHWA-SA-97-070 (~00-06) 
CHAPTER 3 

MICROPILE APPLICATIONS IN TRANSPORTATION 

PROJECTS 

3.A INTRODUCTION 

Micropiles are currently used in two general applications: for structural support and less 

frequently as in-situ reinforcement (Figure 3-l). Structural support includes new foundations, 

underpinning of existing foundations, seismic retrofitting applications and earth retention. Insitu reinforcement is used for slope stabilization, earth retention, and ground strengthening and 

protection; settlement reduction; and structural stability. Table 3-l summarizes the typical 

design behavior and micropile construction type for each application. 

For structural support, micropiles can be used as small-diameter substitutes for conventional 

pile types. Micropiles used for structural support are usually loaded directly and, therefore, 

employ a CASE 1 design philosophy. Piles typically used for these applications include Type 

A (gravity grouted and bonded in soil or rock), Type B (pressure grouted), and Type D 

(postgrouted). These pile types can provide the high individual capacities typically required by 

structural support applications in transportation projects. 

It is important to note that the in-situ reinforcement applications of slope stabilization and earth 

retention can employ either CASE 1 or CASE 2 design philosophies. Micropiles used for these 

applications are typically Type A piles (gravity grouted and fully bonded in soil or rock), 

because high individual pile capacities are not required due to the reinforced composite 

material concept of the CASE 2 approach. Recent research (Pearlman et al., 1992) suggests, 

however, that in certain conditions and for certain pile arrangements, the piles are principally, 

directly, and locally subjected to bending and shearing forces, specifically near the slide plane. 

This direct loading, by definition, is CASE 1 design behavior. Micropiles under these 

conditions are typically heavily reinforced and of Type A or B construction. 

FHWA-Sk-97-070 (~00-06) 3-l 
teview of Applications 

Structural 

Stability 

Underpinning 

of 

Existing 

Foundations 

lSeismic 

Retrofitting 

Scour 

Protection 

Repair/ Arresting/ 

Replacement Prevention 

of Existing of 

( Foundatiok 1 Movement ( 

Figure 3 - 1. Classification of Micropile Applications 

Upgzfding 

Foundation 

Capacity 

3-2 FHWA-SA-97-070 (~00-06) 
Table 3-l. Relationship Between Micropile Application, Design Behavior and 

Construction Type 

Application 

Design 

Behavior 

Construction 

-We 

Estimate of 

Relative 

Application 

FTRUCTURAL 

SUPPORT 

Underpinning of 

Existing Foundations 

New Foundations 

Seismic Retrotittinrr 

CASE 1 

Type A (bond zones 

in rock or stiff clays) 

Type B, C, and D in 

soil 

Probably 95 percent 

of total world 

applications 

IN-SITU EARTH REINFORCEMENT 

Slope 

Stabilization 

and Earth 

Retention 

Ground Settlement 

Strengthening Reduction 

Structural 

Stability 

CASE 1 

and CASE 2 

with 

transitions 

Type A and 

TypeBin 

soil 

CASE 2 with CASE 2 

minor CASE 1 

Types A and B Type A 

in soil in soil 

CASE 2 

Type A 

in soil 

0 to 5 percent 

Source: Federal Highway Administration (1997) 

Other insitu reinforcement applications generally employ CASE 2 concepts. Little commercial 

work has been performed for other CASE 2 major applications beyond the stabilization of high 

towers in historical monuments. An example is the restoration scheme used to improve the 

stability of a tall, slender tower in Mosul, Iraq (Lizzi, 1982), as shown in Figure 3-2. This 

CASE 2 network of reinforced soil is attached to the structure, effectively lowering the center 

of gravity of the combined structure/soil system and improving stability. The potential for 

other in-situ reinforcement applications is being studied and pursued in other countries, 

especially France, Italy, Germany, Austria, and Japan. 

FHWA-SA-97-070 (~00-06) 3-3 
A) ARRANGEMEW ~F~R~OT PILES 

CENTER OF 

GRAVITY 

OF TOWER 

I 

I 

CENTER OF 

GRAVITY 

OF RESULTANT 

g CENTER OF GRAVI 

OF COMPOSITE 

BLOCK 

_/ 

B) GRAVITY BLOCK CONCEPT 

Figure 3 - 2. Restoration Scheme for the Leaning Al Hadba Minaret (Lizzi, 1982) 

3.8 STRUCTURAL SUPPORT 

Micropile applications for structural support include foundations for new structures, 

underpinning of existing structures, scour protection, and seismic retrofitting of existing 

structures. Many of these applications have been used for transportation projects. 

TY 

3-4 FHWA-SA-97-070 (~00-06) 
3.B.l New Foundations 

Micropiles are applicable in new bridge construction in areas that require deep foundation 

alternatives or in difficult ground (cobbles/boulders obstructions) where installation of 

conventional piles or drilled shafts is very difficult/expensive. 

The new I-78 dual highway, designed to cross the Delaware River between Pennsylvania and 

New Jersey (Bruce, 1988), is an example. All of the bridge piers were founded either on driven 

piles or spread footings on rock, with the exception of Pier E-6. At this location, bedrock was 

encountered below the anticipated depth and was found to be extremely variable. Micropiles 

and drilled shafts were proposed as alternative foundations to solve this geological problem, 

and micropiles were selected based on cost, installation time, and test pile performance. 

The replacement of a two-span bridge over the Mahoning Creek in Armstrong County, PA (see 

Photograph 3-l) provides another example. The original stone abutment foundations were 

constructed in cofferdams and founded on erodible soils overlying competent sandstone. 

Micropiles were used to support the new abutments as they could be conveniently drilled 

through the existing stone footings and founded in the underlying sandstone (Pearlman et al., 

1992). 

New bridges may be constructed in areas of existing overhead restrictions, and with traffic 

flow that must be maintained. A major improvement project was undertaken to replace the 

deck of the Brooklyn-Queens Expressway in the Borough of Brooklyn, New York (Bruce and 

Gemme, 1992.). A new center lane and several new entry/exit ramps were also added. Smalldiameter piles were specified and used successfully for the new viaduct and the ramps. Major 

factors in the selection of micropiles were the relative lack of vibration during installation by 

comparison to pile-driving methods that could have affected adjacent old and sensitive 

structures; the variable fluvioglacial deposits; the restricted access; and the need to maintain 

traffic flow in the area. 

FHWA-SA-97-070 (~00-06) 3-5 
Photograph 3 - 1. 175mm Micropiles used under new abutments for bridge over 

Mahoning Creek, Armstrong County, PA (Pearhnan and Wolosick, 1992). 

Other micropile applications used for structural support include buildings, earth-retaining 

structures, and soundwalls. Figure 3-3 shows typical arrangements of micropiles for support of 

common transportation-related structures. 

3.6.2 Underpinning of Existing Foundations 

Micropiles were originally developed for underpinning existing structures (Section 1.C). The 

underpinning of existing structures may be performed for many purposes: 

l To arrest and prevent structural movement. 

l To upgrade load-bearing capacity of existing structures. 

l To repair/replace deteriorating or inadequate foundations. 

l To add scour protection for erosion-sensitive foundations. 

l To raise settled foundations to their original elevation. 

l To transfer loads to a deeper strata. 

3-6 FHWA-SA-97-070 (~00-06) 
NEW BRIDGE STRUCTURE CPLUMN (TYP) 

BRIDGE 

ABUTMENT 

MICROPILE 

FOUNDATION 

SUPPORT (TYP) , 

so~i GROUND TUNNEL 

A) BRIDGE FOUNDATION SUPPORT 6) MICROPILE FOUNDATION SUPPORT 

FOR SOFT GROUND TUNNELING 

BENEATH EXISTING STRUCTURES 

Cl 

SOUNDWALL 

FOUNDATION SUPPORT FOR CAST D) FOUNDATION SUPPORT FOR 

IN PLACE REINFORCED CONCRETE HIGHWAY SOUNDWALLS 

RETAINING WALS 

Figure 3 - 3. Micropiles for Foundation Support of Transportation Applications 

Micropiles can be installed through, and bonded within, existing structures, providing direct 

connection with a competent underlying strata without the need for new pile caps, while at the 

same time reinforcing the structure internally. 

Construction can be executed without reducing the existing foundation capacity. Photograph 

3-2 is of the West Emerson Street Viaduct in Seattle, Washington, where micropiles were 

added to five existing bents to provide additional foundation support. 

FHWA-SA-97-070 (vQO-06) 3-7 
Photograph 3 - 2. Underpinning of West Emerson Street Viaduct, 

Seattle, Washington 

Structural movements can be caused by a variety of factors, including compressible ground 

beneath the existing foundation, dewatering activities, groundwater elevation fluctuations, 

deterioration of existing foundations, and adjacent deep excavations and tunneling activities. 

Micropiles can mitigate this structural movement by being installed to deeper, more competent 

bearing strata, thus providing improved structural support. 

Increased load-bearing capacity of an existing foundation may be required for several reasons. 

Additional vertical, lateral, or vibratory loads may be applied to the foundation due to 

expansion of the existing structure, increased magnitude of applied loads, or the addition of 

vibrating machinery. 

3-8 FHWA-SA-97-070 (~00-06) 
4 

30.4 m Lk 32 m LA 30.4 m 

rT l 

WEST ABUTMENT 

(DRIVEN TIMBER PILES) 

- MICROPILE (lYP) 2 

BASCULE BASCULE 

PIER 3 PIER 4 

EAST ABUTMENT 

(DRIVEN TIMBER PILES) 

Figure 3 - 4. Underpinning Arrangement for Pocomoke River Bridge, Maryland 

(Bruce et al., 1990) 

The 75-year-old Pocomoke River Bridge in Maryland was rehabilitated when the capacity of 

the original wooden piles of the pier foundations was compromised by exposure to river scour 

(Bruce et al., 1990). The underpinning arrangement for this bridge is shown in Figure 3-4. 

These micropiles were installed throughout the existing foundation and were preloaded to 

provide support without allowing additional settlement of this sensitive structure. 

3.6.3 Seismic Retrofit 

Micropiles are being used increasingly for seismic retrofitting of existing highway structures, 

especially in California. Micropiles may be economically feasible for bridge foundation 

retrofits having one or more of the following constraints: 

l Restrictions on footing enlargements. 

l Vibration and noise restrictions. 

l Low headroom clearances. 

FHWA-SA-97-070 (~00-06) 3-9 
l Difficult access. 

l High axial load demands in both tension and compression. 

l Difficult drilling/driving conditions. 

l Hazardous soil sites. 

Micropiles exhibit near equal tension and compression capacities, therefore optimizing the 

additional foundation support elements used (Bruce and Chu, 1995). 

Micropiles were used for the California Department of Transportation’s earthquake retrofit of 

the North Connector over-crossing at I-l 10 in Los Angeles, (Photograph 3-3) where the use of 

the previously specified drilled shafts proved unsuccessful (Pearlman et al., 1993). Difficult 

drilling conditions, including buried concrete obstructions and water-bearing, flowing sand 

layers, low overhead conditions, and limited right-of-way access prohibited the use of the 

originally prescribed drilled shaft system. 

Micropiles have been used for earthquake retrofit of major bridges in the San Francisco Bay 

area, New York City and Southern Illinois: 

l Benicia-Martinez Bridge, CA. 

l 2415801980 Interchanges, Oakland, CA. 

l Williamsburg Bridge, NewYork City, NY. 

l Route 57, Illinois. 

3.C IN-SITU REINFORCEMENT (SLOPE STABILIZATION & EARTH RETENTION) 

The concept of reticulated networks of micropiles (CASE 2) involves the use of an 

appropriately spaced, three-dimensional arrangement of vertical and inclined piles that 

encompass and reinforce the ground, and at the same time are supported by the ground. For 

slope stabilization, Lizzi (1982) suggested that the reticulated network of micropiles creates a 

stable, reinforced-soil, “gravity-retaining wall”, in which the reinforced soil gravity mass 

. 

supplies the essential resisting force, and the piles, encompassed by the soil, supply additional 

3 - 10 FHWA-SA-97-070 (~00-06) 
Photograph 3 - 3. Seismic Retrofit of I-l 10, North 

Connector, Los Angeles, California 

resistance to the tensile and shear forces acting on the “wall”. For such applications, the 

individual piles are engaged as friction piles securing the reinforced soil composite mass in the 

upper soil mass, and as structural elements subject to shear and bending in the lower 

competent material. The function of this structure is to provide a stable block of reinforced soil 

to act as a coherent retaining structure, stabilizing the upper soil mass, while providing 

resistance to shear across the failure plane. Such an application is, therefore, transitional 

between CASE 1 and CASE 2 behavior. 

FHWA-SA-97-070 (~00-06) 3-11 
(a) ROADWAY SLOPE (b) SLOPE ABOVE HIGHWAY 

125mm - 230mm 125mm - 230mm 

STEEL PIPE (100 mm 

TO 175 mm O.D. TYP) 

(c) PILE SECTIONS 

Figure 3 - 5. Typical Configurations for Inclined Micropile Walls 

Conversely, the research by Pearlman et al. (1992) and Palmer-ton (1984) suggests that groups 

of inclined micropiles serve to connect the moving zone (above the failure surface) to the stable 

zone (below the failure surface). These piles provide reinforcement to resist the shearing forces 

that develop along the failure surface and exhibit purely CASE 1 behavior. Typical 

configurations of inclined nonreticulated micropile walls for slope stabilization and earth 

retention are shown in Figure 3-5. 

For rocky, stiff, or dense materials, the shear resistance of the piles across the failure surface, 

i.e., individual capacity, is critical (CASE 1). For loose materials, the piles and soil are 

mutually reinforcing and create a gravity wall, so the individual pile capacities are not as 

significant (CASE 2). 

3 - 12 FHWA-SA-97-070 (~00-06) 
TOP OF 

ROC ‘K 

VARIES 150mm TO 1200mm 

\ 

Figure 3 - 6. State Road 4023 Micropile Slope Stabilization, 

Armstrong County, Pennsylvania 

For example, micropiles were used to stabilize a portion of State Road 4023 (S.R. 4023) in 

Armstrong County, Pennsylvania, as shown in Figure 3-6 (Bruce, 1988a). A 7.5m-long section 

of this road and railroad tracks located up-slope were experiencing damage from slope 

movements towards an adjacent river. Rock anchors and tangent drilled shafts extending into 

rock were the proposed remedial techniques. An alternative bid specification allowed the use 

of inclined CASE 1 nonreticulated micropiles was proposed and accepted, with resultant 

savings of approximately $1 million compared to the lowest bid for the anchored caisson wall 

design. The wall included four rows of Type 1A micropiles extending across the failure plane 

and into competent rock. 

FHWA-SA-97-070 (~00-06) 3- 13 
GEOCOMPOSITE DRAIN 

- EXISTING GRADE 

GRADE BEAM 

\ 

305mm CIP PERMANENT 

WALL FACING 

IOOmm MIN TEMP. 

SHOTCRETE FAflNG 

75mm DIA 

- WATER MAINS 

ORIGINAL COUNTERFORT 

WALL ENVELOPE 

Figure 3 - 7. Wall 600 Permanent Earth Retention, Portland, Oregon 

Micropiles were recently used to provide permanent earth retention along a new section of the 

Portland Westside Lightrail Project in Portland, Oregon, as shown in Figure 3-7 (Ueblacker, 

1996). Wall 600 extends from the east portal of the Westside Lightrail cut and cover, tunnel 

approximately 183 meters to just beneath the Vista Avenue Bridge. Cut heights along the 

retaining wall range from 4 to 9.5 meters. The wall includes CASE 1 nonreticulated micropiles 

installed at varying vertical and subvertical angles that were temporally exposed by excavation 

of the new rail alignment. An architecturally treated cast-in-place reinforced concrete facing is 

structurally attached to the vertical micropiles, forming the permanent wall face. The micropile 

wall was accepted as a contractor-proposed value engineering alternate. The original design 

‘included a counterfort concrete retaining wall supported on a driven-pile foundation. 

3 - 14 FHWA-SA-97-070 (~00-06) 
Construction of the owner-designed wall required temporary excavation support to maintain 

service to Jefferson Street and existing utilities. A micropile retaining wall system was the 

only acceptable option that could be kept within the envelope of the original counter-fort 

structure. 

An earlier example of slope stabilization using micropiles as a demonstration project involved 

networks of reticulated micropiles (CASE 2) used to stabilize Forest Highway 7 in Mendocino 

National Forest, California, as shown in Figure 3-8 and Photograph 3-4 (Palmerton, 1984). 

This two-lane road was constructed across a landslide where slide movement was experienced 

due to excessive rainfall. A 94-m-long section of the road was stabilized using CASE 2 

micropiles to reinforce the soil mass and provide additional shear capacity. It is significant that 

the density of micropiles per lineal meter of wall was significantly higher than that of the 

CASE 1 design approach used at S.R. 4023. S.R.4023 used 2.9 to 4.1 piles per lineal meter 

and FH-7 used 7.4 piles per lineal meter. Both structures have performed acceptably 

(Pearlman, et al., 1992). 

3.D FACTORS INFLUENCING THE CHOICE OF MICROPILES 

Several factors influence the choice of micropiles for structure foundations and slope 

stabilization including: 

l Physical Considerations. 

Restricted access, remote areas. 

Close pile proximity to existing structures. 

l Subsurface Conditions. 

Difficult geologic conditions. 

Susceptibility of ground to liquefaction during pile driving. 

l Environmental Conditions. 

Vibration/noise sensitive areas. 

Hazardous or contaminated soils. 

l Existing Structure Adaptation. 

FHWA-SA-97-070 (~00-06) 3-15 
l Micropile Limitations. 

l Economics. 

Further discussion of these factors and how they influence the designer’s choice is addressed 

below. 

DOWNHILL 

4 

PILE CAP 

1 C 1 VERT. 1 

1 G,G’ 1 10:1.4 1 

SAGE DEPTH 1.22m 

127mm - DIAMETER 

REINFORCED PILE 

I \I I\\ ASSUMED 

1 o.37mc ) 

e4 

Ftk{FE 

G’F’ E’ D’ B’A’ 

Figure 3 - 8. CASE 2 Slope Stabilization, FH-7, Mendocino National Forest, 

California 

3- 16 FHWA-SA-97-070 (~00-06) 
Photograph 3 - 4. FH-7, Mendocino National Forest, California (Slope 

Stabilization) 

3.D.l Physical Considerations 

The drilling and grouting equipment used for micropile installation is relatively small and can 

be mobilized in restrictive areas that would prohibit the entry of conventional pile-installation 

equipment. Photograph 3-5 shows micropiles being installed in low headroom conditions, 

illustrating the maneuverability of the equipment. 

Micropiles can be installed within a few millimeters of existing walls or foundations, provided 

that there is space above for the drill-head and safe work zone or the piles are battered to 

provide this space. Their installation is not as affected by overhead power lines or other 

obstructions as are conventional pile-installation systems. The equipment can be mobilized 

upsteep slopes and in remote locations. Also, drilling and grouting procedures associated with 

micropile installations do not cause damage to adjacent existing structures or affect adjacent 

ground conditions when proper drilling and grouting procedures are utilized. 

FHWA-SA-97-070 (~00-06) 3-17 
Photograph 3 - 5. Low Headroom Micropile Installation 

3.D.2 Subsurface Conditions 

Micropiles can be installed in areas of particularly difficult, variable, or unpredictable geologic 

conditions such as ground with cobbles and boulders, tills with buried utilities and 

miscellaneous debris, and irregular lenses of competent and weak materials. Soft clays, 

running sands, and high ground water not conducive to conventional drilled shaft systems 

cause minimal impacts to micropile installations. Micropiles have been applied throughout the 

world in karstic limestone formations. 

The construction of the new I-78 bridge over the Delaware River described in Section 3 .B. 1 is 

a good example of the use of micropiles for conditions where difficult geology prevents the use 

of more conventional systems. 

3-18 FHWA-SA-97-070 (~00-06) 
3.D.3 Environmental Conditions 

Micropiles can be installed in hazardous and contaminated soils. Their small diameter results 

in less spoil than caused by conventional replacement piles, and the flush effluent can be 

controlled easily at the ground surface through containerization or the use of lined surface pits. 

These factors greatly reduce the potential for surface contamination and handling costs. 

Grout mixes can be designed to withstand chemically aggressive ground water and soils. 

Special admixtures can be included in the grout mix design to reduce and avoid deterioration 

from acidic and corrosive environments. For example, a micropile “screen” was constructed 

from the installation of overlapping (secant) piles adjacent to an existing concrete diaphragm 

wall of an underground parking garage in Barcelona, Spain (Bachy, 1992). The existing wall 

was physically deteriorating due to extremely aggressive ground water (chlorides, sulfates, and 

pH values as low as 1.7) originating from an adjacent metallurgical plant (Figure 3-9). No 

trace of acid was detected in samples of the diaphragm wall collected after construction of the 

micropile screen. 

Micropiles can be installed in environmentally sensitive areas, including areas with fragile 

natural settings. The installation equipment is not as large or as heavy as conventional pile 

driving or shaft drilling equipment and can be used in swampy areas or other areas of wet or 

soft surface soils with minimal impacts to the environment. Portable drilling equipment is 

frequently used in areas of restricted access. 

Micropile installations cause less noise (no more than typical ambient noise levels) and 

vibration than conventional piling techniques, especially driven piles. The vibration from pile 

driving is imparted to the soil and can be transferred through the soil to adjacent structures. The 

use of micropiles in old urban environments and industrial/manufacturing areas, can prevent 

this potential damage to adjacent sensitive structures and equipment. 

Micropiles can be installed in areas where there is a contaminated aquifer overlying a bearing 

strata. Unlike driven piles that may produce a vertical conduit for contaminates transfer, 

micropiles can be installed in a manner preventing contamination of the lower aquifers. 

FHWA-SA-97-070 (~00-06) 3-19 
I 2m 

CAR PARK 

f- DIAPHRAGM WALL 

Figure 3 - 9. Protection of an Existing Diaphragm Wall with a Secant 

Micropile Screen using Anti-acid mortar (Bachy, 1992.) 

3.D.4 Existing Structure Adaptation 

Micropiles can be added into an existing pile cap on existing piling, thereby eliminating the 

need for increased footing dimension. This provides the additional compression, tension and 

moment resistance associated with increased structural loads. Often times adjacent utilities 

and/or structures restrict the possibility of enlarging the existing pile caps, thus eliminating 

more traditional piling systems. 

3-20 FHWA-SA-97-070 (~00-06) 
3.D.5 Micropile Limitations 

Under certain circumstances, vertical micropiles may be limited in lateral capacity and cost 

effectiveness. Traditionally, axial capacity was also deemed a limitation due to the micropile’s 

relatively small diameter. However, micropiles have now been tested to well beyond 4,500 kN 

axial capacity in dense sand, as at Vandenberg Air Force Base in California (Federal Highway 

Administration, 1997), and so one may hope to expect advances in lateral capacity with 

additional research and pile testing. The ability of micropiles to be installed on an incline 

provides designers an option for achieving the required lateral capacity in such applications. 

Because of their high slenderness ratio (length/diameter), micropiles may not be acceptable for 

conventional seismic retrofitting applications in areas where liquefaction may occur, given the 

current standards and assumptions on support required for long slender elements. However, 

the ground improvement which can be induced by micropiles may ultimately yield an 

improved earthquake mitigation foundation system. 

The lineal cost of micropiles usually exceeds that of conventional piling systems, especially 

driven piles. However, under certain combinations of circumstances, micropiles will be the 

cost- effective option, and occasionally will be the only feasible constructible option. 

Use of micropiles for slope stabilization has been applied to limited heights and is based on 

very limited experience to date. Due to the limited number of project applications, it is 

suggested that stabilization applications be instrumented and monitored for performance. On 

Federal Highway projects, it is suggested that initial projects be designated “Experimental 

Features”. This designation allows construction funds to be used to pay for performance 

monitoring and evaluation. 

FHWA-SA-97-070 (~00-06) 3 -21 
3.D.6 Economics of Micropiles 

Costs associated with the use of micropiles are discussed in Chapter 10. Cost effectiveness of 

micropiles is dependent upon many factors. It is important to assess the cost of using 

micropiles in light of the physical, environmental, and subsurface factors described above. For 

example, for an open site with soft, clean, uniform soils and unrestricted access, micropiles 

may not be a competitive solution. However, for the delicate underpinning of an existing 

bridge pier in a heavily trafficked old industrial or residential area, micropiles can provide the 

most cost-effective solution. 

Care should be taken to clearly define the true final cost of a solution based on micropiles. 

Cost analysis should be based on all related costs for the entire project and not just the item 

cost of the piling system. These costs may include: 

. 

. 

. 

. 

. 

. 

. 

. 

. 

. 

. 

Right-of-way acquisition. 

Right-of-way agreements. 

Utility realignment. 

Excavation, shoring and backfill requirements. 

Footing construction. 

Hazardous material handling. 

Dewatering. 

Erosion control. 

Access restrictions. 

Ground improvement. 

Owner and Neighbor disruption. 

Refer to Chapter 10 for further clarification on the cost of micropile systems. 

3-22 FHWA-SA-97-070(vOO-06) 
CHAPTER 4 

CONSTRUCTION TECHNIQUES AND MATERIALS 

4.A INTRODUCTION 

The intent of this chapter is to familiarize designers and construction personnel with the 

different techniques and materials utilized in the construction of micropiles. 

The construction of a micropile involves a succession of processes, the most significant of 

which are drilling, placing the reinforcement, and grouting. There are a large number of 

drilling systems available for both overburden and rock, and many are used for micropile 

construction. This narrow range of drilling systems tends to be utilized worldwide as a result of 

the comprehensive international marketing and sales efforts of the drill rig equipment 

manufacturers and the ongoing exchange of data and experiences in trade and professional 

organizations and their related journals. 

The placement of reinforcement is a fairly standard process, although different countries use 

different grades, sizes, and configurations. In the United States is it common practice to leave 

the drill casing in place from the surface down to the top of the bond length of the pile. 

It is in the process of grouting that the most extreme range of practices and preferences is 

evident. This consideration fosters the use of the grouting method as the basis of the micopile 

type classification, presented in Chapter 2. 

Included in this chapter is an examination of the following: 

1. The types of drill rigs used for micropile drilling; 

2. The various techniques used for overburden and open hole drilling; 

3. Types of grout with methods of mixing and placing; and 

4. Types of pile reinforcement. 

FHWA-SA-97-070 (~00-06) 4-l 
The typical construction sequence for simple Type A and B micropiles (Figure 4-l) includes 

drilling the pile shaft to the required tip elevation, placing the steel reinforcement, placing the 

initial grout by tremie, and placing additional grout under pressure as applicable. In general, 

the drilling and grouting equipment and techniques used for the micropile construction are 

similar to those used for the installation of soil nails, ground anchors, and grout holes. 

4.B DRILLING 

Most of the drilling methods selected by the specialty contractor are likely to be acceptable on a 

particular project, provided they can form a stable hole of the required dimensions and within 

the stated tolerances, and without detriment to their surroundings. It is important not to 

exclude a particular drilling method because it does not suit a predetermined concept of how 

the project should be executed. It is equally important that the drilling contractor be 

knowledgeable of the project ground conditions, and the effects of the drilling method chosen. 

Drilling within a congested urban site in close proximity of older buildings or deteriorating 

foundations has very different constraints than drilling for new foundations on an open field 

site. 

The act of drilling and forming the pile hole may disturb the surrounding ground for a certain 

time and over a certain distance. The drilling method selected by the contractor should avoid 

causing an unacceptable level of disturbance to the site and its facilities, while providing for 

installation of a pile that supports the required capacities in the most cost-effective manner. 

Vigorous water flushing can increase drilling rates and increase the removal of the fine 

components of mixed soils, enlarging the effective diameter in the bond zone and aiding in 

grout penetration and pile capacity. Conversely, the use of higher flush flow rates and 

pressures should be approached with caution, with consideration to the risks of creating voids 

and surface settlement, and the risks of hydrofracturing the ground, leading to heaving. 

4-2 FHWA-SA-97-070 (~00-06) 
ADDITIONAL GROUT 

BEGIN DRILLING COMPLETE REMOVE INNER PLACE REMOVE COMPLETE PILE 

&/OR INSTALLATION DRILLING TO DRILL BIT & REINFORCEMENT & TEMPORARY (CASING MAY BE 

OF TEMPORARY DEPTH ROD (IF USED) GROUT (BY TREMIE) CASING, INJECT LEFT IN PLACE 

CASING FURTHER GROUT THROUGH THE 

UNDER COMPRESSIBLE 

PRESSURE AS STRATUM) 

APPLICABLE 

Figure 4 - 1. Typical Micropile Construction Sequence Using Casing 
It is typically in the best interest of pile quality that the drilling, installation of the 

reinforcement, and grouting of a particular pile be completed in a series of continuous 

processes and executed as expeditiously as possible. Longer duration between completion of 

drilling and placing of the reinforcement and grout can be detrimental to the integrity of the 

surrounding soil. Some materials, such as overconsolidated clays and clay shales, can 

deteriorate, relax, or soften on exposure, resulting in a loss of interfacial bond capacity. In 

these cases, installation of a pile bond zone should be completed within one day to avoid a pile 

hole remaining open overnight. 

Other site-specific conditions may affect selection of the drilling method and flush type. The 

use of a water flush can require the supply, handling, and disposal of large quantities of water. 

In areas where the water supply may be scarce, the setup of a series of ponds or tanks for 

settlement and recirculation of the water may be necessary. Requirements for cleanliness or 

lack of space for water handling and disposal may dictate the use of air flush or augers for hole 

drilling. The presence of hazardous materials in the ground and the need for careful control 

and disposal of the soil cuttings may also necessitate the use of augers for pile installation. 

4.6.1 Drill Rigs 

The drill rigs typically used are hydraulic rotary (electric or diesel) power units. They can be 

track mounted; this allows maneuverability on difficult and sloped terrain. The size of the 

track-mounted drills can vary greatly, as seen in Photographs 4-l and 4-2, with the larger drill 

allowing use of long sections of drill rods and casing in high, overhead conditions, and the 

smaller drill allowing work in lower overhead and harder-to-reach locations. The drill mast 

can be mounted on a frame, allowing work in very limited-access and low-overhead areas, such 

as building basements. A frame-mounted drill, such as the one shown in Photograph 4-3, can 

be connected with long hoses to a separate hydraulic rotary power unit. This allows placement 

of the power unit outside the area of work, reducing space requirements, noise in the work area, 

and problems with exhaust removal. The drill frame can be moved and supported with a fork 

lift, or moved by hand with winches and supported by bolting to a concrete floor, and/or bridge 

footing or bracing from a ceiling or bridge soffit. 

4-4 FHWA-SA-97-070 (~00-06) 
Photograph 4 - 1. Large Track-mounted Rotary Hydraulic Drill Rig 

Photograph 4 - 2. Small Track-mounted Rotary Hydraulic Drill Rig 

FHWA-SA-97-070 (~00-06) 4-5 
Photograph 4 - 3. Small Frame-mounted Rotary Hydraulic Drill Rig 

The rotary head that turns the drill string (casing, augers, or rods) can be extremely powerful on 

even the smallest of rigs, allowing successful installation in the most difficult ground 

conditions. Shortening of the drill mast and the use of short jointed sections of drill string and 

pile reinforcement allows pile installation with less than 3 m of overhead. The pile centerline 

can be located within 0.3 m of the face of an adjacent wall. 

4-6 FHWA-SA-97-070 (~00-06) 
4.B.2 Drilling Techniques 

The drilling method is chosen with the objective of causing minimal disturbance or upheaval to 

the ground and structure, while being the most efficient, economic, and reliable means of 

penetration. Micropiles must often be drilled through an overlying weak material to reach a 

more competent bearing stratum. Therefore, it typically requires the use of overburden drilling 

techniques to penetrate and support weak and unconsolidated soils and fills. In addition, unless 

the bearing stratum is a self-supporting material, such as rock or a cohesive soil, the drill hole 

may need temporary support for its full length, e.g. through the use of temporary casing or 

suitable drilling fluid. If self-supporting material is present for the full depth of the pile, the 

drillhole can possibly be formed by open hole techniques, i.e., without the need for temporary 

hole support by drill casing or hollow stem auger. 

A different drilling method may be used to first penetrate through an existing structure. 

Concrete coring techniques may be used to provide an oversized hole in existing slabs and 

footings, to allow the subsequent drill casing to pass through. In some cases, conventional rock 

drilling methods involving rotary percussive techniques can be used to penetrate existing 

footings or structures with only light reinforcement. Rotary percussive or rotary duplex 

techniques may be used to first penetrate an initial obstruction layer, such as concrete rubble, 

with more conventional single-tube advancement drilling used for completion of the pile shaft 

in the soil layers below. 

Water is the most common medium for cleansing and flushing the hole during drilling, 

followed by air, drill slurries, and foam. Caution should be exercised while using air flush to 

avoid injection of the air into the surrounding ground, causing fracturing and heaving. The use 

of bentonite slurries to stabilize and flush holes is generally believed to impair grout /ground 

bond capacity by creating a skin of clay at the interface; however, this is not an uncommon 

choice in Italian and French practice with Type D piles. Polymer drilling muds have been used 

successfully in micropile construction in all types of ground. This slurry type reduces concern 

for impairment of the bond capacity, and allows for easier cleanup and disposal versus 

bentonite slurry. 

FHWA-SA-97-070 (~00-06) 4-7 
4.B.3 Overburden Drilling Techniques 

There is a large number of proprietary overburden drilling systems sold by drilling equipment 

suppliers worldwide. In addition, specialty contractors often develop their own variations in 

response to local conditions and demands. The result is a potentially bewildering array of 

systems and methods, which does, however, contain many that are of limited application, and 

many that are either obsolete or virtually experimental. Closer examination of this array further 

confirms that there are essentially six generic methods in use internationally in the field of 

specialty geotechnical construction (i.e., diameters less than 300 mm, depths less than 60 m). 

The following is a brief discussion of these six methods. These six methods are also 

summarized in Table 4-1, and simply represented in Figure 4-2. 

Single-tube advancement - external flush (wash boring): By this method, the toe of the drill 

casing is fitted with an open crown or bit, and the casing is advanced into the ground by 

rotation of the drill head. Water flush is pumped continuously through the casing, which 

washes debris out and away from the crown. The water-borne debris typically escapes to 

the surface around the outside of the casing, but may be lost into especially loose and 

permeable upper horizons. Care must be exercised below sensitive structures in order 

that uncontrolled washing does not damage the structure by causing cavitation. 

Air flush is not normally used with this system due to the danger of accidentally 

overpressurizing the ground in an uncontrolled manner, which can cause ground 

disturbance. Conversely, experience has shown that polymer drill flush additives can be 

very advantageous in certain ground conditions, in place of water alone (Bruce, 1992.). 

These do not appear to detrimentally affect grout-to-soil bond development as may be 

the case with bentonite slurries. 

4-8 FHWA-SA-97-070 (~00-06) 
Table 4-l. Overburden Drilling Methods 

Drilling Method Principle Common Diameters 

and Depths Notes 

1 Single-tube Casing with “lost point” percussed 50-100 mm to 30 m Obstructions or very dense soil 

advancement: without flush. Casing, with shoe, 100-250 mm to 60 m problematical. 

a) Drive drilling rotated with strong water flush. Very common for anchor installation. 

b) External flush Needs high torque head and powerful flush 

pump. 

2 Rotary duplex 

3 Rotary percussive 

concentric duplex 

Simultaneous rotation and loo-220 mm 

advancement of casing plus internal to 70 m 

rod, carrying flush. 

As 2, above, except casing and rods 89-175 mm 

percussed as well as rotated. to40m 

Used only in very sensitive soil/site 

conditions. Needs positive flush return. 

Needs high torques. (Internal flushing only) 

Useful in obstructed/rocky conditions. 

Needs powerful top rotary percussive 

hammer. 

4 Rotary percussive 

eccentric duplex 

As 2, except eccentric bit on rod cuts 89-200 mm Expensive and difficult system for difficult 

oversized hole to ease casing to60m overburden. 

advance. 

5 “Double head” As 2 or 3, except casing and rods 

duplex may rotate in opposite directions. 

100-I 50 mm 

to60m 

Powerful, new system for fast, straight 

drilling in very difficult ground. Needs 

significant hydraulic power. 

6 Hollow-stem auger Auger rotated to depth to permit 100-400 mm Obstructions problematical; care must be 

subsequent introduction of grout to 30 m exercised in cohesionless soils. Prevents 

and/or reinforcement through stem. application of higher grout pressures. 

Note: Drive drilling, being purely a percussive method, is not described in the text as it has no application in micropile construction. 

Source: after Bruce et al., 1989. 
SINGL: TUBE ROTAR’fDUPLEX ROARY ROTARY PtRCUSSlVE “DOUBL: HEAD HOLLOk STEM 

ADVANCEMENT (ROD MAY HAVE PERCUSSIVE ECCENTRIC DUPLEX 

(ENDFyLsCHqSING DOWN-THE-HOLE CO;;;;gIC 

DUPLEX. (CASING 

(ROD CAN BE 

AUGER (WITH 

CAN BE PERCUSSED 

HAMMER ABOVE PERCUSSIVE AT TOP, OR 

“KNOCK-OFF” BIT) 

BY TOP HAMMER, 

BIT TO BREAK MAY HAVE DOWN-THE- ROD MAY BE 

BOULDERS, HOLE HAMMER ABOVE PERCUSSED BY TOP 

OBSTRUCTIONS) BIT. IN THIS CASE. HAMMER, OR MAY 

CASING IS NOT HAVE DOWN-THEPERCUSSED AT HEAD) HOLE HAMMER ABOVE 

BIT. IN THIS CASE. 

CASING IS NOT 

PERCUSSED AT HEAD) 

I I 

LEGEND b PERCUSSION ROTATION - FLUSH CASING ROD 

(CASING) (CASING) 

8 PERCUSSION c, ROTATION CROWN BIT 

(ROD) (ROD) 

(SHOE) 

I I 

Figure 4 - 2. Overburden Drilling Methods (Bruce, 1988) 
Rotary Duplex: With the rotary duplex technique, drill rod with a suitable drill bit is placed 

inside the drill casing. It is attached to the same rotary head as the casing, allowing 

simultaneous rotation and advancement of the combined drill and casing string. The 

flushing fluid, usually water or polymer flush, is pumped through the head down through 

the central drill rod to exit from the flushing ports of the drill bit. The flush-borne debris 

from the drilling then rises to the surface along the amrulus between the drill rod and the 

casing. At the surface, the flush exits through ports in the drill head. Although any 

danger with duplex drilling is less than when using the single-tube-method, air flush 

must be used with caution because blockages within the annulus can allow high air 

pressures and volumes to develop at the drill bit and cause ground disturbance. 

Rotary Percussive Duplex (Concentric): Rotary percussive duplex systems are a 

development of rotary duplex methods, whereby the drill rods and casings are 

simultaneously percussed, rotated, and advanced. The percussion is provided by a topdrive rotary percussive drill head. This method requires a drill head of substantial rotary 

and percussive energy. 

Rotary Percussive Duplex (Eccentric or Lost Crown): Originally sold as the Overburden 

Drilling Eccentric (ODEX) System, this method involves the use of rotary percussive 

drilling combined with an eccentric underreaming bit. The eccentric bit undercuts the 

drill casing, which then can be pushed into the oversized drill hole with much less 

rotational energy or thrust than is required with the concentric method just described. In 

addition, the drill casing does not require an expensive cutting shoe and suffers less wear 

and abrasion. 

The larger diameter options, of more than 127 mm in diameter, often involve the use of a 

down-the-hole hammer acting on a drive shoe at the toe of the casing, so that the casing 

is effectively pulled into the borehole as opposed to being pushed by a top hammer. 

Most recently, systems similar to ODEX, which is now sold as TUBEX, have appeared 

from European and Japanese sources. Some are merely mechanically simpler versions 

FHWA-SA-97-070 (~00-06) 4-11 
of TUBEX. Each variant, however, is a percussive duplex method in which a fully 

retractable bit creates an oversized hole to ease subsequent casing advancement. 

Double Head Duplex: With the double head duplex method, a development of conventional 

rotary duplex techniques, the rods and casings are rotated by separate drill heads 

mounted one above the other on the same carriage. These heads provide high torque 

(and so enhanced soil-and obstruction-cutting potential), but at the penalty of low 

rotational speed. However, the heads are geared such that the lower one (rotating the 

outer casing), and the upper one (rotating the inner drill string) turn in opposite 

directions. The resulting aggressive cutting and shearing action at the bit permits high 

penetration rates, while the counter-rotation also discourages blockage of the casing/rod 

annulus by debris carried in the exiting drill flush. In addition, the inner rods may 

operate by either purely rotary techniques or rotary percussion using top-drive or downthe-hole hammers. The counter-rotation feature promotes exceptional hole straightness, 

and encourages penetrability, even in the most difficult ground conditions. 

Hollow-Stem Auger: Hollow-stem augers are continuous flight auger systems with a central 

hollow core, similar to those commonly used in auger-cast piling or for ground 

investigation. These are installed by purely rotary heads. When drilling down, the 

hollow core is closed off by a cap on the drill bit. When the hole has been drilled to 

depth, the cap is knocked off or blown off by grout pressure, permitting the pile to be 

formed as the auger is withdrawn. Such augers are used mainly for drilling cohesive 

materials or very soft rocks. 

Various forms of cutting shoes or drill bits can be attached to the lead auger, but heavy 

obstructions, such as old foundations and cobble and boulder soil conditions, are 

difficult to penetrate economically with this system. In addition, great care must be 

exercised when using augers: uncontrolled penetration rates or excessive “hole cleaning” 

may lead to excessive spoil removal, thereby risking soil loosening or cavitation in 

certain circumstances. 

4-12 FHWA-SA-97-070 (~00-06) 
4.B.4 Open-Hole Drilling Techniques 

When the micropile can be formed in stable and free-standing conditions, the advancement of 

casing may be suspended and the hole continued to final depth by open-hole drilling 

techniques. There is a balance in cost between the time lost in changing to a less-expensive 

open-hole system and continuing with a more expensive overburden drilling system for the full 

hole depth. Contractors need to be cautious with open-hole drilling operations. The micropile 

installation contractor is ultimately responsible for seiection andproper per$ormance of the 

drilling and installation method(s). Open-hole drilling techniques may be classified as follows: 

Rotary Percussive Drilling: Particularly for rocks of high compressive strength, rotary 

percussive techniques using either top-drive or down-the-hole hammers are utilized. For 

the small hole diameters used for micropiles down-the-hole techniques are the most 

economical and common. Air, air/water mist, or foam is used as the flush. 

Top-drive systems can also use air, water, or other flushing systems, but have limited 

diameter and depth capacities, are relatively noisy, and may cause damage to the 

structure or foundation through excessive vibration. 

Solid Core Continuous Flight Auger: In stiff to hard clays without boulders and in some 

weak rocks, drilling may be conducted with a continuous flight auger. Such drilling 

techniques are rapid, quiet, and do not require the introduction of a flushing medium to 

remove the spoil. There may be the risk of lateral decompression or wall 

remolding/interface smear, either of which may adversely affect grout/soil bond. Such 

augers may be used in conditions where the careful collection and disposal of drill spoils 

are particularly important environmentally. 

FHWA-SA-97-070 (~00-06) 4- 13 
Underreaming: Various devices have been developed to enlarge or underream open holes in 

cohesive soils or soft sediments, especially when the piles are to act in tension (e.g,. for 

transmission towers). These tools can be mechanically or hydraulically activated and 

will cut or abrade single or multiple underreams or “bells.” However, this is a timeconsuming process, and it is rarely possible or convenient to verify its effectiveness. In 

addition, the cleaning of the under-reams is often difficult; water is the best cleaning 

medium, but may cause softening of the ground. For all these reasons, .it is rare to find 

underreaming conducted in contemporary micropile practice. Increases in load-holding 

capacity are usually achieved by pressure grouting tecnhiques. 

4.C GROUTING 

As described in Chapter 2, the grouting operations have a major impact on micropile capacity 

and form the most fundamental construction basis for micropile classification. Details of each 

type of grouting operation vary somewhat throughout the world, depending on the origins of 

the practice and the quality of the local resources. However, as general observations, it may be 

noted that: 

l Grouts are designed to provide high strength and stability, but must also be pumpable. 

As shown in Figure 4-3, this implies typical water/cement (w/c) ratios in the range of 

0.40 to 0.50 by weight for micropile grout. 

l Grouts are produced with potable water, to reduce the danger of reinforcement 

corrosion. 

l Type I/II cement conforming to ASTM Cl SOIAASHTO M85 is most commonly used, 

supplied either in bagged or bulk form depending on site condition, job size, local 

availability, and cost. 

4-14 FHWA-SA-97-070 (~00-06) 
l Neat cement-water grout mixes are most commonly used, although sand is a common 

additive in certain countries (e.g., Italy, Britain). Bentonite (which reduces grout 

strength) is used in primary mixes only with extreme caution, while additives are 

restricted only to those that improve pumpability over long distances and/or in hot 

conditions (e.g., high-range water reducers). 

l Design compressive strengths of 28 to 35 MPa can reasonably be attained with properly 

produced neat cement grouts. 

RESISTANCE TO FLOW 

STRENGTH dynes/cm2) 

lkgF=981 x103dynes 

Ol I I I I I I I, 

0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 

WATER I CEMENT RATIO (by weight) 

Figure 4 - 3. Effect of Water Content on Grout Compressive 

Strength and Flow Properties (Barley and Woodward, 1992) 

FHWA-SA-97-070 (~00-06) 4-15 
The critical importance of the grouting operation is underlined by the fact that the placed grout 

is required to serve a number of purposes: 

l It transfers the imposed loads between the reinforcement and the surrounding ground. 

l It may form part of the load-bearing cross section of the pile. 

l It serves to protect the steel reinforcement from corrosion. 

l Its effects may extend beyond the confines of the drill hole by permeation, densification, 

and/or fissuring. 

The grout, therefore, needs to have adequate properties of fluidity, strength, stability, and 

durability. The need for grout fluidity can mistakingly lead to the increase in water content; 

this has a negative impact on the other three properties. Of all the factors that influence grout 

fluidity and set properties, the water/cement ratio is the dominant. Again, Figure 4-3 illustrates 

why this ratio is limited to a range of 0.45 to 0.50, although even then, additives may be 

necessary to ensure adequate pumpability for ratios less than 0.40. 

It is essential to the integrity of the pile that on completion of the grouting operation there is no 

significant loss of grout from any part of the pile that will be relied upon for load bearing or 

corrosion protection. This can be achieved by grouting to refusal during pile formation, i.e. 

continue grouting until no more grout take occurs. Problems with grout loss may necessitate 

the use of a filler such as sand for plugging the permeable layer, or may require grouting the 

hole and redrilling and regrouting after set of the initial grout. 

For a Type B pile, it may not always be possible to attain the desired pressures during grouting; 

the soil seal around the casing may not always be adequate to contain the pressurized grout. 

This may occur after partial pressure grouting of the bond length. If this occurs, the grout 

should be pumped until the level reaches the top of pile, at which time grouting is discontinued. 

Maintaining grout pressures at a reasonable level (0.70 MPa or less) will help prevent this from 

occurring. If the bond lengths of the test piles (that verified the geotechnical capacity) are 

grouted full length with the desired pressure, questions may be raised as to the adequacy of 

piles that are grouted under partial pressure. One benefit of conducting pile tests to a high load 

4-16 FHWA-SA-97-070 (~00-06) 
(150 to 200 percent design load) or to bond failure is that it helps to determine if the piles have 

excess geotechnical capacity, relaxing this concern. Production proof tests may be conducted 

on the suspect piling. 

Because the grout is such a vital component of the micropile, close attention must be paid to 

the control and quality of the product. A grout quality control plan, which at the minimum 

should include cube compression testing and grout density (water /cement ratio) testing, is 

discussed in Chapter 8. 

Comprehensive guides to cement grout mix design, performance, and equipment in general are 

provided by Littlejohn (1982), Gourlay and Carson (1982), and Houlsby (1990). Similar issues 

relating solely to the similar demands of prestressed ground anchors are summarized by 

Littlejohn and Bruce (1977). 

4.C.l Grout Equipment 

As a general statement, any plant suitable for the mixing and pumping of fluid cementitious 

grouts may be used for the grouting of micropiles. The best quality grouts, in terms of both 

fluid and set properties, are produced by high-speed, high-shear colloidal mixers (Figure 4-4) 

as opposed to low-speed, low-energy mixers, such as those that depend on paddles (Figure 

4-5). Mixing equipment can be driven by air, diesel, or electricity, and is available in a wide 

range of capacities and sizes from many manufacturers. 

For grout placement, lower pressure injection (say, to 1 MPa) is usually completed using 

constant pressure, rotary-screw type pumps (Moyno pumps), while higher pressure grouting, 

such as for Type C or D micropiles, usually requires a fluctuating pressure piston or ram pump. 

FHWA-SA-97-070 (~00-06) 4-17 
4.C.2 Grout Mixing 

The measured volume of water is usually added to the mixer first, followed by cement and then 

aggregate or filler if applicable. It is generally recommended that grout be mixed for a 

minimum of two minutes and that thereafter the grout be kept in continuous slow agitation in a 

holding tank prior to being pumped to the pile. Only in extreme cases- for example, where 

exceptionally large takes are anticipated- should ready- mix grout supply be required. The 

gr-ut should be injected within a certain maximum time after mixing. This “safe workability” 

time should be determined on the basis of on- site tests, as it is the product of many factors, but 

is typically not in excess of one hour. 

VORTEX DRUM HANY MIXER 

THE ORIGINAL CHEMGROUT 

COLLOIDAL MIXER 

ROTOR TOR 

CEMIX MIXERS 

Left: MODEL 175 Right: MODEL 200 

DOUBLE-ROTOR CONCRETE MIXER 

Figure 4 - 4. Various Types of Colloidal Mixers 

4-18 FHWA-SA-97-070 (~00-06) 
END VIEW ELEVATION ELEVATION 

HORIZONTAL PADDLE MIXER 

CHEMGROUT PADDLE MIXER 

SECTION AA 

VERTICAL PADDLE MIXER 

PROPELLER 

FIXED IN 

DRUM 

PORTABLE 

PROPELLER 

Figure 4 - 5. Various Types of Paddle Mixers 

The water is typically batched into the mixer by means of a calibrated tank or flow meter. 

Cement is typically batched by weight, either in bags or by bulk from a silo. Sand or fillers are 

also batched by weight from premeasured bags or more commonly, by using a gagebox that has 

previously been checked and weighed. For bulk material, some method must be provided for 

controlling the quantities of components (volume or weight measurement) added to the mix. 

Admixtures are usually provided ready-proportioned to a single bag of cement, or the dosage 

can be adjusted by the mixer operator. 

FHWA-SA-97-070 (~00-06) 4-19 
4.C.3 Grout Placement Techniques 

Four classifications of grouting methodology are established in Chapter 2 of this manual. 

These methods are described in further detail as follows. 

4.C.3.1 Gravity Fill Techniques (Type A Micropiles) 

Once the hole has been drilled to depth, it is filled with grout and the reinforcement is placed. 

Grout should always be introduced into the drill hole through a tremie pipe exiting at the 

bottom of the hole. Grout is pumped into the bottom of the hole until grout of similar quality 

to that being injected is freely flowing from the mouth of the borehole. No excess pressure is 

applied. Steps are taken to ensure that the quality of grout is maintained for the full length of 

the borehole. This type and phase of grouting is referred to as the primary treatment. 

The grout usually comprises a neat cement mix with w/c ratio between 0.45 and 0.50 by 

weight. Additionally, sanded mixes of up to 1: 1 or 2: 1 sand:cement ratio have been used in 

European practice, but they are becoming less common due to a growing trend towards the use 

of higher grouting pressures involving neat cement grouts. Gravity fill techniques tend now to 

be used only when the pile is founded in rock, or when low-capacity piles are being installed in 

stiff or hard cohesive soils, and pressure grouting is unnecessary (Bruce and Gemme, 1992). 

For sanded mixes, the w/c ratio is often extended to 0.60, assuming the resultant mix remains 

stable (Barley and Woodward, 1992). 

4.C.3.2 Pressure Grouting Through the Casing (Type B Micropiles) 

Additional grout is injected under pressure after the primary grout has been tremied, and as the 

temporary casing is being withdrawn. The aim is to enhance the grout/soil bond 

characteristics. This operation can be limited to the load transfer length within the designbearing stratum, or may be extended to the full length of the pile where appropriate. 

Pressure grouting is usually conducted by attaching a pressure cap to the top of the drill casing 

(this is often the drilling head itself) and injecting additional grout into the casing under 

controlled pressure. In the early days, pressurization of the grout was achieved by applying 

4-20 FHWA-SA-97-070 (~00-06) 
compressed air through the grout line, since contemporary drill head details and grout pump 

technology could not accommodate the relatively viscous, sand-cement mortars. This method 

has now been rendered obsolete by the developments in pump capabilities, combined with the 

trend to use stable, neat cement grouts without sand. 

Grout pressures are measured as close to the point of injection as possible, to account for line 

losses between pump and hole. Commonly, a pressure gauge is mounted on the drill rig and 

monitored by the driller as a guide to rate of casing withdrawal during the pressurization phase. 

Alternatively, if a grouting cap is used and the casing is being extracted by means other than 

the drill rig (e.g., by hydraulic jacks), it is common to find a pressure gauge mounted on the cap 

itself. Practitioners acknowledge that there will be line losses in the system, but typically 

record the pressure indicated on the pressure gauge without the correction, reasoning that such 

losses are compensated by the extra pressure exerted by the grout column due to its weight in 

the borehole. 

American practice is to inject additional grout at a typical average pressure between 0.5 to 

IMPa, with the aim of reinstating insitu lateral soil pressures that may have been reduced by 

the drilling process and achieving permeation into coarser grained granular soils or fractured 

rocks. The effective injection pressures (typically 20 kPa per meter of depth in loose soils and 

40 kPa per meter of depth in dense soils) are dictated by the following factors: 

l The need to avoid ground heave or uncontrolled loss of grout. 

l The nature of the drilling system (permissible pressures are lower for augers due to 

leakage at joints and around the flights). 

l The ability of the ground to form a “seal” around the casing during its extraction and 

pressure grouting. 

l The need to avoid “seizing” the casing by flash setting of the grout due to excessive 

pressure, preventing proper completion of the pile. 

l The “groutability” of the ground. 

FHWA-SA-97-070 (~00-06) 4-21 
l The required grout/ground bond capacity. 

l Total pile depth. 

The injection of grout under pressure is aimed at improving grout/ground skin friction, thus 

enhancing the load-carrying capacity of the micropile. Extensive experience with ground 

anchors has confirmed the effect of pressure grouting on ultimate load-holding capacity. This 

is discussed in detail in Chapter 5. 

When pressure grouting in granular soils, a certain amount of permeation and replacement of 

loosened soils takes place. Additionally, a phenomenon known as pressure filtration occurs, 

wherein the applied grout pressure forces some of the integral mixing water out of the cement 

suspension and into the surrounding soil. This process leaves behind a grout of lower water 

content than was injected and is thus quicker setting and of higher strength. It also causes the 

formation of cake-like cement paste along the grout/soil interface that improves bond. In 

cohesive soils, some lateral displacement, compaction, or localized improvement of the soil can 

occur around the bond zone, although the improvement is generally less well marked than for 

cohesionless soils. 

Pressure grouting also appears to cause a recompaction or redensitication of the soil around the 

borehole and increases the effective diameter of the pile in the bond zone. These mechanisms 

effectively enhance grout/soil contact, leading to higher skin friction values and improved 

load/displacement performance. Such pressure grouting may also mechanically improve the 

soil between piles. This is an interesting concept within the CASE 2 pile application but is, as 

yet, untested. 

4.C.3.3 Postgrouting (Type C and D Micropiles) 

It may not be possible to exert sufficiently high grout pressures during the casing removal 

stage. For example, there may be ground hydrafracture or leakage around the casing. 

Alternatively, some micropile construction methods may not use or need a temporary drill 

casing, and so pressure grouting of the Type B method is not feasible. These circumstances 

have led to the development of post-grouting techniques, whereby additional grout can be 

4-22 FHWA-SA-97-070 (~00-06) 
injected via special grout tubes some time after the placing of the primary grout. Such grouts 

are always neat cement-water mixes (for the ease of pumpability) and may therefore have 

higher water contents than the primary grout, being in the range of 0.50 to 0.75 by weight. It is 

reasoned that excess water from these mixes is expelled by pressure filtration during passage 

into the soil, and so the actual placed grout has a lower water content (and therefore higher 

strength). 

As described in the following paragraphs, high postgrouting pressures are typically applied, 

locally, for quite restricted periods; it may only take a few minutes to inject a sleeve. As 

mentioned by the Federal Highway Administration (1997) report, Herbst noted that the 

required aim of providing higher grout/ground bond capacity may, in fact, be more efficiently 

achieved in Type B micropiles, where grouting pressures are lower but are exerted over a larger 

area and a much longer period. This has yet to be evaluated. 

The construction-based classification of Section 2.A.2 identified two types of postgrouted 

piles, namely Type C and Type D. 

Type C: Neat cement grout is placed in the hole as done for Type A. Between 15 and 25 

minutes later, and before hardening of this primary grout, similar grout is injected once 

from the head of the hole without a packer, via a 38- to 50-mm diameter preplaced 

sleeved grout pipe (or the reinforcement) at a pressure of at least 1 MPa. 

Type D: Neat cement grout is placed in the hole as done for Type A. When this primary grout 

has hardened, similar grout is injected via a preplaced sleeved grout pipe. Several 

phases of such injection are possible at selected horizons and it is typical to record 

pressures of 2 to 8 MPa, especially at the beginning of each sleeve treatment when the 

surrounding primary grout must be ruptured for the fast time. There is usually an 

interval of at least 24 hours before successive phases. Three or four phases of injection 

are not uncommon, contributing additional grout volumes of as much as 250 percent of 

the primary volume. 

FHWA-SA-97-070 (~00-06) 4-23 

-- 
Variations on the technique exist. The postgrout tube can be a separate 25- or 38-mm diameter 

sleeved plastic pipe (tube a manchette) placed together with the steel reinforcement (Figure 

4-6), or it can be the reinforcement tube itself, suitably sleeved (Figure 4-7). In each of these 

cases, a double packer may be used to grout through the tubes from the bottom sleeve upwards. 

REINFORCING 

STEEL . 

BOND ZONE 

(SOIL / GROUT . 

INTERFACE) 

CEMENT GROUT 

(FIRST PHASE) 

CLOSEDRUBBER 

SLEEVE 

SLEEVED PIPE 

DOUBLE 

PACKER (TYP) 

_ POST GROUT 

(SECOND PHASE) 

, OPENRUBBER 

SLEEVE 

Figure 4 - 6. Principle of the Tube a Manchette Method of 

Postgrouting Injection 

4-24 FHWA-SA-97-070 (~00-06) 
DRILL HOLE 

DIAMETER 

STEEL TUBE 

I:, i. Ix;*‘; 

, DOUBLE PACKER 

F’; 

ANNULUS GROUT 

(GROUT PRESSURE) 

lti PHASE PRESSURE 

I 2”d PHASE PRESSURE 

GROUT 

i GROUT 

Figure 4 - 7. Use of Reinforcement Tube as a Tube 6 Manchette 

Postgrouting System 

Alternatively, this pressure grouting can be conducted from the surface via a circulating-loop 

arrangement. By this method, grout is pumped around the system and the pressure increased 

steadily by closing the pressurization value on the outlet side. At the critical “break out” 

pressure, dictated by the lateral resistance provided by the adjacent grout, the grout begins to 

flow out of the tube through one or more sleeves and enters the ground at that horizon. When 

using the loop method, it is assumed that with each successive phase of injection, different 

sleeves open, so ultimately ensuring treatment over the entire sleeved length (a feature 

guaranteed by the tube a manchette method using double packers.) 

FHWA-SA-97-070 (~00-06) 4-25 
A slight cautionary note against the use of postgrouting was raised by Dr. Lizzi, who was 

concerned about leaving “foreign materials” (e.g., the postgrouting tubes) in the pile, and about 

contractors being tempted to use low-strength bentonite primary grouts to reduce “break out” 

pressures. Consideration should be given to these issues if the pile structural design places 

dependency on the compressive strength of the grout. 

4.C.4 Top-Off (Secondary) Grouting 

Due to slow grout seepage, bleed, or shrinkage, it is common to find that the grout level drops a 

little prior to stiffening and hardening. In ground anchorage practice, this is simply rectified by 

topping off the hole with the lowest water-content grout practical, at some later phase. 

However, in micropile practice where a permanent casing for reinforcement of the upper pile 

length is not used, such a cold joint is best avoided, since the grout column should be 

continuous for load holding and corrosion protection reasons. Topping off is therefore best 

conducted during the stiffening phase to ensure integrity. Where particularly high interfacial 

bond stresses must be resisted between the pile and an existing structure, the use of a highstrength non-shrink grout may be considered. 

4.D REINFORCING STEEL 

The amount of steel reinforcement placed in a micropile is determined by the loading it 

supports and the stiffness required to limit the elastic displacement. Reinforcement may 

consist of a single reinforcing bar, a group of reinforcing bars, a steel pipe casing or rolled 

structural steel. 

4.D.l Placement of Reinforcement 

Reinforcement may be placed either prior to grouting, or placed into the grout-filled borehole 

before the temporary support (if used) is withdrawn. It must be clean of deleterious substances 

such as surface soil and mud that may contaminate the grout or coat the reinforcement, 

impairing bond development. Suitable centralizers should be fumly fixed to maintain the 

specified grout cover. Pile cages and reinforcement groups, if used, must be sufficiently robust 

4-26 FHWA-SA-97-070 (~00-06) 
to withstand the installation and grouting process and the rotation and withdrawal of the 

temporary casing. 

4.D.2 Reinforcement Types 

Further description of the various types of reinforcement follows. 

Concrete Reinforcing Steel Bars (rebar): Standard reinforcing steel (Table 4-2); conforming 

to ASTM A61 YAASHTO M3 land ASTM A706, with yield strengths of 420 and 520 

MPa, is typically used. Bar sizes range in diameter from 25 mm to 63 mm. A single bar 

is typically used, but a group of bars is possible. For a group, the individual bars can be 

separated by the use of spacers or tied to the helical reinforcement, to provide area for 

grout to flow between the bars and ensure adequate bonding between the bars and grout 

(Figure 4-8). Or bars can be bundled, provided adequate development length for bundle 

is provided. 

For low overhead conditions where placement of full-length bars is not feasible, mechanical 

couplers can be used. Field adjustment of individual bar lengths can be difficult if the coupler 

type requires shop fabrication. 

Continuous-Thread Steel Bars: Steel reinforcing bars that have a continuous full-length 

thread, such as the Dywidag Systems International (DSI) Threadbar or the Williams 

All-Thread Bar. 

The DSI Threadbar system, which is also named a GEWI pile (Figure 4-9 and 

Table 4-3), is a common choice throughout the world for micropile reinforcement. The 

bar has a coarse pitch, continuous ribbed thread rolled on during production. It is 

‘- available in diameters ranging from 19 mm to 63 mm in steel conforming to ASTM 

A61YAASHTO M 3 1, with yield strengths of 420,520, and 550 MPa. The size range of 

44 mm to 63.5 mm is most commonly used. Higher strength bars of steel conforming to 

ASTM A722/AASHTO M 275 with an ultimate strength of 1,035 MPa are also 

available, in diameters of 26,32, and 36 mm. 

FHWA-SA-97-070 (~00-06) 4-27 I 
Table 4-2. Dimensions, Yield, and Ultimate Strengths for Standard Reinforcing Bars 

I Em.04. urn de 420 Grade 520 

Rebar Size (mm> 25 29 32 36 43 57 36 43 57 63 

Area (mm’) 510 645 819 1006 1452 2581 1006 1452 2581 3168 

Yield Strength W 211 267 339 416 600 1068 520 751 1334 1742 

Ultimate Strength (kN) 316 400 508 624 901 1601 690 997 1770 2168 

Notes -- 

1. Grade 420 reinforcing steel has yield stress off,,= 420 MPa and tensile strength off, = 620 MPa. 

2. Grade 520 reinforcing steel has yield stress of fy= 520 MPa and tensile strength off, = 690 MPa. 

3. 63 nun threadbars by DSI have a minimum yield strength off,,= 550 MPa. 

PILE 

c 

245 mm OD x 

THICKNESS ! 

-.- 

/ 

BAR CENTRALIZER 

280 ” 

DRILI 

.- 

- GROUT 

Figure 4 - 8. Multiple Bar Reinforcement with Bar Centralizer/Spacer 

4-28 FHWA-SA-97-070 (~00-06) 
Figure 4 - 9. Details of Continuously Threaded Dywidag Bar (DSI, 1993) 

The Williams All-Thread Bar is available in diameters ranging from 20 mm to 53 mm in steel 

conforming to ASTM A61 S/AASHTO M 3 1 and from 26 mm to 45 mm in steel conforming to 

ASTM A722/AASHTO M 275 , with an ultimate strength of 1,035 MPa. The bar has a finer 

thread than used on the Dywidag bar. 

The thread on the bars not only ensures grout-to-steel bond, but also allows the bar to be cut at 

any point and joined with a coupler to restore full tension/compression capacity. The 

continuous thread also simplifies pile-to-structure connections where the bar is connected to a 

anchor plate. A hex nut is used to connect the plate, with the continuous thread allowing easy 

adjustment of the plate location. 

Continuous-Thread Hollow-Core Steel Bars: Steel reinforcing bars that have a hollow core 

and a continuous full-length thread include the Dwyidag (Type MAI), Ischebeck Titan 

and Chance IBO Injection Boring Rods. These bar types offer the advantages of 

continuous thread, and the hollow core allows the bar to be used to drill the pile hole. A 

drill bit is mounted on the tip of the bar, and the bar is drilled in with grout flush pumped 

to the bit through the hollow core of the bar. Alternately, an air or water flush can be 

used, with the grout placed through the bar after drilling to the final depth. 

FHWA-SA-97-070 (~00-06) 4-29 
Table 4-3 Dywidag Threadbars - Technical Data (Courtesy of DSI) 

DYWIDAG THREAD BARS TECHNICAL DATA [METRIC UNITS] 

Hot-Roiled Reinforcing Threadbar 

(SAME AS ABOVE) 

Ductile Reinforcement 

Cold Region Anchors and Anchor Bolts 

800/900 MPa Cold-Rolled Threadbar 

Form-Use Temp. Soil Nailing 

480/600 MPa DSI-MAI 

Hollow-Core Self-Drilling Injection Anchors 

Temporary Shoring Temp. Soil Nailing 

Micropiles (Gewi-piles) Overburden Drilling 

Without Casing 

20 w 335 270 300 2.50 22.00 

R25N 1” 150 200 2.60 25.00 

R32N 1 ‘A” 230 280 3.50 32.00 

R38N 1 ‘A” 430 500 6.00 38.00 

R51N 2” 630 800 9.63 51 .oo 

Approximate Modulus of Elasticity, E = 200,000 MPa 

4-30 FHWA-SA-97-070 (~00-06) 
The continuous thread allows the bar to be cut to length and coupled, and allows the use 

of a hex nut for the pile top connection. The main drawback of this type of 

reinforcement is the higher cost. Table 4-4 presents various bar sizes and strength 

capacities for the MAI, Titan and IBO bars. (Caution to designers and speczfzcation 

writers: Currently these types ofproprietary bar systems are manufactured outside of 

the United States and are subject to Buy America or Buy American Provisions on 

federally finded transportation projects.) 

Steel Pipe Casing: With the trend towards micropiles that can support higher loads at low 

displacements and for the requirement to sustain lateral loads, steel-pipe reinforcement 

has become more common. Pipe reinforcement can provide significant steel area for 

support of high loading and contribution to the pile stiffness, while providing high shear 

and reasonable bending capacity to resist the lateral loads. 

Pipe reinforcement is placed by either using the drill casing as permanent reinforcement, 

or by placing a smaller diameter permanent pipe inside the drill casing. Use of the drill 

casing for full-length reinforcement is typical only for micropiles founded in rock, where 

extraction of the casing for pressure grouting is not necessary. The length of the pipe 

sections used is dictated by the length of the drill mast and by the available overhead 

clearance. Casing sections are typically joined by a threaded connection, which is 

machined into the pipe. The reduced area of the threaded joint should be considered in 

the structural design of the pile, particularly for the capacity in tension and bending. 

Methods exist for reinforcement of the threaded joints that can provide a strength 

equivalent to the full casing section. 

FHWA-SA-97-070 (~00-06) 4-31 
Table 4-4. Hollow Injection Bars 

INNER .- 

DIAMETER 

CAPACITY (kN) Diameter (mm) 

Rod Yield Ultimate Test 

Design Design Weight 

Size WI Load Load Inner Outer (kg’m) 

100% 80% 

70% 60% 

MA1 

150 200 160 140 120 12 25 2.6 

R25N 

MA1 

230 280 224 196 168 18 32 3.5 

R32N 

MA1 

430 500 400 350 300 19 38 6.0 

R38N 

MA1 

630 800 640 560 480 34 51 9.6 

R51N 

TITAN 

525 660 528 462 396 16 40 6.9 

40116 

TITAN 970 1160 928 812 696 53 73 12.8 

73153 

TITAN 

1570 1950 1560 1365 1170 78 103 24.7 

103178 

IBO 160 190 152 133 114 12 25 2.5 

R2512 

IBO 

220 250 200 175 150 19 30 3.5 

3220 

IBO 

420 490 392 343 294 17 38 6.6 

3817 

4-32 FHWA-SA-97-070 (~00-06) 
Pipe in the sizes typically used for micropile construction are available in steel 

conforming to ASTM A53, A5 19, A252 and Al 06 with a typical yield strength of 241 

MPa. Availability of the desired pipe size may determine the grade of steel used. The 

main drawbacks of using these pipe grades is the relatively low yield strength and a very 

high unit cost per linear meter. 

API 5CT or 5L (N-80) casing may be used. The high yield strength of 55 1 MPa greatly 

aids the micropile’s ability to support high loads, and improves the strength of threaded 

joints machined into the pipe wall. The pipe is also readily available in the form of mill 

secondary material at a reasonable unit cost. The overwhelming majority of higher 

capacity micropiles installed to date in the United States have used the N-80 casing. The 

use of this pipe source requires verification of steel quality through tensile and chemical 

testing of sampled steel rather than through mill certification, which is typically not 

available. 

Due to the high strength and typical chemical composition of the API N-80 casing, 

weldability of the casing requires special welding procedures. Prior to welding the N-80 

casing, welding procedures must be submitted to the owner for approval. 

Pipe dimensions and yield strengths, for various grades of steel, are presented in 

Table 4.5. 

FHWA-SA-97-070 (~00-06) 4-33 

-.. 
Table 4-5. Dimensions and Yield Strength of Common Micropile Pipe Types and Sizes 

API N-60 Pipe - Common Sizes 

Casing OD Wall (mm) 139.7 177.8 177.8 244.5 

Thickness Steel (mm) 9.17 12.65 18.54 11.99 

Area (mm2) 3760 6563 9276 8756 

Yield Strength (kN) 2075 3619 5117 4829 

ASTM A519, Al06 Pipe - Common Sizes 

Casing OD Wall (mm) 141.3 168.3 203.2 

Thickness Steel (mm) 12.7 12.7 12.74 

Area (mm2) 5128 6204 8230 

Yield Strength (kN) 1919 2319 3075 

Notes -. 

273.1 

15.9 

12,820 

4794 

1. Casing outside diameter (OD) and wall thickness (t) are nominal dimensions. 

2. Steel area is calculated as A, = (W4) x (OD2 - ID”) 

3. Nominal yield stress for API N-80 steel is F, = 55 1 MPa 

4. Nominal yield stress for ASTM A519 & A106 steel is F, = 241 MPa 

5. Other pipe sizes are manufactured but may not be readily available. Check for 

availability through suppliers. 

Composite Reinforcement: For micropiles with partial length permanent drill casing (Type 

lC, 2C and 4C), the use of a steel bar for reinforcement of the bottom portion of the pile 

is common, resulting in a composite reinforced pile (Figure 4-10). The reinforcing bar 

may be extended to the top of the micropile for support of tension loading. The use of 

varying reinforcement adds complexity to the pile structural analysis, with particular 

attention needed for the location of reinforcement transition. 

4-34 FHWA-SA-97-070 (~00-06) 
TOP OF FOOTING -\ #I6 REINFORCING BAR 

I 

BAR COUPLER 

BOTTOM OF 

FOOTING 

176 0,. 21 1 DIA CASING 

13mm WALL 

TOP OF DENSE 

SOILS 

MAX TEST LOAD - 4,500 kN ; 

CASING: 176 or 211 mm DIA 

WALL: 13 mm 1 i 

NOTE: ALL DIMENSIONS IN MILLIMETERS 

Figure 4 - 10. Details of Composite High-capacity Type 1B Micropiles, 

Vandenberg Air Force Base, California 

FHWA-SA-97-070 (~00-06) 4-35 
4.D.3 Reinforcement Corrosion Protection 

Traditionally, with the exception of permanent tension piles in aggressive ground conditions, 

little corrosion protection other than the surrounding cement grout has been provided to the 

reinforcement in most countries. 

The most common and simplest tests utilized to measure the aggressiveness of the soil 

environment include electrical resistivity, pH, chloride, and sulfate. Per FHWA-RD-89-198, 

the ground is considered aggressive if any one of these indicators show critical values as 

detailed in Table 4-6. 

Table 4-6. Corrosion Critical Values 

Property 

Resistivity 

PH 

Sulfate 

Chloride 

Test Designation* 

AASHTO T-288 

ASTM G 57 

AASHTO T-289 

ASTMG51 

AASHTO T-290 

ASTM 5 16M 

ASTM D4327 

AASHTO T-29 1 

ASTM 512 

ASTM D4327 

Critical Values’ 

below 2,000 ohm-cm 

below 5 

above 200 ppm 

above 100 ppm 

*Note - Corrosion protection requirements vary between Transportation 

Agencies. The designer should check test standards for latest updates and 

individual transportation agencies may have limits on critical values different 

than those tabulated above. Standard specifications or test methods for any of 

the above items which are common to your agency can be referenced in lieu 

of the above listed AASHTO / ASTM references. 

4-36 FHWA-SA-97-070 (~00-06) 
The various levels of corrosion protection that can be applied to reinforcing bars are discussed 

below. 

Grout Protection Only - Reinforcing Bar: Centralizers are applied along the length of the 

bar (Figure 4-l la) to ensure adequate cover of grout between the bar and the side of the 

borehole. Centering of the reinforcement in the grout is also structurally desirable for 

compression piles. Internationally, various codes require minimum grout cover of 20 to 

30 mm. Alternative approaches regarding this level of protection include “geometric” 

corrosion protection, which relates to the concept that a progressive loss of section with 

time is allowable, and typical rates are widely quoted (Fleming et al., 1985). Corrosive 

potential of the existing ground, magnitude of the tension loading, and the structural 

detailing of the pile must all be considered for this level of protection. 

Protective Barriers (Epoxy coating or Encapsulation) - Reinforcing Bar: Additional 

protection may be required in those cases where a continuous grout cover of adequate 

thickness cannot be guaranteed, where the pile is installed in aggressive ground 

conditions, or the reinforcement may cause tension cracking of the grout, providing a 

corrosion path to the steel. Options for protective barriers include providing a coating 

on the bar, such as an electrostatically applied epoxy coating, or providing a encasing 

sheath (encapsulation), such as corrugated plastic, with the annulus between the bar and 

the sheath filled with high-strength, non-shrink grout. The use of a grout-filled 

corrugated sheath is a common feature of permanent anchor tendons and the DSI 

threadbar and GEWI Bar (Figure 4-l lb), and is often referred to as double corrosion 

protection. 

For micropiles with composite reinforcement (bar and pipe), the permanent grout-filled 

pipe provides protection in a manner similar to the encapsulation method for the upper 

portion of the bar encased by the pipe. Protection may still be necessary for the uncased 

portion of the pile. 

FHWA-SA-9’7-070 (~00-06) 4-37 
ADDITIONAL 

REINFORCEMENT 

(COMPPESSION) 

ADDITIONAL 

REINFORCEMENT 

(TENSION) TORQUED 

ANCHOR 

’ PLATE 

_ CONCRETE 

_ ELASTIC - 

SPACER 

INNER 

CEMEN 

GROUT 

&;ATED 

GEWI - BAR 

(4 (W 

GEWI - PILE with GEWI - PILE with 

Standard Corrosion Protection Double Corrosion Protection 

Figure 4 - 11. GEWI Piles with (a) Grout protection only, and (b) Double 

corrosion protection (Courtesy DSJ) 

4-38 FHWA-SA-97-070 (~00-06) 
Table 4-7. Minimum Dimensions (mm) of Shell Thickness as Corrosion Protection 

Soil Type 

Not Aggressive 

Barely Aggressive 

Very Aggressive 

25 

0.25 

1 .oo 

2.50 

Service Life (years) 

50 75 

0.60 0.70 

1.60 2.00 

4.00 5.00 

100 

0.80 

2.50 

6.00 

Source: CCTG, 1993. 

Corrosion protection of pipe reinforcement can be more difficult, particularly if the drill casing 

has been used as the reinforcement. The various levels of corrosion protection that can be 

applied to permanent casings follow. 

Sacrificial Steel - Casing: According to AASHTO section 4.5.7.4 (16th Edition), for concretefilled pipe piles in installations, where corrosion may be expected, 1.6 mm shall be 

deducted from the shell thickness to allow for reduction in the section due to corrosion. 

The French code CCTG (1993) recommends adoption of the minimum dimensions of 

shell thickness to be sacrificed as corrosion protection in the absence of specific studies 

as summarized in Table 4-7. The effect on pile strength and stiffness should be included 

in the consideration of sacrificial steel as corrosion protection. 

Grout Cover - Casing: The pile shaft annulus around a drill casing can vary typically from 10 

to 75 mm, depending on the soil conditions and methods of drilling and grouting. Grout 

cover may not be present in this annulus if the soil “seal” contains the grout in the lower 

bond length during pressure grouting, preventing it from filling in around the upper 

portion of the pile. This may not be a concern in cohesive soil or rock conditions. The 

method to ensure a minimum grout cover around pipe reinforcement, particu!arly in 

granular soil conditions, may be to place a separate permanent pipe and completely 

retract the drill casing. 

FHWA-SA-97-070 (vOO-06) 4-39 
Protective Barriers - Casing: Where the drill casing provides permanent reinforcement, the 

use of a coating such as epoxy paint is not desirable due to the abrasive action of the soil 

on the outer casing wall and probable resulting damage to or wearing off of the coating. 

An encasing sheath in the form of an additional outer casing provides very effective 

corrosion protection, but at considerable additional cost. 

Protective Barriers - Pipe: Protective coating such as epoxy paint can be applied to pipe 

inserted into a temporarily supported hole so that the coating will not be damaged during 

drilling. 

4-40 FHWA-SA-97-070 (~00-06) 
CHAPTER 5 

DESIGN OF MICROPILES FOR STRUCTURE 

FOUNDATIONS (CASE 1 PILES) 

5.A INTRODUCTION 

This chapter outlines subsurface investigation requirements, and the geotechnical and structural 

design considerations for CASE 1 micropiles. The section on pile geotechnical design includes 

guidance for estimating the grout-to-ground bond capacity of micropiles. A table presenting 

nominal (ultimate) grout-to-ground bond strength values typically attainable for various 

soil/rock types is also included. The section on pile structural design includes methods for 

determination of pile component structural strengths in accordance with the 1996 AASHTO 

Standard Specifications for Highway Bridges, Sixteenth Edition. 

The geotechnical load capacity of a micropile is highly sensitive to the processes used during 

pile construction, principally the techniques used for drilling the pile shafts, flushing the drill 

cuttings, and grouting the pile. Therefore, verification of the grout-to-ground nominal bond 

strength assumed in design via pile load testing during construction is essential to ensure 

structure safety; the construction load testing should be considered an extension of design. 

The basic philosophy of micropile design differs little from that required for any other type of 

pile. The system must be capable of sustaining the anticipated loading conditions with the pile 

components operating at safe stress levels, and with resulting displacements falling within 

acceptable limits. For conventional piling systems, where the large cross sectional area results 

in high structural capacity and stiffness, the design is normally governed by the geotechnical 

load carrying capacity. With a micropile’s smaller cross sectional area, the pile design is more 

frequently governed by structural and stiffness considerations. This emphasis on the structural 

pile design is further increased by the high grout-to-ground bond capacities that can be attained 

using the pressure grouting techniques described in Chapters 2 and 4. Furthermore, the use of 

FHWA-SA-97-070 (~00-06) 5-l 
higher strength steel reinforcement in the micropile can result in a design that is governed by 

stiffness considerations. 

With respect to axial, lateral, or combined loading, the design of micropiles consists of two 

basic aspects: 

. The evaluation of the geotechnical load capacity of the micropile, which 

requires appropriate estimation of the grout/ground interface parameters and the 

state of stress in the ground after micropile installation (effects of pressure 

grouting). 

. The structural load capacity and stiffness performance of the micropile section 

which depends mainly on the area of the composite reinforced micropile and the 

strength of the section materials. 

The following design steps are suggested: 

1. Review available project information. 

(a) Requirements of the job, pile loading requirements, pile layout constraints. 

(b) Special conditions such as available access and overhead clearance, presence of 

hazardous materials, environmental constraints. 

(c) Contractual requirements. 

2. Review geotechnical data. 

(a) Obtain geotechnical/geological subsurface profile. 

(b) Estimate geotechnical design parameters. 

(c) Obtain soil properties that determine corrosion protection requirements. 

(d) Identify problem areas if any. 

5-2 FHWA-SA-97-070 (~00-06) 
3. Complete initial geotechnical pile design. 

(a) Estimate load transfer parameters (grout-to-ground bond) for the different 

subsurface layers and determine the pile bond length required to support the 

loading. 

(b) Evaluate pile spacing for impact to geotechnical capacity from group effects. 

4. Complete pile structural design for the various components: 

(a) Pile cased length structural capacity (bar and/or pipe reinforcement with grout). 

(b) Pile uncased length structural capacity (bar reinforcement with grout). 

(c) Grout to steel bond capacity. 

(d) Transition between reinforcement types (cased to uncased section). 

(e) Strain compatibility between structural components/ductility. 

(f) Reinforcement splice connections (bar and/or pipe reinforcement). 

(g) Pile to footing connection. 

5. Complete combined geotechnical and structural design considerations. 

(a) Anticipated settlement/required stiffness analysis. 

(b) Lateral load capacity/Anticipated lateral displacement and combined stresses 

(axial + bending) due to lateral loading conditions. 

(c) Buckling of the pile/soil lateral support considerations. 

6. Complete additional micropile system considerations. 

(a) Corrosion protection requirements. 

(b) Determine construction load testing and quality control program requirements. 

(c) Examine constructability and cost effectiveness of the design. 

FHWA-SA-97-070 (~00-06) 5-3 
5.B COMMENT ON THE USE OF THIS MANUAL FOR DESIGN 

This manual emphasizes that the geotechnical load capacity of a micropile can be highly 

sensitive to the processes used during pile construction, principally the techniques used for 

drilling the pile shafts, flushing the drill cuttings, and grouting the pile. Problems may occur if 

the designer lacks expertise in micropile design and construction techniques or lacks the 

control of construction on site to avoid methods that may be detrimental to the pile’s capacity. 

Therefore, this chapter is intended to assist a designer in determining the feasibility of 

installing a micropile system that will meet predetermined performance criteria at a given site. 

The most optimum pile design and method of installation may be obtained through the use of a 

design/build type performance specification, allowing the use of experienced micropile 

specialty contractors’ methods and expertise to optimize the system. This chapter also 

provides the designer with the necessary tools to properly evaluate a contractors proposed 

micropile system. 

The information on specifying a design/build project element is included in Chapter 9 and 

provides methods for selecting a contractor who is qualified to do the work. Chapters 7 and 8 

include information on load testing, inspection, and quality control, which intend to verify that 

the installed piles meet the specified criteria. 

5-4 FHWA-SA-97-070 (~00-06) 
5.C EXPLANATION OF SLD AND LFD DESIGN METHODS 

The following explanation of SLD (Service Load Design) and LFD (Load Factor Design) 

design methods and cp calibration procedure was developed from Appendix A of NCHRP 343 

(Procedures for Evaluating Performance Factors-Load Factor Design Criteria for Highway 

Structure Foundations NCHRP 24-4,1991). 

Introduction 

This chapter includes both the SLD-Service Load Design Method (also known as Allowable 

Stress Design) and the LFD-Load Factor Design Method (also known as the Strength Design 

Method) as described in the 1996 AASHTO Standard Specifications for Highway Bridges, 

Sixteenth Edition. Both methods are included in order to address the current state-of-thepractice with structural and geotechnical engineers across the United States. Structural 

engineers are currently using both SLD and LFD methods, however most are moving away 

from SLD toward LFD and its newest version, the LRFD-Load and Resistance Factor Design 

Method (the new AASHTO LRFD Bridge Design Specification). Geotechnical engineers are 

mostly using the SLD method, however they are in the early stages of moving toward the LFD 

and LRFD methods as well. 

All three of these design methods provide factors-of-safety to allow for uncertainties in the 

loads and uncertainties in the structural and geotechnical material strengths due to their 

inherent randomness. The safety factors also account for the natural lack of perfect information 

and knowledge during design. 

FHWA-SA-97-070 (~00-06) 5-5 
DESIGN LOAD = IQ = DL 

i=l 

ALLOWABLE LOAD = RJ FS 

REQUIRED STRENGTH = &&Q, = LF x DL 

i=l LFD 

DESIGN STRENGTH = cp R, 

\ I R, = NOMINAL STRENGTH 

FS 

CI- SLD 

Figure 5 - 1. SLD and LFD Description and Relationships 

Qi = load effect due to load component (dead load, live load, earth pressure, etc.) 

<p = strength reduction factor 

Y = group load factor per AASHTO Table 3.22.1 A 

pi = in d ivi d ua 11 oa d component factor per AASHTO Table 3.22.1 A 

LF = combination of y and pi load factors 

FS = factor-of-safety 

n = number of load components 

See Figure 5-l for a description of SLD and LFD terms and their relationships. In the SLD 

method, the factor-of-safety is selected in a rather intuitive manner and is based on experience, 

tradition, and engineering judgment. SLD treats design loads and material strengths as 

5-6 FHWA-SA-97-070 (~00-06) 
deterministic constants. It reduces the nominal strength (ultimate strength) of the material to an 

allowable load which must not be exceeded by the design load. The design load is the 

controlling group of load components (dead load, live load, earth pressure, etc.) as prescribed 

in Table 3.22.1 A of the AASHTO specification. Table 3.22.1 A governs the combination of all 

loads except seismic loads which are covered under Division I-A of the AASHTO specification 

(see AASHTO section 3.21). SLD corresponds to position 1 in Figure 5-1. 

In the LFD and LRFD methods the factor-of-safety is comprised of two components, load 

factors (LF) which amplify the design load, and a strength reduction (LFD) factor cp, which 

reduces the nominal strength of the material. In LFD and LRFD all the elements of a structure 

are assigned the same probability of failure, thus all the elements are working equally without 

one element being way over or under designed compared with another element of the structure. 

The uncertainty in load is represented by a load factor (LF) and the uncertainty in material 

strength is represented by a strength reduction factor cp. This allows us to adjust (“calibrate”) 

the factors as we get more data and further improve on the design method. LFD is a 

probabilistic approach rather than the SLD deterministic approach toward the randomness of 

design loads and material strengths. It uses probabilistic theory to separate the distributions of 

design loads and material strengths sufficiently in order to maintain an acceptable level of risk 

against failure. LFD requires that the load components be amplified by load factors and 

grouped as prescribed in Table 3.22.1 A of the AASHTO specification. The controlling group 

of these factored loads (required strength) must be less than the design strength which is the 

nominal strength reduced by the strength reduction factor cp. As mentioned previously, Table 

3.22.1 A governs the combination of all loads except seismic loads which are covered under 

Division I-A of the AASHTO specification (see AASHTO section 3.2 1). LFD corresponds to 

position 2 shown in Figure 5-l. 

FHWA-SA-97-070 (~00-06) 5-7 
Special Design Method for Seismic Load Groups 

Division I-A of the AASHTO specification specifies procedures for seismic design which are 

based on using the full nominal foundation strength to resist the controlling group of 

unfactored loads considered to coincide with seismic loads (dead load, buoyancy, stream flow, 

and earth pressure). For the grout-to-ground bond design this manual recommends a LFD 

method for seismic load groups using load factors and cp factor equal to 1. 

Nominal’Strength for Grout-to-Ground Bond 

The nominal strength value for use in both SLD and LFD methods is obtained by picking a unit 

strength per surface area value from Table 5-2. 

SLD Factor-of-Safety for Grout-to-Ground Bond 

The factor-of-safety (FS) recommended in this chapter for the grout-to-ground bond allowable 

value for typical SLD designs is 2.5. This recommendation is based on experience nationwide 

and the design and construction procedures outlined in this manual. It is also tied to the field 

verification and proof test requirements outlined in Chapter 7. Reduction of this value may be 

justified where specific knowledge of the site ground conditions indicate very consistent and 

competent conditions, such as fresh unfractured rock. 

LFD $IpG Factors for Grout-to-Ground Bond 

At this time (ho factors for grout-to-ground bond strengths for micropiles have not been 

determined systematically as required by the probablistic theory. There is, however, a wealth of 

experience with SLD designs for micropiles and ground anchors which are similar in many 

respects. Therefore this chapter recommends the determination of cpc factors for the grout-toground bond strengths by calibration to the SLD factor-of-safety in order to use that body of 

successful knowledge and experience until the probabilistic based values are developed and 

accepted by practitioners for use. This procedure will provide a LFD design that equals a SLD 

5-8 FHWA-SA-97-070 (~00-06) 
one for the grout-to-ground bond. The following procedure shows how to calibrate LFD (Pa 

factors to a SLD FS. It shows that each unique group of loads requires a different (Pi factor. 

SLD Method 

Allowable Load 2: Design Load 

and R, 2 FS x 2 Qi 

i = 1 

LFD Method 

Design Strength 2 Required Strength 

As R,, is the same for SLD and LFD the (Pi required to calibrate LFD to SLD is: 

(PG = 

i = 1 

FS x 2 Qi 

i = 1 

FHWA-SA-97-070 (~00-06) 5-9 
For AASHTO Group I Load Group: 

(PG = 

Y @DQn + B,Q, + B,,QcF + B,Q, + B,Q, + Bs,QsF> 

FS x (QD + QL + QcF + QE + QB + QsF) 

y =1.3 

B, = 1; B, = 1.67; B,, = 1; B, = 1.3 for lateral earth pressure 

B, = 1; BSF = 1 

FS = Factor of Safety 

Q ,, = Dead Load; QL = Live Load; QCF = Centrifugal Force 

Q E = Earth Pressure; QB = Buoyancy; Q SF = Stream Flow Pressure 

Table 5-l shows how qG varies with different combinations of loads for AASHTO Group I 

with dead load, live load, and earth pressure for a FS = 2.5. The (ho value in this table is 

calculated using: 

(PG = 

1.3Qn + 2.174, + 1.694, 

2.5 (QD + Q, + QE) 

It also shows LF = (ho (FS) which is the combination of y and p load factors. These data 

indicate that qG = 0.6 could be used for a conservative value for typical LFD designs. Higher 

qG values may be calibrated for the controlling group of loads with their load factors. 

For AASHTO Load Groups IA - X, 

<pG = 

i = 1 

2 Bi Qi 

where OS = Overstress factor per AASHTO Table 3.22.1A. 

5-10 FHWA-SA-97-070 (~00-06) 
Table 5-l. Calibrated LFD (pG Factor for Various Ratios of Q ,,, Q L and Q E 

QD QL QE cpr, LF 

10 0.52 1.3 

0.9 0.1 0.0 0.55 1.38 

0.0 0.1 0.54 1.35 

0.8 0.1 0.1 0.57 1.43 

0.2 0.0 0.59 1.48 

0.1 0.2 0.59 1.48 

0.7 0.2 0.1 0.61 1.53 

0.1 0.3 0.60 1.50 

0.6 0.2 0.2 0.62 1.55 

0.1 0.4 0.62 1.55 

0.2 0.3 0.64 1.60 

0.5 0.3 0.2 0.66 1.65 

0.4 0.1 0.67 1.68 

0.5 0.0 0.69 1.73 

0.1 0.5 0.63 1.58 

0.2 0.4 0.65 1.63 

0.4 0.3 0.3 0.67 1.68 

0.4 0.2 0.69 1.73 

0.5 0.1 0.71 1.78 

0.1 0.6 0.65 1.63 

0.2 0.5 0.67 1.68 

0.3 0.4 0.69 1.73 

0.3 0.4 0.3 0.71 1.78 

0.5 0.2 0.73 1.83 

0.6 0.1 0.74 1.85 

0.7 0.0 0.76 1.90 

0.1 0.7 0.66 1.65 

0.2 0.6 .0.68 1.70 

0.3 0.5 .0.70 1.75 

0.2 0.4 0.4 0.72 1.80 

0.5 0.3 0.74 1.85 

0.6 0.2 0.76 1.90 

0.7 0.1 0.78 1.95 

0.8 0.0 0.80 2.00 

0.1 0.1 0.8 0.68 1.70 

Note: LF for most designs varies from 1.3 to 1.7 

FHWA-SA-97-070 (~00-06) 5-11 
5.D GEOTECHNICAL DESIGN 

5.D.l Geotechnical Investigation Requirements 

The subsurface investigation required for the design of micropiles is typically no more 

extensive than that which is required for any other type of deep foundation element such as 

drilled shafts or driven piles. The following information is necessary for proper micropile 

design: 

l General geology. 

l Site history (mining, previous excavations, any problems with previous construction, 

construction methods for adjacent utilities or basements or foundations). 

l Description of geologic processes or modes of deposition of strata. 

l Logs of soil borings completed in close proximity to the structure that includes 

description and classification of the soil strata encountered, unit weights, moisture 

contents, standard penetration tests (SPT) or cone penetrometer test (CPT) values, and 

description of groundwater conditions. Boring depths should extend beyond 

anticipated tip elevations of the piles with detailed soil description, particularly for the 

bond zone strata. 

l A subsurface soil profile along the alignment of the structure developed from the soil 

boring information presenting soil type and SPT values as a minimum, must be 

provided for evaluation of the variability of soil strata and determination of the worst 

case soil conditions. 

l An estimation of soil shear strength parameters. Determination of liquid and plastic 

limits for cohesive soils, and determination of the grain size distribution for granular 

soils. 

l If rock is encountered, logs with rock classifications, penetration rates, degree of 

weathering and fracturing, recovery and RQD measurements, unconfined 

compressive strength, and driller’s observations should be provided. 

l Determination and discussion of the presence of hazardous, contaminated, and/or 

corrosive conditions if applicable. This may include resistivity, pH, and the presence 

of lead, sulfates, and chloride content. 

5- 12 FHWA-SA-97-070 (~00-06) 
In general, all geotechnical data interpretations should be provided. The basic character and 

extent of the soil strata determined from the geotechnical investigation can be verified during 

pile installation by monitoring and logging of the penetration rates, drilling action, flush return, 

and soil cuttings. 

50.2 Geotechnical Bond Capacity 

For design purposes, micropiles are usually assumed to transfer their load to the ground 

through grout-to-ground skin friction, without any contribution from end bearing due to the 

following factors: 

l The high grout-to-ground bond capacities that can be attained as a consequence of the 

micropile installation methods. These capacities can reach ultimate values in excess 

of 365 kN per meter of bond length in dense granular soils and 750 kN per meter of 

bond length in competent rock, for typical micropile bond zone diameters (150 - 300 

n-m. 

l The area available for the skin friction is significantly greater than that for end 

bearing. For a pile that is 200 mm in diameter with a 6 m long bonded length, the 

area available for skin friction is 120 times greater than that available for end bearing. 

l The pile movement needed to mobilize frictional resistance is significantly less than 

that needed to mobilize end bearing. 

The dependence on skin friction results in a pile which is considered geotechnicaly equivalent 

in tension and compression. This is a common design assumption for determining the bond 

length for a compression/ tension pile. 

The value typically considered when determining the grout-to-ground bond, either empirically 

or through load testing, is the average value over the entire bond length. Instrumentation of 

tieback anchor and micropile testing has shown that particularly for dense and stiff soils and 

competent rock, the rate of load transfer to the ground is higher at the top of the bond length. 

This is most significant when calculating anticipated pile settlements. A practical 

consideration is that concentration of the reaction to the applied loading in the upper portion of 

FHWA-SA-97-070 (~00-06) 5- 13 
the bond length effectively shortens the length over which the pile deforms elastically, 

reducing the magnitude of the settlement, particularly in stiff soils and rock. 

While the application of micropiles is growing rapidly, the current state of the practice for the 

geotechnical design is primarily based upon the experience and research on drilled shafts, soil 

nails, and tieback anchors. Detailed information on empirical methods used for estimating 

grout-to-ground bond capacities are available in the following publications: 

l ‘<Post-Tensioning Institute (PTl) Recommendations for Prestressed Rock and Soil 

Anchors ” (1996) - Includes a section on determining bond capacity for the various 

types of anchors. 

l “Construction, Carrying Behavior, and Creep Characteristics of Ground Anchors, ” 

H. Ostermayer, 1975 Conference on Diaphragm Walls and Anchorages, Institute of 

Civil Engineers, London - Includes information on the bond capacity of pressure 

grouted and post grouted anchors. 

l “Ground Control and Improvement” (1994) by Xanthakos, Abramson, and Bruce 

(ISBN O-471-5523 l-3, John Wiley & Sons, Inc., New York, NY) - Includes a 

chapter on micropiles with a brief discussion on pile geotechnical design. 

l FHWA Report No.s RD-96-016/017/018/019; Volumes I - IV: “FHWA Drilled and 

Grouted Micropiles State-of-the-Practice Review ” - Includes discussion on micropile 

research developments, construction methods, load test data and case histories 

l FHWA Report No. FHWMRLJ-82/047 “Tiebacks” and FHWA-DP-68-1R “Permanent 

Ground Anchors ” - Includes a section on determining bond capacity for the various 

types of anchors. 

l FHWA Publication No. FHWA-HI-88-042 “Drilled Shafts: Construction Procedures 

and Design Methods ” - Includes a section on determining skin friction and end 

bearing capacities for drilled piers. 

s- 14 FHWA-SA-97-070 (~00-06) 
5.D.3 Summary of Grout-to-Ground CX Bond Nom,na, Strangth Values 

Table 5-2 provides guidance for estimating the unit values for grout-to-ground bond nominal 

(ultimate) strengths. The table includes ranges for the four methods of pile grouting (Type A, 

Type B, Type C, and Type D) in a variety of ground conditions. 

Values for the grout-to-ground nominal bond strength are commonly based on the experience 

of the local Contractors or Geotechnical Engineers. Table 5-2 presents ranges of typical values 

of the nominal bond strength for various installation methods and ground conditions. 

These values can be used to estimate the pile geotechnical tension and compression axial 

design values for SLD and LFD as shown in 5.D.3.1 and 5.D.3.2. As these values vary with 

actual ground conditions, drilling, grouting and pile installation procedures, the final pile 

design should be completed by a specialty contractor who is qualified to perform micropile 

design and construction (see section 5B). The following values are intended to assist a 

designer with preliminary design and general evaluation of specialty contractor final designs. 

Higher bond values may be used based on proper documentation and load test data. 

FHWA-SA-97-070 (~00-06) 5-15 
Table 5-2. Summary of Typical CXbona nominal strength Values (Grout-to-Ground Bond) for 

Preliminary Micropile Design that have been used in Pradtice. 

Soil / Rock Description 

Typical Range of Grout-to-Ground Bond Nominal Strengths @Pa) 

Type A Type B Type C Type D 

Silt & Clay (some sand) 

(soft, medium plastic) 

35-70 35-95 50-120 50-145 

Silt & Clay (some sand) 

(stiff, dense to very dense) 

50-120 70-190 95-190 95-190 

Sand (some silt) 

(fine, loose-medium dense) 

70-145 70-190 95-190 95- 240 

Sand (some silt, gravel) 

(fine-coarse, med.-very dense) 

95-215 120-360 145-360 145-385 

Gravel (some sand) 

(medium-very dense) 

95-265 120-360 145-360 145-385 

Glacial Till (silt, sand, gravel) 

(medium-very dense, cemented) 

95-190 95-310 120-310 120-335 

Soft Shales (fresh-moderate fracturing, 

little to no weathering) 

205-550 N/A N/A N/A 

Slates and Hard Shales (fresh-moderate 

fracturing, little to no weathering) 

515-1,380 N/A N/A N/A 

Limestone (fresh-moderate fracturing, 

little to no weathering) 

1,035-2,070 N/A N/A N/A 

Sandstone (fresh-moderate fracturing, 

little to no weathering) 

520-l ,725 N/A N/A N/A 

Granite and Basalt (fresh-moderate 

fracturing, little to no weathering) 1,380-4,200 N/A N/A N/A 

Type A - Gravity grout only 

Type B - Pressure grouted through the casing during casing withdrawal 

Type C - Primary grout placed under gravity head, then one phase of secondary “global” pressure grouting 

TypeD- Primary grout placed under gravity head, then one or more phases of secondary “global” pressure grouting 

5-16 FHWA-SA-97-070 (~00-06) 
5.D.3.1 Geotechnical Bond Length Tension And Compression Allowable 

Axial Load - SLD 

P 53 

G-allowable = Ond ‘OF;’ strength x 3.14 x DIA bond x (bond length) 

Use FS = 2.5 (soil or rock) for non-seismic load groups. 

5.D.3.2 Geotechnical Bond Length Tension And Compression Design 

Axial Strength - LFD 

P G-design strength = TG x ( Cl bond nominal strength ) x 3.14 x DIAbon, x (bond length) 

Use (Pi = 0.60 for typical designs for non-seismic load groups or calibrated to SLD as 

shown in section 5 .C. 

Use (Pi = 1 .O for seismic load groups. 

SD.4 Geotechnical End Bearing Capacity 

Moderately loaded micropiles have been designed for end bearing on rock. The design may be 

done similar to end bearing drilled shafts or driven piles, or may be based on previous load test 

experience of similar micropiles. 

SD.5 Group Effect For Axially Loaded Micropiles 

The design of foundation support systems incorporating micropiles may require the installation 

of groups of closely spaced piles. With conventional piles, depending on the pile type, 

installation method, and soil conditions, the capacity of a pile group can be significantly 

smaller and the settlement of the group larger than the capacity and settlement of a single pile 

under the same average load per pile in the group. 

FHWA-SA-97-070 (~00-06) 5-17 
This effect is more significant for pile types such as drilled piers, where the opening of the 

large diameter pier shaft reduces the effective stresses acting against the walls and base of the 

adjacent installed piers. The group effect for pile types such as driven friction piles or pressure 

grouted micropiles is much less significant, even beneficial, due to the increase in the effective 

stress of the soil due to the soil displacement by the driven pile or the compaction of the soil 

due to the pressure grouting. 

Model and full scale load testing have verified what has been referred to as a “knot effect” 

(Lizzi, 1982; ASCE 1987) where a positive group effect is achieved in the loading of the soilpile system. This effect is more prominent in granular soils than in cohesive soils. 

For driven piles, AASHTO section 4.5.6.4 recommends no individual pile capacity reduction 

for group considerations with the exception of friction piles in cohesive soils, where an 

efficiency factor of 0.70 should be applied for piles with a center to center spacing of less than 

3.0 times the pile diameter. For pressure grouted micropiles (Type B, C, and D) with a typical 

grouted diameter of 200 mm and typical minimum center to center spacing in the range of 0.75 

to 1 meter, it is unnecessary to consider a group reduction effect under this criteria. 

For gravity grouted (Type A) piles, the soil type and installation method should be examined 

for impact to the effective stress of the soil surrounding the piles and the impact to the capacity 

of the pile group. A method similar to that used for drilled piers may be applicable where the 

pile group is treated as one pile, with the perimeter and base area of the group establishing the 

pile dimension. 

5-18 FHWA-SA-97-070 (~00-06) 
5.E MICROPILE STRUCTURAL DESIGN 

In the structural design of micropiles, reference must be made to local construction regulations 

or building codes. The special considerations of micropile design may not always be 

specifically or adequately addressed in these regulations and codes. In that event, sensible 

interpretation or extrapolation is essential by all parties, backed up by appropriate field testing. 

In this section, design of the various sections of a composite reinforced micropile are examined 

in accordance with the 1996 AASHTO Standard Specifications for Highway Bridges, Sixteenth 

Edition. Both SLD and LFD methods are presented. See Section 5.C for explanation. 

The micropile design examined in this section is represented in Figure 5-2 and consists of an 

upper length reinforced with a permanent steel casing with a center reinforcing bar and a lower 

pressure grouted bond length reinforced with a center reinforcing bar. 

MICROPILE 

TOP OF FOOTING 

----Jx. .,.I ;.. /--- RE’NFoRC’NGBAR 

PILE CAP ANCHORAGE . [ ” i Al 

TOP OF DENSE 

STEEL CASING 

CENTRALIZER 2 

GROUT 

’ Lb-- DIAMETER (DIA 

GROUTED BOND ZONE 

BOND) 

Figure 5 - 2. Detail of a Composite Reinforced Micropile 

FHWA-SA-97-070 (~00-06) 5-19 
SE.1 Notation 

C, = column slenderness ratio (SLD and LFD) 

DL = Design Load (SLD) 

E steel = modulus of elasticity of steel (SLD and LFD) 

FS = factor-of-safety (SLD) 

F, = allowable axial unit stress (SLD) 

F, = nominal axial steel stress (LFD) 

f c-grout = compression strength of grout (SLD and LFD) 

F, = specified minimum yield point of steel (SLD and LFD) 

K = effective length factor (see AASHTO for a table of values) (SLD and LFD) 

L = actual unbraced length (SLD and LFD) 

P e -allowable = allowable structural axial compression load (SLD) 

P t - allowable = allowable structural axial tension load (SLD) 

P c -nominal = nominal structural compression strength (LFD) 

P t-nominal = nominal structural tension strength (LFD) 

P c design = design structural compression strength (LFD) 

P t-design = design structural tension strength (LFD) 

r = governing radius of gyration (SLD and LFD) 

cp = strength reduction factor (LFD) 

a bond nominal strength = geotechnical unit grout-to-ground bond strength (SLD and LFD) 

P G-allowable = allowable geotechnical bond axial load (SLD) 

P G-design strength = geotechnical bond axial design strength (LFD) 

P transfer allowable = cased bond length (plunge zone) allowable transfer load (SLD) 

P transfer design = cased bond length (plunge zone) design strength (LFD) 

5-20 FHWA-SA-97-070 (~00-06) 
5.E.2 Pile Cased Length Structural Capacity 

The tension and compression allowable loads (SLD) and design strength (LFD) for the cased 

upper portion of the pile can be determined with the equations included in the following two 

subsections. Since it is common for the upper section of a pile to be located in a weak upper 

soil, consideration of a laterally unsupported length may be included in determination of the 

compression capacity. See Section 5.F.5 for discussion on lateral stability considerations. See 

Section 5.E.7 regarding compatibility of strain between the grout and various strengths of steel 

used for the bar and casing reinforcement. 

An alternative method for computing allowable loads (SLD) and design strengths (LFD) 

utlizing the transformed section of the pile could be used. It would, however, require careful 

consideration and documentation of the allowable (SLD) and ultimate (LFD) strains for each of 

the component materials. 

Most piles that have soil surrounding the pile have no effective unsupported length and thus no 

reduction for buckling except for piles extending above ground, piles subject to scour, piles 

through mines/caves and piles through soil that may liquefy. The allowable loads formulae for 

piles with unsupported lengths are presented in Section 5.F.5. 

5.E.2.1 Pile Cased Length (Service Load Design) 

For strain compatibility between casing and bar (see section 5.E.7), use the following for steel 

yield stress: 

F y-steel = the minimum of FyVbar & FyWcasing 

Tension - Allowable load 

P t -allowable = 0.55 Fyestee, x ( Areabar + Areacasing) 

(Note: 0.55 is per AASHTO Table 10.32.1A) 

FHWA-SA-97-070 (~00-06) 5-21 
Compression - Allowable load 

F 

Fa = -. . . . . . FS = 2.12 y-steel 

FS 

Allowable load - 

P c -allowable = 0.40f’c-grout Areagout + 0.47Fy-st,,l [ kq,, + heacasing 1 

(Note: 0.40 is per AASHTO 8.15.2.1.1; 0.47 is per AASHTO, i.e. 1 + 2.12 = 0.47) 

5.E.2.2 Pile Cased Length (Load Factor Design) 

For strain compatibility considerations between casing and bar (see section 5.E.7), use the 

following for steel yield stress: Fyesteel = the minimum of Fy-bar and Fy-cming 

Tension - Design strength 

P t -nominal = Fy-steel [ ATeabm + keacaing 1 

Use q+ = 0.90 . . . and with Ptvdesign = qt x Ptenominal 

P t -design = o-90 x Fy-steel [ ATeabm + heacasing 1 

Compression - Design strength 

P c -nominal = O-85 C-grout hea grout + Fy-steel [Area bar + keacasing ] 

Use ‘p, = 0.85 and With PC -&sign = 9, ’ PC -nominal 

P c -design = 0.85 x 0.85 f,‘-,, Area grout + Fymstee,[~eabar + heacasing] 

5-22 FHWA-SA-97-070 (~00-06) 
SE.3 Pile Uncased Length 

The tension and compression allowable loads (SLD) and design strengths (LFD) for the pile 

uncased bond length are shown in the following two subsections. Since the lower uncased 

portion of the bond length is the weakest structurally (typically), an allowance is made for the 

geotechnical grout-to-ground capacity developed along the upper cased portion of the bond 

length (plunge length). This capacity (Ptransfer ai,,,wab,e for SLD and P,,,, design for LFD) adds to 

the structural capacity of the uncased pile for resisting the design load (SLD) or the required 

strength (LFD). Another way to view Ptransfer is that it is the grout-to-ground bond capacity 

developed along the plunge length which can be used to reduce the pile load for design of the 

uncased length of the pile. In the design process the Pcransfer value is typically estimated and 

later verified. Ptransfer may be conservatively ignored and does not apply to end bearing piles. It 

is based on CI bo,,d nomina, stieagth over the plunge zone length (length of casing that is plunged into 

the bond length). See section 5.E.6 for further discussion about the plunge length and Ptransfer 

capacity. The compression ‘pc value for LFD (cp, = 0.75) was selected to provide a similar 

factor-of-safety to the SLD compression value. Higher values of cpc (e.g. 0.85) are justified 

since the uncased length is composed mostly of a steel reinforcing bar surrounded by grout and 

fully supported by the ground. 

See section 5.E.7 regarding strain compatibility between the pile components. This may cause 

the use of less than actual material design strength values. 

5.E.3.1 Pile Uncased Length (Service Load Design) 

Cased bond length (plunge length) allowable load = P transfer al,,,wa,,le 

ci 

P transfer allowable = 

bond “OFF strength x 3.14 x DIA,,,, x (Plunge length) 

I 

The plunge length is typically assumed and later verified. See section 5.E.6 for further 

discussion. 

FHWA-SA-97-070 (~00-06) 5-23 
Tension - Allowable load 

P t-allowable = o*55 F,bar Area bar + p transfer allowable 

(Note: 0.55 is per AASHTO Table 10.32.1A) 

Compression - Allowable load 

P c-allowable = Oa4’ c-grout Area grout + om47 Fy-bar Areakr+ p transfer allowable 

(Note: 0.40 is per AASHTO 8.15.2.1.1; 0.47 per AASHTO, i.e 1 + 2.12 = 0.47) 

5.E.3.2 Pile Uncased Length (Load Factor Design) 

Cased bond length (plunge length) design strength = P transfer design 

P transfer design =Ma bond nominal strength > x 3. l4 ’ DIA bond x (#wze length) 

The plunge length is typically assumed and later verified. See section 5.E.6 for further 

discussion. In the above equation, (~c = 0.60 for typical designs for non seismic load groups or 

calibrated to SLD as shown in 5.C, or (~c = 1 .O for seismic load groups 

Tension - Design strength 

Pt-design = [‘*” ’ Fy-bar x Area bar] ’ Ptmnsferdesign 

(Note: 0.90 is per AISC LRFD, Dl-1) 

Compression - Design strength 

‘c-design = (“‘75) (o*85 rc-gmut ’ Fy-bar Area bar > + Ptransfer design 

Note: The design strength formulas for the pile uncased length are expressed as shown since 

the structural cp values (0.9 for tension and 0.75 for compression) are different from the 

(PG vahles used for Pmsferdesigne The (~c = 1 .O for seismic load groups is only allowed for 

the geotechnical a,,& nominal strensth values. 

5-24 FHWA-SA-97-070 (~00-06) 
5.E.4 Recommended Safety Factors and Test Loads for Field Verification 

and Proof Tests 

Geotechnical 

The verification test load and the proof test load are the maximum test loads computed from 

the following criteria considering both non-seismic and seismic load groups. 

SLD - Use only for non-seismic load groups 

Verification Test Load = 2.5 x DL = Verification Test FS x DL 

Proof Test Load = 1.67 x DL = Proof Test FS x DL 

DL is the unfactored controlling pile load. 

LF’D - Use for both non-seismic and seismic load groups 

Verification Test Load = Required Nominal Strength = (DL x LF) + (Pi = 2.5 x DL 

where DL x LF is thefactored controlling pile load. 

Use (Pi = 0.60 for typical designs for non-seismic load groups or calibrated to SLD as 

shown in Section 5.C. 

Use (Pi = 1.0 for seismic load groups 

Proof Test Load = 1.67 + 2.5 x [Verification Load or Nominal Strength] 

or Proof Test Load = DL x LF 

See Figure 5-3 and worked example problem No. 1 for application of the above. 

The verification test is primarily intended to confirm that the selected bond length for the pile 

and the Contractor’s installation equipment, methods and procedures are capable of producing 

the required grout-to-ground bond nominal strength. The verification test pile is usually a 

FHWA-SA-97-070 (~00-06) 5-25 
sacrificial pile tested prior to the installation of production piles. The proof tests are intended 

to confirm adequate and consistent Contractor pile installation procedures for the production 

piles and to confirm that the production piles will carry the required design service loads 

without excessive, long-term deflection. Proof tests are typically performed on permanent 

production piles. Both tests are required to assure that all permanent production piles 

(including the untested ones) provide the required allowable load for SLD and the required 

design strength for LFD. 

The rationale for selection of the recommended micropile geotechnical safety factors is based 

on comparison to those currently in use for soil nails and permanent ground anchors, both of 

which have similarities to micropiles. A comparison of Industry Standards (U.S.) for 

maximum test loads applied to soil nails and ground anchors, with those recommended in this 

manual for micropiles, is as follows: 

Verification Test Proof Test 

Micropiles 2.5 x DL 1.67 x DL 

Soil Nails 2.0 x DL 1.5 x DL 

Ground Anchors 1.33 x DL 

DL = Design Load = unfactored controlling pile design load 

A graphical representation of the above comparison is also shown in Figure 5-3 that helps 

relate the tests to the SLD and LFD design criteria. It shows that the proof tests for micropiles, 

soil nails, and ground anchors are selected to provide assurance that the DL x LF criterion is 

met or exceeded. The verification test is used, in combination with the proof tests, to provide 

this assurance for micropiles and soil nails as only 5 percent of these elements are tested. The 

combination of verification and proof tests is meant to assure that the distribution of the 

strengths will not exceed <pc and that all untested elements will meet or exceed DL x LF. 

5-26 FHWA-SA-97-070 (~00-06) 
LFD 

DESIGN LOAD (DL) 

I PROOF TEST = 1.67 x DL 

MICROPILES 

(5% TESTED) 

NOMINAL STRENGTH 

DISTRIBUTION 

K!IE: The distribution of loads, which is 

shown by LF, varies from 1.3 to 1.7 for 

most designs. The micropile proof test was 

selected at 1.67 x DL to equal or exceed 

the LF x DL for most designs. FS = LFkp, 

FS = 2.5 

REF: THIS MANUAL 

D ‘ESIGN LOAD (DL) 

SOIL NAILS 

PROOF TEST = 1.5 x DL 

\ ~IZST=~XDL 

(5% TESTED) 

NOMINAL STRENGTH 

FS = 2.5 

REF: FHWA SOIL NAILING 

MANUAL, 1998 

1 rlGN LOAD (DL) 

PROOF TEST = 1.33 x DL 

PERFORMANCE TEST = 1.33 x DL 

GROUND ANCHORS 

(100% TESTED) 

DISTRIBUTION 

_. 

FS = 2.5 

REF: AASHTO STD. SPECS. FOR 

HIGHWAY BRIDGES, 1998 

Figure 5 - 3. Comparison of Maximum Test loads for Micropiles, Soil Nails, and Ground 

Anchors 

FHWA-SA-97-070 (~00-06) 5-27 
With permanent ground anchors, every single production anchor (i.e. 100 percent) is proof 

tested, therefore a lower safety factor is justified. For both soil nails and micropiles, typically 

only up to 5 percent of the production piles / nails are proof tested, therefore higher test loads 

are justified in comparison to ground anchors. Soil nails are typically of lower capacity and 

have greater redundancy than micropiles, especially compared to micropiles used for structural 

foundation designs as set forth in this manual. Thus, higher test loadings are prudent for 

micropiles, compared to soil nails. 

Recall earlier in Section 5 .D.3.1 the recommendation that a geotechnical safety factor of 2.5 

(for both soil and rock) be applied to the estimated “nominal” (ultimate) grout-to-ground bond 

strength, to obtain the “allowable” (SLD) geotechnical grout-to-ground bond value to use for 

design. Section 5.C shows how to calibrate (~c for LFD designs in order to match a SLD 

design with FS = 2.5. 

Therefore, the recommended micropile verification test loading to 2.5 x DL is to verify that the 

micropile’s geotechnical capacity is at least equal to the nominal grout-to-ground strength 

(a bondnominal ,ti,&, estimated for use in the design. The recommended proof test loading to 1.67 

x DL is to provide some assurance that the geotechnical capacity of & the production 

micropiles, including the 95 percent which are not tested, is at least equal to LF x DL. 

Structural 

Since these tests occur under controlled conditions and have a short load duration, the design of 

the pile against structural failure for the verification and proof test loads may utilize a smaller 

factor-of-safety against steel yield in tension, steel yield or buckling in compression, or grout 

crushing in compression, versus the structural safety factors used in the design of the 

production piles. This manual recommends a structural FS = 1.25 against these structural 

modes of failure for the field load tests. Put another way, the micropile structural elements 

should not be stressed to greater than 80 percent (111.25) of the above structural capacities 

during application of the maximum test loads. 

5-28 FHWA-SA-97-070 (~00-06) 
For some designs, the verification test pile may require larger pile casing and rebar than the 

production piles. The resulting stiffer pile can adequately confirm the grout-to-ground bond 

strength for the permanent production piles, but may not provide representative structural 

deflection behavior for the smaller permanent production piles. When this is the case, the 

proof tested production piles (at the 1.0 x Design Load test load increment) will have to be 

relied on to provide the representative structural deflection behavior of the in-service 

production piles. The verification tested pile would only be used to verify the grout-to-ground 

bond nominal strength. 

Guideline criteria for the number of verification and proof tests to perform, along with 

suggested test acceptance criteria, are presented in Chapter 7 - “Load Testing” and in Appendix 

A, “Guide Construction Specifications”. 

This manual recommends a SLD approach for the structural design of the pile for verification 

and proof field test loads with the structural FS = 1.25 as previously discussed. An alternative 

method utilizing the transformed section of the pile could be used. It would, however, require 

careful consideration and documentation of the allowable strains for each of the component 

materials. The allowable load formulas with structural FS = 1.25 are developed next. 

5.E.4.1 Pile Cased Length 

F Y stee, = the minimum of F,, and FyGasing 

Tension - Allowable Field Test Load 

P t-allowable = ‘a80 Fy steel cArea bar + Area casing) 

Compression - Allowable Field Test Load 

F y-steel F,=- . . . FS = 1.25 

FS 

FHWA-SA-97-070 (~00-06) 5-29 
SE.42 Pile Uncased Length 

P transfer allowable = 

‘bond nominal strength 

FS I 

X 3.14 X DIA bond x (Plunge length) 

FS = 1.25 for field test loads 

Tension - Allowable Field Test Load 

Compression - Allowable Field Test Load 

P callowablc = o-68 flc-grout ~CX-I grout $- 0.80 Fy-ba Area bar + Ptransfer allowable 

5.E.5 Grout to Steel Bond Capacity 

The bond between the cement grout and the reinforcing steel allows the composite action of the 

pile, and is the mechanism for transfer of the pile load from the reinforcing steel to the ground. 

Typical ultimate bond values range from 1 .O to 1.75 MPa for smooth bars and pipe, and 2.0 to 

3.5 MPa for deformed bars (AC1 318). 

In the majority of cases, grout-to-steel bond does not govern the pile design. The structural or 

geotechnical pile capacity typically governs. 

As is the case with any reinforcement, the surface condition can affect the attainable bond. A 

film of rust may be beneficial, but the presence of loose debris or lubricant or paint is not 

desirable. Normal methods for the handling and storage of reinforcing bars applies to 

micropile construction. For the permanent casing that is also used to drill the hole, cleaning of 

the casing surface can occur during drilling, particularly in granular soils. 

5-30 FHWA-SA-97-070 (~00-06) 
5.E.6 Design of Plunge Length 

As shown in Figure 5-4, a typical procedure for constructing a composite reinforced micropile 

is to insert the pile casing into the top of the grouted bond length. This detail accommodates 

the transition between the upper cased section and the uncased portion of the bond length. It 

also allows transfer of a portion of the pile load to the ground, reducing the load the uncased 

portion of the pile must support, which is typically the weakest structural portion of the pile. 

This “transfer” load (PtranSfer) occurring through the plunge length of the casing can be 

accounted for as shown in the pile structural calculations included in section 5.E.3 and as 

detailed in Figure 5-4. The value for this load is based on the unit grout-to-ground bond (from 

Table 5-2) acting uniformly over the casing plunge length. 

LOAD IN PILE 

P PILE W) 

CASING PLUNGE 

TO GROUND 

a BOND NOMINAL STRENGTH 

k----k “‘AEON, 

TRANSFER LOAD: 

~,,,~PER WI = (a ~.,~,.,o~,~LsTRE,,~~~) x 3.14 x Dhmv, x PLUNGE LENGTH) 

LOAD CARRIED BY PILE @ DEPTH 1 = PPILE 

LOAD CARRIED BY PILE Q DEPTH 2 = PPILE - PTRANsFER 

Figure 5 - 4. Detail of Load Transfer through the Casing Plunge Length. 

FHWA-SA-97-070 (~00-06) 5-31 
,Reduced P TRANSFER 

P2’- Reduced PmFw 

ZONE I -CASING TOTALLY DEBONDED FROM SURROUNDING GROUND 

ZONE II - CASING TRANSFERRING LOAD TO SURROUNDING GROUND (11) 

ZONE Ill- CASING TRANSFERRING LOAD TO GROUT THROUGH END BEARING (/ \) 

Figure 5 - 5. Change in Load Transfer through Casing Plunge Length as Load 

Increases (Bruce and Gemme, 1992). 

Consideration must also be given to a reduction in the transfer load due to disturbance of the 

bond between the casing and the grout and between the grout and the ground. As shown in 

Figure 5-5, as the pile is subject to higher loading, the casing can debond from the grout and 

the ground, effectively reducing the plunge length and reducing the available transfer load. 

This reduction has been confirmed through field load tests on highly loaded piles. PtranSfer can 

be a significant component in the design of the pile uncased length and therefore its value must 

be carefully evaluated with the micropile specialty contractor’s experience and confirmed by 

field tests. 

5-32 FHWA-SA-97-070 (~00-06) 
5.E.7 Strain Compatibility Between Structural Components 

Strain compatibility between the structural components of a composite reinforced micropile 

should be considered in the pile structural design, particularly when the use of high strength 

reinforcing bars is included. Reinforcing bars are available with yield stress up to 828 MPa 

(1,035 MPa ultimate strength). The strain associated with reaching 85 percent of the bar yield 

stress in compression at the pile ultimate strength (Load Factor Design) may exceed the strain 

that the grout can sustain without fracturing or crushing. Limiting the value of the yield stress 

used in the design may be necessary to avoid grout failure. AASHTO section 8.16.2.3 limits 

the maximum usable concrete compression strain to 0.003, which corresponds to a maximum 

steel stress of 600 MPa. 

Strain compatibility between the grout and casing reinforcement is typically less of a concern 

due to the lower yield strength of the casing steel (typically 55 1 MPa max), and the confined 

state of the grout inside the casing section allows the grout to support higher strain values 

without fracturing. 

Strain compatibility between a high strength bar and the casing needs to be addressed. The 

area of the casing section is typically much greater than that of a high strength bar. This results 

in the distribution of a majority of the pile load to the casing. The strain associated with 

reaching 85 percent of the bar yield stress at the pile ultimate strength (Load Factor Design) 

may result in yielding of the casing, which can reduce the casing threaded joint integrity. 

In summary, strain compatibility requirements dictate the use of the smaller yield stress of the 

reinforcing bar and casing in the calculations, and in compression this value must not exceed 

600 MPa to address the strain compatibility of the grout for the cased portion of the pile. For 

the uncased portion of the pile the reinforcing bar yield stress used in the calculations in 

compression must not exceed 600 MPa. 

The strain compatibility approach used in this manual and described above is intended to be an 

easy to use practical approach for normal designs. A higher tension ultimate strength using a 

higher bar yield stress (compared to the casing) may be utilized if the strains due to the 

FHWA-SA-97-070 (~00-06) 5-33 
working loads are shown to not cause permanent deformations in the threaded casing joints. A 

higher compression ultimate capacity could be utilized for the cased portion if documentation 

was provided to show that the grout within the casing can sustain strains larger than 0.003. 

Other approaches are acceptable with proper documentation. 

SE.8 Reinforcing Bar and Casing Connections 

As discussed in chapter 3, reinforcing bar and casing are commonly installed in coupled 

sections. For installation of piles in very low overhead clearance conditions, the lengths of 

these sections can be 1 meter or shorter. Common reinforcing bar couplers can provide a 

minimum axial capacity of 125 percent of the bar yield strength, which is adequate for 

micropile applications. 

A common method for coupling casing lengths is to machine a thread into the wall of the 

casing at the section ends. If the joint is properly designed, it can provide a compressive 

capacity almost equivalent to the strength of the full casing section. Having the casing filled 

with grout is important to the strength of this type of joint. The grout provides support, 

preventing the male half of the joint from deforming inward. 

Tension and bending have the biggest impact on the strength of the joint due to the reduced 

thickness at the threaded area. The threading may reduce the casing strength in bending and 

tension. The casing joint detail used by a specialty contractor may be a proprietary item. 

Testing data that verifies adequacy of the joint detail may be a necessary component of the 

contractor’s design submittal. 

Bending in a pile is due to lateral loading and the resulting lateral displacement. The bending 

moment in the pile is commonly dissipated within 3 to 5 meters below the bottom of the pile 

cap (see section 5.F.4). The reduced capacity of the joint may be accommodated by locating 

the first joint at an adequate depth where the pile moment is reduced to an acceptable value. 

5-34 FHWA-SA-97-070 (VOO-06) 
SE.9 Pile to Footing Connection 

Unless a single micropile is used, a pile cap (footing) is necessary to spread the structure loads 

and any overturning moments to all the micropiles in the group. Reinforced concrete pile caps 

are designed in accordance with the AASHTO Standard Specifications for Highway Bridges, 

Sixteenth Edition, or AC13 18. The design of reinforced concrete pile caps is not addressed in 

this manual. 

The connection between the top of the micropile and the reinforced concrete pile cap can vary 

depending on the required capacity of the connection, the type of pile reinforcement, and the 

details of the pile cap. Seven examples of the pile-to-footing connections are shown in Figures 

5-6 through 5-12. Figures 5-6 through 5-9 show typical connections for piles that can have 

both tension and compression loads depending on load case. Figures 5-10 through 5-12 show 

simple connections for piles that are only in compression. 

Figure 5-6 shows a composite reinforced pile connected to a new footing. The footing tension 

and compression load is transferred to the pile through the top plate. The stiffener plates 

provide bending strength to the plate, plus provide additional weld length for transferring the 

load from the bearing plate to the pile casing. The stiffener plates can be eliminated if the 

support of the top plate and additional weld length are not required. Additional considerations 

for this connection detail include the following: 

l The portion of the tension load carried by the reinforcing bar can be transferred to the 

top plate through the nut, reducing the plate-to-casing weld requirement. 

l The bond between the pile casing and the footing concrete can be utilized, reducing 

the load capacity required for the top plate and top plate to casing weld. 

l A portion of the compression load can be transferred from the top plate to the casing 

through bearing, reducing the weld capacity requirement. This requires a higher level 

of quality for the fabrication of the bearing surface between the casing and the plate. 

FHWA-SA-97-070 (~00-06) 5-35 
Figure 5 - 6. Pile to Footing Connection Detail. 

Sample problem No. 1 at the end of this chapter includes the design of a pile connection similar 

to that shown in Figure 5-6. The additional considerations listed above are not included in the 

sample problem calculations. 

Figure 5-7 shows a composite reinforced pile connected to an existing footing. The pile is 

installed through an oversized hole cored through the existing footing or slab. After the pile is 

installed, the core hole is cleaned and tilled with non-shrink cement grout. Steel rings are 

welded to the top section of the casing prior to pile installation. These rings transfer the pile 

load from the casing to the non-shrink grout. Adequate spacing must be used between the rings 

to avoid combining bearing stresses in the concrete and grout. The total capacity of the 

connection is controlled by the following: 

5-36 FHWA-SA-97-070 (~00-06) 
REINFORCING 

Figure 5 - 7. Pile to Footing Connection Detail. 

l The sum of the bearing strength of the rings. 

l The capacity of the load transfer across the interface between the non-shrink grout and 

the existing concrete. 

l The shear capacity of the existing concrete. 

Grooves may be chipped into sides of the core hole (typical dimension = 20 mm deep x 32 mm 

wide) to increase the capacity of the grout to existing concrete load transfer. Also, vertical 

reinforcing bars may be drilled and epoxied into the existing concrete around the exterior of the 

connection to increase the punching shear capacity. 

An ultimate capacity in excess of 2,660 kN was demonstrated on a project through load testing 

two connections which were similar to the connection shown in Figure 5-7. The connection 

was installed in a 0.25meter-diameter core hole through a 0.6-meter-thick slab. Completion of 

pre-production load testing for this connection type may be necessary to verify the adequate 

structural capacity. 

FHWA-SA-97-070 (~00-06) 5-37 
For thick existing footings, the shear rings and grooves in Figure 5-7 may be eliminated. Load 

tests on the connections are appropriate to verify the casing to grout bond and grout to existing 

concrete bond for the proposed materials and methods. 

Figure 5-8 shows a composite reinforced pile connected to a new footing. The footing 

compression load is transferred to the pile through bearing on the pile top and reinforcing bar 

plate, and the tension load is transferred through bearing on the reinforcing bar plate. A 

portion of the load transferred from the footing to the pile may be attributed to the bond 

between the pile casing and the footing concrete. 

Figure 5 - 8. Pile to Footing Connection Detail. 

5-38 FHWA-SA-97-070 (~00-06) 

--- 
Figure 5 - 9. Pile to Footing Connection Detail. 

Figure 5-9 shows a bar-reinforced pile cast into a new footing. The footing’s compression and 

tension load is transferred to the pile through bearing on the bar plate, and through bond 

between the footing concrete and the reinforcing bar. Additional compression load can be 

transferred through footing compression on the top of the pile grout column. Competency of 

the construction joint between the pile grout and footing concrete is an important quality 

consideration for this pile type. 

Figure 5-10 shows a typical compression connection for a moderately loaded micropile where 

concrete bearing stress on the pile top is within AASHTO/ACI limits. 

Figure 5-l 1 shows a typical compression connection for a heavily loaded micropile where a 

bearing plate is required for concrete bearing stress on the pile top to be within AASHTO/ACI 

limits. 

FHWA-SA-97-070 (~00-06) 5-39 
Figure 5 - 10. Pile to New Footing Connection Detail - Simple 

Compression, Moderate Load. 

I I 

Figure 5 - 11. Pile to New Footing Connection Detail - Simple 

Compression, High Load. 

5-40 FHWA-SA-97-070 (~00-06) 
Figure 5 - 12. Pile to Existing Footing Connection Detail - 

Compression Only. 

Figure 5-12 shows a compression connection for a pile through an existing footing. The bond 

of the non-shrink grout to the footing is a function of the proposed methods (cored hole, downthe-hole hammered hole, etc.) and materials (non-shrink grout, existing concrete, etc.). 

Connection load tests are appropriate to verify the design. 

5.F ADDITIONAL GEOTECHNICAL / STRUCTURAL CONSIDERATIONS 

5.F.l Prediction of Anticipated Structural Axial Displacements 

For many designs axial pile stiffness is not a concern. Earthquake foundation retrofit work 

requires, however, deflection compatibility between existing and new piles thereby requiring 

special attention to pile stiffness. New structure and earthquake retrofit designs often require 

determination of pile stiffness for inclusion into the overall structure model for earthquake 

analysis. When pile designs require stringent displacement criteria, such as the earthquake 

cases mentioned above, it may be necessary to predict pile stiffness and deflection limits during 

design and confirm through field load tests. 

FHWA-SA-97-070 (~00-06) 5-41 
The design of a micropile can be controlled by stiffness requirements (allowable displacement) 

due to the small cross sectional area of the pile. Pile load testing is typically conducted to 

verify the required stiffness and load capacity. During the pile design phase, methods are 

available for prediction of the pile displacements under compression and tension loading, 

allowing initial estimation of the required pile stiffness. 

One method is referred to as the “t-z” load transfer model, where t represents the interface shear 

stress and z the vertical displacement. This method, which was developed for predicting the 

displacement behavior of large diameter piles, involves modeling the pile as a series of finite 

elements, while the ground is represented as a series of distributed axial springs. Selection of 

proper design parameters and accurate prediction of pile behavior can be difficult given the 

effect of variable soil conditions and the effect on pile performance from installation methods. 

This method is not addressed further in this manual. 

A more simplistic approach included in this manual is similar to the approach used for 

analyzing tieback anchor performance and consists of separately predicting the elastic 

(recoverable) and residual (non-recoverable) portion of the pile displacement. 

The magnitude of the elastic displacement is dictated by the magnitude of the applied load and 

the length of the pile over which it acts. If a pile were acting purely in end bearing, then the 

length over which the pile acts elastically would be constant, and the deflection would always 

be proportional to the load. As the load is increased on a micropile, the bond to the soil in the 

upper portion of the grouted pile length is disturbed, and the location of soil bond resultant 

moves progressively downward. The elastic length of the pile increases with the downward 

movement of the soil bond resultant. 

The magnitude of elastic and residual components of the displacement can be determined 

during load testing, with the residual movement being the zero (or alignment) load 

displacement reading after a load cycle, and the elastic movement being the difference between 

the peak load and zero load displacement readings. 

5-42 FHWA-SA-97-070 (~00-06) 
For an anchor or micropile, the elastic displacement and elastic length can be approximated 

using the formula: 

A PL 

Elastic = - AE 

where 

A(Elastic = elastic component of the total displacement (m) 

P = applied load (kN) 

L = elastic length (m) 

AE = stiffness of section (Area (m*) x Elastic Modulus @Pa)) 

For prediction of the anticipated displacement during the design phase, the pile stiffness must 

be determined, and values must be selected for the residual movement and elastic length. For 

anchors, determination of the stiffness value AE is relatively easy, with its single reinforcement 

and no contribution to the anchor stiffness from the grout. Determination of a micropile’s 

stiffness value is more complex due to: 

l The contribution of the grout to the pile’s stiffness due to the pile acting in 

compression. 

l The varying reinforcement used in some micropiles, with casing reinforcement in the 

upper portion of the pile and bar reinforcement in the lower portion. 

l The stiffening effect on the grout when confined in the casing reinforcement. 

The stiffness of the composite section can be determined using the formula: 

EApile = [ Agrout ’ Egr~ut] + [ Asteel ’ Esteel ] 

FHWA-SA-97-070 (~00-06) 5-43 

-- ..- 
Based on results from pile and material load testing, the use of Egrout =3 1,000 MPa for grout 

confined in a cased length, and E,,, = 23,000 MPa for unconfined grout can provide 

reasonable results. 

The length over which the pile will act elastically must be estimated based on pile installation 

methods, competency of the soils, and experience. A pile grouted full length in competent soils 

will have a short elastic length. For a pile such as shown in the example problem in this 

chapter, with a casing extending into a lower competent layer and the grouted length limited to 

that competent layer, the elastic length may extend to the bottom of the casing. If the bond 

length soils are less competent, the elastic length may extend below the cased length. 

The magnitude of the residual movement must also be predicted based on pile installation 

methods, competency of the soils, and experience. The residual movement will increase with 

an increased applied load, and will increase with the increasing softness of the soil or 

decreasing geotechnical bond capacity. For a pile bonded into medium dense to dense soil 

with applied loads up to 1,300 kN, typical residual movement values may vary from 2 to 5 mm. 

During load testing, this method can be used to examine pile performance by plotting 

development of the elastic length. The elastic length extending into the bottom half or third of 

the bond length may be an indication of pending geotechnical failure. Examination of the 

reasonableness of the results is also necessary. An elastic length extending beyond the bottom 

of the pile would indicate an inaccuracy in the stiffness value used. 

5.F.2 Long Term Ground Creep Displacement 

Long term displacement performance of micropiles and ground anchors depends upon the 

potential of the ground system to creep. Creep is a time dependent deformation of the soil 

structure under a constant or sustained loading. Theoretically, creep can develop in three basic 

components of the system: the grout, the steel, and the ground surrounding the bond length. 

Creep deformations of the grout and the steel material are considered to be insignificant. 

Creep deformations due to loss of bond between grout and steel may be greater, but are still 

insignificant. As the bond decreases and the load transfers deeper, the “creep” also includes 

5-44 FHWA-SA-97-070 (~00-06) 
elastic deflection of the pile. Fine-grained clayey soils may undergo large creep deformation 

that will result in significant time dependent anchor or pile displacement. 

If micropiles are to be installed in creep sensitive cohesive soils, extended load testing similar 

to that specified for ground anchors (Post Tensioning Institute (PTI) - Recommendations for 

Prestressed Rock & Soil Anchors - 1996) can be performed to verify performance within 

acceptable limits. Test load hold duration may be extended to 100 minutes or in extreme cases 

1,000 minutes or more, depending on the magnitude and type of the design loading and creep 

sensitivity of the soil. A maximum creep rate of 2 mm per log cycle of time is a common 

acceptance criteria (PTI, 1996). This criteria is included in the load testing acceptance criteria 

in the Appendix A guide specifications. 

5.F.3 Settlement of Pile Groups 

In addition to the components of axial displacement for a single pile described in the previous 

two subsections, arrangement of piles in a group can cause additional displacement due to the 

consolidation of the soil layer below the pile group. Where a single pile will transfer its load to 

the soil in the immediate vicinity of the pile, a pile group can distribute its load to the soil layer 

below the group. Consideration should be made for this group displacement when the soil 

below the group is cohesive in nature and subject to consolidation. 

To compute pile group settlement design guidance is given in the Federal Highway 

Administration Design and Construction of Driven Pile Foundations Workshop Manual 

(Publication No. FHWA HI-97-O 13 -December 1996). 

FHWA-SA-97-070 (~00-06) 5-45 

-_ 
SF.4 Lateral Load Capacity 

The behavior of a laterally loaded micropile depends on the properties of the micropile such as 

diameter, depth, bending stiffness, furity condition of the pile in the footing, and on the 

properties of the surrounding soils. The effects to the surrounding soil from pile installation 

should also be considered. These effects can include loosening of the soil due to pile drilling 

and densification of the soil due to grout placement. Reference is made to the following 

FHWA published documents and computer analysis program for discussion on the behavior 

and analysis of laterally loaded piles. 

l Behavior of piles and pile groups under lateral load (FHWAKD-85/106). 

l Handbook on design of piles and drilled shafts under lateral load (FHWA-IP-84-11). 

l COM624P - laterally loaded pile analysis program for the microcomputer (FHWASA-91-048). 

Methods available to increase the lateral capacity provided by a micropile include: 

l Installing the pile at an incline or batter. 

l Installation of an oversized upper casing which increases the effective diameter of the 

pile, the lateral support provided by the soil, and the bending strength of the pile 

Consideration must be made to the combined stresses in the micropile due to bending induced 

by the lateral displacement and axial loading. The ability of the pile section to support the 

combined stress must be checked, particularly at the casing joint locations. 

The lateral stiffness and capacity of a micropile is limited due to the smaller diameter. 

Computer programs, such as COM624P mentioned above, are available to determine the lateral 

pile stiffness of a micropile, which is a complex relationship of pile deformation and the 

reaction of the surrounding soil, which usually is nonlinear. A linear approximation of this 

behavior is described in NAVFAC (1982). Also, for an illustration of the NAVFAC procedure, 

5-46 FHWA-SA-97-070 (~00-06) 
see the FHWA Publication No. FHWA-SA-97-010, Seismic Design of Bridges - Design 

Example No. 5. 

The NAVFAC procedure will be demonstrated in the following computations for a 244.5 mm 

OD micropile. 

OD = 244.5 mm; Wall thickness = 11.99 mm; ID = 244.5 - 2 x 11.99 = 220.52 mm 

E casing = 200,000 MPa 

E grout =3 1,000 MPa 

Icasing = 6 [COD4 - ID41 = -& [ 244.54 - 220.524 ] = 59,000,OOO mm4 

I grout = $ [ID41 = $ [ 220.524 ] = 116,000,000 mm4 

EI = Ecasing Icasing + Egrout Igrout =15,396 kN - m2 

Lateral load required to produce 6.35 mm (l/4”) lateral displacement in a dense soil by 

NAVFAC is: 

P= 6 II < ’ [ (EI;‘,I ]’ 

Use f (modulus of subgrade reaction) = 17,600 kN/m3 for dense soil above ground water (for 

this example). 

6, = 6.35 mm (l/4”) 

FHWA-SA-97-070 (~00-06) 5-47 
FS=2.6forL=3T L = pile length and T = 

[ 1 

7 

l/5 

F, = 2.25 for L > 5T 

T= [ 1 15,396 1’5 = og7 m 

17,600 ’ 

For a pile length > 5T = 4.9 m use F, = 2.25 

p=[ 6*3;;)OO]XG!$ =47.1kN 

Displacement limits usually control the allowable load for the soil/pile interaction 

demonstrated by the above NAVFAC procedure. The above 6.35 mm (l/4”) limit is used by 

many designers as the limit for non-seismic load groups. Higher displacement limits are 

usually used for seismic load groups, however the ultimate capacity of the pile must not be 

exceeded and the effects of the pile displacements to the overall structure must be evaluated. 

For many designs batter piles provide sufficient lateral resistance for the lateral loads from the 

horizontal component of their axial load. Where batter piles are not sufficient the additional 

resistance from the soil/pile stiffness, as described above, is added to the batter pile resistance. 

Passive soil resistance against the footing is seldom used for lateral resistance since the lateral 

displacement required to mobilize passive resistance is large. 

Table 5-3 shows pile stiffness values (Load in kN for 6.35 mm lateral displacement) for 3 

different micropile sizes for 6 values of soil modulus (3 above ground water and 3 below 

ground water). Also shown for comparison are 3 different HP pile sections. These values can 

be prorated; for example the lateral load required to displace a 244.5 mm (w/ 11.99 mm wall) 

micropile 12.7 mm (%‘I) in a dense soil above the ground water is 47.1 kN x 2 = 94.2 kN. 

5-48 FHWA-SA-97-070 (~00-06) 
Table 5-3. P = Lateral Load for 6.35 mm (l/4 inch) Lateral Displacement in kN for Coarse 

Grained Soil (Pinned Pile Head Condition) 

Above Ground Water 

Soil f (kN/m3) Loose Medium Dense 

2,199 6,597 17,592 

HP 10x42 

EI = 17,477 kN-m* 14.2 27.5 49.5 

HP 12 x 53 

EI = 32,707 kN-m* 18.3 35.3 63.6 

HP 14 x 89 

EI = 75,235 kN-m* 25.5 49.3 88.8 

MP 139.7 mm, 9.17 mm 

wall, EI = 1,905 l&J-m* 5.9 11.3 20.4 

MP 177.8 mm, 12.65 mm 

wall, EI = 5,237 kN-m* 8.7 17.0 30.6 

MP 244.5 mm, 11.99 mm 

wall, EI = 15,396 kN-m* 13.5 26.1 47.1 

HP = H-pile, MP = Micropile 

Below Ground Water 

Loose Medium Dense 

1,256 4,398 10,681 

10.2 21.6 36.7 

13.1 27.7 47.2 

18.2 38.6 65.8 

4.2 8.9 15.1 

6.3 13.3 22.7 

9.6 20.5 34.9 

[(EI,$’ ]” = 1 6.35 / 1000 1 x 

2.25 [ (EI,:;“’ 1’ 

F, = 2.6 forL=3T L = pile length and T = 

F, = 2.25 forL>5T 

Table 5-3 uses soil modulus (f) values consistent with those used in Figures 25 and 26 for steel 

H-piles in the AISC “Highway Structures Design Handbook,” Volwne I, Section 10. The 

FHWA-SA-97-070 (~00-06) 5-49 
following soil properties were used for the f values in Table 5-3. Refer to the AISC and 

NAVFAC references for more information about f values. 

ye (kN/m3) <p (degrees) f (kNm3) 

Coarse Grained Soil Above Groundwater 

Loose soil 14.9 28 2,199 

Medium dense soil 17.3 30 6,597 

Dense soil 17.3 36 17,592 

Coarse Grained Soil Below Groundwater 

Loose soil 8.6 28 1,256 

Medium dense soil 9.4 30 4,398 

Dense soil 10.2 36 10,681 

5.F.5 Lateral Stability (Buckling) 

Mathematical models and experimental load testing have been applied in the examination of 

buckling of micropiles. Bjerrum (1957), Mascardi (1970, 1982), and Gouvenot (1975) 

concluded that buckling of micropiles is of concern only in soils with the poorest mechanical 

properties such as loose silts, peat, and soft unconsolidated clays (soils that have an elastic 

modulus of less than 0.5 MPa). 

As part of the Caltrans pile load test program of various pile types at a deep Bay mud site in 

1992, a specialty contractor installed several micropiles for load testing. The soil conditions 

consisted of approximately 30 meters of soft Bay mud (very soft clay) over dense sand. The 

piles, reinforced with 178~mm-outside-diameter casing, were tested in compression to loads 

exceeding 1,775 kN with no signs of buckling. Fore more information, see Caltrans (1993). 

Project load testing programs will provide a conservative check of a micropiles buckling 

tendency. A production pile will typically perform under a fixed head condition with the pile 

5-50 FHWA-SA-97-070 (~00-06) 
top embedded in a concrete footing. A load test will typically be conducted under a free head 

condition with the pile top approximately 0.5 meter above ground. 

Consideration of a pile’s unsupported length can be addressed during the design phase through 

the inclusion of values for the effective length factor (K) and the unbraced pile length (L) in 

determination of the allowable or nominal (ultimate) compression of the upper pile length 

(sections 5.E.2.1 & 5.E.2.2). As mentioned above, most pile designs that have soil surrounding 

the pile will have KL = 0 and therefore no reduction for buckling. Piles that are extended 

above the ground or piles that are subject to scour must, for example, be checked for the 

buckling reduction. 

For piles with an unsupported length, the following equations apply: 

Service Load Design (SLD) Method 

cc = 

If E=o 

F y-steel Fa = - FS = 2.12 

r FS 

KL F 

If Ox---- I cc Fa = 

y-steel x 

r FS 

If =*c 7C 2 E steel 

r C Fa = 

FS[KL/r12 

Allowable Load: 

i 

l- (K L / r> 2 Fy -steel 

4x2 Esteel I 

F 

P c -allowable = 0.4Of’,-,,, heagrout 

I 

+*(Areabar+Area Fa xcasmg F y-steel 

FS 

FHWA-SA-97-070 (~00-06) 5-51 
Load Factor Design (LFD) Method 

For strain compatibility considerations between casing and bar (see Section 5.E.7), use the 

following for steel yield stress: Fpsteel = the minimum of F,, and F y-casing* 

-I 

I/2 

If $=o Fa = Fy-steel 

If oi= I c, W L / r12 Fymsteel 

r 4 K 2 Esteel I 

If =tc n 2 Esteel 

r c F, = 

(KL/r)2 

Design strength 

P = c -nominal [ 0.85 fc/-grout hea grout + Fyesteel (hea,, + heacasing )] x 5 F y-steel 

Use ‘p, = 0.85 and with ‘c-&s@ = ‘PC x ‘c-nominal 

P c -design = O-85 X [ O-85 Cgrout hea grout + Qteel ( Areabar + Area )I Fa Xcasmg F y-steel 

To illustrate the buckling reduction the piles for Sample Problem No. 1 will be evaluated for a 

predicted scour depth of 3 meters below the bottom of the footing. 

5-52 FHWA-SA-97-070 (~00-06) 
Service Load Design (SLD) Method 

Use K = 0.65 (fured-fixed condition) 

L = 3m 

E steel = 200,000 MPa 

FS = 2.12 

rcasing = 46mm 

Fycasing = 241 MPa 

F y-bar = 520MPa 

f’ c-gmut = 34.5 MPa 

Area casing = 3,224 mm* 

Area bar = 1,452 mm2 

Area grout = 10,240 mm* 

KL 

- = 42.38 < C, = 2R2 Esteel 

rcasing F y-steel 

Fa = 

l/2 

= 128 

= 108 MPa 

P = c -allowable 0.40 fiTgout Area grout + Area FE3 

casing )I XF v-steel 

FS 

= 636 kN 

FHWA-SA-97-070 (~00-06) 5-53 
Load Factor Design (LFD) Method 

Similar to above except: 

Fa = Fy-steel 1 - 

1 

(J=W2 Fy-steel = 228 ma 

4X2 Esteel I 

P c -nominal = 0.85 f,‘-,,t Area groUt + Fywstee, ( Areabar + Area )] x -..!.L [ casmg F y-steel 

= 1,349 kN 

And with cpc = 0.85 

‘c-design = Cpc Pc-nominal = 17147 kN 

SF.6 Downdrag and Uplift Considerations 

Piling systems may be subjected to additional compression loading due to downdrag forces 

from settling soils and additional tension loading due to uplift forces from expansive soils. A 

good discussion on the consideration of these forces for the design of drilled piers is included 

in the manual: Drilled Shafts: Construction Procedures and Design Methods, (FHWA-HI-88- 

042). 

The use of micropiles for a foundation system on sites where downdrag or uplift forces are of 

concern provides several benefits. The small surface area of a micropile reduces the ability of 

the settling or expansive soil to transfer load to the pile. Further isolation of the pile from the 

moving soils can be accomplished by installation of an additional oversized outer casing 

through the moving soils. The use of battered piling should be avoided in such conditions 

where settlement or expansion will induce excessive lateral loading on the pile. 

5-54 FHWA-SA-97-070 (~00-06) 
5.G SAMPLE PROBLEM NO.1 - BRIDGE ABUTMENT FOUNDATION SUPPORT 

The following sample problem illustrates the design of foundation support for a bridge 

abutment using micropiles. This sample problem is intended to illustrate the quantification of 

typical abutment loads and the design of a micropile foundation for non seismic and seismic 

load groups. It also illustrates SLD and LFD methods (See 5.C). Practitioners typically follow 

different procedures around the United States in the design of bridge abutments, and therefore 

this sample problem is not intended to depict a “standard abutment” or a “standard abutment 

design procedure.” F,or simplicity, this example problem considers only a portion of the 

longitudinal forces and no transverse forces. Abutment designs which include wind or seismic 

forces, for example, would have transverse as well as longitudinal forces. This example also 

does not illustrate construction load cases (e.g., full backfill prior to girder placement) that need 

to be incorporated into actual designs. 

5.G.l Problem Statement 

The structure is a simply supported, single-span bridge, 30 meters long, supported on concrete 

retaining abutments. The superstructure consists of 5 AASHTO Type IV precast - prestressed 

concrete girders with a cast-in-place concrete deck. 

The bridge abutment length is 10.5 meters. The abutment wall backfill material is medium 

dense sand with an angle of internal friction of 35 degrees and a unit weight of 17.5 kN/m3. 

The unit weight of the concrete is 23.6 kN/m3. Figure 5-13 shows the dimensions of the 

abutment. The pile details are shown on Figure 5-14. 

A summary of loading applied to the bridge abutment is shown in Figure 5-15. All load values 

are per 1 -meter length of the abutment. The seismic site design coefficient is 0.1 g. 

The foundation soil conditions are described in the boring log contained in Figure 5-16. These 

soils consist of 2.5 meters of loose sandy gravel underlain by a moderately compressible soft, 

brown, fine sandy silt which is 1.5 meters thick. The silt is underlain by dense to very dense 

gravel with cobbles and boulders which extend to a maximum depth of 30 meters. Ground 

FHWA-SA-97-070 (~00-06) 5-55 
water is 4 meters below the top of footing. Type B3 (pressure grouted through the casing 

during casing withdrawal) micropiles will be used. The bond length will be formed in the 

dense gravel. 

1700 

2900 

2500 

1500 

26 m 

5250 

BACKFILL MATERIAL: 

c- MEDIUM DENSE SAND 

1 am y = 17.5 kN/m3 ; cp = 35” 

MICROPILE 

TYPE B3 

LOOSE TO MEDIUM DENSE SANDY 

GRAVEL (GW) 

SANDY SILT (ML) 

- 

DENSE TO VERY DENSE SANDY 

GRAVEL AND COBBLES WITH 

BOULDERS UP TO l-METER (GW) 

Figure 5 - 13. Sample Problem No. 1 - Abutment Section Detail. 

5-56 FHWA-SA-97-070 (~00-06) 
-j--%---j- ,-ABUTMENT 

TYPICAL SECTION 

ABUTMENT FOOTING 

(NOT TO SCALE) 

REINFORCING 

SAR43mnGRADE520 

7 NEATCEMENTGROLIT 

/ 

DETAIL 1 

(NOT TO SCALE) 

TOP OF DENSE SANDY r*0~~~.4N~ COBBLES 

n4mm I254mrn * 25.4mm 

TOP PLATE 

REINFORCING BAR 

141mm~9.5iwn WALL 

NEATCEMENTGROUT 

SECTION A -A 

(NOT TO SCALE) 

DRILLED AND GROUTED 

MICROPILE SHAFT 

EAT CEMENT GROUT 

REINFORCING BAR 

SECTION B - B 

(NOT TO SCALE) 

REINFORCING i3A> / 

“‘“,z\ , ,-13mmvGROUTHOLE 

n 

6.35mm V 

v I 

STIFFENER PLATE (PIPICAL) 

1SDm-n x 12.7mm 
DC = Dead load of concrete abutment 

Ds = Dead load of soil 

V DL = 178.70 kN/m (dead load from bridge structure) 

VLL = 73.00 kN/m (live load from bridge structure) 

HL = Earth pressure due to live load surcharge V, 

VL = Live load surcharge = 0.8 m thick equivalent soil surcharge 

(not included in this example) 

PE = Active earth pressure 

P EQ = Seismic earth pressure 

I* = 30.9 kN/m (seismic inertia force of concrete abutment and soil weight) 

1s = 26.8 kN/m (seismic inertia force of the superstructure) 

Figure 5 - 15. Sample Problem No. 1 - Summary of Abutment Loads. 

5-58 FHWA-SA-97-070(vOO-06) 
c)I 

FEDERAL HIGHWAY ADMINISTRATION 152 mm H-S AUGER III BEGAN: 28/l 1% 

VANCOUVER, WASHINGTON 203 mm H-S AUGER 0 COMPLETED: 28/l l/95 

GEOTECHNICAL SECTION 

4c/ 

NX CORE 0. DRILL: CME 850 

BORING LOG (METRIC UNITS) HQ CORE II DRILLER: Rod Terzaghi 

OTHER: WEATHER: Wart’tI, Bunny 

s 

2 E $ 

z DESCRIPTION 

ii 

d BLOW Standard Penetration Resistance 

ii 

COUNT z 

;: 2 20 

(13104y per ;l m) 

80 b 

n 

I!;,$ Loose to medium dense, gray sandy 

I ,.do + I** subrounded gravel (GW) 

-,*‘*a .:‘*I 

-4;‘::; R-l 

-*.* ‘1 .o I{ 

-:I 0:: 

>.a’: 

und Water Level 

and cobbles with boulders up to 1 

Drove sampler on boulder 

Some seams at clean medium sand 

up to 150 mm thick. 

Becomes very dense 

Note: Similar material encountered to 

maximum depth explored of 30 

PROJECT: Micropile Example Problem #I BORING B-l 

STATION, OFFSET: Sheet: 1 of 1 

Figure 5 - 16. Sample Problem No. 1 - Soil Boring Log 

FHWA-SA-97-070 (~00-06) 5-59 
The step-by-step procedure used for design of the micropiles is as follows: 

Step 1 - Determine the magnitude and point of application of the design loads acting on the 

abutment. 

Step 2 - Determine the summary of horizontal forces, vertical forces, and overturning 

moments acting on the abutment for each non seismic load group per AASHTO Section 3.22 

and for the seismic load groups per AASHTO Division I-A. Select a pile layout and determine 

the front and rear pile design load (SLD) or required strength (LFD) for each load group. 

Step 3 - Complete the design for the pile including: 

1. The allowable load (SLD) or the design strength (LFD) of the upper cased length 

2. The allowable load (SLD) or the design strength (LFD) of the lower uncased length 

3. The allowable load (SLD) or the design strength (LFD) of the geotechnical bond 

length 

4. Verify that the structural and geotechnical allowable loads exceed the design load for 

SLD or that the design strengths exceed the required strength for LFD as determined 

in step 2. 

Step 4 - Determine and evaluate the anticipated displacements under service loading. 

Step 5 - Complete the design for the pile top detail for connection of the piling to the 

abutment footing. 

Step 6 - Complete drawings showing details of the pile design, pile layout, top connection 

detail, and general notes on materials and installation procedures. 

For simplicity this example uses dead load, live load, earth pressure, and seismic loads. It 

illustrates a non seismic load group and a seismic load group as well as SLD and LFD 

methods. Steps 2,3, and 4 are separated into two sections, one for the SLD method, and one 

for the LFD method. 

5-60 FHWA-SA-97-070 (~00-06) 
5.G.2 Step 1 -Abutment Design Loads 

The magnitude and point of application for the active earth pressure, earth pressure due to live 

load surcharge, and seismic earth pressure is determined in this section. Calculations for the 

remaining components of the abutment loading are not included. The magnitude, point of 

application, and resulting moment on the abutment for all of the load components are 

summarized in Table 5-4. The moments are taken about the center point of the base of the 

abutment footing. 

5.G.2.1 Active Earth Pressure - P, 

Soil internal friction angle cp = 35 degrees 

Unit weight of soil ysoi* = 17.5 kN / m3 

Coefficient of active soil pressure K,=tan2 45”~; =0.27 [ I 

Active earth pressure Kaysoil = 4.74 kN/m3 

Resultant load P, = 0.5 x 4.74 -k?! x (5.25 m)2 = 65.32 kN/m 

m3 

Moment about centroid 65.32 g 

5.25 m X ~ = 114.31 l&m/m 

m 3 . 

FHWA-SA-97-070 (~00-06) 5-61 
Table 5-4. Sample Problem No. l- Summary of Abutment Loads Per Meter Length 

Load Moment Arm 

Load Description Type 

(I& (i) (ii) 

DC 

Dead load of concrete 

abutment D 97.00 0.27 

DS Dead load of soil E 108.68 -0.55 

V Dead load from bridge 

DL superstructure D 178.70 0.58 

VLL 

Live load from bridge 

superstructure L 73.00 0.58 

HL 

Earth pressure due to live 

load surcharge 

L 14.96 2.63 

PE Earth pressure E 65.32 1.75 

P EQ-H Seismic earth pressure EQ 15.91 3.15 

1, 

Seismic inertial force of 

concrete & soil weight EQ 30.9 2.35 

IS 

Seismic inertial force of 

the superstructure EQ 26.8 3.55 

*Moment is calculated about the center of the footing at its base. 

Moment* 

(kt?m) 

26.19 

-59.77 

103.65 

42.34 

39.27 

114.31 

50.12 

72.50 

95.14 

5.G.2.2 Earth Pressure Due to Live Load Surcharge - H, 

Surcharge pressure K, x ysoi, x 0.6 m = 2.84 kPa 

Surcharge load 2.84 kN x 5.25 m = 14.96 kN/m 

Moment about centroid 14.96 Idv x 5*2; m = 39.27 kNm/m 

m 

5-62 FHWA-SA-97-070 (vOO-06) 
5.G.2.3 Seismic Earth Pressure - P,, 

See “Seismic Design of Bridges - Design Example No. 3” Publication No. FHWA-SA-97-008 

for a description of the following Mononobe - Okabe lateral seismic earth over pressure and the 

seismic inertia forces from the abutment self-weight and the soil resting on the abutment 

footing. 

Seismic acceleration coefficient A = 0.10 

Seismic coefficients 

kh = 1.5 x A = 0.15 and kV = 0 (assumedvalue) 

Reference AASHTO Sec. 6.4.3 (A) Div. 1A. 

Slope of soil face p=o” 

Backfill slope angle i=()” 

Friction angle between soil and abutment, 6 = % q = 17.5” 

The wall friction was not used for the active earth pressure to be conservative. It was used for 

the seismic over pressure, however, to reduce some conservatism. 

Seismic inertia angle ktl 8 = atan - I 1 = 8.53 ’ 

1 - kV 

Seismic earth pressure coefficient 

Y=[ 1 +44]‘;2.65 

FHWA-SA-97-070 (~00-06) 5 - 63 
KAE = 

I 

cos2(cp - e> 

Y x cos(0) x cos(6 + 6) 1 = 0.34 

Seismic earth pressure KAE ysoil = 5.95 kN/m3 

Resultant seismic force 

P EQ 0.5 x 5.95 f$ x (5.25 m)2 1 - P, = 16.68 kN/m 

Since this force is at angle 6 to the horizontal, ‘EQ-H = 16.68 E (cos 5) = 15.91 kN/m 

m 

To be conservative this example will neglect the vertical portion of P,, over the footing heel. 

5.G.2.4 Seismic inertia Forces 

I, = k, (DC + Ds) = 0.15 x (97.0 + 108.68) = 30.9 kN/m 

I, = k, (V,,) = 0.15 x 178.70 = 26.8 kN/m 

Moment about centroid 16.68 E x 0.6 x 5.25 m = 52.54 .!?% 

m m 

5-64 FHWA-SA-97-070 (~00-06) 
5.G.3 Service Load Design (SLD) Method 

See 5.C for a general description of the SLD method. The following section will illustrate SLD 

for non seismic loads. See 5.G.4 for the design for seismic loads since the LFD method is used 

for seismic in this manual. 

5.G.3.1 Step 2 (SLD) - Determine Pile Design Loads Per Meter Length 

The Group I loads per AASHTO Table 3.22.1A are calculated from the abutment loads shown 

in Table 5-4. 

F, = sum of vertical loads 

F, = sum of horizontal loads 

M = sum of moments about the center of the footing at its base. 

F, = Dc+D,+Vn,+V, 

F, = 97.0 + 108.7 + 178.7 + 73.0 = 457.4 kN/m 

F, = H,+P, 

F, = 15.0 + 65.3 = 80.3 kN/m 

M = D,+D,+V,,,+V,,+H,+P, 

M = 26.2 - 59.8 + 103.7 + 42.3 + 39.3 + 114.3 = 266.0 kN/m 

Determine Pile Group Properties Per Meter Length 

Centroid of Pile Group (from front pile) 

Front Piles 

Rear Piles spaced 

Front piles: l/1.5 x 0.0 = 0.0 

at 2250 mm 

x 1.85 = 0.822 

Sum: l-l l l 

FHWA-SA-97-070 (~00-06) 5-65 
So centroid is (0.822/l. 111) = 0.74 m from front pile. 

1.85 e = - - 0.74 = 0.185 m 

2 

Pile Group I about centroid 

I = & (0.74)2 + & (1.11)2 = 0.9127 

Rear Pile Vert Load = -!k - 

(M - Fye) 1.11 

1.111 0.9127 I 

Front Pile Vert Load = -% + 

(M - F,,e) 0.74 

1.111 0.9127 I 

Group I Pile Design Loads 

Rear Pile Vert Load = 

[266 - 457.4 x 0.1851 1.11 

I 

= 191 kN 

0.9127 

- Front Pile Vert Load 457.4 x 0.74 

= [266 0.1851 1 = 559 kN 

0.9127 

Front Pile Axial Load = 

559 

cos(20”) = 595 kN compression 

(Controlling Group 1, non-seismic, axial design load per pile) 

5-66 FHWA-SA-97-070 (~00-06) 
A more refined analysis would utilize the vertical stiffness of the batter pile. For this example, 

the batter pile stiffness results in approximately a 7 percent larger front pile axial load. 

5.G.3.2 Step 3 (SLD) - Determine Allowable Structural and Geotechnical Pile Loads 

In the following section the material strength properties and reinforcement dimensions are 

selected for the pile section, and design calculations are done to determine the allowable load 

of the upper cased length, allowable load of the lower uncased length, and determination of the 

allowable geotechnical bond load. 

A single pile design is used that can support the maximum compression load that acts on the 

front row of battered piles. 

For the permanent casing reinforcement, a yield strength of 24 1 MPa is used for design. To 

maintain strain compatibility between the reinforcing bar and casing in the upper pile length, a 

yield strength (F,-,,,) of 241 MPa is used for both members. The use of a higher strength 

casing, such as API N80 (F,, = 552 MPa) would greatly increase the structural capacity of this 

section of the pile. These calculations do not address the capacity of the casing joint. 

If the project incorporated a larger number of piles, savings could be realized by the use of 

separate designs for the front and rear piles. For example, a smaller casing size and a shorter 

bond length may be used on the rear piles due to the lower required capacity. The use of 

separate designs could increase the amount of load testing required at the start of the project, 

which the potential savings on a larger piling job could support. Consideration must also be 

given to maintaining simple pile construction. Mixing different reinforcing sizes and pile 

depths can occasionally lead to errors during pile installation. Also, if the overall stiffness 

(load versus pile top deflection) of the pile is changed, then the centroid and load per pile must 

be reevaluated. 

FHWA-SA-97-070 (~00-06) 5 -67 
5.G.3.2.1 Step 3 (SLD) - Pile Cased Length Allowable Load 

Material dimensions and properties: 

Casing - Use 141 mm outside diameter x 9.5 mm wall thickness. 

Reduce outside diameter by 1.6 mm to account for corrosion. 

Casing outside diameter OD,,i,,=141 mm-2~ 1.6mm=137.8mm 

Pile casing inside diameter IDcasing = 141 mm - 2 x 9.5 mm = 122 mm 

Pile casing steel area ATeacasing - ID,2,i,s ] = 3,224 mm2 

Casing yield strength 

Radius of gyration 

Fy-casing = 24 1 MI% 

r, Jr=== = 46 mm casmg 4 

Reinforcing bar - Use 43 mm grade 520 steel reinforcing bar. 

Bar area ha bar = 1,452 mm* 

Bar steel yield strength F Y-bar = 520 MPa 

Cement Grout - Use neat cement grout. 

Grout area ~eagrollt = $ Idling - Areab, = 10,240 mm2 

Grout compressive strength flcegrout = 34.5 MPa 

5-68 FHWA-SA-97-070 (~00-06) 
For strain compatibility between casing and rebar, use for steel yield stress: 

F y-steel = the minimum of F,, and Fyc-ing = 241 MPa 

Tension - Allowable Load 

P t-allowable = 0.55 Fy-steel cArea bar ’ heacasing I = 620 kN 

Compression - Allowable axial unit steel stress, with consideration for pile lateral 

stability - 

FS = 2.12 

F 

As L=O y-steel Fa = - = 113.7 MPa 

FS 

Allowable Load 

P 0.40 f,‘-,,,, Areagrout + 

F 

c -allowable e ( Areabar + AreaCasing )I FL! x - 

F y-steel 

FS 

= 673 kN 

FHWA-SA-97-070 (~00-06) 5-69 
5.G.3.2.2 Step 3 (SLD) - Pile Uncased Length Allowable Load 

Material Dimensions and Properties 

Soil conditions and the method of pile installation can affect the resulting diameter of the pile 

bond length. For this example, assume a drill-hole diameter of 50mm greater than the casing 

outside diameter (OD). 

Using a casing OD = 141mm 

Therefore, Grout DIA bond = 141 mm + 50 mm = 191 mm (0.191 m) 

Bond length grout area, A=$Tout = f DIA:,, - Area,, = 27,200 mm 2 

Based on previous experience estimate the plunge length allowable load Ptransfer a,,owa,,le = 50 kN. 

This value will be verified later in the design. See section 5.E.6 for discussion about the 

plunge length. 

Tension - Allowable load 

P t-allowable = o.55 Fy-bar h@-%ar+ p transfer allowable = 465 ~ 

Compression - Allowable load 

P c -allowable = Oe4’ f,‘-gTOUt Area grout + oe47 Fy-bar ATeaba + ‘transfer allowable 

= 780 la 

5 - 70 FHWA-SA-97-070 (~00-06) 
- 

5.G.3.2.3 Step 3 (SLD) - Allowable Geotechnical Bond Load 

The pile bond length shall be located in the dense to very dense sandy gravel with cobbles and 

boulders starting approximately 3.35 meters deep below the bottom of footing elevation. The 

load capacity gained in the upper soils is ignored in the design analysis. The pile bond length 

shall be installed using a Type B pressure grouting methodology. 

From Table 5-2 select an ultimate unit grout-to-ground bond strength c1 bond nominal strensth = 335 

kPa. An upper bound value is selected for the Type B micropile iu gravel as the gravel is very 

dense and includes cobbles and boulders. 

From 5.G.3.1, the controlling AASHTO Group 1 abutment, non-seismic, pile loading is 595 

kN per pile. Therefore, an allowable geotechnical bond load PG-allowable r 595 kN/pile must be 

provided to support the structural loading. 

Provide: PGallowable r 595 kN/pile 

Compute the geotechnical grouted bond length required to provide Poallowable as follows: 

P G-allowable = 

‘bond nominal strength 

FS I x 3.14 x DIA,,, x (bond length) 

2 595 kN (design load) 

Bond Length 2 595 

‘bond nominal strength x 3 14 x Du 

FS bond 

Bond Length r 595 kN 2 7.4 m 

335 kPa x 3.14 x 0.191 m 

2.5 

Select Bond Length = 7.5 m 

FHWA-SA-97-070 (~00-06) 5-71 
P 335 kPa 

G-allowable = X 3.14 x 0.191 m x 7.5 m 

2.5 

= 603 kN (> 595 kN) 

Select a trial plunge length = lm and verify Ptransferallowable assumed at 50kN. 

P 335 kPa 

transfer allowable = X 3.14 x 0.191 m 

2.5 

= 80 kN > 5okN 

5.G.3.2.4 Step 3 (SLD) - Verify Allowable Axial Loads 

From 5.G.3.1 the controlling axial design load for the pile is 595 kN (compression). The 

summary of the pile allowable loads is: 

Structural Upper Cased Length = 673 kN (compression) 

Structural Lower Uncased Length = 780 kN (compression) 

Geotechnical Bond Length = 603 kN (compression) 

The allowable loads are all greater than the 595 kN design load, therefore the pile axial design 

is OK. 

5.G.3.2.5 Step 2 3 (SLD) - Verify Allowable Lateral Loads 

From 5.G.3.1 the controlling lateral design load is 80.3 

kN/m. Check to see if the batter piles carry lateral loads. 

12 The front row of piles are battered at 20” from vertical 

and each pile carries the following ratio of the vertical 

pile load as a lateral resisting load. 

5-72 FHWA-SA-97-070 (~00-06) 

-----.--_ 
= 204 kN per pile 

The lateral design load per pile = 80.3 E x (1.5 m) = 120 kN per pile 

The lateral resisting load is greater than the lateral design load therefore the pile lateral design 

is OK. 

5.G.3.2.6 Step 3 (SLD) - Compute Field Test Loads 

From 5.G.3.1 the controlling axial design load for the pile is 595 kN (compression). 

Verification Test Load = 2.5 x Design Load 

= 2.5 x 595 kN = 1,500 kN 

Proof Test Load = 1.67 x Design Load 

= 1.67 x 595 kN = 1,000 kN 

See chapter 7 for pile load testing guidelines 

FHWA-SA-97-070 (~00-06) 5-73 
5.G.3.2.7 Step 3 (SLD) - Pile Structural Design for Field Test Loads 

See section 5.E.4 for more information about structural design for field test loads. 

The design pile has the following properties: 

141mm outside diameter x 9.5mm wall thickness 

Cased Length 

OD’ = 141mm 

Uncased Length 

LD = 122mm 

Area casing = 3,925 mm2 

Fy-casing = 241 MPa 

(use for casing and bar) 

Area bar = 1,452 mm* 

F y-bar = 520 MPa 

Area grout = 10,240 mm* 

f’ c-grout = 34.5 MPa 

Arei grout = 27,200 mm* 

f’ c-grout = 34.5 MPa 

Areabar = 1,452 mm* 

F y-bar = 520 MPa 

* Do not reduce by 1.6mm for corrosion as is done for permanent pile design. 

Cased Length Allowable Compression Load 

As the unsupported pile length = 0, F, = Fy-casing / 1.25 

F 

P Fa 

c-allowable 0.68 f,!r+ Areagrout + i-7: ( Areabar + Area casing 

)I 

X 

F y-casing 

1.25 

= 1,277 kN 

5-74 FHWA-SA-97-070 (~00-06) 
Uncased Length Allowable Compression Load 

P transfer allowable 

= 

I 

‘bond nominal strength x 3 14 x DIA 

FS = 1.25 . I 

bond ’@lunge length) 

19 lmxlm 

P c-allowable = Oe6* f ‘c-grout ATeagr~nt + ‘**’ Fy-bar Area bar ’ ‘transfer allowable 

= 0.68 x 34,500 (27.2 x 1O-3 ) + 0.8 ( 520 x lo3 ) ( 1.452 x 1O-3 ) + 160 

= 1,402 kN 

The design pile is OK for the proof test at 1000 kN, but does not have the capacity for the 

verification test at 1500 kN; so try increasing the verification test pile to a 141mm OD with a 

12.7mm wall and also increase the reinforcing bar to a 57mm size, grade 520. 

Cased Length: 

OD = 141 mm 

ID = 141 - 2 x 12.7 = 115.6 mm 

ATeacasing = ; [ 1412 - 115.62] = 5,119 mm2 

Fy-casing = 241 MPa (use for casing and bar) 

FHWA-SA-97-070 (~00-06) 5-75 
AXa IX, = 2,581 mm’ 

AyoLlt = ; [ 115.6*] - 2,581 = 7,915 mm* 

flc-grout = 34.5 MPa 

Uncased Length: 

DJ&O”d = 141 + 50 = 191 mm (0.191m) 

*=&rout = $ [ 191*] - 2,581 = 26,071 mm* 

Area ,,ar = 2,581 mm* 

F, -bar = 520 MPa 

Cased Length Allowable Compression Load: 

As the unsupported pile length = 0, F, = Fyqcasing / 1.25 

P c-allowable = 0.68 fLw,oUt Areagrout + I F 

Y,-yF ( Areabar + AreacaSing 

)I 

X Fa F 

y-casing 

1.25 

= 1,670 kN > 1,500 kN OK 

Uncased Length Allowable Compression Load: 

P c-allowable = o.68 fc/-grout Areagrout + o-8’ Fy-bar ATeabar + ptransfer allowable 

5-76 FHWA-SA-97-070 (~00-06) 
P transfer allowable 

%ond nominal strength x 3 14 x DLA 

FS=1.25 1 * bond x @lunge length) 

= 335 x 3.14 x 0.191 m x (1) 

1.25 

= 160kN 

P c-a,,owa,,le = 1,845 kN > 1,500 kN OK 

This upsized pile is OK for the verification test load of 1,500 kN. 

5.G.3.3 Step 4 (SLD) - Anticipated Axial Displacement 

Pile axial stiffness and displacement checks usually are not required for a bridge abutment but 

the following is shown to illustrate how it is done for the seismic pile loads. 

For an approximate estimation of the anticipated axial displacement under maximum load, the 

pile is assumed to act elastically from the bottom of the abutment footing to the tip of the 

permanent casing. 

Casing Length: 

Front pile batter angle CD batter = 20” 

Max pile length from footing to top of gravel L = 

3.35m 

UPPer = 3.6 m 

‘OS ( @batter ) 

Casing insertion into bond length Lingd = 1-O m 

Pile casing length L casing = Lupper + Linsert = 4.6 m 

FHWA-SA-97-070 (~00-06) 5 - 77 
Pile stiffness values: 

Steel modulus of elasticity I& = 200,000 MPa 

Grout modulus of elasticity E grout = 31,000 MPa 

From Section 5.G.3.b. 1 heagTout = 10,240 mm2 

heastee* = Are%, +Area . casmg = 4,676 mm2 

Pile tension stiffness value AEtension = [ Astee* x Esteel ] = g35~ooo kN 

Pile compression stiflhess value 

AE compression = Agrou* x Egrout] + [ Asteel ’ Esteel] = 1~250~ooo IcN [ 

Elastic Displacement: 

Group VII Required strength 

Ptensio,, =56.4 kN and P compression = 614.3 kN 

Elastic displacement in tension * te*astic = 

‘tension ’ Leasing _ 

- 0.3 mm 

AEtension 

Elastic displacement in compression 

P XL Acelastic = co~;io” casing = 2.2 - 

compression 

5-78 FHWA-SA-97-070 (~00-06) 
Pile Total Displacements: 

The total pile displacement consists of the elastic (recoverable) and residual (permanent) 

displacements. The magnitude of the residual displacement can be estimated based on 

experience from previous pile testing with consideration of the total pile length, soil type, and 

the magnitude of the applied loads. From previous experience, use the following values for the 

inelastic displacement. 

Inelastic displacement *residual = O-2 mm (Tension) 

*residual = 2.5 mm (Compression) 

Total displacement, tension *L31 = Ate*astic + *residual = Oe5 mm 

Total displacement, compression AC,, = Acelastic + Aresidual =4.7 mm 

These calculations illustrate how to estimate the design dispacements. When refined 

displacements are required, a careful evaluation of elastic lengths and residual displacements 

will be necessary. 

5.G.3.4 Step 5 (SLD) - Pile Connection Design 

Design calculations are completed for the detail shown in Figure 5-17 for connection of the pile 

top to the abutment footing. 

Required Design Loads, Dimensions 

Group I service load, compression P c-tice = 594.6 kN 

Abutment concrete compressive strength f’, = 27.6 MPa 

Casing outside diameter ODcasing = 141 lllfn 

Pile area kea*ik = %OD,Z,i,, = 15,615 llltn’ 

FHWA-SA-97-070 (~00-06) 5-79 
Figure 5 - 17. Pile Top to Abutment Footing Connection Detail. 

Required Plate Area 

Assume a 254 mm square plate ~eap*at e = Plate:, = 64,516 mm2 

Check Cone Shear 

d ,=286+2x400=1,006mm 

L d,=286mm 4 

P coneallowable 2 PC-service = 594.6 kN 

5 - 80 FHWA-SA-97-070 (~00-06) 
Equivalent diameter for 254 mm2 plate, d 1 = 

J 

4 x 254* 

= 286 mm 

x 

A CP = a [d; - d;] = ; [ 1086* - 286*] = 862,053 mm* 

Use AC1 349 Appendix B for cone shear 

P cone design strength =4cp/wxACP 

=4(p 

J 

fc’ psi (6.89476) E x A,, 

psi 

= 10.5 cp dw x A,, 

P cone nominal strength = 10.5 ,/m x A,, 

= 10.5~~xAcpx 1O-6 

= 1,504 kN 

P cone allowable 

P 

= cone nominal strength 

FS 

FHWA-SA-97-070 (~00-06) 
LF = Combined load factor = 

Y[B,Q, + B,Q, + BEQJ 

CQD + QL + QJ 

where: y = 1.3; B, = 1; B, = 1.67; B, = 1.3 

and with QD=0.62; QL=0.16; QE= 0.22 

QD 

= 0.62 

QD + QL + QE 

QL = 0.16 

QE = 0.22 

QD + Q, + QE 

LF = 1.3 [l.O x 0.62 + 1.67 x 0.16 + 1.3 x 0.221 = 1.53 

<p = 0.65 for unreinforced shear cone per AC1 349 Appendix B 

FS = 1.53 = 2.35 

0.65 

P 1504 kN 

cone allowable = 

= 640 kN > 594.6 kN . . . . OK 

2.35 

Note - edge distance limitations and other requirements of AC1 349 Appendix B must be 

satisfied. 

5 - 82 FHWA-SA-97-070 (~00-06) 
Required Plate Thickness 

Actual bearing stress, compression Bearingmqression 

Plate bending moment for 10 mm width 

KXlX 1 ‘latewidth - oDcasing 

=lOmmxz [ 2 

P c-service 

= Areapk3te 

= 9.22 MPa 

I 

2 

x Bearingcomr,,essio,, = 0.147 kNm 

Note - use of this cantilever moment was determined to be a conservative approximation of the 

maximum moment in the plate due to a compression pile load by comparison to a more 

accurate formula shown in Roark and Young (1975). Plate bending from a tension pile load or 

with different plate support details must be further analyzed. 

Allowable bending stress - F,, = 345 MPa and F, = 0.55FY 

Note - Allowable stresses should be increased by the appropriate % factor (AASHTO Table 

3.22.lA column 14) depending on which load group produces controlling load condition. For 

this example, the Group I load produces the controlling condition (% = 100%). 

M 

S Z-232 

x -req 

= 775 mm3 

Fl.3 

Required plate thickness - t,, = = 21.6 mm 

Use 254 mm square x 25.4 mm thick top plate. 

FHWA-SA-97-070 (~00-06) 5-83 
Required Weld Size: 

Tensile strength of E70 electrode, Fu,+.~,, = 483 MPa 

Minimum tensile strength of connected parts (ASTM A53 Grade B Casing) 

FUpa* = 414 MPa 

Fillet weld strength @F = 0.27Fupart = 111.8 MPa 

Top weld size t weld -top = 6.35 mm 

Stiffener plate size tstiff = 12.7 mm Wstiff = 100 mm Lsti* = 150 mm 

Top weld length 

Top weld strength 

L weld = x ODcasing -4 tstiff + 8 Wstiff = 1,190 mm 

P weld-top = 0.707 tweld-top <DF Lwe, = 598 kN > 595 kN . . . OK 

Stiffener plate side weld dimensions t weld -side = 6.35 mm 

Stiffener plate side weld strength 

L weld = 8 Lstiff = 1,200 mm 

P weld-side = 0.707 tweld-side ‘Fwe,d Lweld = 603 kN 

> 0.707 tweld-top (9F x 8 Wstiff = 402 kN . . . . OK 

Use 6.35 mm fillet weld for welding top and stiffener plates. 

5-84 FHWA-SA-97-070 (~00-06) 
Note - this example ignored bending stresses on the welds which is considered appropriate for 

a compression pile load with the weld details shown. Weld bending stress must be analyzed 

for other conditions. 

56.4 Load Factor Design (LFD) Method 

See 5.C for a general description of the LFD method. The following section will illustrate LFD 

for non seismic and seismic loads. 

5.G.4.1 Step 2 (LFD) - Determine Pile Required Strengths Per Meter Length. 

The Group I factored loads per AASHTO Table 3.22.1A are calculated from the abutment 

loads shown in Table 5-4. 

F, = sum of factored vertical loads 

F, = sum of factored horizontal loads 

M = sum of factored moments about the center of the footing at its base 

F, = Y[B,D~+B,D,+B,V,,+B,V~ 

F,, = 1.3 [(l.O) (97.0) + (1.0) (108.7) + (1.0) (178.7) + (1.67) (73.0)] = 658.2 kN/m 

F, = 1.3 [(1.3) (14.96) + 1.3 (65.32)] = 135.7 kN/m 

M = y[B,Dc+B,D,+B,V,,+B,V,,+B,H,+B,P,] 

M = 1.3 [(l.O) (26.2) - (1.0) (59.8) + (1.0) (103.7) + (1.67) (42.3) + (1.3) (39.3) 

+ (1.3) (114.3)] = 442.5 kNm/m 

The Group VII seismic loads per AASHTO Division I-A (typical section is 6.2.1) are 

calculated from the abutment loads shown in Table 5-4. 

FHWA-SA-97-070 (~00-06) 5-85 
F, = sum of vertical loads 

F, = sum of horizontal loads 

M = sum of moments about the center of the footing at its base 

Note: All load factors are 1 .O for load groups that include seismic loads. 

F,=D,+D,+V,, 

F, = 97.0 + 108.7 + 178.7 = 384.4 kN/m 

F, = 65.32 + 15.91 + 30.9 + 26.8 = 138.9 kN/m 

M=D,+D,+V,,+P,+P,, + I* + I, 

M = 26.2 - 59.8 + 103.7 + 114.3 + 50.12 + 72.5 + 95.1 = 402.1 kNm/m 

Determine Pile Group Properties Per Meter Length 

1 FY 

Pile Footing 

7 

Centroid of Pile Group (from front pile) 

So centroid is at (0.8220 .l 11) = 0.74 m from front pile. 

5-86 FHWA-SA-97-070 (~00-06) 
1.85 

e = - - 0.74 = 0.185 m 

2 

Pile Group I about centroid 

I = & (o.74)2 + -j& (1.11)2 = 0.9127 m4 

Rear Pile Vert Load= FY - - 

(M - FY e) 1.11 

1.111 0.9127 I 

Front Pile Vert Load = - FY + 

(M - FY e) 0.74 

1.111 0.9127 1 

Group I Pile Required Strengths (Non Seismic) 

Rear Pile Vert Load = I. 658.2 C442.5 - 

1111 

- 658.2(0.185)] 1.11 1 = 202 4 w 

0.9127 

Front Pile Vert Load = 

[442.5 - 658.2(0.185)] 0.74 

0.9127 1 = 852.7 kN 

Front Pile Axial Load = 

852.7 

cos (20 “) = 907.4 kN Compression 

(Controlling Group 1, non-seismic, required axial design strength per pile) 

FHWA-SA-97-070 (~00-06) 5-87 
A more refmed analysis would utilize the vertical stiffness of the batter pile. For this example, 

the batter pile stiffness results in approximately a 7 percent larger front pile axial load. 

Group VII Pile Required Strengths (Seismic) 

[402 - 384.4(0.185)] x 1.11 1 = -56.4 kN Tension 

0.9127 

384.4 - Front Pile Vert Load = 1.111 + [402 384.4(0.185)] 0.9127 0.74 I = 614 3 w 

614.3 

Front Pile Axial Load = 

cos (20 “) 

= 654 kN Compression 

(Controlling Group VII, seismic, required axial design strength per pile) 

5.G.4.2 Step 3 (LFD) - Determine Structural and Geotechnical Pile Design Strengths 

In the following section the material strength properties and reinforcement dimensions are 

selected for the pile section, and design calculations are done to determine the design strength 

of the upper cased length, design strength of the lower uncased length, and determination of the 

design geotechnical bond strength. 

A single pile design is used that can support the maximum compression load that acts on the 

front row of battered piles and the maximum tension load which acts on the rear row of vertical 

piling (only required for seismic). 

For the permanent casing reinforcement, a yield strength of 241 MPa is used. To maintain 

strain compatibility between the reinforcing bar and casing in the upper pile length, a yield 

5-88 FHWA-SA-97-070 (~00-06) 
strength (F,,-Steel) of 241 MPa is used for both members. The use of a higher strength casing, 

such as API N80 (F,, = 552 MPa) would greatly increase the structural capacity of this section 

of the pile. These calculations do not address the capacity of the casing joint. 

If the project incorporated a larger number of piles, savings could be realized by the use of 

separate designs for the front and rear piles. For example, a smaller casing size and a shorter 

bond length may be used on the rear piles due to the lower required capacity. The use of 

separate designs could increase the amount of load testing required at the start of the project, 

which the potential savings on a larger piling job could support. Consideration must also be 

given to maintaining simple pile construction. Mixing different reinforcing sizes and pile 

depths can occasionally lead to errors during pile installation. 

5.G.4.2.1 Step 3 (LFD) - Pile Cased Length Design Strength 

Material dimensions & properties - 

Casing - Use 141 mm outside diameter x 9.5 mm wall thickness. Reduce outside 

diameter by 1.6 mm to account for corrosion. 

Casing outside diameter ODcasing = 141 mm - 2 x 1.6 mm = 137.8 mm 

Pile casing inside diameter IDcasing = 141 mm - 2 x 9.5 mm = 122 mm 

Pile casing steel area Area casnlg = ;[ OD2 - IDc;si,] = 3,224 mm2 

Casing yield strength F y-casing = 241 MPa 

FHWA-SA-97-070 (~00-06) 5 - 89 
Radius of gyration r casing 

=d- =4emm 

4 

Reinforcing bar - Use 43 mm grade 520 steel reinforcing bar. 

Bar area &ea,ar = 1,452 mm2 

Bar steel yield strength F y-bar = 520 MPa 

Cement Grout - Use neat cement grout. 

Grout area Area grout = 3 U)c2asing - Areabar = 10,240 mm 2 

Grout compression strength f’c-gro”t = 34.5 MPa 

For strain compatibility between casing and rebar, use for steel yield stress: 

F y-steel = the minimum of F,,bar & F y-casing = 241 MPa 

Tension - Design Strength 

P t -nominal = FyTsteet [ Areabar + Areacasing ] = 1,127 kN 

(Pt = 0.90 P t -design = (PtPt-nomind = 1,o14 kN 

Compression - Design Strength 

As L=O, F,=F,St,,1=241 MPa 

5-90 FHWA-SA-97-070 (~00-06) 
Design Strength 

P c -nominal = 0’85fc-gr0nt ATeagrout + Fy-steel( AIeaLmr + keacasing) [ 

= 1,427 kN 

and with ‘p, = 0.85, Pcmdesign = ‘p, Pc-nominal = 1,213 kN 

5.G.4.2.2 Step,3 (LFD) - Pile Uncased Length Design Strength 

Material Dimensions and Properties 

1 

Fa XV 

F y-steel 

Soil conditions and the method of pile installation can affect the resulting diameter of the pile 

bond length. For this example, assume a drill-hole diameter of 50mm greater than the casing 

outside diameter (OD). 

Using a casing OD = 141mm 

Therefore, Grout DIA,,, = 141 mm+5Omm= 191 mm (0.191 m) 

Bond length grout area A=ag,o”t = a DIAiond - Areabar = 27,200 mm* 

Based on previous experience estimate the plunge length design strength Pmsfer design = 110 kN. 

This value will be verified later in the design. See section 5.E.6 for discussion about the 

plunge length. 

Tension - Design Strength 

Pt-design = (O-90) Fy-bar Area,,, + P transfer design = 790 k~ 

Compression - Design Strength 

Pt-design = (0.75) (0.85 flcegrout hea gout + Fy-bar Area,,) + Peansfer design = 1,275 k~ 

FHWA-SA-97-070 (~00-06) 5-91 
5.G.4.2.3 Step 3 (LFD) - Geotechnical Bond Design Strength 

The pile bond length shall be located in the dense to very dense sandy gravel with cobbles and 

boulders starting approximately 3.35 meters deep below the bottom of footing elevation. The 

load capacity gained in the upper soils is ignored in the design analysis. The pile bond length 

shall be installed using a Type B pressure grouting methodology. From Table 5-2 select an 

ultimate unit grout-to-ground bond strength, c1 bond nominal strength = 335 kPa. An upper bound value 

is selected for the Type B micropile in gravel as the gravel is very dense and includes cobbles 

and boulders. 

From section 5.G.4.1, the controlling AASHTO Group 1, non-seismic, required pile axial 

strength is 907 kN per pile. Therefore, a geotechnical bond axial strength, PG-de,ign strengtll r 907 

kN per pile, must be provided to support the structural loading. The required geotechnical 

grouted bond length may be computed as follows: 

Provide: PGdesign strength 2 907 kN/pile 

P G-design strength =‘PG[a bond nominal strength 1 x 3.14 x DIA,,, x (Bond Length) 

2 907 kN (required strength) 

Bond Length 2 907 

‘PC Ia bond nominal strength 1 x 3*14 x DIA bond 

Bond Length ;1 907 2 7.5 m 

0.6 (335 kPa) x 3.14 x 0.191 m 

Select Bond Length = 7.5 m 

5 - 92 FHWA-SA-97-070 (~00-06) 
For Group I (non-seismic) use (pG = 0.60 

P G-design strength = 0.60 X 335 kPa X 3.14 x 0.191 m x 7.5 m 

= 904 kN (= 907 kN) 

For Group VII (seismic) use ‘pc = 1.00 

P G-design strength = 1.0 x 335 kPa x 3.14 x 0.191 m x 7.5 m 

= 1507 kN 

Select a trial plunge length = lm and verify Pbansferdesign assumed at 110 kN. 

For Group I (non seismic) use tpG = 0.60 

P transfer design = 0.60 x (335 kPa) x 3.14 x 0.191 m x (1 m) 

= 120 kN > 110 kN . . . OK 

For Group VII (seismic) use (pc = 1.0 

P transfer design = 1.0 x (335 kPa) x 3.14 x 0.191 m x (1 m) 

= 2OOkN> 11OkN.. . OK 

FHWA-SA-97-070 (~00-06) 5-93 
5.G.4.2.4 Step 3 (LFD) - Verify Axial Design Strengths 

From 5.G.4.1, the controlling axial required strengths for the pile are: 

Group I (non seismic) = 907 kN (compression) 

Group VII (seismic) = 654 kN (compression) and -56 kN (tension) 

The summary of the pile design strengths is: 

Structural Upper Cased Length 

Structural Lower Cased Length 

Geotechnical Bond Length 

Non Seismic Seismic 

1213 kN (C) 1213 kN (C) 1014 kN (T) 

1275 kN (C) 1275 kN (C) 790 kN (T) 

904 kN (C) 1507 kJ!J (C) 1507 kN (T) 

The pile design strengths are all equal to (904 =907) or greater than the required strengths; 

therefore the pile axial design is OK. 

Figure 5-l 8 shows the SLD (at position 1) and LFD (at position 2) to illustrate that both 

methods (for this example) provide the same factor-of-safety (2.5) for the nominal strength of 

the geotechnical grout-to-ground bond. As discussed in 5.C this condition was obtained by 

calibrating (ho for LFD to SLD with FS = 2.5. See 5.C for more discussion about SLD, LFD, 

and (Pi calibration. Figure 5-18 also shows that both SLD and LFD methods produce the same 

requirements for the verification field test load and the proof field test load. 

5-94 FHWA-SA-97-070 (~00-06) 
DISTRIBUTION 

DESIGN LOAD = 595 kN 

Note: Design Strength z Required 

SLD Strength but for this example 904 kN 

ALLOWABLE LOAD = 600 kN is close enough to 907 kN. 

REQUIRED STRENGTH = 907 kN ‘1 LFD 

DESIGN STRENGTH = 904 kN 

PROOF TEST LOAD = 1.67 x 595 = 1,000 kN 

rj 

\ 

SLD AND LFD 

NOMINAL STRENGTH = 1.507 kN 

FS = 2.5 

Figure 5 - 18. Comparison of SLD and LFD for Geotechnical Bond Length Design. 

5.G.4.2.5 Step 3 (LFD) - Verify Lateral Design Strength 

From 5 .G.4.1, the controlling lateral required strengths for the pile per meter are: 

Group I (non seismic) = 135.7 kNm 

Group VII (seismic) = 138.9 kN/m 

E 

Check to see if the batter piles have sufficient strength to carry 

the lateral required strengths. The front row of piles are battered 

12 

20” at 20” from vertical and each pile carries the following ratio of 

the vertical pile required strength. 

4.37 

FHWA-SA-97-070 (~00-06) 5-95 
Group I (non seismic) 

4.37 Lateral design strength = 853 kN x 12 = 311 kN per pile 

The lateral required strength per pile = 135.7 z x (1.5 m) = 204 kN per pile 

Pile OK for Group I 

Group VII (seismic) 

4.37 Lateral design strength = 614 kN x 12 = 224 kN per pile 

The lateral required strength per pile = ( 138.9 z ) x (1.5 m) = 208 kN Per Pile 

Pile OK for Group VII 

5.G.4.2.6 Step 3 (LFD) - Compute Field Test Load. 

From 5.G.4.1, the controlling axial required strengths for the pile are: 

Group I (non seismic) = 907 kN (compression) 

Group VII (seismic) = 654 kN (compression) and - 56 kN (tension) 

The verification test load is equal to the nominal strength required for the controlling required 

strength which is 907 kN for this example. 

(~c (Nominal Strength) = 907 kN 

(~c = 0.60 for Group I, so Nominal Strength = 

907 kN 

o 6 = 1500 kN 

5-96 FHWA-SA-97-070 (~00-06) 
X (Nominal Strength) 

so the proof test load = $y x 1500 = 1000 kN. 

Note that these are the same as the SLD since the LFD was calibrated to SLD. See chapter 7 

for pile load testing discussion and requirements. 

5.G.4.2.7 Step 3 (LFD) - Pile Structural Design for Field Test Loads 

Section 5.E.4 describes a SLD method with a FS = 1.25 for the pile structural design for the 

field test loads. Section 5.G.3.2.7 illustrates this method for this Sample Problem. 

5.G.4.2.8 Step 4 (LFD) - Anticipated Axial Displacement 

See section 5 .G.3.3 for discussion of displacement checks and sample calculations for this 

Sample Problem. 

5.G.4.2.9 Step 5 (LFD) - Pile Connection Design 

Design calculations are completed for the detail shown in Figure 5-19 for connection of the 

pile top to the abutment footing. 

Required Design Strength, Dimensions 

Required strength, tension P t-required = 56 kN Group VII (seismic) 

Required strength, compression P c-required = 907 kN Group I (non seismic) 

Abutment concrete compressive strength fc’ = 27.6 MPa 

Casing outside diameter ODcasing = 141 mm 

Pile area heapile = 3 OD~,i,g = 15,615 mm2 

FHWA-SA-97-070 (~00-06) 5-97 
Figure 5 - 19. Pile Top to Abutment Footing Connection Detail. 

Required Plate Area 

Assume a 254 mm square plate- A=$late = Plate:, = 64,516 mm2 

Check cone shear for compression loads: 

d ~=206+2x400=1,006mm 

Equivalent diameter for 254 mm2 plate 

cone design strength compression 

d, =JTx:jql2f$6- 

L d,=286mm 4 

A CP = 2 7~ [d; - d;] = 862,053 mm2 

5-98 FHWA-SA-97-070 (~00-06) 
Use AC1 349, Appendix B 

P cone design strength compression = 10.5 x (0.65) 4 27.6 x 1000 x [AJ x 1O-6 

= 977 kN > 907 kN . . . . OK 

Check cone shear for tension loads: 

d ~=286+2x250=786mm 

A,, = $[ d; - d;] = 536,000 mm2 

P cone design strength tension = 10.5 x (0.65) 4 27.6 x 1000 x [AcP] x 1O-6 

= 608 kN > 56 kN . . . . OK 

Note - edge distance limitations and other requirements of AC1 349 Appendix B must be 

satisfied. 

Required Plate Thickness 

P 

Actual bearing stress, tension Bearingtension =

t-required Area = 1.15 MPa 

plate - Areapile 

FHWA-SA-97-070 (~00-06) 5-99 
P 

Actual bearing stress, compression Bearingcomp,essi,,,, = 

c-required Area = 14.06 MPa 

plate 

Plate bending moment for 1 cm width 

M max I 

2 

P1ate,dfi - ODcasing 

2 

x Bearingcomp,essio,, = 0.224 kNm 

Note - use of this cantilever moment was determined to be a conservative approximation of the 

maximum moment in the plate due to a compression pile load by comparison to a more 

accurate formula shown in “Formulas for Stress and Strain,” Sh Edition, 1975, by Roark and 

Young. Plate bending from a tension pile load or with different plate support details must be 

further analyzed. 

Design bending stress FY = 345 MPa 

M 

S =.-!T!E 

x-req = 649.3 mm3 

FY 

Required plate thickness 6s t,, = 

x-req = 19.7 mm 

10 mm 

Use 254 mm square x 25.4 mm thick top plate. 

5-100 FHWA-SA-97-070 (~00-06) 
Required Weld Size: 

Tensile strength of E70 electrode FLI,,,,,, = 483 MPa 

Minimum tensile strength of connected parts (ASTM A53 Grade B Casing) 

FUp- = 414 MPa 

Fillet weld strength <DF = 0.45Fupart = 186 MPa 

Top weld size t weld -top = 6.35 mm 

Stiffener plate size ‘stiff = 12.7mm Wstiff = 100 mm L,, = 150 mm 

Top weld length L weld = 7~ ODcasj,s - 4 tstiff + 8 Wstiff = 1,190 mm 

Top weld strength 

P weld -top = 0.707 tweldwmp @F Lweld = 995 kN > Pcerequired . .. . OK 

Stiffener plate side weld dimensions t weld -side = 6.35 mm 

Stiffener plate side weld strength - 

L weld = 8 Lstiff = 1,200 mm 

P weld -side = 0.707 tweldeside QFwe, = 1,004 kN 

’ O-707 tweld-top @Fx8W,,,=669kN... OK 

Use 6.35 mm fillet weld for welding top and stiffener plates. 

FHWA-SA-97-070 (~00-06) 5 - 101 
Note - this example ignored bending stresses on the welds which is considered appropriate for 

a compression pile load with the weld details shown. Weld bending stress must be analyzed 

for other conditions. 

5.G.5 Step 6 (SLD & LFD) - Complete Detail Drawings I 

The following detail drawings (Figures 5-20 and 5-21) show the pile design determined by the 

Service Load and Load Factor design methods in the previous sections. (The two methods 

yielded the same design in this case.) 

5-102 FHWA-SA-97-070 (~00-06) 
I.7Sm I 6 SPACES fB l.Sm O.C. I .75m I 

0.3m 

FRONT - -4 - 4. - & - 

(INCLINED) 

4 I 

I 

--------- 

,..~I”~~ 4SPACESQZZS”,O.C. ~.75m~ Am 

ABUTMENT FOOTING - PLAN VIEW 

NOT TO SCALE 

025m 

ABUTMENT SECTION 

NOT TO SCALE 

GENERAL NOTES 

MICROPILE INSTALLATION PROCEDURE 

1. Secure area for work and survey locations for piling installatton. 

2Advance 141 mm outside diameter casing to full pile depth required (7.5 meters 

minimum into dense gravel soils or 11.2 meters total below bottom of footing whichever 

is greater) utfliztng rotary drilling techniques. 

3.Tremie casing full with neat cement grout 

4. Place 43 mm reinforcing threadbar with centralizers 

S.Reattach drill head to the top of the casing and Pressure groutthe 7.5 meter-long pile 

bond length by pumping neat cement grout under pressure while extracting casing. 

Minimum grout pressure should be 0.35 MPa. 

6. Upon completion of pressure grouting, reinsert the casing 1.5 meters into the top of the 

bond length. 

7.Trfm top of casing to its proper elevation, end weld the top bearing plate with stiffener 

plates. 

6.The quality of the grout shell be monitored by collecting grout cubes for later 

compression testing and by measuring the grout specific gravity from one batch per 

day. 

9. Consistency of pile installation shall be monitored and recorded as described In the pile 

installation quality control document. Monitored and recorded data shall include total 

pile depth, grout pressures and quantities. sotls /rock encountered during installation 

and any obstructions or Irregularities. 

PILE LOAD TESTfNG 

1 .The pile load test program shall be conducted as described in the specifications. 

Testing procedures and results will be Inspected and reviewed by DOT representative, 

and are subject to DOT approval. An expeditious response to the load test subrnlttal Is 

needed from DOT so as not to delay the progress of the Contractor. 

MATERIAL SPECIFICATIONS 

Grout-A neat mix of Portland Cement (Type I I II ) conforming to ASTM Cl 56 with a 

water cement ratio of approximately 0.45. The minimum 26 day compressive strength 

of the grout shall be 34.5 MPa. 

Reinforcing Bar-The reinforcing bar shall be a 43 mm Grade 520 Dywidag Threadbar (01 

equivalent) conforming to ASTM A - 615 (F, = 520 MPa). Length of couple bar section! 

shell be determined based on the overhead clearance available at each pile location. 

Bearhrg Plate-Steel for the top bearing plate with dde stiffeners shall conform to 

AASHTO M270 Grade 350 (FY = 345 MPa). 

Casing -The steel casing shall be 141 mm outside diameter, 9.5mm wall thickness 

conforming to ASTM designation: A108 Grade B. A252 Grade 2, A519 with a minimum 

yield strength of 241 MPa. or A53 Grade B. 

CONTRACTOR DESIGN I BUILD 

WORKING DRAWING SUBMITTAL 

CHAPTER 5, ABUTMENT MICROPILES 

SAMPLE PROBLEM #I 
-j--+----l- /ABUTMENT 

REINFORCING 

BAR 43mm GRADE 520 

2~ CASING 11 

x&~~~~~~~~ COBBLES 

TOP OF DENSE SANDY 

/- NEAT CEMENT GROUT 

r 

TYPICAL SECTION 

ABUTMENT FOOTING 

(NOT TO SCALE) 

DETAIL 1 

(NOT TO SCALE) 

2S4mmr254mmr254mm 

TOP PLATE 

REINFORCING BAR7 / F.=34SMPsREINFORCING BAR 

14lmnrSSmmWALL 

NEAT CEMENT GROUT 

SECTION A -A 

(NOT TO SCALE) 

DRILLED AND GROUTED 

MICROPILE SHAFT 

EAT CEMENT GROUT 

REINFORCING BAR 

SECTION B - B 

(NOT TO SCALE) 

13mm 9 GROUT HOLE 

44.51~ P CENTER HOLE 

FOR 43mm RESAR 

L-STIFFENER PLATE (rfPlcAL) 

1SOmm x 12.7nw1 

CONTRACTOR DESIGN I BUILD 

WORKING DRAWING SUBMllTAL 

CHAPTER 5, ABUTMENT MICROPILES 

SAMPLE PROBLEM #I 
DESIGN METHODOLOGY: MICROPILES FOR SLOPE 

STABILIZATION AND EARTH RETENTION 

6.A SPECIAL NOTE 

Chapter 6 was intended to cover slope stabilization applications. Due to a lack of consensus on 

design procedures, Chapter 6 is still under preparation. When completed, Chapter 6 will be 

made available as a supplement to this manual. 

FHWA-SA-97-070 (~00-06) 6-l 
[BLANK PAGE] 

FHWA-SA-97-070 (~00-06) 
CHAPTER 7 

PILE LOAD TESTING 

7.A INTRODUCTION 

It is industry standard, as a first order of work on micropile foundation support projects, to 

perform load testing of at least one micropile to the ultimate design load in order to verify the 

design assumptions and the adequacy of the contractor’s installation methods. Load testing 

may consist of installation and verification load testing of one or more piles prior to 

commencement of production pile installation. Additional confirmation of the pile load 

capacity may be obtained through (proof) testing on production piles during the course of 

construction. 

Micropiles are tested by the static axial load testing of individual piles. These tests usually 

feature incremental axial loading until the pile either sustains a predetermined maximum test 

load, reaches a predetermined structural axial displacement limit, or reaches a predetermined 

ground creep threshold. With the trend towards higher capacity CASE 1 piles, failure may 

occur in the form of the sudden loss of load and increase in displacement associated with 

structural failure. This aspect requires careful consideration in design of higher capacity 

micropiles (see Chapter 5). 

In this manual, the close link between micropiles, ground anchors and soil nails has been noted 

regarding installation methods and geotechnical performance. Please refer to the more detailed 

discussion in Chapter 5, section 5.E.4. This link is also reflected in micropile load-testing 

methods and acceptance criteria. Many references address the load testing of driven piles, 

drilled piers, ground anchors and soil nails, but none specifically address micropiles. The loadtesting procedures presented in this section conform to the requirements of ASTM D 1143 and 

D 3689 for testing individual piles under static axial compression and tension load, with 

modifications that reflect micropile testing practices. 

FHWA-SA-97-070 (~00-06) 7-l 
7.B TYPES AND PURPOSE OF LOAD TEST 

Micropiles are field tested to verify that the micropile design loads can be carried without 

excessive movements and with an adequate factor of safety for the service life of the structure. 

In addition, testing is used to verify the adequacy of the contractor’s drilling, installation and 

grouting operations prior to and during construction of production piles. Therefore, the 

soil/rock conditions, as well as the method, equipment and operator used for installing 

production piles m be similar to those used for installing test piles. If ground and/or 

installation procedures change, additional testing may be required. If test results indicate faulty 

construction practice, or grout-to-ground load capacities less than required, the contractor is 

required to alter the micropile installation/construction methods. In the event that the required 

design grout-to-ground bond capacities are still not achievable, redesign may be necessary. 

Testing criteria will be part of the specifications and may include ultimate and/or verification 

tests which are conducted to validate the contractors installation methods and to verify 

compliance with the micropile load carrying capacity and grout-to-ground bond values used in 

design. These tests usually require loading to a maximum test load that includes the factor of 

safety assigned to the design grout-to-ground bond and/or that which results in failure (i.e. 

inability to maintain constant test load without excessive micropile movement). The number of 

tests will vary, depending on the size of project and the major different ground types in which 

micropiles will be installed. On smaller projects, one or two ultimate, or verification, tests are 

commonly conducted prior to beginning production pile installation, and then one or more 

additional such tests may be conducted in each major different ground type encountered as 

construction proceeds. A larger number of ultimate, or verification, tests may be specified for 

larger projects. Ultimate and verification tests are typically performed on “‘sacrificial” test 

piles. 

During production installation, “proof’ testing may be conducted on a specified percentage of 

the total production piles installed. Some specifications allow proof testing to be performed on 

production piles that will be incorporated into the structure while other specifications require 

7-2 FHWA-SA-97-070 (~00-06) 
the proof-tested piles to be extra “sacrificial” piles that will not be incorporated into the 

structure. For economy, the first approach is strongly recommended. 

Creep tests are typically performed as part of ultimate, verification, and proof tests. Creep is a 

time dependent deformation of the soil structure under a sustained loading. Creep is primarily 

of concern in organic and cohesive (clayey) soils. The creep test consists of measuring the 

movement of the pile at constant load over a specified period of time. This test is done to 

ensure that the pile design loads can be safely carried throughout the structure’s service life 

(typically 75 to 100 years) without causing movements that could damage the structure. 

Micropile testing is conducted by incrementally loading (and if specified, unloading) the pile 

and measuring the movement of the pile head at each load increment. Typically, the pile-head 

movement reading is recorded just after the next load increment has been applied. The loading 

increments, the time that each load increment is held and the number of measurements for each 

load increment are determined by the type of test being performed and will be specified in the 

contract documents. If not specified, recommended practice is to obtain a pile-head movement 

reading just after the load has been applied, and a second reading after the load has been 

maintained for a sufficient amount of time to ensure that pile-head movement has stabilized. 

Testing procedures were not ‘standardized” at the time this manual was written and vary 

among different Highway Agencies. Check specifications for test procedures applicable to 

your projects. 

Most micropile load tests completed to date have been performed in accordance with ASTN 

D 1143 “Quick” test procedures. 

7.B.l Ultimate Test 

Y Ultimate tests (if used) are performed on non-production, “sacrificial” micropiles and provide 

the following information: 

l Determination of the ultimate grout-to-ground bond capacity (if carried to failure). 

FHWA-SA-97-070 (~00-06) 7-3 
l Verification of the design grout-to-ground bond factor of safety. 

l Determination of the load at which excessive creep occurs. 

A true “ultimate” test is performed by loading the micropile until failure takes place along the 

grout-ground interface. Failure is the inability to maintain constant test load without excessive 

movement. Excessive movement is often taken as the slope of the load-deflection curve 

exceeding 0.15 mm/kN. True ultimate tests, taken to failure, are usually only specified as part 

of a research project or on very large projects where a design phase test program can be 

justified. The design phase test program will allow the micropile design to be optimized. 

7.6.2 Verification Test 

Verification tests are conducted to verify that installation methods will provide a micropile 

capable of achieving the specified grout-to-ground bond capacity with a specified factor of 

safety. Verification maximum test loading will be defined by the grout-to-ground bond factor 

of safety and the chosen design grout-to-ground bond capacity. If the design grout-to-ground 

bond factor of safety is 2.5, the maximum test load will verify 250 percent of the design bond 

value. Verification tests are generally completed on non-production, “sacrificial” micropiles as 

a first order of work prior to construction of production piles. In addition, “verification” testing 

may be required during production to verify capacities for different soil/rock conditions and/or 

drilling/installation methods. Verification tests may or may not test the micropile to the point 

of failure. The test is a rigorous multiple cycle test with the test load progressively increased 

during each loading cycle until the final specified maximum test load is reached. 

7.B.3 Proof Test 

A proof test is typically performed on a specified number of the total number of production 

micropiles installed. The test is usually a single cycle test in which the load is applied in 

increments until a maximum test load, recommended to be 167 percent, or more, of the design 

grout-to-ground bond value is reached. Proof tests provide information necessary to evaluate 

the ability of production micropiles to safely withstand in-service design loads without 

excessive structural movement or long-term creep over the structure’s service life. 

7-4 FHWA-SA-97-070 (~00-06) 
7.8.4 Creep Test 

If micropiles are to be bonded in creep susceptible cohesive soil, creep tests are typically 

performed as part of the ultimate, verification, or proof test. Creep testing is conducted at a 

specified, constant test load, with movement recorded at specified time intervals. The 

deflection versus log-time results are plotted on a semi-log graph, and are compared with the 

acceptance criteria presented in the contract documents. A maximum creep rate of 2 mm per 

log cycle of time is a common acceptance criteria. Creep tests should utilize a calibrated load 

cell during the creep test load hold increment to monitor and adjust for small changes in load 

caused by jack-bleed, ram friction, or other factors. 

7.C DETERMINATION OF PROJECT LOAD TESTING REQUIREMENTS 

Load testing of foundation support elements confirms the bond capacities of the underlying 

soils, pile structural design capacities, and the movement characteristics of the micropile itself. 

Procedures used for installation of successfully tested pre-production piles establish the 

procedures to be used for installation of the production piles. The factors to be considered 

when determining the project testing requirements include the following: 

1. Total number of production micropiles. 

2. Magnitude and type of design loading. 

3. Sensitivity and importance of the supported structure. 

4. Variance in ground subsurface conditions across the installation site. 

5. Types of subsurface conditions. 

6. Site access and headroom/installation constraints. 

7. Contractor experience. 

FHWA-SA-97-070 (~00-06) 7-5 
A project specification for foundation support elements should include requirements for the 

following components of a load test program. These requirements are discussed in detail 

below, and include: 

l The number of pre-production verification load tests. 

l The number of production pile proof load tests. 

l The magnitude of the test loads. 

l The method of load application (single vs. multiple cycle, order of testing). 

l The duration for which the test loads are applied. 

l Acceptance criteria for maximum total displacement and maximum creep 

displacement at a specified load. 

7.C.l Number of Load Tests 

For a smaller micropiling project, the load testing required will usually consist of testing at 

least one pre-production verification pile to some factor times the design loading. This pile 

may be a sacrificial (non-production) pile or a production pile. Loading beyond the required 

minimum capacities may be conducted in an attempt to determine the pile’s ultimate capacity. 

For large projects, additional pre-production tests and production pile proof testing will usually 

be required. For example, load testing specified for Caltrans seismic retrofit micropiles 

typically consists of at least one pre-construction performance (verification) test on a nonproduction pile 

For a specific large commercial project involving strengthening the foundations under existing 

grain silos with 800 high-capacity micropiles, the required load testing initially included three 

successful sacrificial preproduction tests and 12 production pile tests, all to 200 percent of the 

service design compression loading. The required number of production load tests was later 

reduced to six after consistent and successful performance of the piles tested. 

Testing requirements can add considerable cost to a project. Sacrificial pile compression tests, 

including reaction piles, can add between $10,000 and $30,000 per test to the project cost. 

7-6 FHWA-SA-97-070 (~00-06) 
Production pile tests that do not involve high-capacity ultimate testing and where adjacent piles 

may be used to provide reaction to compression test loads will add between $2,000 to $10,000 

per test. Considering these costs, efforts should be made to provide a load-test specification 

that ensures that all project piles have adequate capacity while keeping total testing costs to a 

reasonable amount. 

The magnitude of the maximum test load may be reduced for production pile proof testing, 

therefore reducing the cost. Performance at the lower test loads can be compared to results of 

the pre-production verification testing. Alternatively, production pile testing may consist of 

tension only tests, which are less expensive because the ground may be used for reaction to the 

test load rather than adjacent piles or reaction tie-down anchors. This is reasonable if the pile’s 

critical load condition is in tension, or if results of the production tension testing can be 

correlated to the pre-production tension and compression testing results. 

For a project involving a very small number of piles, load testing may be foregone if the 

structural and geotechnical design capacities provide sufficient redundancy (factor of safety) 

over the required loading. Some verification of the pile capacity could be provided from 

results on previous projects completed in similar conditions. The functional importance of the 

supported structure should also be considered. 

7X.2 Micropile Load-Testing Guidelines 

7.C.2.1 Micropile Load-Testing Guidelines for New Users (Owners/Contractors) 

The following are suggested guidelines to establish micropile load-testing requirements for a 

structural foundation project. These guidelines are based on the following assumptions: 

l Micropiles are required for foundation support of bridge structures. (e.g., critical 

structures). 

l Project specifications require pre-qualified micropile specialty contractors. 

l Micropile proof load testing is in critical loading direction only. 

FHWA-SA-97-070 (~00-06) 7-7 

-.- 
Table 7-l. Suggested Micropile Load Testing Guidelines 

Number of Piles 

Proposed for Project 

Number of Tests 

Pre-Production Pile Production Pile 

Verification Testing Proof Testing 

l-249 1 5% 

250 - 499 2 5% 

more than 500 3 

I 5% I 

Additional factors to be considered when determining a project’s pre-production bond testing 

requirements are listed in Table 7-2. To establish the testing requirements for each project, 

multiply the assigned testing amplification factor for each category by the number of above 

referenced number of tests (i.e. from Table 7-l) according to size of project. The final 

verification testing requirements will be the summation of the number of pile tests required by 

Table 7-l added to the number required by Table 7-2 based on the size of the project and each 

of the applicable pile-testing factors. For economy, round down to the nearest whole number of 

load tests when computing the required number of verification and proof tests. 

Verification tests may be performed on production piles provided that: 

l the piles are designed with a structural factor of safety of at least 1.25 at maximum 

test load, 

l the piles are not failed or overloaded during testing, and 

l the pile can be replaced if the pile fails. 

Previous tests by the Owner/Contractor on Micropiles may be used to reduce the number of 

verification and proof tests provided the previous tests had: 

l similar ground conditions 

l similar construction methods (drilling and grouting) 

l similar design loads and/or design bond stresses 

7-8 FHWA-SA-97-070 (~00-06) 
Table 7-2. Amplification Factors for Micropile Verification Load Testing 

Pile Load-Testing Factor Amplificatio 

n 

1. Criticality of Structure Supported 

Lifeline structure* . . . . . . . . . . . . . . . . . . . 2.0 

Non-lifeline structure . . . . . . . . . . . . . . . . 0.0 

2. Micropile Ultimate (Nominal) Capacity 

1-1299 kN . . . . . . . . . . . . . . . . . . . . . . 0.0 

>1300 kN . . . . . . . . . . . . . . . . . . . . . . . . 0.5 

3. Subsurface Material Type (Pile Bond Zone) 

Rock . . . . . . . . . . . . . .._............ 0.0 

Soil . . . . sands, gravels . . . . . . . . . . 0.0 

. . . clays, silts . . . . . . . . . . . . . 0.5 

4. Variance in Subsurface Conditions 

Little to no variance . . . . . . . . . . . . . . . . . 0.0 

Mild variance . , . . . . . . . . . . . . . . . . . . . . 0.5 

Substantial variance . . . . . . . . . . . . . . . . . 1 .O 

*Lifeline structure is a structure on a designated lifeline 

transportation route which must remain in service after an 

earthquake or other extreme design event. 

7.C.2.2 Micropile Load-Testing Guidelines for Experienced Users 

(Owners/Contractors) 

Many projects have been done with less testing than presented in Section 7.C.2.1. Reference 

[26] by Pearlman, et al., presents a summary of bridge projects and number of load tests 

performed. This list shows that: 

l Typically only 1 or 2 pile load tests have been performed, especially for piles founded 

in rock. 

l More tests have been performed on larger jobs with variable ground ( e.g., 

Williamsburg Bridge and Brooklyn-Queens Expressway) 

FHWA-SA-97-070 (~00-06) 7-9 
For experienced users and/or contractors, fewer tests than suggested in Section 7.C.2.1 may be 

appropriate and should be evaluated on a project-by-project basis. 

Verification tests may be performed on production piles provided that: 

l the piles are designed with a structural factor of safety of at least 1.25 at maximum 

test load, 

l the piles are not failed or overloaded during testing, and 

l the pile can be replaced if the pile fails. 

Previous tests by the Owner/Contractor on Micropiles may be used to reduce the number of 

verification and proof tests provided the previous tests had: 

l similar ground conditions 

l similar construction methods (drilling and grouting) 

l similar design loads and/or design bond stresses 

7.C.3 Sample Problem No. 1 (Chapter 5) - Pile Load Testing Requirements 

Given: Single-span bridge (not lifeline structure) located in mountainous terrain approximately 

125 km northwest of Denver, Colorado. Each abutment is to be supported on 12 

micropiles (a total of 24), approximately 12 meters in length. The controlling axial 

design load for the pile is 595 kN in compression Access to each abutment work bench 

must be established. Ground conditions are as shown in the Chapter 5 example problem. 

Table 7-3 summarizes the verification testing multipliers that have been selected for this 

example. 

7-10 7 FHWA-SA-97-070 (~00-06) 
Table 7-3. Sample Problem No. 1 - Pile Testing Requirements 

Load Test Factor 

Amplification Number of 

Factor Verification Tests 

24 Micropiles (from Table 7-1) n/a 1 

Non-lifeline Structure 0.0 1 x 0.0 = 0 

1,500 kN (ultimate capacity) 0.5 1 x 0.5 = 0.5 

Sands and Gravels 0.0 1 x 0.0 = 0 

Little Variance in Subsurface Conditions 0.0 1 x o.o= 0 

Total: 1.5 

Therefore, for this project oYte initial verification test is recommended on a non-production 

sacrificial test pile, to verify the contractor’s design and installation procedure. Also, one proof 

test (5% of 24) will be required on the production piles. The verification test can be conducted 

at abutment one and the proof test at abutment two. Test in compression to a maximum test 

load of 1,500 kN (2.5 x DL) for the verification test pile and 1,000 kN (1.67 xDL) for the proof 

test pile. 

7.C.4 Test Load Magnitude 

The factor applied to determine the test-load magnitude is typically controlled by the 

geotechnical factor of safety desired. This geotechnical factor is usually greater than factors 

applied for ultimate structural capacity considerations. Therefore, excess structural capacity 

may be required for the pile to support the test loads based on geotechnical capacities. 

For magnitude of test loading, this manual recommends: 

Verification load testing to . . . . . . 2.5 x Design Load, and 

Proof load testing to . . . . . . . . . . . 1.67 x Design Load. 

Refer to Section 5.E.4 for a detailed discussion of the rationale for these recommended test 

loads. 

FHWA-SA-97-070 (vOO-06) 7-11 
For ground-anchor testing tension, extra reinforcement is sometimes added to the initial 

verification test anchors, saving cost on the less-reinforced production anchors. It is much 

more difficult to correlate the test results of piles with differing reinforcement, particularly for 

composite-reinforced piles acting in compression. Also, a pile’s geotechnical capacity can 

have an effect on the ultimate structural capacity, calling for the same factor for both 

geotechnical and structural considerations. Whether or not the reinforcement can be varied for 

the test piles should be considered carefully, and should be addressed in the project 

specifications. Refer to Chapter 5 for more detailed coverage of this design aspect. 

7.D MICROPILE LOAD TESTING METHODS AND PROCEDURES 

7.D.l Method of Load Application 

If the micropiles are designed for tension and compression loads, then both loading conditions 

should be tested. If the same micropile is to be tested in both tension and compression, it is 

suggested that the tension test be conducted first. This will allow the pile to be reseated during 

compression testing in the event some net upward residual movement occurs during the tension 

test. 

The method of applying the load can vary, either in one cycle, incrementally advancing to the 

required capacity, or in multiple cycles where the load increments are applied and removed 

gradually until the maximum load is attained. The use of multiple cycles may be preferable if 

an attempt is made to reach the ultimate capacity of the pile. As discussed in Chapter 5, the 

analysis of the elastic and residual displacements measured during cyclic loading will provide 

valuable insight into the pile performance and mode of failure, and the extra costs are minimal. 

It is usually not necessary to conduct the load test on an inclined pile, even if the project 

includes them. Installing the pile on an incline has little, if any, effect on difficulty of 

construction and resulting capacity, particularly for piles installed by the cased hole method. 

However, testing an inclined pile can be difficult and increase the testing costs, particularly for 

compression testing. 

7-12 FHWA-SA-97-070 (~00-06) 
7.D.2 Load-Hold Duration 

The duration for which the applied loads are held (testing the tendency of a pile to creep) is 

another important consideration. If a pile is installed in non-creep-sensitive soils, such as 

sands, gravels, or rock, the maximum test load may be held for only ten minutes, with the hold 

duration extended if the acceptance criterion is not met. For piles in a creep-sensitive soil, such 

as plastic silt or clay, the maximum load hold duration may range from 100 minutes to as long 

as 24 hours, depending on the type and magnitude of design loading, nature of the soil, and 

sensitivity of the supported structure. 

7.D.3 Load Test Acceptance Criteria 

The magnitude and direction of load a pile must support is determined in the design of the 

structure. Maximum pile displacement is also determined during design of the structure, 

considering its sensitivity to movement, or considering the allowable displacements and footing 

rotations specified during seismic analysis. Criteria for allowable creep displacements can be 

based on standard criteria for ground-anchor testing (PTI, 1996) and soil nail testing (FHWA, 

1996), which is based on an allowable displacement of 2 mm per log cycle of time in minutes. 

Load test acceptance criteria is structure specific and typically includes the following: 

l The verification test pile shall support a load in tension and/or compression equal to 

250 percent of the specified service design loading (i.e. 2.5 x DL) without failure. 

The proof test pile shall support a load in tension and/or compression equal to 167 

percent of the specified service design loading (i.e. 1.67 x DL) without failure. Pile 

failure is defined as continued pile top displacement without supporting an increase in 

applied load. 

l The test pile shall support the service design load values with a total pile top 

displacement of not greater than mm. For combined tension and compression 

testing, the total displacement shall be measured relative to the pile top position at the 

start of initial testing. (Commentary: Structural designer should determine the 

FHWA-SA-97-070 (~00-06) 7- 13 
maximum allowable total pile-head displacement based on structural design 

requirements. Refer to Chapter 5 for more detailed design guidance.) 

l While holding the applied test load at 133 percent of the design service load, increase 

in the pile top displacement (creep) measured between 1 minute and 10 minutes shall 

not exceed 1 mm. If the increase in displacement exceeds the criterion value, the load 

hold duration shall be extended to 60 minutes or longer. The creep rate between 6 

and 60 minutes shall not exceed 2 mm per log cycle of time in minutes. 

The load test acceptance criterion is structure specific and must be verified by the designer. 

Seismic retrofit applications may allow higher displacements for compatibility with the 

existing pile system. 

7.E LOAD-TEST SETUP AND INSTRUMENTATION 

For axial and lateral testing, the most common and convenient system includes a hydraulic jack 

and reaction arrangement. The reaction system may be adjacent micropiles, ground anchors, 

and/or footings or mats providing ground-bearing reaction for tension testing. Test setup 

arrangements are shown in Figures 7-1, 7-2, and 7-3 for compression, tension, and lateral 

testing. Additional test setups are shown in Photographs 7-l through 7-3. The load is applied 

with the hydraulic jack, with the magnitude of the load determined through the correlation with 

the hydraulic pressure read from the gauge attached to the jack. This correlation is established 

through calibration of the jack and gauge, which is usually required within 90 days prior to the 

date of the load test. Figure 7-4 shows the load versus gauge pressure relationship established 

by jack calibration. 

7-14 FHWA-SA-97-070 (~00-06) 
STIFFENER PLATE 

BOLTS 4 EACH AS 

REQ. ON A PROJECT REACTION BEAM 

BY PROJECT BASIS 

/ / 

TYPICAL COMPRESSION LOAD TEST ARRANGEMENT (ASTM 01143) 

Figure 7 - 1. Compression Load Test Arrangement 

TRANSITION BEAM 

W/STIFFENER PLATE 

STIFFENER PLATE 

EXTENSION CASI 

WIENDPIATES 

STIFFENERS (TYP) AS 

REQUIRED ON A PROJ 

BT PROJECT BASIS 

THREAD BAR 

REACTION MI 

PLATE W I ST 

DIAL GAUGES MOUNTED 

ON AN INDEPENDENT 

REFERENCE FRAME 

(PER ASTM D3689) 

REACTION 

MICROPILE 

WP) 

TYPICAL TENSION LOAD TEST ARRANGEMENT (ASTM D3669) 

Figure 7 - 2. Tension Load Test Arrangement 

FHWA-SA-97-070 (~00-06) 7- 15 
MICROPILE 

STEEL JACK 

DIAL 

I ENCE 

I 

DIAL GAUGE 

LATERAL LOAD TEST SETUP 

Figure 7 - 3. Lateral Load Test Arrangement 

Photograph 7 - 1. Compression Load Test Setup 

7-16 FHWA-SA-97-070 (~00-06) 
Photograph 7 - 2. Tension Load Test Setup 

FHWA-SA-97-070 (~00-06) 7-17 
Photograph 7 - 3. Typical Load Test Jack 

Reference Gauge: RGH 

2.225 

1.780 

g 1.335 

2 

2 0.890 

0.445 

0 

14 28 42 56 70 

PRESSURE MPa 

Figure 7 - 4. Typical Jack Calibration Curve. 

7-18 FHWA-SA-97-070 (~00-06) 
Instnunentation for recording pile performance may include the following (refer to 

Photographs 7-4 to 7-6): 

l Dial Gauges - The vertical and lateral displacement is usually measured with dial 

gauges, with reading sensitivity to 0.025 mm. For an axial load test, the gauges are 

mounted on an independent reference beam whose supports are located a minimum of 

2.13 meters from the test pile. The averaged readings of three gauges are used to 

compensate for tipping of the pile head. The gauges are placed around the pile at an 

equal distance from the pile center. 

l Wire with Mirror and Scale - Pile top displacement may be determined using a 

mirror with a scale mounted on the pile, and a wire mounted on a separate reference 

strung in front of the scale. The scale is read by adjusting the line of sight until the 

wire lines up with its reflection. This method may be used as a backup to the dial 

gauge system, but it is less sensitive. 

l Survey Method - The displacement of the pile may be gauged using survey 

instruments (level, theodolite). This method may be used as a backup to the dial 

gauge system, but it is less sensitive. 

. Load Cell - Load cells may be used as an additional backup method of measuring the 

load applied to the pile. They can be used to more accurately maintain a constant test 

load throughout the creep-test load hold for verification test piles. When load cells 

are used, care should be taken to ensure that the cell is properly aligned with the axis 

of the micropile and jack. Load cells are used mainly to detect small changes in load 

and allow load adjustment and maintenance of constant holding load during creep 

testing. As an example, assuming that the load cell reads “440” once the creep test 

load is reached, it is important that the “440” reading on the load cell be maintained 

through jack pressure adjustments for the duration of the test. This provides 

assurance that a constant load was indeed maintained throughout the creep test. 

FHWA-SA-97-070 (~00-06) 7-19 
Photograph 7 - 4. Micropile Load Test Instrumentation 

7-20 FHWA-SA-97-070 (~00-06) 
Photograph 7 - 5. Micropile Load Test Instrumentation 

Photograph 7 - 6. Micropile Load Test Instrumentation 

FHWA-SA-97-070 (~00-06) 7-21 
l Telltales - Telltales are metal or fiberglass rods anchored into the pile grout at 

discrete elevations to measure axial displacement at these points in the pile. The rod 

tip is embedded into the grout at the point of interest, with the remainder sheathed to 

allow free movement of the pile. Movement at the top of the rod is measured with a 

dial gauge or electronic measuring device. Multiple telltales can be installed to 

measure movement at several points. Care must be taken to avoid damage to the 

telltales during pile installation, such as the rods becoming wrapped and damaged 

during drill string rotation, or the sheath becoming partially filled with grout. 

l Strain Gauges - Strain gauges may be mounted on the reinforcing steel, allowing 

measurement of the level of stress in the reinforcement at various levels in the pile. 

Again, care must be taken to avoid damage to the gauges and connecting wires during 

installation. If used, vibrating-wire-type strain gauges are recommended. 

l Inclinometer - An inclinometer may be installed in a pile to measure the deflected 

shape of the pile during lateral load testing. 

7.F DATA RECORDING AND PRESENTATION 

Shown in Tables 7-4,7-5, and 7-6 are schedules for cyclic tension and compression tests. The 

schedules shown are arranged to show testing that may be appropriate for Sample Problem No. 

1, included in Chapter 5. The figures that follow present data from completed micropile 

projects in several formats. 

Figure 7-5 - Pile Top Displacement vs. Load Displacement data for axial and lateral tests 

can be presented as shown. The data shown is for a five-cycle compression test. The pile was 

well grouted into an upper layer of concrete rubble and dense sand, resulting in a very stiff pile. 

Figure 7-6 - Pile Top Elastic/Permanent Displacement vs. Load The elastic and 

permanent displacement for a cyclic compression and tension test are shown. The permanent 

displacements are to the left, and the elastic to the right. The readings plotted are for the peak 

load of each cycle. The tension test included three cycles, and the compression test five. 

7-22 FHWA-SA-97-070 (~00-06) 
Table 7-4. Example Tension Cyclic Load Test Schedule (Verification Test) for a 322 kN, 

Seismic Service Design Load 

Initial Tension Test 

Dial 

It: I 

I.1 

Jage “3 Average 

adina Readma DisDlaceme 

Tension Tension Dial Dial 

Load Gage Hold I 

Load Load Pressure Duration Gage ‘1” Gac- “On ’ 

Cycle Reading Rez _ 

(% Design) WJ) Wa) (minutes) (mm) (mm) (mm) nt 

TI-1 AL 45.0 2.64 0 

25% 80.5 4.72 1 

Service 

Design 

Tension 

322 kN 

50% 161.0 9.44 0 

2 

75% 241.5 14.17 1 

3 

5 

FHWA-SA-97-070 (~00-06) 7-23 
Table 7-4. . . . . . . continued 

Initial Tension Test 

Set-via ! Tension Tension 

Load Gage Hold Dial 

Design Load Load Pressure Duration Gage “1” 

Tensiol 

Cycle Reading 

n (% Design) W) (Mpa) (minutes) 

I! (mm) 

75% 241.5 14.17 0 

2 

100% 322.0 18.89 0 

Average 

Displaceme 

nt 

II I 

*Note - Hold durations not necessary unless the displacement creep measured 

from 1 to 10 minutes exceeds 1 mm. 

7-24 FHWA-SA-97-070 (~00-06) 
Table 7-5. Example Compression Cyclic Load Test Schedule (Verification Test) for the 

72 1 kN Non-seismic Service Design Load 

Initial Compression Test 

Desil 

Tensi 

Service Tension Tension Gage Hold Dial Dial Dial 

Load Average 

In Load Load Pressure Duration 

Gage ‘1” Gage ‘2” Gage “3” 

Cycle Reading Reading Reading Displacemen 

Ion (% Design) W) Wa) (minutes) Imm\ II-i-m-.\ t 

721 Cl-l AL 45.0 2.64 0 

25% 180.3 10.57 1 

3 

5 

FHWA-SA-97-070 (~00-06) 7-25 
Table 7-5. . . . . . Continued 

Initial Compression Test 

Service 

Design 

Tension 

Tension Tension Gage Hold 

Load 

Dial Gage Dial Dial Average 

Load Load Pressure Duration ‘1” Gage Gage 

Cycle 

Displacemen 

I% Design) CkN) (MPa) (minutes) Reading ‘2” ‘3 (mm) I I . -, ., . I I I , 

75% 540.8 31.72 0 

7-26 FHWA-SA-97-070 (~00-06) 
Table 7-5. . . . . . Continued 

Initial Compression Test 

AL 45 2.64 0 

2 

* Note - Hold durations not necessary unless the displacement creep measured from 1 to 10 

minutes exceeds 1 mm. 

FHWA-SA-97-070 (~00-06) 7-27 
Table 7-6. Example Ultimate Compression Load Test Schedule (Verification Test) for the 

72 1 kN, Non-seismic Service Design Load 

Ultimate Compression Test 

7-28 FHWA-SA-97-070 (~00-06) 
MICROPILE VERIFICATION LOAD TEST PROGRAM 

Compression Test - l/25/93 

Pile Top Downward Displacement vs. Load 

2500 

I I I I I I 

I 

0 2 4 6 8 IO 12 

TOTAL MOVEMENT (mm) 

Figure 7 - 5. Pile top displacement vs. pile load curve 

FHWA-SA-97-070 (~00-06) 7-29 
2500 

2000 

0 

MICROPILE VERIFICATION LOAD TEST PROGRAM 

Pile Top Permanent & Elastic Displacement vs. Load 

Tension & Compression Testing 

I l Tension Test A Compression Test I I 

10 8 6 4 2 0 2 4 6 8 10 

PERMANENT ELASTIC 

MOVEMENT (mm) 

Figure 7 - 6. Pile Top Elastic/permanent Displacement Vs. Pile Load Curve 

7-30 FHWA-SA-97-070 (~00-06) 
Figure 7-7 Displacement Creep vs. Time The increase in displacement (creep) at constant 

load versus time is shown in this figure, with the time shown on a logarithmic scale. The 

criterion line for creep of 2 mm per log cycle is also shown. The long duration tests shown 

were for high-capacity piles installed in medium-dense to dense sands and gravels. 

Figure 7-8 - Elastic Length vs. Load The pile elastic length is determined based on the 

measured elastic displacement per load cycle and the calculated stiffness (EA) value. Again, 

note the stiffness of the pile due to it being well grouted into the upper layers. 

MICROPILE VERIFICATION LOAD TEST PROGRAM 

Test Piles TP - 1 through TP - 6 

Displacement Creep vs. Time at 2667 kN Load 

10 100 

TIME (minutes) 

Figure 7 - 7. Displacement Creep Vs. Time Curve 

FHWA-SA-97-070 (~00-06) 7-31 
2500 

2000 

s 1500 

Y 

2 0 J 1000 

500 

0 

MICROPILE VERIFICATION LOAD TEST PROGRAM 

Pile Calculated Elastic Length vs. Load 

Calculated EA = 1,723,OOO kN tension 

2,448,OOO kN compression 

I l Tension Test A Compression Test I I I 

I I I I It I I I I I I 

0 2 4 6 8 IO 12 14 16 18 20 

ELASTIC LENGTH (m) 

Figure 7 - 8. Elastic Length vs. Pile Load Curve 

7-32 FHWA-SA-97-070 (~00-06) 
7.G PILE LOAD-TEST REPORT 

For each load test, a report must typically be written and submitted to the Owner, usually 

within 24 to 48 hours of the load test completion. Suggestions for the contents of this report 

are as follows: 

1. 

2. 

3. 

4. 

5. 

6. 

7. 

8. 

9. 

10. 

Brief project description. 

Description of site and subsurface conditions. 

Key personnel. 

Pile installation data. 

Results of load test, including data and data presentation. 

Statement of load-test requirements and acceptance criteria. 

Comparison of load-test requirements, acceptance criteria. 

Summary statement on the load test results. 

Hydraulic jack/load cell calibration report. 

Material certification, including grout compressive strength testing, steel mill 

certification. 

Pile installation data includes detailed information about the test micropile, such as length of 

the pile (cased and uncased), number of bags of cement, size and type of casing and 

reinforcement, material encountered during installation, and actual drilling and grouting 

records. The description of site and subsurface conditions restates the anticipated ground 

conditions at the location of the load test, and compares actual with foreseen conditions. 

Personnel listed should include the drill rig operator, the superintendent, the grout plant 

operator, and any other key personnel involved in the installation and testing of the micropile. 

Results of the load test should include the actual sheets used during the load-test as well as 

tabulated and graphical presentations of the load-test results. A summary of the load-test 

requirements and acceptance criteria, in addition to the load-test results, should also be 

included for comparison. 

FHWA-SA-97-070 (~00-06) 7-33 
7.H TEST PILE FAILURE 

In the event a micropile fails to meet the specified acceptance criteria during pre-production 

testing, the contractor can modify the design of the micropile and/or the construction procedure 

in order to provide the required capacity. These modifications may include modifying the 

installation procedures, increasing the pile bond length, or changing the micropile type. If 

failure occurs during load testing of a production pile, the contractor shall modify the design, 

the construction procedure, or both. These modifications may include installing replacement 

micropiles, modifying installation methods, increasing the bond length, or changing the 

micropile type. Additional testing may be required to verity adequacy of the piling system 

depending on the details of the failure, results of the previous testing, and specifics of the load 

capacity requirements. 

7-34 FHWA-SA-97-070 (~00-06) 
CHAPTER 8 

CONSTRUCTION INSPECTION/QUALITY CONTROL 

8.A INTRODUCTION 

A major consideration in promoting new construction technology is implementing and 

achieving quality assurance and control (QA/QC). The importance of this construction control 

is magnified with specialty construction techniques, as in the use of micropiles. In such 

contractor- designed and built‘situations, the owner’s project engineer and inspector must have 

a clear understanding of the items to be controlled by specification and the items left to the 

discretion of the specialty contractor. Recommended contracting methods and construction 

specification guidelines are discussed in Chapter 9. This chapter is devoted to the operations 

and procedures that require the attention of the owner’s project engineer and/or construction 

inspector. Its purpose is to provide field inspectors with the knowledge necessary to effectively 

monitor and document the construction of micropile support systems. 

The QA/QC process begins before the specialty micropile contractor appears on the job site. 

QA/QC begins in the plans and specification development stage, and is ensured by adherence 

to the intended specifications. The proper installation of micropiles is a process that 

requires the expertise of an experienced specialty contractor. The most important section 

of the specifications to be enforced by the owner deals with the experience qualifications 

of the micropile contractor and personnel hired to perform the project work. Failure to 

enforce the specified qualifications opens the doors for inexperienced contractors trying to cut 

costs, often with negative consequences to the owner. The results often are inferior 

workmanship, project delays, and project claims that often substantially increase project costs. 

Results like these often discourage project owners from implementing new technology, and 

draws them back to more traditional methods at any cost. This can be avoided with the proper 

specification implementation and, as importantly, enforcement to ensure a mutually successful 

project. 

FHWA-SA-97-070 (~00-06) 8-1 
In addition to the expertise of the specialty contractor, the quality of the individual construction 

elements is directly related to the final products overall quality. As with other drilled pile 

systems, the actual ultimate capacity of a micropile can only be definitively proven by pile load 

tests. It is not practical or economical to test every element installed. It is therefore imperative 

that close attention is paid to the quality of the materials, including their storage, handling, and 

preinstallation preparation, and the construction at all stages of the work. Inspection begins 

with the arrival on the job site of the micropile materials. 

8.6 QUALITY CONTROL INSPECTION 

8.B.l Material Handling and Storage 

Grout 

High-quality grout starts with proper material storage (Photograph 8-l). This primarily 

consists of preventing hydration by keeping the cement dry. Cement is typically supplied 

either in bagged (43 kg/sack) or bulk (1320 kg/bulk bag) form, depending on site conditions, 

job size, local availability, and cost. Cement should be shipped on wooden pallets, and handled 

and stored in a protected location to avoid moisture absorption. It should not be stored directly 

on the ground or anywhere it may be subject to significant moisture absorption. Avoid 

stacking the bags too high to prevent overcompaction of the cement. Prior to use, the cement 

should be visually checked for lumps or other indications of hydration or contamination by 

foreign matter. 

All cement shall conform to the proper standards (Chapter 4, Section 4.C). Handling of 

admixtures should follow a similar course, with primary attention paid to manufacturer’s 

recommendations. Admixtures that have exceeded the manufacturer’s recommended shelf life 

shall not be used. 

Water used for grout mixing must be potable, and not contain impurities harmful to the steel or 

the grout. Potable water is typically obtained from adjacent hydrants and/or other sources. 

8-2 FHWA-SA-97-070 (~00-06) 
Photograph 8 - 1. Cement Storage 

Reinforcing Steel 

The micropile contractor should use care in unloading and storing all reinforcing steel material. 

The storage area should be away from construction traffic and in a protected location. When 

reinforcing steel material is delivered to the job site, it should be visually checked for size and 

any signs of corrosion. Reinforcing steel elements should be accompanied by their mill 

certificates. Mill secondary pipe material, for which mill certificates are not available, require 

verification of steel quality through tensile and chemical testing of steel samples. For Buy 

America Contracts, United States manufacturing of materials must be veriJied All steel 

reinforcing materials shall conform to the proper standards (Chapter 4, Section 4.D.2), as 

identified in the project specifications. 

FHWA-SA-97-070 (~00-06) 8-3 
Photograph 8 - 2. Storage of Reinforcing Steel 

Properly stored steel reinforcing materials (see Photograph 8-2) prevents corrosion or 

contamination by dirt, oils, and/or organics. Wood dunnage placed between the ground and the 

steel materials will prevent slight rusting that can occur if steel is exposed to the ground. A 

light coating of surface rust on the steel is normal and indicates that oil and grease are not 

present. Deep flaky corrosion or deep pitting of the steel is cause for rejection. 

Double corrosion-protected and/or epoxy-coated steel bars should be delivered to the job site 

pre-manufactured (Photograph 8-3). Extra care in handling and storage shall be given to these 

materials. Epoxy-coated bars shall be properly wrapped with padded bands and placed on 

dunnage. Pre-manufactured corrosion-protected steel bars must be stacked with care to prevent 

any damage to the corrugated tube and/or epoxy coating. 

8-4 FHWA-SA-97-070 (~00-06) 
Photograph 8 - 3. Double Corrosion Protected (Encapsulated) 

Reinforcing Steel 

Mechanical couplers, hex nuts, centralizers, and spacers are typically delivered to the job site in 

boxes. Mill certificates will be provided for the steel couplers and hex nuts. Storage of these 

materials shall follow the same procedures as the other materials. 

Inspection of all associated reinforcing elements should include: 

l Checking for damage. 

l Checking for corrosion of steel. 

l Checking proper size. 

l Checking proper corrosion protection. 

l Obtaining mill certificates and any independent material test results. 

FHWA-SA-97-070 (~00-06) 8-5 
8.B.2 Construction Monitoring 

Prior to the commencement of any construction, the micropile contractor’s plan of work should 

be established by submittal of their project-specific working drawings. The work plan should 

describe all anticipated aspects of construction, including any anticipated difficulties. 

Following is a compilation of installation submittal requirements. 

1. 

2. 

3. 

4. 

5. 

6. 

7. 

8. 

9. 

Contractor and employee qualifications. 

Description of Contractor’s understanding regarding materials anticipated to be drilled 

from geotechnical report. (List any potential problems). 

Performance criteria and tolerances. 

Location and orientation of the micropiles. 

Micropile size and configuration. 

Micropile capacity. 

Drilling equipment, including manufacturer and model numbers, flushing media, and 

precautions against drilling deviation. 

Anticipated equipment loads on structure or adjacent ground during construction. 

General installation plan, including proposed sequence of installation, phasing, and 

scheduling. 

10. Grout design mix, along with batching, mixing, and injection techniques. 

11. Reinforcement, including sizing, configuration, and corrosion protection. 

12. Postgrouting methods, procedure, and equipment (if any). 

13. Documentation and protection of existing utilities and other sensitive elements in the 

built-environment, and proposed measures to anticipate the conditions. 

8-6 FHWA-SA-97-070(vOO-06) 
14. Plan to accommodate low headroom or nearby obstructions. 

15. Testing criteria including: maximum design and proof loads, allowable deformations 

under test loads, and testing procedures. 

16. Location of load cells, proper gauge calibration, and any other testing or monitoring 

devices. 

17. Details of connection to existing structures. 

18. Criteria for implementing remedial procedures. 

19. Spoil handling (typically performed by general contractor). 

Most importantly, protection of the existing environment, including underground utilities, 

must be considered. The existing site conditions should be examined by the micropile 

contractor with the owner’s inspector. The contractor is responsible for furnishing equipment 

suitable for the specific site conditions of each project. 

Drilling 

As outlined in Section 4.B (Drilling), a wide range of methods can be utilized for micropile 

drilling. Most drilling techniques are likely to be acceptable, provided they can form a stable 

hole of the required dimensions within the permitted tolerances, and without detriment to the 

surroundings. 

The physical nature of drilling and forming a hole may disturb the surrounding ground for a 

certain time and over a certain distance. It is important that the owner’s inspector is aware of 

this and that he/she is assured the micropile contractor’s equipment and drilling and grouting 

methods and procedures will produce the desired results. Ground disturbance is an important 

issue; the use of high pressures in poorly controlled flushing operations should always be 

undertaken with care. 

FHWA-SA-97-070 (~00-06) 8-7 

__-. 
The inspector must ensure that drilling operations are not causing unacceptable loss of ground. 

Obvious signs are the inability to withdraw drill casing, large quantity of soil removal for little 

or no casing advancement, and subsidence of the ground above drilling location. 

The inclination and position of micropiles may deviate somewhat from their designed 

specifications. The following normal tolerances are typical of those proposed for larger 

diameter elements. 

Inplan .......................... 75 mm in any direction 

Verticality ....................... 1 in50 

Between vertical or inclined up to 1:6 . 1 in25 

Inclined greater than 1:6 ............ 1 in 15 

Although the pile tolerances above are realistic for most sites, tolerances may have to be 

relaxed where the ground contains obstructions and appropriate allowances made in the design 

of micropiles and their associated structural members. 

In all cases, drilling must not allow collapse of the borehole. When temporary casing is to be 

fully extracted during the grouting process, it should be removed in such a way that the pile 

reinforcement is not disturbed, damaged, or allowed to contact the soil. It should also be kept 

full of grout during the extraction operation, to minimize the danger of pile hole collapse. 

(Note: Fluid levels in the hole must always be kept above the ground water level so that the 

internalfluidpressure adequately balances external ground water pressure and active earth 

pressure throughout all potentially unstable soils.) 

The drilling, installation of reinforcement, and grouting of any given micropile should be 

completed in a series of continuous processes as expeditiously as possible. Some materials, 

such as consolidated clays and weak rock, can deteriorate and soften on exposure. A good rule 

of thumb in these conditions is to make sure drilling of the pile bond zone be undertaken on the 

same day as reinforcement installation and grouting. 

8-8 FHWA-SA-97-070 (~00-06) 
Unless pile construction is continuous, the pile hole should be temporarily plugged or covered 

at the surface to prevent debris from falling into the borehole and for safety of the personnel 

working on the site. It is also recommended that any casing (temporary or permanent) project a 

minimum of 0.3 m above the head elevation to avoid drill spoils and flushing water from other 

operations flowing down and contaminating the completed hole. 

The micropile contractor shall provide for proper disposal and containment of the drilling 

spoils from the site in accordance with the approved working plan. (Note: Handling and 

disposal of the drill spoils may be the responsibility of the general contractor.) Beware of 

uncontrolled drill water causing erosion or freezing on/off the site. 

Any of a great number of problems may develop during the drilling process, and thereby 

require close supervision. It is imperative that comprehensive pile installation logs be 

maintained. This is discussed later in the chapter. 

Grouting 

Before any grouting begins, the condition of the equipment should be checked and the 

approved grout-mix design verified. Care in execution of the grouting process can greatly 

impact pile load performance. Some guidelines to proper grouting practices follow. 

1. 

2. 

3. 

4. 

5. 

Select equipment to ensure continuous grout placement. 

Select a mixer capable of colloidally mixing the cement, and a storage tank that will then 

permit continuous grout agitation. 

Prevent the presence of air in the grout lines by expelling all air and checking that the 

suction circuit is airtight. 

During grouting, do not draw down the level of grout in the supply tank below the crown 

of the exit pipe. 

Ensure the exclusion of any foreign matter during grout placement for the full length of 

the pile. 

FHWA-SA-97-070 (~00-06) 8-9 
6. Use tremie methods. 

7. Prevent heaving in horizontally stratified soil by limiting grout pressures or by limiting 

the quantity of grout pumped. Both may be necessary. 

8. Prevent the soil at the bottom of the hole from blowing in. 

Prior to grouting, the diameters of micropiles drilled without casings should be verified. 

Ideally, micropiles should be grouted immediately after drilling the bond zone. If this is not 

possible, then grouting should be performed before degradation of the drillhole. 

The primary grouting operation involves injecting cement grout at the lowest point of the 

borehole so the hole will fill evenly without air voids. A tremie tube is typically tied to the 

reinforcing steel before insertion in the borehole. Care must be taken to ensure that the tube is 

tied loose enough to be removed from the borehole on completion of the grouting. Contractors 

typically use duct tape to wrap the tremie tube and reinforcing steel loosely together. After 

placement of the reinforcement steel and tremie tube, the grout is pumped to the bottom of the 

pile until grout of suitable quality (of the same consistency as that being injected) returns to the 

top of the pile. The tremie tube is then removed fi-om the pile hole. 

Grouting should be performed in one continuous operation. Care should be taken to maintain a 

positive head at the grout holding tank, to avoid drawing air into the injected grout. 

Pressure grouting may be applied to the primary grout during casing withdrawal. Care must be 

taken with such pressures to avoid distress to the ground or adjacent structures. The pressure 

grouting process should be stopped if grout is observed escaping at the ground surface. 

Grout pressures are measured as close to the point of injection as possible to account for line 

losses between the pump and the borehole. Typically, a pressure gauge is mounted on the drill 

rig and monitored by the drill rig operator as a guide during pressure grouting and casing 

withdrawal (Photograph 8-4). 

8-10 FHWA-SA-97-070 (~00-06) 
Photograph 8 - 4. Drill Rig Pressure Gauge 

For micropiles installed with a hollow-stem auger, the grout should be pumped under 

continuous pressure with a grout head maintained above the tip of the augers at controlled and 

monitored withdrawal rates to ensure that no “necking” of the borehole occurs. 

If postgrouting is performed, it is very important to monitor grout pressures and volumes 

throughout the injection of each sleeve. This will help prevent dangerous or needless 

over-injection and progressively verify the effectiveness of the treatment. Postgrouting 

pressures have been documented in excess of 6.5 MPa. Typical range during injection is 2 to 4 

MPa. 

FHWA-SA-97-070 (~00-06) 8-11 
Grouting records are of vital importance on every project and are described in detail later in 

this chapter. 

Installation of Reinforcement 

Reinforcement may be placed either prior to grouting, or placed into the grout-filled borehole 

before temporary casing (if used) is withdrawn. In any case, the inspector shall record the total 

pile length and bond zone length for each installed micropile. The reinforcement shall be 

capable of being inserted to its prescribed length in the hole without force. Inability to achieve 

this penetration in uncased holes usually means caving of soil into the hole has occurred, and 

withdrawal of the reinforcement and redrilling is necessary. (Note: Inspectors should be 

cautioned to prevent the micropile contractorfiom driving the reinforcement to full length or 

cutting off the reinforcement.) 

Care must be taken not to damage any corrosion protection or centralizers during installation. 

Neither epoxy-coated, double corrosion-protected, or uncoated steel reinforcements should be 

dragged across abrasive surfaces or through the surface soil. All splices and couplings should 

be checked for proper seating. The reinforcement must be clean of deleterious substances such 

as surface soil, oil, and mud, which may contaminate the grout or coat the reinforcement. 

Centralizers and spacers shall be checked to ensure placement at specified intervals, typically 

2.5 to 3 meters (Photograph 8-S.) Attachment of the centralizers to the reinforcement should 

also be checked to ensure that the reinforcement remains centered in the borehole. Typically 

centralizers are attached with tie-wire or duct tape. (Note: Inspectors should be cautioned to 

make sure tremie grout tubes can be placed to the bottom of the borehole without interference 

with the centralizers, tfplaced after bar installation.) 

8- 12 FHWA-SA-97-070 (~00-06) 
Photograph 8 - 5. Reinforcement Centralizers 

8.C QUALITY CONTROL DOCUMENTATION 

8.C.l. Pile Load Testing 

Quality assurance and control often begins with the verification pile load testing and continues 

during production pile installation and production pile proof load testing. During the 

verification load testing, the inspector must understand the reasons for requiring careful control 

of such testing and the practical meaning of the acceptance criteria. Verification pile testing is 

done to verify the design capacity and construction processes of the micropiles. 

The inspector must ensure the test pile installation method reflects the method planned for the 

installation of the production piles, the pile load test setup is in accordance with the approved 

working drawings, and the pile is load tested in accordance with the project specifications. 

Items to check include testing apparatus alignment, use of properly calibrated jacks and gauges, 

dial gauge arm alignment and travel length, dial gauge point of contact, application of the 

proper loads, and potential interference points due to pile deflection. 

FHWA-SA-97-070 (~00-06) 8- 13 
It is important for the owner’s project engineer and the micropile design engineer to be 

prepared to evaluate potential pile load test failure. Solutions for alternative pile types and/or 

layouts will need to be developed and implemented quickly so as not to delay the project 

schedule. 

KC.2 Production Piles 

Comprehensive records of the pile installation and grouting operations are of vital importance 

in establishing the basis of payment and highlighting any deviation that may be significant to 

pile performance at a later stage. Table 8-l provides a recommended micropile installation log, 

to be used by the micropile contractor and the owner’s inspector. 

Because the grout is such a vital component of the micropile, close attention should be paid to 

the control and quality of the product. Grout production and consumption records must be kept 

daily. It is important that the actual pressure and volume of grout pumped in each pile be 

recorded. The grout take itself will tell a lot about the success of the procedure. 

Compressive Strength: Unconfined compressive grout strength is determined according to 

AASHTO T106/ASTM C-109. Compressive strength is checked through a set of three each, 

50-mm grout cubes (Photograph 8-6.) Tests are typically performed at 3,7, and 28 days after 

grouting. Seven-day tests are considered the most crucial, as the grout will typically attain the 

required design strength within this time period. Anticipated strength will depend upon project 

need, but compressive cube strengths can be very high, as much as 24 MPa after only 24 hours 

of set time. The unconfined compressive strength is largely dependent on project needs, but a 

28-day strength requirement between 25 and 40 MPa is considered common for micropiles. 

The inspector needs to verify that the grout cube break strengths comply with the project 

specifications. Grout samples should be taken directly from the grout plant. It is recommended 

that one set of three grout cubes be taken for every 10 piles installed, or every day for each 

grout plant in operation, whichever occurs more frequently. 

8- 14 FHWA-SA-97-070 (~00-06) 
Table 8-1. Micropile Installation Log 

Micropile Installation Log 

Project Name: 

Cantract Na.: 

II Pile Designation # 1 I Time @ ~1 

II Installation Date I I start of Diilli, II 

Drill Rig/Drill Method 

Drill Rig/ #, Operator 

Start of Grouting 

Pile Completion 

I Grout Plant #, Operator I Total Duration 

Drill Bit Type and Size Cement Type 

Casing Dia./Wall Thickness Admixtures 

Pile Inclination w/c Ratio 

c Reinforcement Size/Length 

Pile Length Above B.O.F. 

Upper Cased Length 

Cased and Bond Length (Plunge) 

Bond Length Below Casing 

Tremie Grout Quantity (bags) 

Pressure Grout Quantity 

(bags) 

Grouting after plunge (bags) 

Total Grout Quantity (bags) 

Total Pile Length I Grout Ratio (bags/m bond) 

Comments - Pile Drilling 

Depth from B.0.F (m) Soil I Rock 

Description Flush Description Comments 

Depth from B.0.F 

(ml 

Comments - Pile Grouting 

Pressure Range 

Max/Average (MPa) Comments 

B.O.F. = Bottom of Footing (ref. FHWA-SA-97-070) 

FHWA-SA-97-070 (~00-06) 8-15 
Table 8-2. Completed Micropile Installation Log 

Micropile Installation Log 

II Project Name: Keeley Palace -II 

Contract No.: 0064 

Pile Designation # Pile 1 Time @ 

installation Date 12-25-96 Start of Drilling 

Drill Rig/Drill Method Klemm 806, Cased Rotary Start of Grouting 

Drill Rig/ #, Operator 1 O-l / Kilian Pile Completion 

8:OO a.m. 

9:00 a.m. 

920 a.m. 

I Grout Plant #, Operator 8-I 1 Holder Total Duration I:20 

II Drill Bit Type and Size 1 Casing Teeth - 175 mm 1 Cement Typa I I II II 

Casing Dia.NVall Thickness 175mm/50mm Admixtures None 

Pile Inclination 0 degrees WIG Ratio 0.45 

1 

Reinforcement Size/Length 57mm/155m I 

Pile Length Above B.O.F. 0.5 m 

Upper Cased Length 6 m 

Tremie Grout Quantity (bags) 

Pressure Grout Quantity 

(bags) 

12 

13 

Cased and Bond Length (Plunge) 2 m Grouting after plunge (bags) 2 

Bond Length Below Casing 6 m Total Grout Quantity (bags) 27 

Total Pile Length 1 16.5 m I Grout Ratio (bags/m bond) 1 12/16.5 + 15/8 = 2.6 

Comments - Pile Drilling 

Depth from B.0.F Soil I Rock Description Flush 

ON Description Comments 

o-5 Gravel and Cobbles Brown, full return 

5-10 Sand Gravel w/Cobbles Brown, full return 

IO-15 Cobbles w/Gravel Gray, full return Occasional Sand Seams 

Comments - Pile Grouting 

II Depth from B.0.F (ml I 

Pressure Range 

Max I Average (MPa) I 

Comments 

II 

II 15-10 1 0.55 I 0.45 I Grouted first 3 m under pressure, pulled remainder under static head II 

II IO-5 1 0.5510.47 1 Grouted first 3 m under pressure, pulled remainder ~ ~~~- under stabc head ---It 

Plunge, 5 - 6.5 1.2 After plunging casing, pumped additional 2 bags at max pressure = 1.2 MPa 

B.O.F. = Bottom of Footing (ref. FHWA-SA-97-070) 

S- 16 FHWA-SA-97-070 (~00-06) 
Photograph 8 - 6. Grout Cubes for Compressive Strength Testing 

Specific Gravity: The water content in grout is the prime control over grout properties and is 

most frequently checked through specific gravity measurements. Neat cement grout density 

can be determined per the API Recommended Practice 13B-I by the Baroid Mud Balance Test 

(Photograph 8-7). The test is extremely quick and inexpensive. Frequent checking is 

recommended - at least once per pile. By monitoring the water/cement ratio during grouting, 

the inspector can ensure that the grout is being prepared according to the specified project mix 

design. A common specific gravity for many types of micropiles is between 1.8 and 1.9 kg/m3. 

This equates to a 0.45 water/cement ratio. 

Tests that permit quality assessment prior to installation are more valuable since they permit 

immediate reaction in case of anomalies. Unfortunately, other factors like compressive 

strength values may only be established 48 hours after the sampling at the earliest. It is good 

practice that such initial tests be made on the design grout mix in pre-construction trials to give 

the owner’s representative confidence prior to production drilling. The design mix of the grout 

can be verified prior to and/or during the micropile verification load testing. 

FHWA-SA-97-070 (~00-06) 8- 17 
Photograph 8 - 7. Baroid Mud Balance Test 

8-18 FHWA-SA-97-070 (~00-06) 
CHAPTER 9 

CONTRACTING METHODS 

9.A INTRODUCTION 

To date, methods of design, specification, and installation of micropile systems in the United 

States have been developed by specialty geotechnical contractors. Most public agencies and 

consulting engineers presently have little or no knowledge regarding micropiles and their 

application. The goal of this manual is to provide guidelines that will help owners and 

engineers implement the most technically and economically feasible application of micropile 

technology in everyday use. 

As discussed at the beginning of Chapter 8, preparation and enforcement of the contract 

documents are important steps in the introduction of new technologies. For innovation to 

flourish, there must be a need and a reward for those who take the risk of funding and 

implementing the technology. One of the biggest constraints on this is our most commonly 

used delivery system for construction projects, the traditional “low-bid” owner-designed 

system. This process limits innovation, promotes the use of unqualified contractors and poor 

quality, may result in an increase in end-product costs, and retards the implementation of new 

technology. With this in mind, the implementation of project specifications incorporating the 

use of alternative contracting methods is vital to the adoption of micropile technology. 

In order to insure quality micropile construction, it is strongly recommended that projects 

utilizing micropiles be specified to place a large amount of responsibility on the specialty 

micropile contractor. This is the basis for the remaining portions of this chapter. Alternative 

contracting methods are described and compared to provide guidelines for owners and 

consulting engineers when specifying or allowing micropiles. Also, information to be included 

on the contract plans and the specialty contractor working plan submittal is discussed. 

FHWA-SA-97-070 (~00-06) 9-1 
9.6 SPECIFICATIONS 

As most specialty geotechnical work is performed in the public sector, fair contracting 

practices are always an issue. The U.S. construction industry is very strongly geared to the 

owner- designed low-bid contracting process to promote these fair contracting practices. Some 

owners are reluctant to move away from this system, for fear of potential litigation caused by 

unfair contracting. However, there are proven alternative contracting methods that can be used 

to encourage good quality innovation and still protect the owners’ interest, even within the 

confines of the traditional low-bid system. 

The specifications can be used to mandate methodology or allow alternative designs. The 

degree of detail will be based on the designer’s experience with micropile installations, owner 

confidence with the pre-qualified micropile contractors, and the critical nature of the 

application. An important note to always remember when preparing the plans, specifications, 

and cost estimate for micropile projects is that the specialty contractor installing the micropiles 

is often a subcontractor on the project. Support services are necessary from the general 

contractor and will have to be included in the micropile pricing (e.g., access, spoils handling, 

footing excavation and backfill, etc.). 

Two general options for micropile design are set forth in Appendix A-l and A-2 Guide 

Specifications of this manual. These are summarized in Table 9-l. 

As stated earlier in this chapter, micropile specifications must place a large amount of 

responsibility on the specialty geotechnical contractor. With this in mind, pre-qualification of 

micropile contractors is highly recommended for both owner-controlled design and contractor 

design/build specifications. It is not the intent of this document to develop a pre-qualification 

system for every project owner, but it is recommended that the pre-qualification process be 

performed prior to contract advertisement. The pre-qualified micropile contractors (minimum 

of two or as determined by state law or owner agency policy) can be listed in the contract 

specifications. If the micropile work is a subcontracted item, each general contractor must be 

required to provide the name of its micropile subcontractors with the bid submittal. 

9-2 FHWA-SA-97-070 (~00-06) 
Table 9-l. Contractor Design/Build Options 

Option 1: 

Contractor 

Design/Build of 

Micropiles 

Option 2: 

Contractor 

Desigfluild of 

Foundations 

(Micropiles and 

Footing) 

. . ‘<., ~nf&n&~~~ prdvided by :. 

‘,,, ~w+-$Ehgineef ,“, 

1. Footing Design & Pile Layout 

2. Pile working load (axial tension, axial 

compression and lateral) and ultimate/seismic 

load and tolerable deflections 

3. Geotechnical information (preferably 

Geotechnical Baseline Report) 

4. Ground zones which can not be used for pile 

capacity (weak soil, scour and liouefiable zones) 

1. Load combination (vertical, lateral and 

moments) for working and seismic cases at top 

of footing 

2. Geotechnical information (preferably 

Geotechnical Baseline Report) 

3. Ground zones which can not be used for pile 

capacity (weak soil, scour and liquefiable zones) 

f&qiQn by h$&pilb 

t Spfsi$t$ Cqtitracltor , 

1. Pile structural design 

2. Pile geotechnical design 

(length, diameter etc.) 

3. Pile connection design 

1. Footing design 

2. Pile structural design 

3. Pile geotechnical design 

(length, diameter etc.) 

4. Pile connection design 

The following minimum pre-qualification requirements are recommended for each micropile 

contractor: 

1. 

2. 

3. 

4. 

The company and personnel’s previous successful experience - evidenced by owner 

references-in the design and installation of micropiles of similar scope to those proposed 

for the application project. Documentation shall include at least five successful projects 

performed in the last five years. 

The contractor shall assign an engineer to supervise the work who has at least three years 

of experience in the micropile design and construction of at least 3 successfully 

completed projects over the past 5 years. 

Micropile contractor must have previous drilling experience in soil/rock similar to 

project conditions. Contractor must submit at least three successful load-test reports 

from different projects of similar scope to project. 

Project superintendents and drill operators responsible for installation of micropile 

system must have micropile installation experience on at least 3 successfully completed 

projects over the past 5 years. 

FHWA-SA-97-070 (~00-06) 9-3 
Alternative specifications for both owner controlled design and contractor design/build projects 

are described in the following sections. (Note: All spec@ation recommendations assumeprequaliJied micropile contractors are listed in the contract specifications and the general 

contractors are required to name theirpre-qualljied micropile subcontractor of choice at bid 

time.) 

9.B.l Owner-Controlled Design Methods 

Owner-controlled design specifications vary in the amounts of the design to be performed by 

the owner’s design engineer and the micropile contractor. The owner typically establishes the 

following: 

l Scope of work. 

l Micropile design loadings and spacing. 

l Footing details. 

l Corrosion protection. 

l Micropile testing procedures and requirements. 

l Instrumentation requirements. 

l Special design consideration (e.g., scour and liquefaction potential). 

l Performance criteria. 

The micropile contractor specifies the following: 

l Micropile construction process. 

l Micropile type. 

l Micropile design. 

. Pile top footing connection design. 

9-4 FHWA-SA-97-070 (~00-06) 
This division of work allows the pre-qualified specialty contractors to provide an economical 

micropile design, while satisfying the owner’s engineer’s design requirements. This method 

also allows responsibility for the work to be shared between the owner and the micropile 

contractor. While this is slightly more restrictive than the contractor design/build specification 

method, it is still flexible enough to allow for some innovation and cost savings. Various types 

of owner-controlled design specifications include the following. 

Standard Design 

The contract documents for the Standard Design Type are prepared to allow for various prequalified micropile designs. The owner’s engineer provides the micropile design loadings, 

footing design, and pile layout for foundation support projects. In addition, the owner 

provides the following: 

l Geotechnical reports and data. 

l Micropile design parameters, including (foundation support projects only): 

Maximum pile sizes. 

Axial pile loads. 

Lateral pile loads. 

Displacement requirements. 

Ductility requirements. 

l Existing utility plans. 

l Site limitations, including: 

Access limitations. 

Right of way. 

scour. 

Liquefaction. 

Noise requirements. 

FHWA-SA-97-070 (~00-06) 9-5 
Vibration requirements. 

Hazardous/contaminated soils. 

l Contractor working drawing/design submittal and review requirements, including: 

Time frame. 

Penalties. 

l Material specifications. 

l Testing requirements. 

l Instrumentation requirements (if any). 

l Acceptance criteria. 

l Method of measurement and payment. 

It is recommended that micropile measurement and payment be performed on a unit-permicropile basis separately for 1) furnished and installed production piles, 2) verification 

testing, and 3) proof testing, except for piles founded in rock, for which an add/deduct footage 

price should be included (see Appendix A, Guide Specifications Commentary). 

During the bidding process, the pre-qualified micropile contractors prepare a preliminary 

design and a firm cost proposal based on the owner’s plans and specifications. If the project is 

to be subcontracted, general contractors will receive bids from each pre-qualified specialty 

subcontractor and includes the best offer in the proposal. The name of the recommended 

micropile subcontractor is included in the general contractor’s bid, which is then submitted. 

Once the contract has been awarded, the selected specialty contractor prepares their working 

drawings and design calculations and submits them to the engineer for review and approval. 

After acceptance of the design, construction begins. 

Appendix A-l contains sample contract plans and guide construction specifications utilizing 

this method. 

9-6 FHWA-SA-97-070 (~00-06) 
Alternate Micropile Design 

The contract documents for the Alternate Micropile Design Type are also prepared to allow for 

various pre-qualified micropile designs. The major difference in this method is the owner 

provides a design in the contract documents utilizing a more traditional foundation support 

system. The contract documents allow alternate micropile designs to be submitted by the listed 

specialty contractors on a one-for-one pile replacement of the owner-designed pile system. The 

information described under Standard Design Type is also required. 

An attractive option to this one-for-one pile replacement method is where the owner provides 

three alternative pile-supported footing designs in the contract documents. Alternate 1 would 

be one-for-one pile replacement, Alternate 2 could be one-for-two pile replacement, and 

Alternate 3 could be one-for-one-and-one-half pile replacement. This allows the pre-qualified 

micropile contractor to provide fewer higher capacity micropiles than the conventional pile 

footing design requires, and allows the owner’s engineer to maintain control of the footing 

design. 

The information necessary in the contract documents is similar to that previously mentioned, 

except the owner provides two to three alternative pile footing designs on the plans with the 

associated pile design and load criteria. 

Cost Reduction Incentive Proposal (Value Engineering) 

The Cost Reduction Incentive Proposal is another long-established form of alternate proposal 

used in the United States. 

When Cost Reduction Incentive Proposals are permitted, it is important that the owner specify 

any additional restrictions that may apply, such as right-of-way restrictions. Foundation 

support elements are usually the first order of work, so the approval time required for review 

and approval of the Cost Reduction Incentive Proposal requires quick response by the 

Contractor and commensurate responsive review by the Owner. 

FHWA-SA-97-070 (~00-06) 9-7 
9.62. Contractor Design/Build Methods 

According to the owner-controlled design specification method, when the use of micropiles is 

stipulated for a project, the owner’s engineer defines the scope of work and shares 

responsibility in the design and installation of the micropile system. With the design/build 

method, by contrast, the owner outlines the project’s ultimate needs and the specialty 

contractor is responsible for the micropile system detailed design and installation. The owner’s 

engineer typically establishes the following: 

l Scope of work. 

l Total structure loads. 

l Footing details. 

l Corrosion protection. 

l Micropile testing procedures and requirements. 

l Instrumentation requirements. 

l Special design consideration (e.g., scour and liquefaction potential). 

. Performance criteria. 

The micropile contractor specifies the following: 

l Micropile construction process. 

l Micropile type and quantity. 

l Micropile design. 

l Revisions to footing details to accomodate micropile design. 

l Pile top footing connection design. 

Based upon specified limitations and requirements, a design/build proposal is submitted, either 

before the bid advertisement @e-bid), or after contract award (post-bid). Measurement and 

payment is typically made on a lump-sum basis. 

9-8 FHWA-SA-97-070 (~00-06) 
In design/build construction, the owner is committed to a team approach whereby the specialty 

contractor becomes an important part of the team, contributing to all foundation and groundsupport aspects of the project. Risk sharing is integral: the micropile contractor is responsible 

for the adequacy of the design and its construction, the owner is responsible for the accuracy of 

the information upon which the design is based. Costs are reduced, as the contractor includes 

fewer contingencies. Innovation is encouraged, since the contractor is rewarded for economies 

of design and installation. Lastly, quality is enhanced due to pre-qualified contractors working 

with the project owner in a partnering approach. The two recommended types of contractor 

design/build specifications follow. 

Postbid 

The contract documents for the postbid design/build method are prepared to allow for various 

pre-qualified-contractor-designed alternatives. The owner’s engineer provides the design and 

detailing of ancillary structures, and the performance criteria and objectives necessary for the 

micropile system design and installation. This information includes, as a minimum, the 

following: 

l Geotechnical reports and data. 

l Structure loadings (axial, lateral and moment) 

. Existing utility drawings.. 

l Design criteria and parameters. 

l Site limitations (e.g., right of way). 

l Design and details of ancillary structures. 

l Contractor working drawing/design submittal and review requirements. 

l Acceptance criteria. 

During the bidding process, the pre-qualified micropile contractors prepare a preliminary 

design and a firm cost proposal based on the owner’s plans and specifications. For 

subcontracted items, general contractors will receive bids from each pre-qualified specialty 

FHWA-SA-97-070 (VOO-06) 9-9 
subcontractor and include the best cost proposals in their bids. The name of the recommended 

micropile subcontractors are included in the general contractor’s bid, which is then submitted. 

Once the contract has been awarded, the selected specialty contractor prepares their detailed 

design calculations and working drawing and submits them to the engineer for review and 

approval. After acceptance of the design, construction begins. 

Appendix A-2 contains sample contract plans and specifications utilizing this method. 

Prebid 

The contract documents for the prebid design/build method are also prepared to allow for prequalified-contractor-designed micropiling alternatives. The major difference in this method is 

the timing of the design and the bidding. Performance criteria and necessary project design 

information are usually made available 60 to 90 days prior to the contract advertisement date. 

Pre-qualified micropile contractors prepare and submit final design calculations and working 

drawings for the owner’s review and approval. Once the designs (typically two to three total) 

are approved, a list of pre-qualified specialty contractors with approved designs are included in 

the contract documents. Often the specialty contractors’ proprietary working drawings are 

included in the contract bid documents. These drawings illustrate the proposed construction 

and assist the general contractors in understanding and coordinating their other project tasks 

with the proposed micropile construction. General contractors then receive bids from each prequalified subcontractor, bidding only on their own proprietary design, and include the best cost 

proposal in their bid. The name of the recommended micropile subcontractor is included in the 

general contractor’s bid, which is then submitted. Once the contract is awarded, the selected 

general contractor and specialty micropile contractor can begin work immediately. 

9-10 FHWA-SA-97-070 (~00-06) 
9.B.3 Other Methods 

Technical and Cost Proposals 

Weighted technical cost proposals require the contractor to submit two very detailed 

documents, one for the price to perform the work and the other for the proposed work plan. 

Each is independently assessed. The first portion is a plan of how the contractor proposes to 

perform the work. This portion contains the contractor’s design and construction experience, 

proposed scope of work and work plan, preliminary design and construction schedule, 

preliminary design calculations and drawings and proposed quality control and safety plans. 

This document is evaluated for technical competence, personnel, and corporate experience and 

safety. The second portion is a lump-sum cost proposal. Each element is given a rating; 70/30 

or 60/40 for the technical-to-price ratio is not unusual. The contract award is made to the 

contractor who provides the best overall proposal to the owner. 

This two-pronged contracting procedure is slowly gaining popularity with Federal and State 

agencies. In the technical cost proposal process as with the value engineering process, bidding 

contractors incur a lot of time and expense. This fact alone, however, will defer all but the 

most serious contractors. This process also involves considerable effort by the owner, and so is 

really viable only for particularly large and/or complex projects. 

9.C. CONTRACT PLANS 

For all of the specification methods mentioned, the contract plans must include the necessary 

bid information in order to protect the owner’s best interests. They also need to describe the 

owner’s objectives in enough detail that the micropile contractor can provide an adequate 

design and bid. It is recommended that the contract plans be prepared in a three-line format for 

both owner-controlled design and contractor design/build specification methods. Concept-only 

plans are provided for the contractor design/build method. 

The quality of the subsurface inforrnation, existing utility plan, and micropile design criteria are 

very important for a mutually successful project. Inadequate subsurface information and 

FHWA-SA-97-070 (~00-06) 9-11 
conservative pile design criteria may create expensive contractor contingencies, higher pile 

prices and increase claim potential. 

Sample contract plans and guide construction specifications are provided in Appendix A-l 

(Contractor Design/Build of Micropiles) and Appendix A-2 (Contractor Design/Build of 

Foundation - Micropiles and Footing). 

9- 12 FHWA-SA-97-070 (~00-06) 
CHAPTER 10 

FEASIBILITY AND COST DATA 

10.A FEASIBILITY 

Micropiles are used for structural support of foundations and in-situ earth reinforcement. 

Chapters 1 (Introduction), 3 (Applications), and 4 (Construction Techniques) demonstrated the 

wide range of conditions under which micropiles can be used. Micropiles are practical in any 

soil, fill, or rock condition, and can be installed at any angle. They can accommodate restrictive 

access and environmental problems, and have wide application both for new construction and 

rehabilitation of existing structures and/or marginally stable or failing slopes. 

Technology-selection criteria for each foundation support and slope stabilization project must 

be site specific. Besides price, some standard criteria issues include environmental concerns, 

settlement sensitivity, soil disturbance, scheduling, physical access, noise sensitivity, pile 

tension, compression, and positioning capabilities. Sometimes micropiles are the only 

alternative. Other times, extenuating circumstances make them more economical than the 

more traditional systems. 

Owners and engineers in the United States are gaining confidence with the development and 

implementation process of micropile technology, as attested by this manual. The American 

transportation construction industry must continue to build from the success of past research 

and project installations. It must continue to improve micropile design and installation 

methodologies and contracting methods to achieve the most optimal foundation and ground 

support solutions. 

FHWA-SA-97-070 (~00-06) lo- 1 
1O.B MICROPILE COST DATA 

Micropile costs are the product of many factors such as: 

1. Physical access and environmental conditions. 

2. Subsurface conditions. 

3. Mobilization/demobilization. 

4. Project location. 

5. Pile quantity. 

6. Pile capacities. 

7. Pile length. 

8. Pile inclination. 

9. Pile testing requirements. 

10. Pile installation schedule. 

11. Local labor regulations. 

12. Contractor overhead and margin percentages. 

13. Risk assessment. 

14. Contractual arrangement. 

Because of these many cost factors, micropile pricing varies widely on every project. As a 

guideline, however, and assuming the below listed constraints, the contract bid price range for 

micropiles in the United States is typically $150.00 to $300.00 per lineal meter of pile (1996 

costs). To take it a step further, the various cost factors must be analyzed in a “best case vs. 

worst case scenario” to determine a realistic micropile contract price range. Table 10-l 

illustrates this cost analysis. 

10-2 FHWA-SA-97-070 (~00-06) 
For example, price constraints may include the following factors: 

1. 

2. 

3. 

4. 

5. 

6. 

7. 

8. 

9. 

No physical, environmental, or access restrictions. 

No unusual subsurface conditions. 

Average pile load capacities and lengths (1,000 kN, 15m). 

Average pile quantity (50 piles). 

One verification pile load test and proof testing 5 percent of the production piles. 

One mobilization/demobilization. 

Continuous pile drilling operations. 

Prevailing labor rates. 

Typical contractor overhead and margin percentages. 

The sample problem from Chapter 5, along with a seismic retrofit sample project for 

comparison, are presented below in a discussion of budget ranges by micropile project type, 

using Table lo- 1. 

Table 10-l. Micropile Cost Influence Analysis 

Cost Factor Influence Range 

Physical and access conditions Very easy to very difficult 

Cost Influence (%) 

0% to +loo% 

Geology/soil conditions ) Very easy to very difficult 0% to + 50% 

Pile capacity Very low to very high -30% to + 30% 

Pile lengths 

Pile quantities 

Testing requirements 

Very short to very long 

Very high to very low 

Very low to very high 

-25% to + 25% 

-50% to +loo% 

-10% to + 10% 

Mobilization/demobilization One to multiple I 0% to + 10% 

Continuous drilling operations Continuous to not I 0% to + 25% 

Union agreements 

continuous 

Nonunion to very strong -15% to + 30% 

O/H and profit margins (risk evaluation) Very low to very high -10% to + 10% 

FHWA-SA-97-070 (~00-06) 10-3 
1O.C SAMPLE PROBLEMS-COST ESTIMATES 

1O.C.l Sample Problem No. 1 (Chapter 5-Bridge Abutment Support) 

Given: Single-span bridge located in mountainous terrain approximately 125 km northwest 

of Denver, Colorado. Each abutment is to be supported on 12 CASE 1, Type B 

micropiles with ultimate capacities of 1,190 kN in compression, and 490 kN in 

tension, and approximately 12 meters in length. Testing requirements are one 

verification pile load test (compression only) on a sacrificial pile and two proof tests. 

Access to each work bench must be established. Soil conditions are as shown in the 

Chapter 5 example problem. Drilling of bond zone is expected to be very difficult, 

due to cobbles and boulders. Drilling will be continuous at each location with reset 

up required at location of Abutment 2. 

Solution: Evaluate each influence factor and multiply by the average of the two base prices as 

shown in Table 10-2, below: 

Table 10-2. Sample Problem No. 1 - Cost Analysis (Chapter 5-Bridge Abutment Support) 

Cost Factor Influence Cost Influsnce (%) 

Physical and access conditions 

Geology/soil conditions 

Pile capacity 

Pile lengths 

Pile quantities 

Testing requirements 

Mobilization/demobilization 

Easy access 

Difficult drilling 

Moderate pile capacity 

Moderate pile lengths 

Low pile quantities 

Moderate testing 

One mobilization 

10 

30 

0 

0 

25 

10 

0 

Continuous drilling operations Two setups I 5 -1 

Union agreements 

O/H and profit margins (risk evaluation) 

Standard union agreements 0 

High-risk project 5 

Total: 85% 

10-4 FHWA-SA-97-070 (vOO-06) 
10.C.2 Sample Problem No. 2 (Seismic Retrofit) 

Given: Seismic retrofit of a 200-meter-long concrete viaduct located in San Francisco, 

California. A total of 30 footings need retrofitting with 12 CASE 1, Type B 

micropiles (360 micropiles) per footing. Ultimate capacities are 2,500 kN in 

compression and 1,500 kN in tension. Subsurface profile includes approximately 10 

to 12 meters of medium-dense silty sands over weathered to moderately weathered 

serpentine bedrock. Micropiles average approximately 20 meters in length. Testing 

requirements include one each verification pile load test (compression only) on a 

sacrificial pile and 30 each proof tests. Access to each pile location is good, and 

overhead clearance varies between 5 to 12 meters for each footing. Drilling is 

assumed to be continuous allowing setups between each footing. 

FHWA-SA-97-070 (~00-06) 10-5 
Table 10-3. Sample Problem No. 2 (Seismic Retrofit) 

Mobilization/demobilization 

Continuous drilling operations 

Union agreements 

O/H and profit margins (risk evaluation) 

One mobilization 0 

Continuous drilling 0 

Strong union 10 

Lower risk project -5 

Total 10% 

Therefore, 

I II 

I+10 x $150 + $300 

100 2 

= 

$250.00/m or $5,000.00/each (average unit) 

or 

_ $3,330.00/each to $6,660.00/each (range) 

Applying an unrealistic combination of the factors in Table 10-l might raise the unit price by 

almost 400 percent beyond the “typical price,” to approximately $700.00 per lineal meter. In 

such a case, micropiles may be technically feasible but may not appear cost effective, so an 

alternate technology may be investigated. Usually, however, the same factors that raised the 

micropile pricing will have the same cost implication on other piling options, which may not be 

technically feasible in any case. In addition, when comparing costs of alternative solutions, 

care should always be taken to clearly define the total end-product costs. 

10-6 FHWA-SA-97-070 (~00-06) 
There are several methods of measuring and paying for micropiles. Table 10-4 lists 

recommendations for both owner-controlled design and contractor design/build specification 

methods. 

Table 10-4. Micropile Measurement and Payment Units 

Item 

Measurement/Payment Utiit 

Owner-Controlled Design Contractor 

Design/Build 

Mobilization/demobilization Lump sum 

Pile load testing Per each 

Lump sum 

Lump sum 

Furnish and install piles 

(foundation support) 

Furnish and install micropile slope 

stabilization 

Per each 

Per lineal meter of structure 

Proportioning the micropile unit costs (furnish and install only) typically results in the 

following breakdown: 

Labor . . . . . . . . . 30 - 50 percent 

Equipment . . . . . 20 - 30 percent 

Materials . . . . . . 25 - 40 percent 

In closing, the unit price of micropiles usually exceeds that of conventional piles, especially 

driven piles. However, under certain combinations of circumstances, such as difficult ground 

conditions, site access constraints, low headroom/limited work area, etc., micropiles are cost 

effective and occasionally, represent the only technically feasible option. 

FHWA-SA-97-070 (~00-06) 10-7 
GLOSSARY OF TERMS 

Admixture: Substance added to the grout to control bleed and/or shrinkage, improve 

flowability, reduce water content, or retard setting time. 

Alignment Load (AL): A minimum load (5 percent DL maximum) applied to micropile 

during testing to keep the testing equipment correctly positioned. 

Allowable Geotechnical Bond Load: For SLD, computed as the nominal grout-to-ground 

bond strength (a bond nominal strength), divided by the geotechnical safety factor and then 

multiplied by the grouted bond surface area (bond length times drill hole circumference). 

Apparent Free Micropile Length: The length of pile that is apparently not bonded to the 

surrounding ground, as calculated from the elastic load extension data during testing. 

Bonded Length: The length of the micropile that is bonded to the ground and conceptually 

used to transfer the applied axial loads to the surrounding soil or rock. Also known as 

the load transfer length. 

Bond-breaker: A sleeve placed over the steel reinforcement to prevent load transfer. 

CASE 1 Micropile: A pile designed to accept vertical or lateral load directly, and transfer it to 

an appropriate bearing stratum. Usually includes significant steel reinforcement. 

CASE 2 Micropile: One of a network of low-capacity piles used to delineate and internally 

reinforce a volume of soil. 

Casing: Steel tube introduced during the drilling process in overburden soil to temporarily 

stabilize the drill hole. This is usually withdrawn as the pile is grouted, although in 

certain types of micropiles, some casing is permanently left in place to reinforce the 

unbonded length or provide additional capacity. 

Centralizer: A device to support and position the reinforcing steel in the drill hole and/or 

casing so that a minimum grout cover is provided. 

FHWA-SA-97-070 (~00-06) Glossary - 1 

-.- 
Coarse-grained Soils: Soils with more than 50 percent of the material by weight, larger than 

the No. 200 (0.075 mm) sieve size. 

Cohesive Soils: Fine-grain soils that exhibit plasticity. Atterberg limits are commonly used to 

determine plasticity and better define a soil as cohesive or noncohesive. 

Contractor: The person&m responsible for performing the micropile work. 

Corrosion-inhibiting Compound: Material used to protect against corrosion and/or lubricate 

the reinforcing steel. 

Coupler: A device by which the pile load capacity can be transmitted from one partial length 

of reinforcement to another. 

Creep Movement: The movement that occurs during the creep test of a micropile under a 

constant load. 

Design Load (DL): The maximum unfactored load expected to be applied to the micropile 

during its service life. 

Duplex Drilling: An overburden drilling system involving the simultaneous advancement of 

(inner) drill rod and (outer) drill casing. Flush from the inner drill rod exits the hole via 

the annulus between rod and casing. 

Elastic Movement: The recoverable movement measured during a micropile test. 

Elastic Ratio: A measure of pile stiffness calculated as elastic movement divided by pile load. 

Encapsulation: A grout filled corrugated or deformed tube protecting the reinforcing steel 

against corrosion. 

Fine-grained Soils: Soils with at least 50 percent of the material, by weight, smaller than the 

No. 200 (0.075 mm) sieve size. 

Free (unbonded) Length: The designed length of the micropile that is not bonded to the 

surrounding ground or grout during stressing. 

Glossary - 2 FHWA-SA-97-070 (~00-06) 
Geotechnical Bond Design Strength: For LFD, computed as the nominal grout-to-ground 

bond strength (01 ,,ond), multiplied by a geotechnical resistance factor qG. Use (ho = 0.6 for 

typical designs and non-seismic load groups; use (ho = 1 .O for seismic loads groups 

LFD: Load Factor Design 

Maximum Test Load: The maximum load to which the micropile is subjected during testing. 

Micropile: A small-diameter (typically less than 300 mm) drilled and grouted replacement 

pile which is typically reinforced. 

Non-cohesive Soils: Granular soils that are generally nonplastic. 

Overburden: Material, natural or placed, that requires cased drilling methods to provide an 

open borehole to underlying strata. 

Permanent Micropile: Any micropile for permanent use, generally with more than a 24- 

month service life. May require special design, corrosion protection, and supervision 

during installation. 

Plunge Length: The length that the pile casing is inserted into the grout-to-ground bond zone. 

Post-grouting: The injection of additional grout into the load transfer length of a micropile 

after the primary grout has set. Also known as regrouting or secondary grouting. 

Preloading: Loading the micropile, prior to the micropile’s connection to the structure, to 

minimize or eliminate any structural movement in service. 

Primary Grout: Portland-cement-based grout injected into the micropile hole prior to or after 

the installation of the reinforcement to direct the load transfer to the surrounding ground. 

Proof Test: Incremental loading of a production micropile, recording the total movement at 

each increment. 

Reinforcement: The steel component of the micropile that accepts and/or resists applied 

loadings. 

FHWA-SA-97-070 (~00-06) Glossary - 3 
Residual Movement: The nonelastic (nonrecoverable) movement of a micropile measured 

during load testing. 

Safety Factor: The ratio of the ultimate capacity and design working load; used for the design 

of any component or interface. 

Sheath: A smooth or corrugated pipe or tube that protects the reinforcing steel against 

corrosion. 

Single-tube Drilling: The advancement of a steel casing through overburden, usually aided by 

water flushing through the casing. The water may or may not return to the surface 

around the casing, depending largely on the permeability of the overburden. 

Spacer: A device to separate elements of a multiple-element reinforcement to ensure full bond 

development of each steel element. 

SLD: Service Load Design 

Temporary Micropile: Any micropile for temporary use, generally with less than a 24-month 

service life. Temporary micropiles installed in corrosive environments may require 

corrosion protection. 

Type A-D: Classification of micropiles based on method and pressure of grouting. 

Verification Load Test: Non-production (sacrificial) pile load test performed to verify the 

design of the pile system and the construction methods proposed, prior to installation of 

production piles. 

Working Load: Equivalent term of Design Load. 

Glossary - 4 FHWA-SA-97-070 (~00-06) 
REFERENCES 

ASCE Committee on Placement and Improvement of Soils, 1987. “Soil Improvement, A TenYear Update,” Proceedings, Symposium at ASCE Convention, Atlantic City, NJ, April 28. 

Bachy, 1992. “Interception of Pollution by Impervious Barrier;” Sancho de Avila Car Park, 

Barcelona, Spain, Promotional literature, Paris, France. 

Barley, A. D., and Woodward, M. A., 1992. “High Loading of Long Slender Minipiles,” 

Proceedings, ICE Conference on Piling European Practice and Worldwide Trends, 

Thomas Telford, London, pp. 131-136. 

Bjerrum, L., 1957. “Norwegian Experiences with Steel Piles to Rock,“, Geotechnique, Vol. 7, 

pp. 73 - 96. 

Bruce, D. A., 1992. “Recent Progress in American Pin Pile Technology,” Proceedings, ASCE 

Conference, Grouting, Soil Improvement, and Geosynthetics, New Orleans, Louisiana, 

Feb. 25-28, pp. 765-777. 

Bruce, D. A., 1989. “Aspects of Minipiling Practice in the United States,” Ground 

Engineering, Vol. 22, No. 1, pp. 35-39. 

Bruce, D. A., 1988a. “Developments in Geotechnical Construction Processes for Urban 

Engineering”, Civil Engineering Practice, Vol. 3, No. 1, Spring, pp. 49-97. 

Brnce, D. A., 1988b. “Aspects of Minipiling Practice in the United States,” Ground 

Engineering, Vol. 21, No. 8, pp. 20-33. 

Bruce, D. A., and Chu, E. K., 1995. “Micropiles for Seismic Retrofit, Proceedings,” National 

Seismic Conference on Bridges and Highways, Sponsored by FHWA and Caltrans, San 

Diego, California, December 10-13, 17 pages. 

Bruce, D. A., DiMillio, A. F., and Juran, I., 1995. “A Primer on Micropiles,” CiviE 

Engineering, American Society of Civil Engineers, New York, December, pp. 5 l-54. 

FHWA-SA-97-070 (~00-06) References - 1 
Bruce, D. A. and Gemme, R., 1992. “Current Practice in Structural Underpinning Using 

Pinpiles,” Proceedings, New York Met Section, ASCE Seminar, New York, April 21-22, 

46 pages. 

Bruce, D. A., Pearlman, S. L., and Clark, J. H, 1990. “Foundation Rehabilitation of the 

Pocomoke River Bridge, Maryland, Using High Capacity Preloaded Pinpiles,” 

Proceedings, Th Annual International Bridge Conference, Pittsburgh, Pennsylvania, June 

18-20, Paper IBC-90-42,9 pages. 

Caltrans, 1993. “Caltrans Pile Load Test Results at a Deep Bay Mud Site Using Various Pile 

Types - I-280 San Francisco: Results and Seismic Design Guidelines,” DFI and Caltrans 

Specialty Seminar, Sacramaneto, CA, October, 44 pages. 

CCTG, 1992. Technical Rules for the Design and Calculation of the Foundations of the Civil 

Engineering Works, Publication 62, Title V, September. 

Federal Highway Administration, 1997. Drilled and Grouted Micropiles, State-of-Practice 

Review. Report No. FHWA-RD-96-016/O 19, United States Department of Transportation, 

July 1997. Four Volumes. 

Fleming, W. G. K., Weltman, A. J., Randolph, M. F., and Elson, W. K., 1985, PiZing 

Engineering, Second Edition, Surrey University Press, Glasgow, 380 pages. 

Gouvenot, D., 1975. “Essais de chargement et de plambement de pieux aiguelles,” Annals de 

ITdB et de Travau Publics, No. 334, December. 

Littlejohn, G. S., and Bruce, D. A., 1977. Rock Anchors State-of-the-Art, Foundation 

Publications Ltd., Brentwood, Essex, England, 50 pages. 

Lizzi, F., 1982. “The Pali Radice (Root Piles),” Symposium on Soil and Rock Improvement 

Techniques including Geotextiles, Reinforced Earth and Modern Piling Methods, 

Bangkok, December, Paper Dl . 

Mascardi, C. A., 1982. “Design Criteria and Performance of Micropiles,” Symposium on Soil 

and Rock Improvement Techniques, Including Geotextiles, Reinforced Earth, and Modern 

Piling Methods, Bangkok, December, Paper D3. 

Mascardi, C. A., 1970. “11 Comportamento dei Micropali Sottoposti a Sforzo Assiale, 

Moment0 Flettente e Taglio,” Verlag Leeman, Zurich. 

References - 2 FHWA-SA-97-070 (~00-06) 
NAVFAC, 1986. Soil Mechanics, Soil Dynamics, Foundations and Earth Structures, and Deep 

Stabilization, and Special Geotechnical Construction, Design Manual DM-7.02 

Department of the Navy, Naval Facility Engineering Command, Alexandria, Virginia, 

September. 

Palmerton, J. B., 1984. Stabilization of Moving Land Masses By Cast-In-Place Piles, Geotech 

Lab, USACOE, WES, Vicksburg, Mississippi, Final Report GL-84-4, 134 pages. 

Pearlman, S. L., Campbell, B. D., and Withiam, J. L., 1992. “Slope Stabilization Using 

InSitu Earth Reinforcements,” ASCE Specialty Conference on Stability and Pe@ormance 

of Slopes and Embankments-II, June 29-July 1, Berkeley, California, 16 p. 

Pearlman, S. L., Richards, T. D., Wise, J. D. and Vodde, W. F., 1997. “Pin Piles for Bridge 

Foundations - A Five Year Update,” Proceedings, 14th Annual International Bridge 

Conference, Pittsburgh, Pennsylvania, June. 

Pearlman, S. L., and Wolosick, J. R., 1992. “Pin Piles for Bridge Foundations,” Proceedings, 

9’ Annual InternationaE Bridge Conference, Pittsburgh, Pennsylvania, June 15-l 7. 

Pearlman, S. L., Wolosick, J. R. and Gronek, P. B., 1993. “Pin Piles for Seismic Rehabilitation 

of Bridges,” Proceedings, 10” International Bridge Conference, Pittsburgh, PA, June 15- 

17. 

PTI, 1996. Post-Tensioning Institute (PTl) Recommendations for Prestressed Rock and Soil 

Anchors, PTI, 1717 West Northern Avenue, Suite 114, Phoenix, AZ 85021,70 pages. 

Roark & Young, 1975. Formulas for Stress and Strain,” 5th Edition, McGraw-Hill, New York, 

NY, ISBN O-07-053031-9 

Ueblacker, G., 1996. “Portland Westside Lightrail Corridor Project Micropile Retaining 

Wall,” Foundation Drilling, November, pp. 8-12. 

FHWA-SA-97-070 (~00-06) References - 3 
[BLANK PAGE] 

References - 4 FHWA-SA-97-070 (~00-06) 
APPENDIX A-1 

Micropile Guide Construction Specification 

and Sample Plans 

[Foundation Support Projects] 

Contractor Design/Build of Micropiles 

Metric (SI) Units 

(With Commentary) 

(Commentary: Owner-Controlled design spectfications can vary in the amount of the design to 

be pet$ormed by the Owner’s design engineer and the amount per$ormed by the micropile 

specialty Contractor. This guide specification is set up for the Owner-controlled design 

(Standard Design) method wherein the Owner provides preliminary plans showing the pile 

design loadings, footing design, and pile layout for each footing location. The Owner also 

provides related design criteria and requirements, subsurface data, rights-of-way limits, 

utility locations, site limitations, construction material and testing specifications, and 

required Contractor working drawing/design and construction submittals and review 

requirements. The micronile Contractor designs the individual microuile elements and vile 

tov footing connections and selects the micronile construction process and eauivment, This 

approach is very similar to that commonly used by many highway agencies for Owner design 

ofpermanent tieback andpermanent soil nail walls. During the bidding process, the 

prequalified micropile contractors prepare a preliminary micropile design and a firm cost 

proposal based on the Owner’s preliminary plans and specifications. If the micropile portion 

of the project is to be subcontracted, general contractors will receive bids from the 

prequaltfted micropile contractors and include the best ogler and name of the selected 

micropile contractor in their bid submittal. Once the contract is awarded, the selected 

micropile Contractor prepares detailed micropile design calculations and working drawings 

and submits them to the Engineer for review. After acceptance of the design, construction 

begins. For more detailed discussion on various contracting methods, refer to Chapter 9.) 

FHWA-SA-97-070 (~00-06) Al - 1 
1 .O DESCRIPTION 

This work shall consist of constructing micropiles as shown on the contract plans and approved 

working drawings and as specified herein. The micropile specialty Contractor is responsible for 

furnishing of all design, materials, products, accessories, tools, equipment, services, 

transportation, labor and supervision, and manufacturing techniques required for design, 

installation and testing of micropiles and pile top attachments for this project. 

The selected micropile Contractor shall select the micropile type, size, pile top attachment, 

installation means and methods, estimate the ground-grout bond value and determine the 

required grout bond length and final micropile diameter. The micropile Contractor shall design 

and install micropiles that will develop the load capacities indicated on the contract plans. The 

micropile load capacities shall be verified by verification and proof load testing as required and 

must meet the test acceptance criteria specified herein. 

Where the imperative mood is used within this specification, “The Contractor shall” is implied. 

(Commentary: Successjul design and installation of high-quality micropiles require 

experienced Contractors having the necessary specialty drilling and grouting equipment and 

expertise and experienced work crews. The most important section of the specifications to be 

enforced by the Owner deals with the experience qualifications of the micropile Contractor. 

Failure to enforce the spect#ied experience qual$cations opens the door for inexperienced 

Contractors trying to cut costs. The results often are inferior workmanship, project delays, 

andproject claims that, more often than not, substantially increase project costs. Results like 

these often discourage project Owners porn implementing new technology and draws them 

back to more traditional methods at any cost. This can be avoided with the proper 

spectfication implementation and, as importantly, enforcement to ensure a mutually successful 

project.) 

Al-2 FHWA-SA-97-070 (~00-06) 
1 .I Micropile Contractor’s Experience Requirements And Submittal. 

The micropile Contractor shall be experienced in the construction and load testing of 

micropiles and have successfully constructed at least 5 projects in the last 5 years involving 

construction totalling at least 100 micropiles of similar capacity to those required in these 

plans and specifications. 

The Contractor shall have previous micropile drilling and grouting experience in soil/rock 

similar to project conditions. The Contractor shall submit construction details, structural 

details and load test results for at least three previous successful micropile load tests from 

different projects of similar scope to this project. 

The Contractor shall assign an Engineer to supervise the work with experience on at least 3 

projects of similar scope to this project completed over the past 5 years. The Contractor shall 

not use consultants or manufacturers’ representatives to satisfy the supervising Engineer 

requirements of this section. The on-site foremen and drill rig operators shall also have 

experience on at least 3 projects over the past 5 years installing micropiles of equal or greater 

capacity than required in these plans and specifications. 

The micropiles shall be designed by a Registered Professional Engineer with experience in the 

design of at least 3 successfully completed micropile projects over the past 5 years, with 

micropiles of similar capacity to those required in these plans and specifications. The micropile 

designer may be either a employee of the Contractor or a separate Consultant designer meeting 

the stated experience requirements. (Commentary: If the Owner prepares a fully detailed 

design, this paragraph can be deleted). 

At least 45 calendar days before the planned start of micropile construction, the Contractor 

shall submit 5 copies of the completed project reference list and a personnel list. The project 

reference list shall include a brief project description with the owner’s name and current phone 

number and load test reports. The personnel list shall identify the micropile system designer (if 

applicable), supervising project Engineer, drill rig operators, and on-site foremen to be 

assigned to the project. The personnel list shall contain a summary of each individual’s 

FHWA-SA-97-070 (~00-06) Al-3 
experience and be complete enough for the Engineer to determine whether each individual 

satisfies the required qualifications. The Engineer will approve or reject the Contractor’s 

qualifications within 15 calendar days after receipt of a complete submission. Additional time 

required due to incomplete or unacceptable submittals will not be cause for time extension or 

impact or delay claims. All costs associated with incomplete or unacceptable submittals shall 

be borne by the Contractor. 

Work shall not be started, nor materials ordered, until the Engineer’s written approval of the 

Contractor’s experience qualifications is given. The Engineer may suspend the Work if the 

Contractor uses non-approved personnel. If work is suspended, the Contractor shall be frilly 

liable for all resulting costs and no adjustment in contract time will result from the suspension. 

1.2 Pre-approved List 

(Commentary: The intent of this section, ifused, is to reduce the administrative burden on 

specialty contractors who have previously satisfactorily demonstrated to the Owner Agency 

that they meet the Section A I. I experience qua1iJications.j 

Listed below are the micropile specialty Contractors pre-qualified to design, furnish and install 

micropiles on this project, based on previous Contractor experience submittals verified and 

accepted by the Owner: 

1. Contractor Name 

Mailing Address 

Contact Name 

Phone Number 

2. Contractor Name 

Mailing Address 

Contact Name 

Phone Number 

Al-4 FHWA-SA-97-070 (~00-06) 
3. Contractor Name 

Mailing Address 

Contact Name 

Phone Number 

The above named micropile specialty Contractors do not have to submit the experience 

qualification requirements called for in Section 1.1. The Section 1.8 design and Section 1.9 

construction submittals are still required. The prime Contractor shall name the intended 

micropile specialty Contractor in the bid submittal documents under 

(Commenlary: Be careful not to automaticallypre-approve Contractors for allprojects based 

on pre-approval for one project. The projects may be very d@erent.) 

The Section Al. 1 experience qualifications and submittal requirements shall apply to other 

micropile specialty contractors not named on the pre-approved list. 

1.3 Related Specifications 

(Commentary: Engineer to speciJL all related specifications.) 

1.4 Definitions 

(Commentary: Engineer to speciJL any additional de$nitions.) 

Admixture: Substance added to the grout to control bleed and/or shrinkage, improve 

flowability, reduce water content, or retard setting time. 

Alignment Load (AL): A minimum initial load (5 percent DL maximum) applied to 

micropile during testing to keep the testing equipment correctly positioned. 

Allowable Geotechnical Bond Load: For Service Load Design (SLD), computed as the 

nominal grout-to-ground bond strength (a bond nominal strengt&, divided by the geotechnical 

safety factor and then multiplied by the grouted bond surface area (bond length times 

drillhole circumference). 

FHWA-SA-97-070 (~00-06) Al-5 
Bonded Length: The length of the micropile that is bonded to the ground and conceptually 

used to transfer the applied axial loads to the surrounding soil or rock. Also known as the 

load transfer length. 

Bond-breaker: A sleeve placed over the steel reinforcement to prevent load transfer. 

Casing: Steel tube introduced during the drilling process in overburden soil to temporarily 

stabilize the drill hole. This is usually withdrawn as the pile is grouted, although in certain 

types of micropiles, some casing is permanently left in place to provide added pile 

reinforcement. 

Centralizer: A device to support and position the reinforcing steel in the drill hole and/or 

casing so that a minimum grout cover is provided. 

Contractor: The person/firm responsible for performing the micropile work. 

Coupler: The means by which load capacity can be transmitted from one partial length of 

reinforcement to another. 

Creep Movement: The movement that occurs during the creep test of a micropile under a 

constant load. 

Design Load (DL): The maximum unfactored load expected to be applied to the micropile 

during its service life. 

Encapsulation: A corrugated or deformed tube protecting the reinforcing steel against 

corrosion. 

Engineer: The Owner or Owner’s authorized agent, 

Free (unbonded) length: The designed length of the micropile that is not bonded to the 

surrounding ground or grout. 

Al-6 FHWA-SA-97-070 (~00-06) 
Geotechnical Bond Design Strength: For Load Factor Design (LFD), computed as the 

nominal grout-to-ground bond strength (cl bond nominal stre,,gth), multiplied by a geotechnical 

resistance factor (pc. Use (~c = 0.6 for typical designs and non-seismic load groups; use 

<pc = 1 .O for seismic loads groups 

Micropile: A small-diameter, bored, cast-in-place composite pile, in which the applied load is 

resisted by steel reinforcement, cement grout and frictional grout/ground bond. 

Maximum Test Load: The maximum load to which the micropile is subjected during testing. 

Recommended as 2.5 x DL for verification load tests and as 1.67 x DL for proof load tests. 

Nominal Grout-to-Ground Bond Strength: The estimated ultimate geotechnical unit groutto-ground bond strength selected for use in design. Same as (11 bond nominal strenfl (SLD and 

LFD) 

Overburden: Material, natural or placed, that may require cased drilling methods to provide 

an open borehole to underlying strata. 

Post-grouting: The injection of additional grout into the load transfer length of a micropile 

after the primary grout has set. Also known as regrouting or secondary grouting. 

Primary Grout: Portland-cement-based grout injected into the micropile hole prior to or after 

the installation of the reinforcement to direct the load transfer to the surrounding ground 

along the micropile. 

Proof Load Test: Incremental loading of a production micropile, recording the total 

movement at each increment. 

Reinforcement: The steel component of the micropile that accepts and/or resists applied 

loadings. 

Sheathing: Smooth or corrugated piping or tubing that protects the reinforcing steel against 

corrosion. 

Spacer: A device to separate elements of a multiple-element reinforcement. 

FHWA-SA-97-070 (~00-06) Al-7 

-- 
Verification Load Test: Pile load test performed to verify the design of the pile system and the 

construction methods proposed, prior to installation of production piles. 

1.5 Referenced Codes and Standards. 

The following publications’form a part of this specification to the extent indicated by the 

references. The latest publication as of the issue date of this specification shall govern, unless 

indicated otherwise. 

1.51 American Society for Testing and Materials (ASTM) 

American Association of State Highway and Transportation Officials (AASHTO) 

ASTM 

A36, A572 

A82 

A252 

A615 

A722 

A775 

A934 

c 33 

c 109 

C 188 

Cl44 

c 150 

c 494 

D 1143 

D 1784 

D 3350 

D 3689 

D 3966 

AASHTO SPECIFICATION / TEST 

M183, M223 Structural Steel 

M55 Cold-Drawn Steel Wire for Concrete Reinforcement 

Welded and Seamless Steel Pipe Piles 

M31 Deformed and Plain Billet Steel Bars for Concrete Reinforcement 

M275 Uncoated High-Strength Steel Bar for Prestressing Concrete 

- Epoxy -Coated Reinforcing Steel Bars 

Epoxy-Coated Prefabricated Steel Reinforcing Bars 

MS0 Concrete Aggregates 

T106 Compressive Strength of Hydraulic Cement Mortar 

T133 Density of Hydraulic Cement 

M45 Aggregate for Masonry Mortar 

M 85 Portland Cement 

Ml94 Chemical Admixtures for Concrete 

Method of Testing Piles Under Static Axial Compressive Load 

- Polyvinyl Chloride (PVC) Pipe (Class 13464-B) 

M 252 Polyethylene Corrugated Tubing 

Method of Testing Individual Piles Under Static Axial Tensile Load 

Standard Test Method for Piles Under Lateral Load 

T26 Quality of Water to be Used in Concrete 

1.52 American Welding Society (AWS) 

Al-8 FHWA-SA-97-070 (~00-06) 
D 1.1 Structural Welding Code-Steel 

D 1.2 Structural Welding Code-Reinforcing Steel 

1.5.3 American Petroleum institute (API) 

5CT (N-80) Specification for casing and tubing 

RP 13B-1 Recommended Practice - Standard Procedure for Field Testing 

Water Based Drilling Fluids 

1.6 Available Information. 

Available information developed by the Owner, or by the Owner’s duly authorized 

representative include the following items: 

1. Plans prepared by , dated . The plans include the plan view, 

profile and typical cross sections for the proposed micropile locations. (Commentary: 

Refer to chapter 9 of the FHWA “Micropile Design and Construction Guidelines 

Manual “‘, Report No. FHWA- SA-97-070 for detailed guidance on plan information to 

provide on the Owner-Controlled Design preliminary plans. An example preliminary 

plan for the bridge foundation support design example no. I is included at the end of 

this guide spect$cation.) 

2. Geotechnical Report No.(s) titled , dated , included 

or referenced in the bid documents, contains the results of test pits, exploratory borings 

and other site investigation data obtained in the vicinity of the proposed micropile 

locations. 

(Commentary: The subsurface conditions expected can significantly impact the contractor’s 

choice of procedures, methods, or equipment, the biddingprocess, and contract 

administration. Experience has proven that use of a geotechnical summary is advantageous 

FHWA-SA-97-070 (~00-06) Al-9 
toward achieving a success$xl contract. It is recommended that advisory wording be inserted 

into the contract special provisions to “redflag” the conditions to bidders. Preparation and 

use of a “Summary of Geotechnical Conditions”, such as used by the Washington State DOT, is 

recommended. WSDOT’s in-house guide for preparation of said summary, along with an 

example writeup for the Sample Problem No. I contained in this manual, are included in 

Appendix B. WSDOT inserts the Summary into the contract special provisions, making it a 

legal part of the contract documents. The purpose is to alert and be fair to bidders, and thus 

prevent/minimize differing site condition construction claims and dispute.) 

1.7 Construction Site Survey 

Before bidding the Work, the Contractor shall review the available subsurface information and 

visit the site to assess the site geometry, equipment access conditions, and location of existing 

structures and above ground facilities. 

The Contractor is responsible for field locating and verifying the location of all utilities shown 

on the plans prior to starting the Work. Maintain uninterrupted service for those utilities 

designated to remain in service throughout the Work. Notify the Engineer of any utility 

locations different from shown on the plans that may require micropile relocations or structure 

design modification. Subject to the Engineer’s approval, additional cost to the Contractor due 

to micropile relocations and/or structure design modification resulting fi-om utility locations 

different from shown on the plans, will be paid as Extra Work. 

(Commentary: The location of both active and abandoned buried utilities within the ground 

mass to receive micropiles can have a profound impact on the design and construction of the 

micropiles. Careful consideration of the presence and location of all utilities is requiredfor 

successfil design and installation of micropiles.) 

Prior to start of any micropile construction activity, the Contractor and Engineer shall jointly 

inspect the site to observe and document the pre-construction condition of the site, existing 

structures and facilities. 

Al-10 FHWA-SA-97-070 (~00-06) 
1.8 Micropile Design Requirements. 

The micropiles shall be designed to meet the specified loading conditions, as shown on the 

contract plans and approved working drawings. Design the micropiles and pile top to footing 

connections using the Service Load Design (SLD) procedures contained in the FHWA 

“Micropile Design and Construction Guidelines Manual”, Report No. FHWA- SA-97-070. 

(Commentary: The FHWA micropile manual- Chapter .5- also presents Load Factor Design 

(LFD) procedures for micropile foundations. Revise specification if LFD design is required.) 

The required geotechnical safety factors/strength factors (for SLD Design) or load and 

resistance factors (for LFD Design) shall be in accord with the FHWA manual, unless specified 

otherwise. Estimated soil/rock design shear strength parameters, unit weights, applied 

foundation loadings, slope and external surcharge loads, corrosion protection requirements, 

known utility locations, easements, right-of-ways and other applicable design criteria will be as 

shown on the plans or specified herein. Structural design of any individual micropile structure 

elements not covered in the FHWA manual shall be by the service load design method in 

conformance with appropriate articles of the most current Edition of the AASHTO Standard 

Specifications for Highway Bridges, including current interim specifications. 

Steel pipe used for micropile permanent casing shall incorporate an additional mm 

thickness of sacrificial steel for corrosion protection. (Commentary: This paragraph is 

optional and to be selected by the designer or specified by the owner. AASHTO Section 4.5.7.4 

Cross-Section Adjustment for Corrosion, states - “For concrete-filledpipepiles where 

corrosion may be expected I/16 inch (1.6mm) shall be deductedfiom the shell thickness to 

allow for reduction in section due to corrosion. “) 

Where required as shown on the contract plans, corrosion protection of the internal steel 

reinforcing bars, consisting of either encapsulation, epoxy coating, or grout, shall be provided 

in accordance with Materials Section 2.0. Where permanent casing is used for a portion of the 

micropile, encapsulation shall extend at least 1.5 m into the casing. 

FHWA-SA-97-070 (~00-06) Al-11 
(Commentary: when installation of micropiles provides additional support for existing 

structures, such as for seismic retroftt or underpinning applications, the structural designer 

should add appropriate specification verbage and design criteria into this specification to 

cover the penetration of the existing structural elements and the top ofpile anchorage to the 

existing structure.) 

1.8.1 Micropile Design Submittals. 

At least 21 calendar days before the planned start of micropile structure construction, submit 

complete design calculations and working drawings to the Engineer for review and approval. 

Include all details, dimensions, quantities, ground profiles, and cross-sections necessary to 

construct the micropile structure. Verify the limits of the micropile structure and ground survey 

data before preparing the detailed working drawings. 

The drawings and calculations shall be signed and sealed by the contractor’s Professional 

Engineer or by the Consultant designer’s Professional Engineer (if applicable), previously 

approved by the owner’s Engineer. If the micropile contractor uses a consultant designer to 

prepare the design, the micropile contractor shall still have overall contract responsibility for 

both the design and the construction. 

1.82 Design Calculations. 

Design calculations shall include, but not be limited to, the following items: 

1. A written summary report which describes the overall micropile design. 

2. Applicable code requirements and design references. 

3. Micropile structure critical design cross-section(s) geometry including soil/rock strata 

and piezometric levels and location, magnitude and direction of design applied loadings, 

including slope or external surcharge loads . 

Al - 12 FHWA-SA-97-070 (~00-06) 
4. Design criteria including, soil/rock shear strengths (friction angle and cohesion), unit 

weights, and ground-grout bond values and micropile drillhole diameter assumptions 

for each soil/rock strata. 

5. Safety factors/strength factors (for Service Load Design) or load and resistance factors 

(for Load Factor Design) used in the design on the ground-grout bond values, 

surcharges, soil/rock and material unit weights, steel, grout, and concrete materials. 

6. Seismic design earthquake acceleration coefftcient. 

7. Design calculation sheets (both static and seismic) with the project number, micropile 

structure location, designation, date of preparation, initials of designer and checker, and 

page number at the top of each page. Provide an index page with the design calculations. 

8. Design notes including an explanation of any symbols and computer programs used in 

the design. 

9. Pile to footing connection calculations. 

1.8.3 Working Drawings. 

The working drawings shall include all information required for the construction and quality 

control of the piling. Working drawings shall include, but not be limited to, the following 

items unless provided in the contract plans: 

1. A plan view of the micropile structure(s) identifying: 

(a) A reference baseline and elevation datum. 

(b) The offset from the construction centerline or baseline to the face of the micropile 

structure at all changes in horizontal alignment. 

(c) Beginning and end of micropile structure stations. 

FHWA-SA-97-070 (~00-06) Al - 13 
(d) Right-of-way and permanent or temporary construction easement limits, location 

of all known active and abandoned existing utilities, adjacent structures or other 

potential mterferences. The centerline of any drainage structure or drainage pipe 

behind, passing through, or passing under the micropile structure. 

(e) Subsurface exploration locations shown on a plan view of the proposed micropile 

structure alignment with appropriate reference base lines to fix the locations of the 

explorations relative to the micropile structure. 

2. An elevation view of the micropile structure(s) identifying: 

(a) Elevation view showing micropile locations and elevations; vertical and 

horizontal spacing; batter and alignment and the location of drainage elements (if 

applicable). 

(b) Existing and finish grade profiles both behind and in front of the micropile 

structure. 

3. Design parameters and applicable codes. 

4. General notes for constructing the micropile structure including construction sequencing 

or other special construction requirements. 

5. Horizontal and vertical curve data affecting the micropile structure and micropile 

structure control points. Match lines or other details to relate micropile structure 

stationing to centerline stationing. 

6. A listing of the summary of quantities on the elevation drawing of each micropile 

structure showing pay item estimated quantities. 

7. Micropile typical sections including micropile spacing and inclination; minimum 

drillhole diameter; pipe casing and reinforcing bar sizes and details; splice types and 

locations; centralizers and spacers; grout bond zone and casing plunge lengths (if used); 

Al-14 FHWA-SA-97-070 (~00-06) 
corrosion protection details; and connection details to the substructure footing, 

anchorage, plates, etc. 

8. A typical detail of verification and production proof test micropiles defining the 

micropile length, minimum drillhole diameter, inclination, and load test bonded and 

unbonded test lengths. 

9. Details, dimensions, and schedules for all micropiles, casing and reinforcing steel, 

including reinforcing bar bending details. 

10. Details for constructing micropile structures around drainage facilities (if applicable). 

The working drawings and design calculations shall be signed and sealed by the Contractor’s 

Professional Engineer or by the Consultant designer’s Professional Engineer (if applicable), 

previously pre-qualified by the Owner. If the micropile Contractor uses a Consultant designer 

to prepare the design, the micropile Contractor shall still have overall contract responsibility 

for both the design and the construction. 

Submit 5 sets of the working drawings with the initial submission, Drawing sheet size shall be 

550 by 850 mm. One set will be returned with any indicated corrections. The Engineer will 

approve or reject the Contractor’s submittal within 15 calendar days after receipt of a complete 

submission. If revisions are necessary, make the necessary corrections and resubmit 5 revised 

sets. When the drawings are approved, furnish 5 sets and a Mylar sepia set of the approved 

drawings. The Contractor will not be allowed to begin micropile structure construction or 

incorporate materials into the work until the submittal requirements are satisfied and found 

acceptable to the Engineer. Changes or deviations from the approved submittals must be resubmitted for approval. No adjustments in contract time or delay or impact claims will be 

allowed due to incomplete submittals. (Commentary: Submittals procedures shall be 

coordinated with Owner/Agency procedures). 

Revise the drawings when plan dimensions are changed due to field conditions or for other 

reasons. Within 30 days after completion of the work, submit as-built drawings to the 

FHWA-SA-97-070 (~00-06) Al - 15 
Engineer. Provide revised design calculations signed by the approved Registered Professional 

Engineer for all design changes made during the construction of the micropile structure. 

1.9 Construction Submittals. 

The Contractor shall prepare and submit to the Engineer, for review of completeness, 5 copies 

of the following for the micropile system or systems to be constructed: 

1. 

2. 

3. 

4. 

5. 

Detailed step-by-step description of the proposed micropile construction procedure, 

including personnel, testing and equipment to assure quality control. This step-by-step 

procedure shall be shown on the working drawings in sufficient detail to allow the 

Engineer to monitor the construction and quality of the micropiles. 

Proposed start date and time schedule and micropile installation schedule providing the 

following: 

Micropile number 

Micropile design load 

Type and size of reinforcing steel 

Minimum total bond length 

Total micropile length 

Micropile top footing attachment 

If welding of casing is proposed, submit the proposed welding procedure, certified by a 

qualified welding specialist. 

Information on headroom and space requirements for installation equipment that verify 

the proposed equipment can perform at the site. 

Plan describing how surface water, drill flush, and excess waste grout will be controlled 

and disposed. 

Al - 16 FHWA-SA-97-070 (~00-06) 
6. Certified mill test reports for the reinforcing steel or coupon test results for permanent 

casing without mill certification. The ultimate strength, yield strength, elongation, and 

material properties composition shall be included. For API N-80 pipe casing, coupon 

test results may be submitted in lieu of mill certification. 

7. Proposed Grouting Plan. The grouting plan shall include complete descriptions, details, 

and supporting calculations for the following: 

(a) Grout mix design and type of materials to be used in the grout including certified 

test data and trial batch reports. 

(b) Methods and equipment for accurately monitoring and recording the grout depth, 

grout volume and grout pressure as the grout is being placed. 

(c) Grouting rate calculations, when requested by the Engineer. The calculations 

shall be based on the initial pump pressures or static head on the grout and losses 

throughout the placing system, including anticipated head of drilling fluid (if 

applicable) to be displaced. 

(d) Estimated curing time for grout to achieve specified strength. Previous test results 

for the proposed grout mix completed within one year of the start of grouting may 

be submitted for initial verification and acceptance and start of production work. 

During production, grout shall be tested in accord with Section 3.4.5. 

(e) Procedure and equipment for Contractor monitoring of grout quality. 

8. Detailed plans for the proposed micropile load testing method. This shall include all 

drawings, details, and structural design calculations necessary to clearly describe the 

proposed test method, reaction load system capacity and equipment setup, types and 

accuracy of apparatus to be used for applying and measuring the test loads and pile top 

movements in accordance with Section 3.6, Pile Load Tests. 

FHWA-SA-97-070 (~00-06) Al - 17 
9. Calibration reports and data for each test jack, pressure gauge and master pressure gauge 

and electronic load cell to be used. The calibration tests shall have been performed by 

an independent testing laboratory, and tests shall have been performed within 90 

calendar days of the date submitted. Testing shall not commence until the Engineer has 

reviewed and accepted the jack, pressure gauge, master pressure gauge and electronic 

load cell calibration data. 

Work other than test pile installation shall not begin until the construction submittals have been 

received, reviewed, and accepted in writing by the Engineer. Provide submittal items 1 through 

5 at least 2 1 calendar days prior to initiating micropile construction, item 7 as the work 

progresses for each delivery and submittal items 6,8 and 9 at least 7 days prior to start of 

micropile load testing or incorporation of the respective materials into the work. The 

Contractor shall allow the Engineer 7 calendar days to review the construction submittals after 

a complete set has been received. Additional time required due to incomplete or unacceptable 

submittals shall not be cause for delay or impact claims. All costs associated with incomplete 

or unacceptable Contractor submittals shall be the responsibility of the Contractor. 

1 .I0 Pre-construction Meeting. 

A pre-construction meeting will be scheduled by the Engineer and held prior to the start of 

micropile construction. The Engineer, prime Contractor, micropile specialty Contractor, 

micropile designer, excavation Contractor and geotechnical instrumentation specialist (if 

applicable) shall attend the meeting. Attendance is mandatory. The pre-construction meeting 

will be conducted to clarify the construction requirements for the work, to coordinate the 

construction schedule and activities, and to identify contractual relationships and delineation of 

responsibilities amongst the prime Contractor and the various Subcontractors - specifically 

those pertaining to excavation for micropile structures, anticipated subsurface conditions, 

micropile installation and testing, micropile structure survey control and site drainage control. 

Al - 18 FHWA-SA-97-070 (1700-06) 
2.0 MATERIALS. 

Furnish materials new and without defects. Remove defective materials from the jobsite at no 

additional cost. Materials for micropiles shall consist of the following: 

Admixtures for Grout: Admixtures shall conform to the requirements of ASTM C 

494/AASHTO M194. Admixtures that control bleed, improve flowability, reduce water 

content, and retard set may be used in the grout, subject to the review and acceptance of the 

Engineer. Admixtures shall be compatible with the grout and mixed in accordance with the 

manufacturer’s recommendations. Expansive admixtures shall only be added to the grout used 

for filling sealed encapsulations and anchorage covers. Accelerators are not permitted. 

Admixtures containing chlorides are not permitted. 

Cement: All cement shall be Portland cement conforming to ASTM C lSO/AASHTO M85, 

Types II, III or V. 

Centralizers and Spacers: Centralizers and spacers shall be fabricated from schedule 40 PVC 

pipe or tube, steel, or material non-detrimental to the reinforcing steel. Wood shall not be used. 

Centralizers and spacers shall be securely attached to the reinforcement; sized to position the 

reinforcement within 10 mm of plan location fiorn center of pile; sized to allow grout tremie 

pipe insertion to the bottom of the drillhole; and sized to allow grout to freely flow up the 

drillhole and casing and between adjacent reinforcing bars. 

Encapsulation: Encapsulation (double corrosion protection) shall be shop fabricated using 

high-density, corrugated polyethylene tubing conforming to the requirements of ASTM 

D3350/AASHTO M252 with a nominal wall thickness of 0.8 mm. The inside annulus between 

the reinforcing bars and the encapsulating tube shall be a minimum of 5mm and be fully 

grouted with non-shrink grout conforming to Materials Section 2.0. 

Epoxy Coating: The minimum thickness of coating applied electrostatically to the reinforcing 

steel shall be 0.3 mm. Epoxy coating shall be in accordance with ASTM A775 or ASTM 

FHWA-SA-97-070(vOO-06) Al - 19 
A934. Bend test requirements are waived. Bearing plates and nuts encased in the pile concrete 

footing need not be epoxy coated. 

Fine Aggregate: If sand - cement grout is used, sand shall conform to ASTM C 

144/AASHTO M45. 

Grout: Neat cement or sand/cement mixture with a minimum 3-day compressive strength of 14 

MPa and a 28day compressive strength of 28 MPa per AASHTO T106/ASTM C109. 

(Commentary: Note to designers/speciJiers. A 28-day grout strength of 28 MPa is common for 

micropiles. If the micropile design calls for a higher grout strength, revise the spebjication 

accordingly.) 

Grout Protection: Provide a minimum 25 mm grout cover over bare or epoxy coated bars 

(excluding bar couplers) or minimum 12 mm grout cover over the encapsulation of 

encapsulated bars. 

Permanent Casing Pipe: Permanent steel casing/pipe shall have the diameter and at least 

minimum wall thickness shown on the approved Working Drawings. The permanent steel 

casing/pipe: 

1. shall meet the Tensile Requirements of ASTM A252, Grade 3, except the yield strength 

shall be a minimum of 345 MPa to 552 MPa as used in the design submittal. 

2. may be new “Structural Grade” (a.k.a. “Mill Secondary” ) steel pipe meeting above but 

without Mill Certification, free from defects (dents, cracks, tears) and with two coupon 

tests per truckload delivered to the fabricator. 

For permanent casing/pipe that will be welded, the following material conditions apply: 

1. the carbon equivalency (CE) as defined in AWS D 1 .l, Section Xl 5.1, shall not exceed 

0.45, as demonstrated by mill certifications 

2. the sulfur content shall not exceed 0.05%, as demonstrated by mill certifications 

For permanent casing/pipe that will be shop or field welded, the following fabrication or 

construction conditions apply: 

Al -20 FHWA-SA-97-070 (~00-06) 
1. the steel pipe shall not be joined by welded lap splicing 

2. welded seams and splices shall be complete penetration welds 

3. partial penetration welds may be restored in conformance with AWS Dl . 1 

4. the proposed welding procedure certified by a welding specialist shall be submitted for 

approval 

Threaded casing joints shall develop at least the required nominal resistance used in the design 

of the micropile. 

(Commentary: From a practical standpoint, the adequacy ofpipe and reinforcing bar splices 

and threadedjoint connections will be vertj?ed by the verification andproof load testing). 

Plates and Shapes: Structural steel plates and shapes for pile top attachments shall conform to 

ASTM A 36/AASHTO M183, or ASTM A 572/AASHTO M223, Grade 350. 

Reinforcing Bars: Reinforcing steel shall be deformed bars in accordance with ASTM 

A 61 YAASHTO M3 1, Grade 420 or Grade 520 or ASTM A 7221AASHTO M275, Grade 

1035. When a bearing plate and nut are required to be threaded onto the top end of reinforcing 

bars for the pile top to footing anchorage, the threading may be continuous spiral deformed 

ribbing provided by the bar deformations (e.g., Dywidag or Williams continuous threadbars) or 

may be cut into a reinforcing bar. If threads are cut into a reinforcing bar, the next larger bar 

number designation from that shown on the Plans shall be provided, at no additional cost. 

Bar tendon couplers, if required, shall develop the ultimate tensile strength of the bars without 

evidence of any failure. 

Reinforcing Bar Corrosion Protection: 

(Commentary: Corrosion protection requirements vary between Transportation Agencies. The 

most common and simplest tests utilized to measure the aggressiveness of the soil environment 

in&de electrical resistivity, pH, chloride, and sulfate. Per FHWA-RD-89-198, the ground is 

considered aggressive ifany one of these indicators show critical values as detailed below: 

FHWA-SA-97-070 (~00-06) Al-21 
PROPERTY TEST DESGhlATlON* 

Resistivity AASHTO T-288, ASTMG 57 

PH AASHTO T-289, ASTM G 51 

CRITICAL VALUES* 

below 2,000 ohm-cm 

below 5 

Sulfate AASHTO T-290, ASTM D516M above 200 ppm 

ASTMD4327 

Chloride AASHTO T-291, ASTMD512, above IO0 ppm 

ASTM 04327 

* SpeciJier should check test standards for latest updates and individual transportation 

agencies may have limits on critical values dtyerent than tabuiated above. Standard 

specifications or test methods for any of the above items which are common to your 

agency can be referenced in lieu of the above listed AASHTOiASTM references. 

Sheathing: Smooth plastic sheathing, including joints, shall be watertight. Polyvinyl chloride 

(PVC) sheathing shall conform to ASTM D 1784, Class 13464-B. 

Water: Water used in the grout mix shall conform to AASHTO T 26 and shall be potable, 

clean, and free from substances that may be injurious to cement and steel. 

3.0 CONSTRUCTION REQUIREMENTS 

3.1 Site Drainage Control. 

The Contractor shall control and properly dispose of drill flush and construction related waste, 

including excess grout, in accord with the standard specifications and all applicable local codes 

and regulations. Provide positive control and discharge of all surface water that will affect 

construction of the micropile installation. Maintain all pipes or conduits used to control surface 

water during construction. Repair damage caused by surface water at no additional cost. Upon 

substantial completion of the Work, remove surface water control pipes or conduits from the 

site. Alternatively, with the approval of the Engineer, pipes or conduits that are left in place, 

may be fully grouted and abandoned or left in a way that protects the structure and all adjacent 

facilities from migration of fines through the pipe or conduit and potential ground loss. 

Al -22 FHWA-SA-97-070 (~00-06) 
Immediately contact the Engineer if unanticipated existing subsurface drainage structures are 

discovered during excavation or drilling. Suspend work in these areas until remedial measures 

meeting the Engineer’s approval are implemented. Cost of remedial measures or repair work 

resulting from encountering unanticipated subsurface drainage structures, will be paid for as 

Extra Work. 

3.2 Excavation 

Coordinate the work and the excavation so the micropile structures are safely constructed. 

Perform the micropile construction and related excavation in accordance with the Plans and 

approved submittals. No excavations steeper than those specified herein or shown on the Plans 

will be made above or below the micropile structure locations without written approval of the 

Engineer. 

3.3 Micropile Allowable Construction Tolerances 

1. Centerline of piling shall not be more than 75 mm from indicated plan location. 

2. Pile shall be plumb within 2 percent of total-length plan alignment. 

3. Top elevation of pile shall be plus 2.5 mm or minus 50 mm maximum from 

vertical elevation indicated. 

4. Centerline of reinforcing steel shall not be more than 15 mm from indicated 

location. 

3.4 Micropile Installation 

The micropile Contractor shall select the drilling method, the grouting procedure, and the 

grouting pressure used for the installation of the micropiles. The micropile Contractor shall 

also determine the micropile casing size, final drillhole diameter and bond length, and central 

tendon reinforcement steel sizing necessary to develop the specified load capacities and load 

testing requirements. The micropile Contractor is also responsible for estimating the grout 

take. There will be no extra payment for grout overruns. (Commentary: Note, extra payment 

for grout takes is appropriate for micropiles in Karst. Otherwise, the bidprice of these piles 

will be artiJcially high to cover risk of high grout loss.) 

FHWA-SA-97-070 (~00-06) Al-23 
3.4.1 Drilling 

The drilling equipment and methods shall be suitable for drilling through the conditions to be 

encountered, without causing damage to any overlying or adjacent structures or services. The 

drillhole must be open along it’s full length to at least the design minimum drillhole diameter 

prior to placing grout and reinforcement. (Commentary: when micropile construction will 

occur in close proximity to settlement sensitive structures, recommend including the following 

sentence in the spectfication - Vibratory pile driving hammers shall not be used to advance 

casing.) 

Temporary casing or other approved method of pile drillhole support will be required in caving 

or unstable ground to permit the pile shaft to be formed to the minimum design drillhole 

diameter. The Contractor’s proposed method(s) to provide drillhole support and to prevent 

detrimental ground movements shall be reviewed by the Engineer. Detrimental ground 

movement is defined as movement which requires remedial repair measures. Use of drilling 

fluid containing bentonite is not allowed. (Commentary: The specifzcation verbage related to 

drillhole support methods and d@culty of drilling may vary project to project depending on 

the subsur$ace conditions revealed by the subsurface investigation data. It is the micropile 

specialty contractor’s responsibility to select the proper drilling equipment and methods for 

the site conditions. It is the owner’s responsibility to provide the available subsurface 

information. For projects with d@cult ground conditions, use of an ‘bdvisory specification ” 

included in the contract documents is recommended. Refer to Appendix B for an example.) 

Costs of removal or remedial measures due to encountering unanticipated subsurface 

obstructions will be paid for as Extra Work. 

3.4.2 Ground Heave or Subsidence. 

During construction, the Contractor shall observe the conditions vicinity of the micropile 

construction site on a daily basis for signs of ground heave or subsidence. Immediately notify 

the Engineer if signs of movements are observed. Contractor shall immediately suspend or 

modify drilling or grouting operations if ground heave or subsidence is observed, if the 

Al-24 FHWA-SA-97-070 (~00-06) 
micropile structure is adversely affected, or if adjacent structures are damaged from the drilling 

or grouting. If the Engineer determines that the movements require corrective action, the 

Contractor shall take corrective actions necessary to stop the movement or perform repairs. 

When due to the Contractor’s methods or operations or failure to follow the specified/approved 

construction sequence, as determined by the Engineer, the costs of providing corrective actions 

will be borne by the Contractor. When due to differing site conditions, as determined by the 

Engineer, the costs of providing corrective actions will be paid as Extra Work. 

3.4.3 Pipe Casing and Reinforcing Bars Placement and Splicing. 

Reinforcement may be placed either prior to grouting or placed into the grout - filled drillhole 

before temporary casing (if used) is withdrawn. Reinforcement surface shall be free of 

deleterious substances such as soil, mud, grease or oil that might contaminate the grout or coat 

the reinforcement and impair bond. Pile cages and reinforcement groups, if used, shall be 

sufficiently robust to withstand the installation and grouting process and the withdrawal of the 

drill casings without damage or disturbance. 

The Contractor shall check pile top elevations and adjust all installed micropiles to the planned 

elevations. 

Centralizers and spacers (if used) shall be provided at 3-m centers maximum spacing. The 

upper and lower most centralizer shall be located a maximum of 1.5 m from the top and bottom 

of the micropile. Centralizers and spacers shall permit the free flow of grout without 

misalignment of the reinforcing bar(s) and permanent casing. The central reinforcement bars 

with centralizers shall be lowered into the stabilized drill hole and set. The reinforcing steel 

shall be inserted into the drill hole to the desired depth without difficulty. Partially inserted 

reinforcing bars shall not be driven or forced into the hole. Contractor shall redrill and reinsert 

reinforcing steel when necessary to facilitate insertion. 

Lengths of casing and reinforcing bars to be spliced shall be secured in proper alignment and in 

a manner to avoid eccentricity or angle between the axes of the two lengths to be spliced. 

Splices and threaded joints shall meet the requirements of Materials Section 2.0. Threaded pipe 

FHWA-SA-97-070 (~00-06) Al -25 
casing joints shall be located at least two casing diameters (OD) from a splice in any 

reinforcing bar. When multiple bars are used, bar splices shall be staggered at least 0.3 meters. 

3.4.4 Grouting. 

Micropiles shall be primary grouted the same day the load transfer bond length is drilled. The 

Contractor shall use a stable neat cement grout or a sand cement grout with a minimum 28-day 

unconfined compressive strength of 28 MPa. Admix&es, if used, shall be mixed in 

accordance with manufacturer’s recommendations. The grouting equipment used shall produce 

a grout free of lumps and undispersed cement. The Contractor shall have means and methods 

of measuring the grout quantity and pumping pressure during the grouting operations. The 

grout pump shall be equipped with a pressure gauge to monitor grout pressures. A second 

pressure gauge shall be placed at the point of injection into the pile top. The pressure gauges 

shall be capable of measuring pressures of at least 1 MPa or twice the actual grout pressures 

used, whichever is greater. The grout shall be kept in agitation prior to mixing. Grout shall be 

placed within one hour of mixing. The grouting equipment shall be sized to enable each pile to 

be grouted in one continuous operation. The grout shall be injected from the lowest point of the 

drill hole and injection shall continue until uncontaminated grout flows from the top of the pile. 

The grout may be pumped through grout tubes, casing, hollow-stem augers, or drill rods. 

Temporary casing, if used, shall be extracted in stages ensuring that, after each length of casing 

is removed the grout level is brought back up to the ground level before the next length is 

removed. The tremie pipe or casing shall always extend below the level of the existing grout in 

the drillhole. The grout pressures and grout takes shall be controlled to prevent excessive 

heave or fracturing of rock or soil formations. Upon completion of grouting, the grout tube 

may remain in the hole, but must be filled with grout. 

If the Contractor elects to use a postgrouting system, Working Drawings and details shall be 

submitted to the Engineer for review in accordance with Section 1.8, Pre-installation 

Submittals. 

Al -26 FHWA-SA-97-070 (~00-06) 
3.4.5 Grout Testing 

Grout within the micropile verification and proof test piles shall attain the minimum required 

3-day compressive strength of 14 MPa prior tb load testing. Previous test results for the 

proposed grout mix completed within one year of the start of work may be submitted for initial 

verification of the required compressive strengths for installation of pre-production verification 

test piles and initial production piles. During production, micropile grout shall be tested by the 

Contractor for compressive strength in accordance with AASHTO T106/ASTM Cl09 at a 

frequency of no less than one set of three 50-mm grout cubes from each grout plant each day of 

operation or per every 10 piles, whichever occurs more frequently. The compressive strength 

shall be the average of the 3 cubes tested. 

Grout consistency as measured by grout density shall be determined by the Contractor per 

ASTM C 188/AASHTO T 133 or API RP-13B-1 at a frequency of at least one test per pile, 

conducted just prior to start of pile grouting. The Baroid Mud Balance used in accordance with 

API RP-13B-1 is an approved device for determining the grout density of neat cement grout. 

The measured grout density shall be between kg/m3 and kg/m3. 

Grout samples shall be taken directly from the grout plant. Provide grout cube compressive 

strength and grout density test results to the Engineer within 24 hours of testing. 

(Commentary: If the Engineer will perform the grout testing, revise this section accordingly). 

3.5 Micropile Installation Records. 

Contractor shall prepare and submit to the Engineer full-length installation records for each 

micropile installed. The records shall be submitted within one work shift after that pile 

installation is completed. The data shall be recorded on the micropile installation log included 

at the end of this specification. A separate log shall be provided for each micropile. 

(Commentary: In addition to the expertise of the micropile specialty Contractor, the quality of 

the individual construction elements is directly related to the$nal product overall quality. As 

with other drilledpile systems, the actual load carrying capacity of a micropile can only be 

de3nitivelyproven by pile load tests. It is not practical or economical to test every pile 

FHWA-SA-97-070 (~00-06) Al -27 
installed. Therefore, aggressive inspection by the Contractor and Owner’s Engineer is needed 

to assure that each individual micropile is well constructed and to justtfy load testing only a 

small number, e.g, 5%, of the total number ofproductionpiles installed) 

3.6 Pile Load Tests 

Perform verification and proof testing of piles at the locations specified herein or designated 

by the Engineer. Perform compression load testing in accord with ASTM D1143 and tension 

load testing in accord with ASTM D3689, except as modified herein. 

(Commentary on number of load tests: Spectj?er/designer need to determine and write into 

this portion of the specification the number and location of required vertftcation andproof, 

tests. The total number of load tests and maximum test loads to be specified can vary on a 

project-by-project basis. They are dependent on ground type and variability, requiredpile 

capacity, pile loading type (i.e., static or seismic), total number ofpiles, criticality of the 

structure and available site access and work space. Guideline criteria for estimating the total 

number of vertjication andproof test piles are given in Chapter 7. For structure foundations, 

the following is recommended as a minimum. Perform verification testing of at least one 

sacrtjicial test pile per structure, prior to installation of any production piles. New users 

shouldperform proof tests on production piles at afiequency of 5percent (1 in 20). For 

experienced users, number of tests are to be determined by Owner/Engineer on a project by 

project basis. Ifpile capaciv demands are greatest in compression, the piles should be load 

tested in compression. If the pile capacity demands are equal for both compression and 

tension, or greater in tension, it is recommended that tension testing alone be conducted, to 

reduce costs). 

(Commentary: Spectfier - Indicate here whether compression or tension testing, or both are 

required for your project). 

Al-28 FHWA-SA-97-070 (~00-06) 
3.6.1 Verification Load Tests 

Perform pre-production verification pile load testing to verify the design of the pile system and 

the construction methods proposed prior to installing any production piles. 

sacrificial verification test piles shall be constructed in conformance with the approved 

Working Drawings. Verification test pile(s) shall be installed at the following locations 

Verification load tests shall be performed to verify that the Contractor installed micropiles will 

meet the required compression and tension load capacities and load test acceptance criteria 

and to verify that the length of the micropile load transfer bond zone is adequate. The 

micropile verification load test results must verify the Contractor’s design and installation 

methods, and be reviewed and accepted by the Engineer prior to beginning installation of 

production micropiles. 

The drilling-and-grouting method, casing length and outside diameter, reinforcing bar lengths, 

and depth of embedment for the verification test pile(s) shall be identical to those specified for 

the production piles at the given locations. The verification test micropile structural steel 

sections shall be sized to safely resist the maximum test load. (Commentary: Note that if 

additional steel area is provided in the verification test, the measured deflection will be lower 

than production piles.) 

The maximum verification and proof test loads applied to the micropile shall not exceed 80 

percent of the structural capacity of the micropile structural elements, to include steel yield in 

tension, steel yield or buckling in compression, or grout crushing in compression. Any 

required increase in strength of the verification test pile elements above the strength required 

for the production piles shall be provided for in the contractor’s bid price. 

The jack shall be positioned at the beginning of the test such that unloading and repositioning 

during the test will not be required. When both compression and tension load testing is to be 

performed on the same pile, the pile shall be tested under compression loads prior to testing 

under tension loads. 

FHWA-SA-97-070 (~00-06) Al -29 
3.62 Testing Equipment and Data Recording. 

Testing equipment shall include dial gauges, dial gauge support, jack and pressure gauge, 

electronic load cell, and a reaction frame. The load cell is required only for the creep test 

portion of the verification test. (Commentary: Thepurpose and value of an electronic load 

cell is to measure small changes in loadfor load tests where the load is heldfor a long 

duration, such as during verification or creep testing. It is not intended to be used during 

proof testing, including the short term creep portion. Experience has proven that load cells 

have been problematic under field conditions, yet even with errors resulting@om cell 

construction, off-center loading, and other eflects, a load cell is very sensitive to small changes 

in load and is strongly recommendedfor creep testing.) The contractor shall provide a 

description of test setup and jack, pressure gauge and load cell calibration curves in 

accordance with the Submittals Section. 

Design the testing reaction fkame to be suff&ziently rigid and of adequate dimensions such that 

excessive deformation of the testing equipment does not occur. Align the jack, bearing plates, 

and stressing anchorage such that unloading and repositioning of the equipment will not be 

required during the test. 

Apply and measure the test load with a hydraulic jack and pressure gauge. The pressure guage 

shall be graduated in 500 kPa increments or less. The jack and pressure gauge shall have a 

pressure range not exceeding twice the anticipated maximum test pressure. Jack ram travel 

shall be sufficient to allow the test to be done without resetting the equipment. Monitor the 

creep test load hold during verification tests with both the pressure gauge and the electronic 

load cell. Use the load cell to accurately maintain a constant load hold during the creep test 

load hold increment of the verification test. 

Measure the pile top movement with a dial gauge capable of measuring to 0.025 mm. The dial 

gauge shall have a travel sufficient to allow the test to be done without having to reset the 

gauge. Visually align the gauge to be parallel with the axis of the micropile and support the 

gauge independently from the jack, pile or reaction frame. Use a minimum of two dial gauges 

Al-30 FHWA-SA-97470 (~00-06) 
when the test setup requires reaction against the ground or single reaction piles on each side of 

the test pile. (Commentary: Experience with testing piles reacting against the ground, or 

agqisst single reaction piles on each side of the test pile, has resulted in rackingpnd 

misalignment of the system on some projects. Two dial guages are recommendedfor this test 

setup to determine tfracking is occurring and to provide a more accurate average micropile 

head movement measurement). 

The required load test data shall be recorded by the Engineer. 

3.6.3 Verification Test Loading Schedule. 

Test verification piles designated for compression or tension load testing to a maximum test 

load of 2.5 times the micropile Design Load shown on the Plans or Working Drawings. 

(Commentary: See Section 5.E.4 for more detailed veriJication load testing information.) The 

verification pile load tests shall be made by incrementally loading the micropile in accordance 

with the following cyclic load schedule for both compression and tension loading: 

r - 

- 

1 

2 

3 

- 

4 

5 

6 

7 

- 

8 

9 

10 

11 

12 

- 

AL = Alignment Load 

LOAD 

AL (0.05 DL) 

DL = Design Load 

HOLD TIME 

1 minute 

0.25 DL 1 minute 

0.50 DL 1 minute 

AL 1 minute 

0.25 DL 1 minute 

0.50 DL 1 minute 

0.75 DL 1 minute 

AL 1 minute 

0.25 DL 1 minute 

0.50 DL 1 minute 

0.75 DL 1 minute 

1.00 DL 1 minute 

1 

FHWA-SA-97-070 (~00-06) Al -31 
r 

13 

14 

15 

16 

17 

18 

19 

20 

21 

22 

23 

AL = Alignment Load 

LOAD 

AL 

0.25 DL 

0.50 DL 

0.75 DL 

1 .OO DL 

1.33 DL 

1.75 DL 

2.00 DL 

2.25 DL 

2.50 DL 

(Maximum Test Load) 

AL 

DL = Design Load 

HOLD TIME 

1 minute 

1 minute 

1 minute 

1 minute 

1 minute 

60 minutes 

(Creep Test Load Hold) 

1 minute 

1 minute 

1 minute 

10 minutes 

1 minute 

The test load shall be applied in increments of 25 percent of the DL load. Each load increment 

shall be held for a minimum of 1 minute. Pile top movement shall be measured at each load 

increment. The load-hold period shall start as soon as each test load increment is applied. The 

verification test pile shall be monitored for creep at the 1.33 Design Load (DL). Pile 

movement during the creep test shall be measured and recorded at 1,2,3,4, $6, 10,20,30, 

50, and 60 minutes. The alignment load shall not exceed 5 percent of the DL load. Dial gauges 

shall be reset to zero after the initial AL is applied. 

The acceptance criteria for micropile verification load tests are: 

1. The pile shall sustain the first compression or tension 1 .O DL test load with no more than 

mm total vertical movement at the top of the pile, relative to the position of the 

top of the pile prior to testing. (Commentary: Structural designer to determine 

maximum allowable total pile top structural axial displacement at I.0 DL test load 

based on structural design requirements. Also, if the ver@cation test pile has to be 

upsized structurally to accommodate the maximum required verification test load this 

Al-32 FHWA-SA-97-070 (~00-06) 
provision will not apply. Om’y the proof testedproduction piles will then be subject to 

this criteria. Refer to Chapter 5 for more design guidance). 

2. At the end of the 1.33 DL creep test load increment, test piles shall have a creep rate not 

exceeding 1 mm/log cycle time (1 to 10 minutes) or 2 mm/log cycle time (6 to 60 

minutes or the last log cycle if held longer).The creep rate shall be linear or decreasing 

throughout the creep load hold period. 

3. Failure does not occur at the 2.5 DL maximum test load. Failure is defined as load at 

which attempts to further increase the test load simply result in continued pile 

movement. 

The Engineer will provide the Contractor written confirmation of the micropile design and 

construction within 3 working days of the completion of the verification load tests. This 

written confirmation will either confirm the capacities and bond lengths specified in the 

Working Drawings for micropiles or reject the piles based upon the verification test results. 

3.6.4 Verification Test Pile Rejection 

If a verification tested micropile fails to meet the acceptance criteria, the Contractor shall 

modify the design, the construction procedure, or both.These modifications may include 

modifying the installation methods, increasing the bond length, or changing the micropile type. 

Any modification that necessitates changes to the structure shall require the Engineer’s prior 

review and acceptance. Any modifications of design or construction procedures or cost of 

additional verification test piles and load testing shall be at the Contractor’s expense. At the 

completion of verification testing, test piles shall be removed down to the elevation specified 

by the Engineer. 

FHWA-SA-97-070 (~00-06) Al-33 
3.6.5 Proof Load Tests 

Perform proof load tests on the first set of production piles installed at each designated 

substructure unit prior to the installation of the remaining production piles in that unit. The 

first set of production piles is the number required to provide the required reaction capacity for 

the proof tested pile. The initial proof test piles shall be installed at the following substructure 

units . Proof testing shall be conducted at a frequency of 5% (1 in 20) of the 

subsequent production piles installed, beyond the first 20, in each abutment and pier. Location 

of additional proof test piles shall be as designated by the Engineer. (Commentary: The above 

is a guideline for new users. Experienced users may go with a lesser number of proof load 

tests as determined by the Owner/Engineer.) 

3.6.6 Proof Test Loading Schedule 

Test piles designated for compression or tension proof load testing to a maximum test load of 

1.67 times the micropile Design Load shown on the Plans or Working Drawings. 

(Commentary: See Section 5.E.4 for more detailedproof load testing information.) Proof tests 

shall be made by incrementally loading the micropile in accordance with the following 

schedule, to be used for both compression and tension loading: 

AL = Alignment Load DL = Design Load 

LOAD HOLD TIME 

AL 1 minute 

0.25 DL 1 minute 

0.50 DL 1 minute 

0.75 DL 1 minute 

1 .OO DL 1 minute 

1.33 DL 10 or 60 minute 

Creep Test 

1.67 DL 1 minute 

(Maximum Test Load) 

AL 1 minute 

Al-34 FHWA-SA-97-070 (~00-06) 
Depending on performance, either a 10 minute or 60 minute creep test shall be performed at the 

1.33 DL Test Load. Where the pile top movement between 1 and 10 minutes exceeds 1 mm, 

the Maximum Test Load shall be maintained an additional 50 minutes. Movements shall be 

recorded at 1,2,3,5,6, 10,20,30,50 and 60 minutes. The alignment load shall not exceed 5 

percent of DL. Dial guages shall be reset to zero after the ititial AL is applied. 

The acceptance criteria for micropile proof load tests are: 

1. The pile shall sustain the compression or tension 1 .O DL test load with no more than 

mm total vertical movement at the top of the pile, relative to the position of the 

top of the pile prior to testing. (Commentary: Structural designer to determine 

maximum allowable total pile top structural axial displacement at the 1.0 DL test load 

based on structure design requirements. Refer to Chapter 5 for more design guidance.) 

2. At the end of the 1.33 DL creep test load increment, test piles shall have a creep rate not 

exceeding 1 mm/log cycle time (1 to 10 minutes) or 2 mm/log cycle time (6 to 60 

minutes).The creep rate shall be linear or decreasing throughout the creep load hold 

period. 

3. Failure does not occur at the 1.67 DL maximum test load. Failure is defined as the load 

at which attempts to further increase the test load simply result in continued pile 

movement. 

3.6.7 Proof Test Pile Rejection 

If a proof-tested micropile fails to meet the acceptance criteria, the Contractor shall 

immediately proof test another micropile within that footing. For failed piles and further 

construction of other piles, the Contractor shall modify the design, the construction procedure, 

or both. These modifications may include installing replacement micropiles, incorporating piles 

at not more than 50% of the maximum load attained, postgrouting, modifying installation 

methods, increasing the bond length, or changing the micropile type. Any modification that 

necessitates changes to the structure design shall require the Engineer’s prior review and 

acceptance. Any modifications of design or construction procedures, or cost of additional 

verification test piles and verification and/or proof load testing, or replacement production 

micropiles, shall be at the Contractor’s expense. 

FHWA-SA-97-070 (~00-06) Al-35 
4.0 METHOD OF MEASUREMENT. 

Measurement will be made as follows for the quantity, as specified or directed by the Engineer: 

l Mobilization will be measured on a lump-sum basis. 

l Micropiles will be measured per each, installed, and accepted. 

l Micropile verification load testing will be measured per each. 

l Micropile proof load testing will be measured per each. 

The final pay quantities will be the design quantity increased or decreased by any changes 

authorized by the Engineer. 

5.0 BASIS OF PAYMENT 

The quantities accepted for payment will be paid for at the contract unit prices for the following 

items: 

Mobilization and Demobilization . . . . . . . . . . . . . . Lump sum 

Micropile Verification Load Test . . . . . . . . . . . . . . Each 

Micropile Proof Load Test Each 

Micropiles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Each* 

Micropiles Variations in Length to Top of Rock . . LF** 

Unexpected Obstruction Drilling . . . . . . . . . . . . . . Hour*** 

*For the option where the contractor designs the footing and number of piles, the foundation 

system should be bid as lump sum and a schedule of values established for progress payments 

after award. 

**Where piles are founded in rock, micropiles will be paid on a per each basis assuming Rock at 

Elevation -. Additional length or shorter length due to variations in the top of rock will be 

paid on a add or deduct lineal foot basis where the linear footage = Elevation minus - 

Elevation of As-Built Rock. 

***If “obstructions” are not defined in the Standard Specifications, a definition should be added. 

Al-36 FHWA-SA-97-070 (vOO-06) 
The contract unit prices for the above items will be full and complete payment for providing all 

design, materials, labor, equipment, and incidentals to complete the work. 

Where verification test piles are designated as sacrificial, the micropile verification load test 

bid item shall include the cost of the sacrificial micropile. 

The unit contract amount for ‘Micropiles” shall include the drilling, furnishing, and placing the 

reinforcing steel and casing, grouting, and pile top attachments. The micropile Contractor is 

also responsible for estimating the grout take. There will be no extra payment for grout 

overruns. 

FHWA-SA-97-070 (~00-06) Al-37 
.75m l-r 6SPACES~I.SmO.C. ;blI B,Sm 

MICROPILE TO FOOTING CONNECTION TO BE 

DESIGNED EYMICROPILE CONTRACTOR 

1 

FRONT - 4 - & a 1. - $I. 

INCUNED) E 

w 

a-------- 2 MICROPILE 

REAR - 

IIERTlCAL) .‘p - -- --__ 

rk +1,-- 

f 

cplp) 

FOOTING REINFORCEMENT 

~- I 

SHOWN ON SHEET NO _ 

f 

0.3m _ 

4SPACES@Z.ZmD.C. j.7Smb 

MICROPILE ALTERNATNE 

ABUTMENT FOOTING - PLAN 

NOT TO SCALE 

GROUP 1 NON-SEISMIC UNFACTORED LOADS PER PILE 

1. MICROPILE DESIGN TO BE PERFORMED BY PREQUALIFIED MICROPILE 

CONTRACTOR IN ACCORDANCE WITH PROJECT SPEClFlCATlONS & 

PILE CAPACITY TABLE ON THIS SHEET 

2. MICROPILE BEARING PLATE SHALL EFFECTIVELY DISTRIBUTE THE 

DESIGN FORCE TO THE FOOTlNG CONCRETE SUCH THAT THE CONE 

SHEAR REQUIREMENTS OF ACI 349, APPENMX B. ARE MET AND THE 

BENDING STRESS DOES NOT EXCEED 0.55f, FOR STEEL. 

3. MICROPILE BOND ZONE TO BE COMPLETELY FOUNDED IN DENSE TO 

VERY DENSE SANDY GRAVEL. 

COMMENTARY: THIS ISTHE MINIMUM AMOUNT OF INFORMATlON THAT THE 

OWNER NEEDS TO PROVIDE IN THE CONTRACT PLANS FOR 

THE OWNER CONTROLLED DESIGN SPECIFICATION METHOD. 

AS AN OPTION, OTHER FOOTlNQ DESIGNS WITH ALTERNATE 

MICROPILE LAYOUTS CAN BE PROVIDED IN TABLE FORM. 

CONTRACTOR DESIGN/BUILC 

MICROPILE ALTERNATlVE 

ABUTMENT TYPICAL SECTION 

OF MICROPILES 

NOT TO SCALE 

SANDY GRAVEL AND 

COBBLES WIT!4 

BOULDERS UP TO Im 

SAMPLE CONTRACT PLANS 
Micropile Installation Log 

Project Name: 

Drill Rig/ #, Operator 

1 Grout Plant #, Operator Total Duration 

II Casing Dia.iWall Thickness I Admixtures] ~ ~~ 7 

II Pile Inclination I w/c Ratio 1 

R Reinforcement Size/Length I 

II Pile Length Above B.O.F. ) ) Tremie Grout Quantity (bags) ) II 

Upper Cased Length Pressure Grout Quantity (bags) 

1 I I I 

Cased and Bond Length (Plunge) 

Bond Length Below Casing 

Grouting after plunge (bags) 

Total Grout Quantity (bags) 

Grout Ratio (bags/m bond) 

Comments - Pile Drilling 

Flush Description Comments 

Comments - Pile Grouting 

Depth from B.0.F 

b-4 

Pressure Range 

Mex/Average (MPa) Comments 

B.O.F. = Bottom of Footing (ref. FHWA-SA-97-070) 

FHWA-SA-97-070 (~00-06) Al-39 
[BLANK PAGE] 

Al -40 FHWA-SA-97-070 (~00-06) 
APPENDIX A-2 

Micropile Guide Construction Specification 

and Sample Plans 

[Foundation Support Projects] 

Contractor Design/Build of Foundation 

(Micropiles and Footings) 

Metric (SI) Units 

(With Commentary) 

(Commentary: This guide specification is set up for a post-bid design solicitation to solicit 

micropile structure designs where the Owner has selected a micropile system as the desired 

system for the given structure location(s). It can be mod$ed as appropriate to also serve as a 

pre-bid design solicitation and /or for a solicitation where alternate foundation or structure 

types are allowed by the Owner, with the. Contractor allowed to select and submit a design for 

the foundation or structure type which the Contractorfeels is most cost-efective. This guide 

specification is set up for the method wherein the Owner provides preliminary plans 

showing a pile footing design and total footing loads and moments for foundation support 

projects. Owner also provides related design criteria and requirements, subsurface data, 

rights-of-way limits, utility locations, site limitations, construction material and testing 

specifications and required Contractor working drawing/design and construction submittals 

and review requirements. The microzGle Contractor designs the individual microuile 

elements. including their svacing and lavout, and Dile tou footing connections and selects 

the microuile construction process and eauipment. As comvared to the Avvendix A-I 

contracting method, with this avvroach the Desigtiuild Contractor has the flexibility to 

provide fewer higher cat.racitv microviles. During the bidding process, the pre-qualified 

micropile contractors prepare a micropile design and afimt cost proposal based on the 

Owner s preliminary plans and spect@cations. If the micropile portion of the project is to be 

subcontracted, general contractors will receive bids from the pre-qualified micropile 

contractors and include the best offer and name of the selected micropile Contractor in their 

bid submittal. Once the contract is awarded the selected micropile Contractor prepares 

detailed micropile design calculations and working drawings and submits them to the Engineer 

for review. After acceptance of the design, construction begins. For more detailed discussion 

on various contracting methods, refer to Chapter 9.) 

FHWA-SA-97-070 (~0-06) A-2- 1 
1 .O DESCRIPTION 

This work shall consist of constructing micropiles as shown on the contract plans and approved 

working drawings and as specified herein. The micropile specialty Contractor is responsible for 

furnishing of all design, materials, products, accessories, tools, equipment, services, 

transportation, labor and supervision, and manufacturing techniques required for design, 

installation and testing of micropiles and pile top attachments for this project. 

The selected micropile Contractor shall select the micropile type, size, pile top attachment, 

installation means and methods, estimate the ground-grout bond value and determine the 

required grout bond length and final micropile diameter. The micropile Contractor shall design 

and install micropiles that will develop the load capacities indicated on the contract plans. The 

micropile load capacities shall be verified by verification and proof load testing as required and 

must meet the test acceptance criteria specified herein. 

Where the imperative mood is used within this specification, “The Contractor shall” is implied. 

(Commentary: Successful design and installation of high-quality micropiles require 

experienced Contractors having the necessary specialty drilling and grouting equipment and 

expertise and experienced work crews. The most important section of the specifications to be 

enforced by the Owner deals with the experience qualzQ%ations of the micropile Contractor. 

Failure to enforce the specified experience qualifications opens the door for inexperienced 

Contractors trying to cut costs. The results often are inferior workmanship, project delays, 

and project claims that, more often than not, substantially increase project costs. Results like 

these often discourage project Owners from implementing new technology, and draws them 

back to more traditional methods at any cost. This can be avoided with the proper 

specijication implementation and as importantly, enforcement to ensure a mutually successful 

project.) 

A-2-2 FHWA-SA-97-070 (~00-06) 
1 .I Micropile Contractor’s Experience Requirements And Submittal. 

The micropile Contractor shall be experienced in the construction and load testing of 

micropiles and have successfully constructed at least 5 projects in the last 5 years involving 

construction totalling at least 100 micropiles of similar capacity to those required in these 

plans and specifications. 

The Contractor shall have previous micropile drilling and grouting experience in soil/rock 

similar to project conditions. The Contractor shall submit construction details, structural 

details and load test results for at least three previous successful micropile load tests from 

different projects of similar scope to this project. 

The Contractor shall assign an Engineer to supervise the work with experience on at least 3 

projects of similar scope to this project completed over the past 5 years. The Contractor shall 

not use consultants or manufacturers’ representatives to satisfy the supervising Engineer 

requirements of this section. The on-site foremen and drill rig operators shall also have 

experience on at least 3 projects over the past 5 years installing micropiles of equal or greater 

capacity than required in these plans and specifications. 

The micropiles shall be designed by a Registered Professional Engineer with experience in the 

design of at least 3 successfully completed micropile projects over the past 5 years, with 

micropiles of similar capacity to those required in these plans and specifications. The micropile 

designer may be either a employee of the Contractor or a separate Consultant designer meeting 

the stated experience requirements. (Commentary: If the Owner prepares a fully detailed 

design, this paragraph can be deleted). 

At least 45 calendar days before the planned start of micropile construction, the Contractor 

shall submit 5 copies of the completed project reference list and a personnel list. The project 

reference list shall include a brief project description with the owner’s name and current phone 

number and load test reports. The personnel list shall identify the micropile system designer (if 

applicable), supervising project Engineer, drill rig operators, and on-site foremen to be 

assigned to the project. The personnel list shall contain a summary of each individual’s 

FHWA-SA-97-070 (v&06) A-2-3 
experience and be complete enough for the Engineer to determine whether each individual 

satisfies the required qualifications. The Engineer will approve or reject the Contractor’s 

qualifications within 15 calendar days after receipt of a complete submission. Additional time 

required due to incomplete or unacceptable submittals will not be cause for time extension or 

impact or delay claims. All costs associated with incomplete or unacceptable submittals shall 

be borne by the Contractor. 

Work shall not be started, nor materials ordered, until the Engineer’s written approval of the 

Contractor’s experience qualifications is given. The Engineer may suspend the Work if the 

Contractor uses non-approved personnel. If work is suspended, the Contractor shall be fully 

liable for all resulting costs and no adjustment in contract time will result from the suspension. 

1.2 Pre-approved List 

(Commentary: The intent of this section, ifused, is to reduce the administrative burden on 

specialty contractors who have previously Satisfactorily demonstrated to the Owner Agency 

that they meet the Section Al.1 experience qual~jkations.) 

Listed below are the micropile specialty Contractors pre-qualified to design, furnish and install 

micropiles on this project, based on previous Contractor experience submittals verified and 

accepted by the Owner: 

1. Contractor Name 

Mailing Address 

Contact Name 

Phone Number 

2. Contractor Name 

Mailing Address 

Contact Name 

Phone Number 

A-2-4 FHWA-SA-97-070 (~00-06) 
4. Design criteria including, soiVrock shear strengths (friction angle and cohesion), unit 

weights, and ground-grout bond values and micropile drillhole diameter assumptions 

for each soil/rock strata. 

5. Safety factors/strength factors (for Service Load Design) or load and resistance factors 

(for Load Factor Design) used in the design on the ground-grout bond values, 

surcharges, soil/rock and material unit weights, steel, grout, and concrete materials. 

6. Seismic design earthquake acceleration coefficient. 

7. Design calculation sheets (both static and seismic) with the project number, micropile 

structure location, designation, date of preparation, initials of designer and checker, and 

page number at the top of each page. Provide an index page with the design calculations. 

8. Design notes including an explanation of any symbols and computer programs used in 

the design. 

9. Pile to footing connection calculations. 

10. Calculations for footing reinforcement different from that on the Owner provided 

preliminary plans, if required to accommodate the Contractor designed pile layout. 

I .8.3 Working Drawings. 

The working drawings shall include all information required for the construction and quality 

control of the piling. Working drawings shall include, but not be limited to, the following 

items unless provided in the contract plans: 

1. A plan view of the micropile structure(s) identifying: 

(a) A reference baseline and elevation datum. 

(b) The offset from the construction centerline or baseline to the face of the micropile 

structure at all changes in horizontal alignment. 

FHWA-SA-97-070 (~0-06) A-2 - 13 
Bonded Length: The length of the micropile that is bonded to the ground and conceptually 

used to transfer the applied axial loads to the surrounding soil or rock. Also known as the 

load transfer length. 

Bond-breaker: A sleeve placed over the steel reinforcement to prevent load transfer. 

Casing: Steel tube introduced during the drilling process in overburden soil to temporarily 

stabilize the drill hole. This is usually withdrawn as the pile is grouted, although in certain 

types of micropiles, some casing is permanently left in place to provide added pile 

reinforcement. 

Centralizer: A device to support and position the reinforcing steel in the drill hole and/or 

casing so that a minimum grout cover is provided. 

Contractor: The person/firm responsible for performing the micropile work. 

Coupler: The means by which load capacity can be transmitted from one partial length of 

reinforcement to another. 

Creep Movement: The movement that occurs during the creep test of a micropile under a 

constant load. 

Design Load (DL): The maximum unfactored load expected to be applied to the micropile 

during its service life. 

Encapsulation: A corrugated or deformed tube protecting the reinforcing steel against 

corrosion. 

Engineer: The Owner or Owner’s authorized agent. 

Free (unbonded) length: The designed length of the micropile that is not bonded to the 

surrounding ground or grout. 

A-2-6 FHWA-SA-9’7-070 (~00-06) 
Geotechnical Bond Design Strength: For Load Factor Design (LFD), computed as the 

nominal grout-to-ground bond strength (a bo,,d nominal strength), multiplied by a geotechnical 

r$sistance factor (po. Use ‘pc = 0.6 for typical designs and non-seismic load groups; use 

(ho = 1 .O for seismic loads groups 

Micropile: A small-diameter, bored, cast-in-place composite pile, in which the applied load is 

resisted by steel reinforcement, cement grout and frictional grout&round bond. 

Maximum Test Load: The maximum load to which the micropile is subjected during testing. 

Recommended as 2.5 x DL for verification load tests and as 1.67 x DL for proof load tests. 

Nominal Grout-to-Ground Bond Strength: The estimated ultimate geotechnical unit groutto-ground bond strength selected for use in design. Same as clbond nominal magth (SLD and 

LFD) 

Overburden: Material, natural or placed, that may require cased drilling methods to provide 

an open borehole to underlying strata. 

Post-grouting: The injection of additional grout into the load transfer length of a micropile 

after the primary grout has set. Also known as regrouting or secondary grouting. 

Primary Grout: Portland-cement-based grout injected into the micropile hole prior to or after 

the installation of the reinforcement to direct the load transfer to the surrounding ground 

along the micropile. 

Proof Load Test: Incremental loading of a production micropile, recording the total 

movement at each increment. 

Reinforcement: The steel component of the micropile that accepts and/or resists applied 

loadings. 

Sheathing: Smooth or corrugated piping or tubing that protects the reinforcing steel against 

corrosion. 

FHWA-SA-97-070 (~0-06) A-2-7 
Spacer: A device to separate elements of a multiple-element reinforcement. 

Verification Load Test: Pile load test performed to verify the design of the pile system and the 

construction methods proposed, prior to installatioh of production piles. 

1.5 Referenced Codes and Standards. 

The following publications form a part of this specification to the extent indicated by the 

references. The latest publication as of the issue date of this specification shall govern, unless 

indicated otherwise. 

1.51 American Society for Testing and Materials (ASTM) 

American Association of State Highway and Transportation Officials (AASHTO) 

ASTM AASHTO SPECIFICATION / TEST 

A36, AS72 M183, M223 Structural Steel 

A82 MS5 Cold-Drawn Steel Wire for Concrete Reinforcement 

A252 - Welded and Seamless Steel Pipe Piles 

A615 M31 Deformed and Plain Billet Steel Bars for Concrete Reinforcement 

A722 M275 Uncoated High-Strength Steel Bar for Prestressing Concrete 

A775 - Epoxy -Coated Reinforcing Steel Bars 

A934 Epoxy-Coated Prefabricated Steel Reinforcing Bars 

c33 

c 109 

C 188 

Cl44 

c 150 

c 494 

MS0 

T106 

T133 

M45 

M 85 

Ml94 

Concrete Aggregates 

Compressive Strength of Hydraulic Cement Mortar 

Density of Hydraulic Cement 

Aggregate for Masonry Mortar 

Portland Cement 

Chemical Admixtures for Concrete 

D 1143 

D 1784 

D 3350 

D 3689 

D 3966 

- 

M 252 

T26 

Method of Testing Piles Under Static Axial Compressive Load 

Polyvinyl Chloride (PVC) Pipe (Class 13464-B) 

Polyethylene Corrugated Tubing 

Method of Testing Individual Piles Under Static Axial Tensile Load 

Standard Test Method for Piles Under Lateral Load 

Quality of Water to be Used in Concrete 

A-2-8 FHWA-SA-97-070 (~00-06) 
1.52 American Welding Society (AWS) 

D1.l Structural Welding Code-Steel 

D 1.2 Structural Welding Code-Reinforcing Steel 

1.5.3 American Petroleum Institute (API) 

5CT (N-80) Specification for casing and tubing 

RP 13B-1 Recommended Practice - Standard Procedure for Field Testing 

Water Based Drilling Fluids 

1.6 Available Information. 

Available information developed by the Owner, or by the Owner’s duly authorized 

representative include the following items: 

1. Plans prepared by , dated . The plans include the plan view, 

profile and typical cross sections for the proposed micropile locations. (Commentary: 

Refer to chapter 9 of the FHWA “‘Micropile Design and Construction Guidelines 

Manual”, Report No. FHWA- SA-97-070 for detailed guidance on plan information to 

provide on the Owner-Controlled Design preliminary plans. An example preliminary 

plan for the bridge foundation support design example no. I is included at the end of 

this guide specification.) 

2. Geotechnical Report No.(s) titled , dated , included 

or referenced in the bid documents, contains the results of test pits, exploratory borings 

and other site investigation data obtained in the vicinity of the proposed micropile 

locations. 

(Commentary: The subsurface conditions expected can significantly impact the contractor’s 

choice of procedures, methods, or equipment, the biddingprocess, and contract 

administration. Experience has proven that use of a geotechnical summary is advantageous 

FHWA-SA-97-070 (~0-06) A-2-9 
toward achieving a successJiz1 contract. It is recommended that advisory wording be inserted 

into the contract special provisions to “redfag” the conditions to bidders. Preparation and 

use of a ‘Summary of Geotechnical Conditions”, such as used by the Washington State DOT, is 

recommended WSDOT’s in-house guide for preparation of said summary, along with an 

example writeup for the Sample Problem No.1 contained in this manual, are included in 

Appendix B. WSDOT inserts the Summary into the contract special provisions, making it a 

legal part of the contract documents. The purpose is to alert and be fair to bidders, and thus 

prevent/minimize dtFering site condition construction claims and dispute.) 

1.7 Construction Site Survey 

Before bidding the Work, the Contractor shall review the available subsurface information and 

visit the site to assess the site geometry, equipment access conditions, and location of existing 

structures and above ground facilities. 

The Contractor is responsible for field locating and verifying the location of all utilities shown 

on the plans prior to starting the Work. Maintain uninterrupted service for those utilities 

designated to remain in service throughout the Work. Notify the Engineer of any utility 

locations different from shown on the plans that may require micropile relocations or structure 

design modification. Subject to the Engineer’s approval, additional cost to the Contractor due 

to micropile relocations and/or structure design modification resulting from utility locations 

different from shown on the plans, will be paid as Extra Work. 

(Commentary: The location of both active and abandoned buried utilities within the ground 

mass to receive micropiles can have a profound impact on the design and construction of the 

micropiles. Careful consideration of the presence and location of all utilities is requiredfor 

successfil design and installation of micropiles.) 

Prior to start of any micropile construction activity, the Contractor and Engineer shall jointly 

inspect the site to observe and document the pre-construction condition of the site, existing 

structures and facilities. 

A-2 - 10 FHWA-SA-97-070 (~00-06) 
1.8 Micropile Design Requirements. 

The micropiles shall be designed to meet the specified loading conditions, as shown on the 

contract plans and approved working drawings. Design the micropiles and pile top to footing 

connections using the Service Load Design (SLD) procedures contained in the FHWA 

“Micropile Design and Construction Guidelines Manual”, Report No. FHWA- SA-97-070. 

(Commentary: The FHWA micropile manual- Chapter 5- also presents Load Factor Design 

(LFD) procedures for micropile foundations. Revise specijkation if LFD design is required.) 

The required geotechnical safety factors/strength factors (for SLD Design) or load and 

resistance factors (for LFD Design) shall be in accord with the FHWA manual, unless specified 

otherwise. Estimated soil/rock design shear strength parameters, unit weights, applied 

foundation loadings, slope and external surcharge loads, corrosion protection requirements, 

known utility locations, easements, right-of-ways and other applicable design criteria will be as 

shown on the plans or specified herein. Structural design of any individual micropile structure 

elements not covered in the FHWA manual shall be by the service load design method in 

conformance with appropriate articles of the most current Edition of the AASHTO Standard 

Specifications for Highway Bridges, including current interim specifications. 

Steel pipe used for micropile permanent casing shall incorporate an additional mm 

thickness of sacrificial steel for corrosion protection. (Commentary: This paragraph is 

optional and to be selected by the designer or specified by the owner. AASHTU Section 4.5.7.4 

Cross-Section Adjustmentfor Corrosion, states - “For concrete-filledpipe piles where 

corrosion may be expected, IN6 inch (1.6mm) shall be deductedporn the shell thickness to 

allow for reduction in section due to corrosion. ‘7 

Where required as shown on the contract plans, corrosion protection of the internal steel 

reinforcing bars, consisting of either encapsulation, epoxy coating, or grout, shall be provided 

in accordance with Materials Section 2.0. Where permanent casing is used for a portion of the 

micropile, encapsulation shall extend at least 1.5 m into the casing. 

FHWA-SA-97-070 (~0-06) A-2- 11 
(Commentary: when installation of micropiles provides additional support for existing 

structures, such as for seismic retrofit or underpinning applications, the structural designer 

should add appropriate speciJcation verbag,e and design criteria into this specification to 

cover the penetration of the existing structural elements and the top ofpile anchorage to the 

existing structure.) 

1.8.1 Micropile Design Submittals. 

At least 21 calendar days before the planned start of micropile structure construction, submit 

complete design calculations and working drawings to the Engineer for review and approval. 

Include all details, dimensions, quantities, ground profiles, and cross-sections necessary to 

construct the micropile structure. Verify the limits of the micropile structure and ground survey 

data before preparing the detailed working drawings. 

The drawings and calculations shall be signed and sealed by the contractor’s Professional 

Engineer or by the Consultant designer’s Professional Engineer (if applicable), previously 

approved by the owner’s Engineer. If the micropile contractor uses a consultant designer to 

prepare the design, the micropile contractor shall still have overall contract responsibility for 

both the design and the construction. 

1.8.2 Design Calculations. 

Design calculations shall include, but not be limited to, the following items: 

1. A written summary report which describes the overall micropile design. 

2. Applicable code requirements and design references. 

3. Micropile structure critical design cross-section(s) geometry including soil/rock strata 

and piezometric levels and location, magnitude and direction of design applied loadings, 

including slope or external surcharge loads. 

A-2 - 12 FHWA-SA-97-070 (~00-06) 
4. Design criteria including, soiVrock shear strengths (friction angle and cohesion), unit 

weights, and ground-grout bond values and micropile drillhole diameter assumptions 

for each soil/rock strata. 

5. Safety factors/strength factors (for Service Load Design) or load and resistance factors 

(for Load Factor Design) used in the design on the ground-grout bond values, 

surcharges, soil/rock and material unit weights, steel, grout, and concrete materials. 

6. Seismic design earthquake acceleration coefficient. 

7. Design calculation sheets (both static and seismic) with the project number, micropile 

structure location, designation, date of preparation, initials of designer and checker, and 

page number at the top of each page. Provide an index page with the design calculations. 

8. Design notes including an explanation of any symbols and computer programs used in 

the design. 

9. Pile to footing connection calculations. 

10. Calculations for footing reinforcement different from that on the Owner provided 

preliminary plans, if required to accommodate the Contractor designed pile layout. 

I .8.3 Working Drawings. 

The working drawings shall include all information required for the construction and quality 

control of the piling. Working drawings shall include, but not be limited to, the following 

items unless provided in the contract plans: 

1. A plan view of the micropile structure(s) identifying: 

(a) A reference baseline and elevation datum. 

(b) The offset from the construction centerline or baseline to the face of the micropile 

structure at all changes in horizontal alignment. 

FHWA-SA-97-070 (~0-06) A-2 - 13 
2. 

3. 

4. 

5. 

6. 

7. 

(c) Beginning and end of micropile structure stations. 

(d) Right-of-way and permanent or temporary construction easement limits, location 

of all known active and abandoned existing utilities, adjacent structures or other 

potential interferences. The centerline of any drainage structure or drainage pipe 

behind, passing through, or passing under the micropile structure. 

(e) Subsurface exploration locations shown on a plan view of the proposed micropile 

structure alignment with appropriate reference base lines to fix the locations of the 

explorations relative to the micropile structure. 

An elevation view of the micropile structure(s) identifying: 

(a) Elevation view showing micropile locations and elevations; vertical and 

horizontal spacing; batter and alignment and the location of drainage elements (if 

applicable). 

(b) Existing and finish grade profiles both behind and in front of the micropile 

structure. 

Design parameters and applicable codes. 

General notes for constructing the micropile structure including construction sequencing 

or other special construction requirements. 

Horizontal and vertical curve data affecting the micropile structure and micropile 

structure control points. Match lines or other details to relate micropile structure 

stationing to centerline stationing. 

A listing of the summary of quantities on the elevation drawing of each micropile 

structure showing pay item estimated quantities. 

Micropile typical sections including micropile spacing and inclination; minimum 

drillhole diameter; pipe casing and reinforcing bar sizes and details; splice types and 

locations; centralizers and spacers; grout bond zone and casing plunge lengths (if used); 

A-2 - 14 FHWA-SA-97-070 (~00-06) 
corrosion protection details; and connection details to the substructure footing, 

anchorage, plates, etc. 

8. 

9. 

10. 

A typical detail of verification and production proof test micropiles defining the 

micropile length, minimum drillhole diameter, inclination, and load test bonded and 

unbonded test lengths. 

Details, dimensions, and schedules for all micropiles, casing and reinforcing steel, 

including reinforcing bar bending details. 

Details for constructing micropile structures around drainage facilities (if applicable). 

The working drawings and design calculations shall be signed and sealed by the Contractor’s 

Professional Engineer or by the Consultant designer’s Professional Engineer (if applicable), 

previously pre-qualified by the Owner. If the micropile Contractor uses a Consultant designer 

to prepare the design, the micropile Contractor shall still have overall contract responsibility 

for both the design and the construction. 

Submit 5 sets of the working drawings with the initial submission. Drawing sheet size shall be 

550 by 850 mm. One set will be returned with any indicated corrections. The Engineer will 

approve or reject the Contractor’s submittal within 15 calendar days after receipt of a complete 

submission. If revisions are necessary, make the necessary corrections and resubmit 5 revised 

sets. When the drawings are approved, furnish 5 sets and a Mylar sepia set of the approved 

drawings. The Contractor will not be allowed to begin micropile structure construction or 

incorporate materials into the work until the submittal requirements are satisfied and found 

acceptable to the Engineer. Changes or deviations from the approved submittals must be resubmitted for approval. No adjustments in contract time or delay or impact claims will be 

allowed due to incomplete submittals. (Commentary: Submittals procedures shall be 

coordinated with Owner/Agency proceduks). 

FHWA-SA-97-070 (~0-06) A-2 - 15 
Revise the drawings when plan dimensions are changed due to field conditions or for other 

reasons. Within 30 days after completion of the work, submit as-built drawings to the 

Engineer. Provide revised design calculations signed by the approved Registered Professional 

Engineer for all design changes made during the construction of the micropile structure. 

1.9 Construction Submittals. 

The Contractor shall prepare and submit to the Engineer, for review of completeness, 5 copies 

of the following for the micropile system or systems to be constructed: 

1. Detailed step-by-step description of the proposed micropile construction procedure, 

including personnel, testing and equipment to assure quality control. This step-by-step 

procedure shall be shown on the working drawings in sufficient detail to allow the 

Engineer to monitor the construction and quality of the micropiles. 

2. Proposed start date and time schedule and micropile installation schedule providing the 

following: 

Micropile number 

Micropile design load 

Type and size of reinforcing steel 

Minimum total bond length 

Total micropile length 

Micropile top footing attachment 

3. If welding of casing is proposed, submit the proposed welding procedure, certified by a 

qualified welding specialist. 

4. Information on headroom and space requirements for installation equipment that verify 

the proposed equipment can perform at the site. 

5. Plan describing how surface water, drill flush, and excess waste grout will be controlled 

and disposed. 

A-2 - 16 FHWA-SA-97-070(vOO-06) 
6. 

7. 

8. 

Certified mill test reports for the reinforcing steel or coupon test results for permanent 

casing without mill certification. The ultimate strength, yield strength, elongation, and 

material properties composition shall be included. For API N-80 pipe casing, coupon 

test results may be submitted in lieu of mill certification. 

Proposed Grouting Plan. The grouting plan shall include complete descriptions, details, 

and supporting calculations for the following: 

(a) 

@> 

w 

Cd> 

(4 

Grout mix design and type of materials to be used in the grout including certified 

test data and trial batch reports. 

Methods and equipment for accurately monitoring and recording the grout depth, 

grout volume and grout pressure as the grout is being placed. 

Grouting rate calculations, when requested by the Engineer. The calculations 

shall be based on the initial pump pressures or static head on the grout and losses 

throughout the placing system, including anticipated head of drilling fluid (if 

applicable) to be displaced. 

Estimated curing time for grout to achieve specified strength. Previous test results 

for the proposed grout mix completed within one year of the start of grouting may 

be submitted for initial verification and acceptance and start of production work. 

During production, grout shall be tested in accord with Section 3.4.5. 

Procedure and equipment for Contractor monitoring of grout quality. 

Detailed plans for the proposed micropile load testing method. This shall include all 

drawings, details, and structural design calculations necessary to clearly describe the 

proposed test method, reaction load system capacity and equipment setup, types and 

accuracy of apparatus to be used for applying and measuring the test loads and pile top 

movements in accordance with Section 3.6, Pile Load Tests. 

FHWA-SA-97-070 (~0-06) A-2 - 17 
9. Calibration reports and data for each test jack, pressure gauge and master pressure gauge 

and electronic load cell to be used. The calibration tests shall have been performed by 

an independent testing laboratory, and tests shall have been performed within 90 

calendar days of the date submitted. Testing shall not commence until the Engineer has 

reviewed and accepted the jack, pressure gauge, master pressure gauge and electronic 

load cell calibration data. 

Work other than test pile installation shall not begin until the construction submittals have been 

received, reviewed, and accepted in writing by the Engineer. Provide submittal items 1 through 

5 at least 21 calendar days prior to initiating micropile construction, item 7 as the work 

progresses for each delivery and submittal items 6,8 and 9 at least 7 days prior to start of 

micropile load testing or incorporation of the respective materials into the work. The 

Contractor shall allow the Engineer 7 calendar days to review the construction submittals after 

a complete set has been received. Additional time required due to incomplete or unacceptable 

submittals shall not be cause for delay or impact claims. All costs associated with incomplete 

or unacceptable Contractor submittals shall be the responsibility of the Contractor. 

1 .I 0 Preconstruction Meeting. 

A pre-construction meeting will be scheduled by the Engineer and held prior to the start of 

micropile construction. The Engineer, prime Contractor, micropile specialty Contractor, 

micropile designer, excavation Contractor and geotechnical instrumentation specialist (if 

applicable) shall attend the meeting. Attendance is mandatory. The pre-construction meeting 

will be conducted to clarify the construction requirements for the work, to coordinate the 

construction schedule and activities, and to identify contractual relationships and delineation of 

responsibilities amongst the prime Contractor and the various Subcontractors - specifically 

those pertaining to excavation’for micropile structures, anticipated subsurface conditions, 

micropile installation and testing, micropile structure survey control and site drainage control. 

A-2- 18 FHWA-SA-97-070 (~00-06) 
2.0 MATERIALS. 

Furnish materials new and without defects. Remove defective materials from the jobsite at no 

additional cost. Materials for micropiles shall consist of the following: 

Admixtures for Grout: Admixtures shall conform to the requirements of ASTM C 

494/AASHTO M194. Admixtures that control bleed, improve flowability, reduce water 

content, and retard set may be used in the grout, subject to the review and acceptance of the 

Engineer. Admixtures shall be compatible with the grout and mixed in accordance with the 

manufacturer’s recommendations. Expansive admixtures shall only be added to the grout used 

for filling sealed encapsulations and anchorage covers. Accelerators are not permitted. 

Admixtures containing chlorides are not permitted. 

Cement: All cement shall be Portland cement conforming to ASTM C lSO/AASHTO M85, 

Types II, III or V. 

Centralizers and Spacers: Centralizers and spacers shall be fabricated from schedule 40 PVC 

pipe or tube, steel, or material non-detrimental to the reinforcing steel. Wood shall not be used. 

Centralizers and spacers shall be securely attached to the reinforcement; sized to position the 

reinforcement within 10 mm of plan location from center of pile; sized to allow grout tremie 

pipe insertion to the bottom of the drillhole; and sized to allow grout to freely flow up the 

drillhole and casing and between adjacent reinforcing bars. 

Encapsulation: Encapsulation (double corrosion protection) shall be shop fabricated using 

high-density, corrugated polyethylene tubing conforming to the requirements of ASTM 

D3350/AASHTO M252 with a nominal wall thickness of 0.8 mm. The inside annulus between 

the reinforcing bars and the encapsulating tube shall be a minimum of 5mm and be fully 

grouted with non-shrink grout conforming to Materials Section 2.0. 

Epoxy Coating: The minimum thickness of coating applied electrostatically to the reinforcing 

steel shall be 0.3 mm. Epoxy coating shall be in accordance with ASTM A775 or ASTM 

FHWA-SA-97-070(vO-06) A-2 - 19 
A934. Bend test requirements are waived. Bearing plates and nuts encased in the pile concrete 

footing need not be epoxy coated. 

Fine Aggregate: If sand - cement grout is used, sand shall conform to ASTM C 

144/AASHTO M45. 

Grout: Neat cement or sand/cement mixture with a minimum 3-day compressive strength of 14 

MPa and a 28-day compressive strength of 28 MPa per AASHTO T106/ASTM C109. 

(Commentary: Note to designers/specifiers. A 28-day grout strength of 28 MPa is common for 

micropiles. If the micropile design calls for a higher grout strength, revise the speciJcation 

accordingly.) 

Grout Protection: Provide a minimum 25 mm grout cover over bare or epoxy coated bars 

(excluding bar couplers) or minimum 12 mm grout cover over the encapsulation of 

encapsulated bars. 

Permanent Casing Pipe: Permanent steel casing/pipe shall have the diameter and at least 

minimum wall thickness shown on the approved Working Drawings. The permanent steel 

casing/pipe: 

1. shall meet the Tensile Requirements of ASTM A252, Grade 3, except the yield strength 

shall be a minimum of 345 MPa to 552 MPa as used in the design submittal. 

2. may be new “Structural Grade” (a.k.a. “Mill Secondary” ) steel pipe meeting above but 

without Mill Certification, free from defects (dents, cracks, tears) and with two coupon 

tests per truckload delivered to the fabricator. 

For permanent casing/pipe that will be welded, the following material conditions apply: 

1. the carbon equivalency (CE) as defined in AWS D 1 .I, Section Xl 5.1, shall not exceed 

0.45, as demonstrated by mill certifications 

2. the sulfur content shall not exceed 0.05%, as demonstrated by mill certifications 

For permanent casing/pipe that will be shop or field welded, the following fabrication or 

construction conditions apply: 

A-2 - 20 FHWA-SA-97-070 (~00-06) 
1. the steel pipe shall not be joined by welded lap splicing 

2. welded seams and splices shall be complete penetration welds 

3. partial penetration welds may be restored in conformance with AWS D1.l 

4. the proposed welding procedure certified by a welding specialist shall be submitted for 

approval 

Threaded casing joints shall develop at least the required nominal resistance used in the design 

of the micropile. 

(Commentary: From a practical standpoint, the adequacy ofpipe and reinforcing bar splices 

and threadedjoint connections will be verified by the verification andproof load testing;). 

Plates and Shapes: Structural steel plates and shapes for pile top attachments shall conform to 

ASTM A 36/AASHTO M183, or ASTM A 572/AASHTO M223, Grade 350. 

Reinforcing Bars: Reinforcing steel shall be deformed bars in accordance with ASTM 

A 6 1 S/AASHTO M3 1, Grade 420 or Grade 520 or ASTM A 722/AASHTO M275, Grade 

1035. When a bearing plate and nut are required to be threaded onto the top end of reinforcing 

bars for the pile top to footing anchorage, the threading may be continuous spiral deformed 

ribbing provided by the bar deformations (e.g., Dywidag or Williams continuous threadbars) or 

may be cut into a reinforcing bar. If threads are cut into a reinforcing bar, the next larger bar 

number designation from that shown on the Plans shall be provided, at no additional cost. 

Bar tendon couplers, if required, shall develop the ultimate tensile strength of the bars without 

evidence of any failure. 

Reinforcing Bar Corrosion Protection: 

(Commentary: Corrosion protection requirements vary between Transportation Agencies. The 

most common and simplest tests utilized to measure the aggressiveness of the soil environment 

include electrical resistivity, pH, chloride, and sulfate. Per FHWA-RLI-89-198, the ground is 

considered aggressive tfany one of these indicators show critical values as detailed below: 

FHWA-SA-97-070 (~0-06) A-2-21 
PROPERTY TEST DESIGNATION* CRITICAL VALUES* 

Resistivity AASHTO T-288, ASTM G 57 below 2,000 ohm-cm 

PH 

Sulfate 

AASHTO T-289, ASTM G 5 I 

AASHTO T-290, ASTM D516M, 

ASTMD4327 

below 5 

above 200 ppm 

Chloride AASHTO T-291, ASTh4D512, 

ASTMD4327 

above 100 ppm 

* Specifier should check test standards for latest updates and individual transportation 

agencies may have limits on critical values dyferent than tabulated above. Standard 

spectf?cations or test methods for any of the above items which are common to your 

agency can be referenced in lieu of the above listed AASHTO/ASTMreferences. 

Sheathing: Smooth plastic sheathing, including joints, shall be watertight. Polyvinyl chloride 

(PVC) sheathing shall conform to ASTM D 1784, Class 13464-B. 

Water: Water used in the grout mix shall conform to AASHTO T 26 and shall be potable, 

clean, and free from substances that may be injurious to cement and steel. 

3.0 CONSTRUCTION REQUIREMENTS 

3.1 Site Drainage Control. 

The Contractor shall control and properly dispose of drill flush and construction related waste, 

including excess grout, in accord with the standard specifications and all applicable local codes 

and regulations. Provide positive control and discharge of all surface water that will affect 

construction of the micropile installation. Maintain all pipes or conduits used to control surface 

water during construction. Repair damage caused by surface water at no additional cost. Upon 

substantial completion of the Work, remove surface water control pipes or conduits from the 

site. Alternatively, with the approval of the Engineer, pipes or conduits that are left in place, 

may be fully grouted and abandoned or left in a way that protects the structure and all adjacent 

facilities from migration of fines through the pipe or conduit and potential ground loss. 

A-2 - 22 FHWA-SA-97-070 (~00-06) 
Immediately contact the Engineer if unanticipated existing subsurface drainage structures are 

discovered during excavation or drilling. Suspend work in these areas until remedial measures 

meeting the Engineer’s approval are implemented. Cost of remedial measures or repair work 

resulting from encountering unanticipated subsurface drainage structures, will be paid for as 

Extra Work. 

3.2 Excavation 

Coordinate the work and the excavation so the micropile structures are safely constructed. 

Perform the micropile construction and related excavation in accordance with the Plans and 

approved submittals. No excavations steeper than those specified herein or shown on the Plans 

will be made above or below the micropile structure locations without written approval of the 

Engineer. 

3.3 Micropile Allowable Construction Tolerances 

1. Centerline of piling shall not be more than 75 mm from indicated plan location. 

2. Pile shall be plumb within 2 percent of total-length plan alignment. 

3. Top elevation of pile shall be plus 25 mm or minus 50 mm maximum from 

vertical elevation indicated. 

4. Centerline of reinforcing steel shall not be more than 15 mm from indicated 

location. 

3.4 Micropile Installation 

The micropile Contractor shall select the drilling method, the grouting procedure, and the 

grouting pressure used for the installation of the micropiles. The micropile Contractor shall 

also determine the micropile casing size, final drillhole diameter and bond length, and central 

tendon reinforcement steel sizing necessary to develop the specified load capacities and load 

testing requirements. The micropile Contractor is also responsible for estimating the grout 

take. There will be no extra payment for grout overruns. (Commentary: Note, extra payment 

for grout takes is appropriate for micropiles in Karst. Otherwise, the bidprice of these piles 

will be artiJicially high to cover risk of high grout loss.) 

FHWA-SA-97-070 (~0-06) A-2 - 23 
3.4.1 Drilling 

The drilling equipment and methods shall be suitable for drilling through the conditions to be 

encountered, without causing damage to any overlying or adjacent structures or services. The 

drillhole must be open along it’s full length to at least the design minimum drillhole diameter 

prior to placing grout and reinforcement. (Commentary: When micropile construction will 

occur in close proximity to settlement sensitive structures, recommend including the following 

sentence in the specz$cation - Vibratory pile driving hammers shall not be used to advance 

casing.) 

Temporary casing or other approved method of pile drillhole support will be required in caving 

or unstable ground to permit the pile shaft to be formed to the minimum design drillhole 

diameter. The Contractor’s proposed method(s) to provide drillhole support and to prevent 

detrimental ground movements shall be reviewed by the Engineer. Detrimental ground 

movement is defined as movement which requires remedial repair measures. Use of drilling 

fluid containing bentonite is not allowed. (Commentary: The specification verbage related to 

drillhole support methods and dt&ulty of drilling may vary project to project depending on 

the subsurface conditions revealed by the subsurface investigation data. It is the micropile 

specialty contractor’s responsibility to select the proper drilling equipment and methods for 

the site conditions. It is the owner’s responsibility to provide the available subsurface 

information. For projects with dtflcult ground conditions, use of an “advisory spectjication ” 

included in the contract documents is recommended Refer to Appendix B for an example.) 

Costs of removal or remedial measures due to encountering unanticipated subsurface 

obstructions will be paid for as Extra Work. 

3.4.2 Ground Heave or Subsidence. 

During construction, the Contractor shall observe the conditions vicinity of the micropile 

construction site on a daily basis for signs of ground heave or subsidence. Immediately notify 

the Engineer if signs of movements are observed. Contractor shall immediately suspend or 

modify drilling or grouting operations if ground heave or subsidence is observed, if the 

A-2 - 24 FHWA-SA-97-070 (~00-06) 
micropile structure is adversely affected, or if adjacent structures are damaged from the drilling 

or grouting. If the Engineer determines that the movements require corrective action, the 

Contractor shall take corrective actions necessary to stop the movement or perform repairs. 

When due to the Contractor’s methods or operations or failure to follow the specified/approved 

construction sequence, as determined by the Engineer, the costs of providing corrective actions 

will be borne by the Contractor. When due to differing site conditions, as determined by the 

Engineer, the costs of providing corrective actions will be paid as Extra Work. 

3.4.3 Pipe Casing and Reinforcing Bars Placement and Splicing. 

Reinforcement may be placed either prior to grouting or placed into the grout - filled drillhole 

before temporary casing (if used) is withdrawn. Reinforcement surface shall be free of 

deleterious substances such as soil, mud, grease or oil that might contaminate the grout or coat 

the reinforcement and impair bond. Pile cages and reinforcement groups, if used, shall be 

sufficiently robust to withstand the installation and grouting process and the withdrawal of the 

drill casings without damage or disturbance. 

The Contractor shall check pile top elevations and adjust all installed micropiles to the planned 

elevations. 

Centralizers and spacers (if used) shall be provided at 3-m centers maximum spacing. The 

upper and lower most centralizer shall be located a maximum of 1.5 m from the top and bottom 

of the micropile. Centralizers and spacers shall permit the free flow of grout without 

misalignment of the reinforcing bar(s) and permanent casing. The central reinforcement bars 

with centralizers shall be lowered into the stabilized drill hole and set. The reinforcing steel 

shall be inserted into the drill hole to the desired depth without difficulty. Partially inserted 

reinforcing bars shall not be driven or forced into the hole. Contractor shall redrill and reinsert 

reinforcing steel when necessary to facilitate insertion. 

Lengths of casing and reinforcing bars to be spliced shall be secured in proper alignment and in 

a manner to avoid eccentricity or angle between the axes of the two lengths to be spliced. 

Splices and threaded joints shall meet the requirements of Materials Section 2.0. Threaded pipe 

FHWA-SA-97-070 (~0-06) A-2 - 25 
casing joints shall be located at least two casing diameters (OD) from a splice in any 

reinforcing bar. When multiple bars are used, bar splices shall be staggered at least 0.3 meters. 

3.4.4 Grouting. 

Micropiles shall be primary grouted the same day the load transfer bond length is drilled. The 

Contractor shall use a stable neat cement grout or a sand cement grout with a minimum 2%day 

unconfined compressive strength of 28 MPa. Admixtures, if used, shall be mixed in 

accordance with manufacturer’s recommendations. The grouting equipment used shall produce 

a grout free of lumps and undispersed cement. The Contractor shall have means and methods 

of measuring the grout quantity and pumping pressure during the grouting operations. The 

grout pump shall be equipped with a pressure gauge to monitor grout pressures. A second 

pressure gauge shall be placed at the point of injection into the pile top. The pressure gauges 

shall be capable of measuring pressures of at least 1 MPa or twice the actual grout pressures 

used, whichever is greater. The grout shall be kept in agitation prior to mixing. Grout shall be 

placed within one hour of mixing. The grouting equipment shall be sized to enable each pile to 

be grouted in one continuous operation. The grout shall be injected from the lowest point of the 

drill hole and injection shall continue until uncontaminated grout flows from the top of the pile. 

The grout may be pumped through grout tubes, casing, hollow-stem augers, or drill rods. 

Temporary casing, if used, shall be extracted in stages ensuring that, after each length of casing 

is removed the grout level is brought back up to the ground level before the next length is 

removed. The tremie pipe or casing shall always extend below the level of the existing grout in 

the drillhole. The grout pressures and grout takes shall be controlled to prevent excessive 

heave or fracturing of rock or soil formations. Upon completion of grouting, the grout tube 

may remain in the hole, but must be filled with grout. 

If the Contractor elects to use a postgrouting system, Working Drawings and details shall be 

submitted to the Engineer for review in accordance with Section 1.8, Pre-installation 

Submittals. 

A-2 - 26 FHWA-SA-97-070 (~00-06) 
3.4.5 Grout Testing 

Grout within the micropile verification and proof test piles shall attain the minimum required 

3-day compressive strength of 14 MPa prior to load testing. Previous test results for the 

proposed grout mix completed within one year of the start of work may be submitted for initial 

verification of the required compressive strengths for installation of pre-production verification 

test piles and initial production piles. During production, micropile grout shall be tested by the 

Contractor for compressive strength in accordance with AASHTO T106/ASTM Cl09 at a 

frequency of no less than one set of three 50-mm grout cubes from each grout plant each day of 

operation or per every 10 piles, whichever occurs more frequently. The compressive strength 

shall be the average of the 3 cubes tested. 

Grout consistency as measured by grout density shall be determined by the Contractor per 

ASTM C 188/AASHTO T 133 or API RP-13B-1 at a frequency of at least one test per pile, 

conducted just prior to start of pile grouting. The Baroid Mud Balance used in accordance with 

API RP-13B- 1 is an approved device for determining the grout density of neat cement grout. 

The measured grout density shall be between kg/m3 and kgIm3. 

Grout samples shall be taken directly from the grout plant. Provide grout cube compressive 

strength and grout density test results to the Engineer within 24 hours of testing. 

(Commentary: If the Engineer will perform the grout testing, revise this section accordingly). 

3.5 Micropile Installation Records. 

Contractor shall prepare and submit to the Engineer full-length installation records for each 

micropile installed. The records shall be submitted within one work shift after that pile 

installation is completed. The data shall be recorded on the micropile installation log included 

at the end of this specification. A separate log shall be provided for each micropile. 

(Commentary: In addition to the expertise of the micropiie speciahy Contractor, the quality of 

the individual construction elements is directly related to thefinalproduct overall quality. As 

with other drilledpile systems, the actual load carrying capacity of a micropile can only be 

dejinitivelyproven by pile load tests. It is not practical or economical to test every pile 

FHWA-SA-97-070 (~0-06) A-2 - 27 
installed. Therefore, aggressive inspection by the Contractor and Owner’s Engineer is needed 

to assure that each individual micropile is well constructed and to justify, load testing only a 

small number, e.g, S%, of the total number ofproduction piles installed) 

3.6 Pile Load Tests 

Perform verification and proof testing of piles at the locations specified herein or designated 

by the Engineer. Perform compression load testing in accord with ASTM D 1143 and tension 

load testing in accord with ASTM D3689, except as modified herein. 

(Commentary on number of load tests: Specijieridesigner need to determine and write into 

this portion of the spectjication the number and location of required verification andproof 

tests. The total number of load tests and maximum test loads to be specijied can vary on a 

project-by-project basis. They are dependent on ground type and variability, requiredpile 

capacity, pile loading type (i.e., static or seismic), total number ofpiles, criticality of the 

structure and available site access and work space. Guideline criteria for estimating the total 

number of vertjication andproof test piles are given in Chapter 7. For structure foundations, 

the following is recommended as a minimum. Perform verification testing of at least one 

sacrificial test pile per structure, prior to installation of any production piles. New users 

should perform proof tests on production piles at a frequency of 5 percent (I in 20). For 

experienced users, number of tests are to be determined by Owner/Engineer on a project by 

project basis. Ifpile capacity demands are greatest in compression, the piles should be load 

tested in compression. If the pile capacity demands are equal for both compression and 

tension, or greater in tension, it is recommended that tension testing alone be conducted to 

reduce costs). 

(Commentary: Specifier - Indicate here whether compression or tension testing, or both are 

required for your project). 

A-2 - 28 FHWA-SA-97-070 (~00-06) 
3.6.1 Verification Load Tests 

Perform pre-production verification pile load testing to verify the design of the pile system and 

the construction methods proposed prior to installing any production piles. 

sacrificial verification test piles shall be constructed in conformance with the approved 

Working Drawings. Verification test pile(s) shall be installed at the following locations 

Verification load tests shall be performed to verify that the Contractor installed micropiles will 

meet the required compression and tension load capacities and load test acceptance criteria 

and to verify that the length of the micropile load transfer bond zone is adequate, The 

micropile verification load test results must verify the Contractor’s design and installation 

methods, and be reviewed and accepted by the Engineer prior to beginning installation of 

production micropiles. 

The drilling-and-grouting method, casing length and outside diameter, reinforcing bar lengths, 

and depth of embedment for the verification test pile(s) shall be identical to those specified for 

the production piles at the given locations. The verification test micropile structural steel 

sections shall be sized to safely resist the maximum test load. (Commentary: Note that if 

additional steel area is provided in the verification test, the measured deflection will be lower 

than production piles.) 

The maximum verification and proof test loads applied to the micropile shall not exceed 80 

percent of the structural capacity of the micropile structural elements, to include steel yield in 

tension, steel yield or buckling in compression, or grout crushing in compression. Any 

required increase in strength of the verification test pile elements above the strength required 

for the production piles shall be provided for in the contractor’s bid price. 

The jack shall be positioned at the beginning of the test such that unloading and repositioning 

during the test will not be required. When both compression and tension load testing is to be 

performed on the same pile, the pile shall be tested under compression loads prior to testing 

under tension loads. 

FHWA-SA-97-070 (~0-06) A-2 - 29 

- 
3.6.2 Testing Equipment and Data Recording. 

Testing equipment shall include dial gauges, dial gauge support, jack and pressure gauge, 

electronic load cell, and a reaction frame. The load cell is required only for the creep test 

portion of the verification test. (Commentary: The purpose and value of an electronic load 

cell is to measure small changes in loadfor load tests where the load is heldfor a long 

duration, such as during verification or creep testing. It is not intended to be used during 

proof testing, including the short term creep portion. Experience has proven that load cells 

have been problematic under field conditions, yet even with errors resultingj-om cell 

construction, off-center loading, and other eflects, a load cell is very sensitive to small changes 

in load and is strongly recommendedfor creep testing.) The contractor shall provide a 

description of test setup and jack , pressure gauge and load cell calibration curves in 

accordance with the Submittals Section. 

Design the testing reaction frame to be sufficiently rigid and of adequate dimensions such that 

excessive deformation of the testing equipment does not occur. Align the jack, bearing plates, 

and stressing anchorage such that unloading and repositioning of the equipment will not be 

required during the test. 

Apply and measure the test load with a hydraulic jack and pressure gauge. The pressure guage 

shall be graduated in 500 kPa increments or less. The jack and pressure gauge shall have a 

pressure range not exceeding twice the anticipated maximum test pressure. Jack ram travel 

shall be sufficient to allow the test to be done without resetting the equipment. Monitor the 

creep test load hold during verification tests with both the pressure gauge and the electronic 

load cell. Use the load cell to accurately maintain a constant load hold during the creep test 

load hold increment of the verification test. 

Measure the pile top movement with a dial gauge capable of measuring to 0.025 mm. The dial 

gauge shall have a travel sufficient to allow the test to be done without having to reset the 

gauge. Visually align the gauge to be parallel with the axis of the micropile and support the 

gauge independently from the jack, pile or reaction frame. Use a minimum of two dial gauges 

A-2 - 30 FHWA-SA-97-070 (~00-06) 
when the test setup requires reaction against the ground or single reaction piles on each side of 

the test pile. (Commentary: Experience with testingpiles reacting against the ground, or 

against single reaction piles on each side of the test pile, has resulted in racking and 

misalignment of the system on some projects. Two diaiguages are recommendedfor this test 

setup to determine if racking is occurring and to provide a more accurate average micropile 

head movement measurement). 

The required load test data shall be recorded by the Engineer. 

3.6.3 Verification Test Loading Schedule. 

Test verification piles designated for compression or tension load testing to a maximum test 

load of 2.5 times the micropile Design Load shown on the Plans or Working Drawings. 

(Commentary: See Section LE.4 for more detailed veriJcation load testing information.) The 

verification pile load tests shall be made by incrementally loading the micropile in accordance 

with the following cyclic load schedule for both compression and tension loading: 

r - 

- 

1 

2 

3 

- 

4 

5 

6 

7 

- 

8 

9 

10 

11 

12 

- 

AL = Alignment Load 

LOAD 

AL (0.05 DL) 

0.25 DL 

0.50 DL 1 minute 

AL 1 minute 

0.25 DL 1 minute 

0.50 DL 1 minute 

0.75 DL 1 minute 

AL 1 minute 

0.25 DL 1 minute 

0.50 DL 1 minute 

0.75 DL 1 minute 

1.00 DL 1 minute 

DL = Design Load 

HOLD TIME 

1 minute 

1 minute 

1 

FHWA-SA-97-070 (~0-06) A-2-31 
13 

14 

15 

16 

17 

18 

19 

20 

21 

22 

23 

AL = Alignment Load 

LOAD 

AL 

0.25 DL 

0.50 DL 

0.75 DL 

1.00 DL 

1.33 DL 

1.75 DL 

2.00 DL 

2.25 DL 

2.50 DL 

(Maximum Test Load) 

AL 

DL = Design Load 

HOLD TIME 

1 minute 

1 minute 

1 minute 

1 minute 

1 minute 

60 minutes 

(Creep Test Load Hold) 

1 minute 

1 minute 

1 minute 

10 minutes 

1 minute 

The test load shall be applied in increments of 25 percent of the DL load. Each load increment 

shall be held for a minimum of 1 minute. Pile top movement shall be measured at each load 

increment. The load-hold period shall start as soon as each test load increment is applied. The 

verification test pile shall be monitored for creep at the 1.33 Design Load (DL). Pile 

movement during the creep test shall be measured and recorded at 1,2,3,4,5,6, 10,20,30, 

50, and 60 minutes. The alignment load shall not exceed 5 percent of the DL load. Dial gauges 

shall be reset to zero after the initial AL is applied. 

The acceptance criteria for micropile verification load tests are: 

1. The pile shall sustain the first compression or tension 1 .O DL test load with no more than 

mm total vertical movement at the top of the pile, relative to the position of the 

top of the pile prior to testing. (Commentary: Structural designer to determine 

maximum allowable total pile top structural axial displacement at I. 0 DL test load 

based on structural design requirements. Also, ifthe ve@cation test pile has to be 

upsized structurally to accommodate the maximum required verfxation test load, this 

A-2 - 32 FHWA-SA-97-070 (~00-06) 
provision will not apply. Only the proof tested production piles will then be subject to 

this criteria. Refer to Chapter 5 for more design guidance). 

2. At the end of the 1.33 DL creep test load increment, test piles shall have a creep rate not 

exceeding 1 mm/log cycle time (1 to 10 minutes) or 2 mm/log cycle time (6 to 60 

minutes or the last log cycle if held longer).The creep rate shall be linear or decreasing 

throughout the creep load hold period. 

3. Failure does not occur at the 2.5 DL maximum test load. Failure is defined as load at 

which attempts to further increase the test load simply result in continued pile 

movement. 

The Engineer will provide the Contractor written confirmation of the micropile design and 

construction within 3 working days of the completion of the verification load tests. This 

written confirmation will either confirm the capacities and bond lengths specified in the 

Working Drawings for micropiles or reject the piles based upon the verification test results. 

3.6.4 Verification Test Pile Rejection 

If a verification tested micropile fails to meet the acceptance criteria, the Contractor shall 

modify the design, the construction procedure, or both.These modifications may include 

modifying the installation methods, increasing the bond length, or changing the micropile type. 

Any modification that necessitates changes to the structure shall require the Engineer’s prior 

review and acceptance. Any modifications of design or construction procedures or cost of 

additional verification test piles and load testing shall be at the Contractor’s expense. At the 

completion of verification testing, test piles shall be removed down to the elevation specified 

by the Engineer. 

FHWA-SA-97-070 (~0-06) A-2 - 33 
3.6.5 Proof Load Tests 

Perform proof load tests on the first set of production piles installed at each designated 

substructure unit prior to the installation of the remaining production piles in that unit. The 

first set of production piles is the number required to provide the required reaction capacity for 

the proof tested pile. The initial proof test piles shall be installed at the following substructure 

units . Proof testing shall be conducted at a frequency of 5% (1 in 20) of the 

subsequent production piles installed, beyond the first 20, in each abutment and pier. Location 

of additional proof test piles shall be as designated by the Engineer. (Cammentaly: The above 

is a guideline for new users. Experienced users may go with a lesser number ofproof load 

tests as determined by the Owner/Engineer,) 

3.6.6 Proof Test Loading Schedule 

Test piles designated for compression or tension proof load testing to a maximum test load of 

1.67 times the micropile Design Load shown on the Plans or Working Drawings. 

(Commentary: See Section 5.E.4 for more detailedproof load testing information.) Proof tests 

shall be made by incrementally loading the micropile in accordance with the following 

schedule, to be used for both compression and tension loading: 

AL = Alignment Load DL = Design Load 

LOAD HOLD TJME 

1 AL 1 minute 

2 0.25 DL 1 minute 

3 0.50 DL 1 minute 

4’ 0.75 DL 1 minute 

5 1.00 DL 1 minute 

6 1.33 DL 10 or 60 minute 

Creep Test 

7 1.67 DL 1 minute 

(Maximum Test Load) 

8 AL 1 minute 

A-2 - 34 FHWA-SA-97-070 (~00-06) 
Depending on performance, either a 10 minute or 60 minute creep test shall be performed at the 

1.33 DL Test Load. Where the pile top movement between 1 and 10 minutes exceeds 1 mm, 

the Maximum Test Load shall be maintained an additional 50 minutes. Movements shall be 

recorded at 1,2,3,5,6, 10,20,30,50 and 60 minutes. The alignment load shall not exceed 5 

percent of DL. Dial guages shall be reset to zero after the ititial AL is applied. 

The acceptance criteria for micropile proof load tests are: 

1. The pile shall sustain the compression or tension 1 .O DL test load with no more than 

mm total vertical movement at the top of the pile, relative to the position of the 

top of the pile prior to testing. (Commentary: Structural designer to determine 

maximum allowable total pile top structural axial displacement at the 1.0 DL test load 

based on structure design requirements. Refer to Chapter 5 for more design guidance.) 

2. At the end of the 1.33 DL creep test load increment, test piles shall have a creep rate not 

exceeding 1 mm/log cycle time (1 to 10 minutes) or 2 mm/log cycle time (6 to 60 

minutes).The creep rate shall be linear or decreasing throughout the creep load hold 

period. 

3. Failure does not occur at the 1.67 DL maximum test load. Failure is defined as the load 

at which attempts to further increase the test load simply result in continued pile 

movement. 

3.6.7 Proof Test Pile Rejection 

If a proof-tested micropile fails to meet the acceptance criteria, the Contractor shall 

immediately proof test another micropile within that footing. For failed piles and further 

construction of other piles, the Contractor shall modify the design, the construction procedure, 

or both. These modifications may include installing replacement micropiles, incorporating piles 

at not more than 50% of the maximum load attained, postgrouting, modifying installation 

methods, increasing the bond length, or changing the micropile type. Any modification that 

necessitates changes to the structure design shall require the Engineer’s prior review and 

acceptance. Any modifications of design or construction procedures, or cost of additional 

verification test piles and verification and/or proof load testing, or replacement production 

micropiles, shall be at the Contractor’s expense. 

FHWA-SA-97-070 (~0-06) A-2 - 35 

_--..-. 
4.0 METHOD OF MEASUREMENT. 

Measurement will be made as follows for the quantity, as specified or directed by the Engineer: 

l Mobilization will be measured on a lump-sum basis. 

l Micropiles will be measured on a lump-sum basis. 

l Micropile verification load testing will be measured on a lump-sum basis. 

l Micropile proof load testing will be measured on a lump-sum basis. 

The final pay quantities will be the design quantity increased or decreased by any changes 

authorized by the Engineer. 

5.0 BASIS OF PAYMENT 

The quantities accepted for payment will be paid for at the contract unit prices for the following 

items: 

Mobilization and Demobilization . . . . . . . . . . . . . . Lump sum 

Micropile Load Tests . . . . . . . . . . . . . . . . . . . . . . . Lump sum 

Micropiles and Footings . . . . . . . . . . . . . . . . . . . . Lump sum* 

Micropiles Variations in Length to Top of Rock . . LF** 

Unexpected Obstruction Drilling . . . . . . . . . . . . . . Hour*** 

*For the option where the contractor designs the footing and number of piles, the foundation 

system should be bid as lump sum and a schedule of values established for progress payments 

after award. 

**Where piles are founded in rock, micropiles will be paid on a per each basis assuming Rock at 

Elevation . Additional length or shorter length due to variations in the top of rock will be 

paid on a add or deduct lineal foot basis where the linear footage = Elevation _ minus 

Elevation of As-Built Rock. 

***If “obstructions” are not defined in the Standard Specifications, a definition should be added. 

A-2 - 36 FHWA-SA-97-070 (~00-06) 
The contract unit prices for the above items will be full and complete payment for providing all 

design, materials, labor, equipment, and incidentals to complete the work. 

The unit contract amount for “Micropiles” shall include the drilling, furnishing, and placing the 

reinforcing steel and casing, grouting, and pile top attachments. The micropile Contractor is 

also responsible for estimating the grout take. There will be no extra payment for grout 

overruns. 

FHWA-SA-97-070 (~0-06) A-2 - 37 
ABUTMENT PLAN VIEW 

NOT TO SCALE 

ABUTMENT SECTION 

NOT TO SCALE 

BORING 

I LOAD DATA (SLD EXAMPLE) I 

J, ;:: I 1 ’ . . 

I ’ & 

O.&% 

20” _I 

~cggt&mJM ‘[ 

DENSE TO VERY DENSE SANDY 26.om 

GRAVELAND COBBLES W,TH 

BOULDERS UP TO I,,, ,GW, 

TYPICAL SECTION 

NOT TO SCALE 

COMMENTARY: THIS IS THE MINIMUM AMOUNT OF Cl 

DESIGN CRITERIA 

DESIGN: 199B AASHTO STANDARD SPECIFICATIONS, 

16lli EDlTlON 

REINFORCED CONCRETE: Fy = 520 MPa 

f; = 27.6 MPa 

STRUCTURAL STEEL MICROPILES: f, = 34.5 TO 55.2 MPa 

PILE TOP AITACHMENTS: f, - 24.9 TO 34.5 MPa 

NOTES 

1. PILE DESIGN AND LAYOUT TO BE DESIGNED BY MICROPILE 

CONTRACTOR IN ACCORDANCE WITH PROJECT 

SPECIFICATIONS AND DESIGN CRITEFUA & LOAD DATA ON 

THIS SHEET. 

2. FOOTING REINFORCEMENT HAS BEEN DESIGNED FOR A 

FACTORED DESIGN LOAD OF __ kN!m. ANY ADDITIONAL 

FOOTING REINFORCEMENT REQUIRED BY CON-fFzACTOR 

DESIGNED PILE LAYOUT SHALL BE SUBMIlTED FOR 

APPROVAL. 

3. MICROPILE BEARING PLATE SHALL EFFECTIVELY 

DlSTRlBLJTE THE DEBIGN FORCE TO THE FOOTING 

CONCRETE SUCH THAT THE CONE SHEAR REQUIREMENTS 

OF ACI 349. APPENDIX 8. ARE MET AND THE BENDING 

STRESS DOES NOT EXCEED OSSf, FOR STEEL. 

4. PILE TO FOOTlNG CONNECTlON TO BE DESIGNED BY 

MICROPILE CONTRACTOR. 

5. SEE PROJECT SPECIFICATIONS FOR SOILS INFORMATION 

CONTRACTOR DESIGN/BUILD 

OF FOUNDATIONS 

(MICROPILES AND FOOTINGS) 

I SAMPLE CONTRACT PLANS 
Micropile Installation Log 

I Project Name: 

Pile Designation # ( I Time @ II 

Installation Date Start of Drilling I II 

Drill Rig/Drill Method 

Drill Rig/ #, Operator 

Start of Grouting 

Pile Completion 

Grout Plant #, Operator Total Duration 

Drill Bit Type and Size 

Casing Dia./Wall Thickness 

Cement Type 

Admixtures 

II Pile Inclination 

Reinforcement Size/Length 1 

II Pile Length Above B.O.F. I ( Tremie Grout Quantity (bags) I II 

1 Upper Cased Length Pressure Grout Quantity (bags) 

Cased and Bond Length (Plunge) 

Bond Length Below Casing 

Total Pile Length 

Grouting after plunge (bags) 

Total Grout Quantity (bags) 

Grout Ratio (bags/m bond) 

Comments - Pile Drilling 

Depth from B.0.F 

(4 

Soil I Rock 

Description 

Flush Description Comments 

Comments - Pile Grouting 

B.O.F. = Bottom of Footing (ref. FHWA-SA-97-070) 

FHWA-SA-97-070 (~0-06) A-2 - 39 
[BLANK PAGE] 

A-2-40 FHWA-SA-97-070 (~00-06) 
1. 

2. 

Washington State DOT 

Guide for Preparation of a 

“Summary of Geotechnical Conditions” 

Describe subsurface conditions in plain English. Avoid use of geologic jargon and/or 

nomenclature which the contractors (and their lawyers) will not understand. Identify 

depths/thicknesses of the-soil or rock layers and their moisture state and density 

condition. Identify the depth/elevation of groundwater and state its nature (e.g. perched, 

regional, artesian, etc.). If referring to an anomalous soil, rock or groundwater condition, 

refer to boring log designation where the anomaly was encountered. 

For each structure, if necessary, state the impact the soil, rock or groundwater condition 

may (will) have on construction. Where feasible, refer to the boring log(s) or data which 

provides the indication of the risk. Be sure to mention the potential of risk for: 

Caving ground. 

Slope instability due to temporary excavation, or as a result of a project element 

(e.g. buttress, tieback wall, soil nail cuts). 

Groundwater flow and control, if anticipated, in construction excavations or drill 

holes. 

Dense layers (may inhibit pile driving, shaft or tunnel excavation, drilling for 

nails, micropiles, dowels or anchors). 

Obstructions, including cobbles or boulders, if applicable. 

FHWA-SA-97-070 (~00-06) B-l 
3. Where design assumptions and parameters can be affected by the manner in which the 

structure is built, or if the assumptions or parameters can impact the contractor’s 

construction methods, draw attention to these issues. This may include: 

Soil or rock strengths (e.g. point load tests, RQD, UCS, UU, CU tests, etc.). 

Whether shafts or piles are predominantly friction or end bearing by design. 

The reasons for minimum tip elevations or bond lengths specified in the contract. 

Downdrag loads and the effect on design/construction. 

If certain construction methods are required or prohibited, state the (geotechnical). 

reason for the requirement. 

Liquefaction potential and impact on design/construction. 

List of geotechnical reports or information. This should include the project 

specific report and memoranda (copies available at the Project office) as well as 

pertinent reports which may be historical or regional in nature. 

4. List of geotechnical reports or information. This should include the project specific 

report and memoranda (copies available at the Project office) as well as pertinent reports 

which may be historical or regional in nature. 

5. The intent of the Summary is to inform the contractor of what we, as the geotechnical 

designers, know or strongly suspect about the subsurface conditions. The Summary 

should be brief (1 or 2 pages maximum). It should not include tabulations of all 

available data (e.g. boring logs, lab tests, etc.). Only that data which is pertinent to the 

adverse construction condition you are anticipating should be mentioned. It should not 

include sections or commentary about structures or project elements about which we 

have no real concerns. 

B-2 FHWA-SA-97-070 (~00-06) 
EXAMPLE 

SUMMARY OF GEOTECHNICAL CONDITIONS 

Micropile Manual-Sample Problem No. 1 

Site and Subsurface Conditions 

The soils at the site consist of loose to very dense sandy gravel with cobbles and boulders. The 

soil is highly permeable in nature. Boulders up to 1 meter in diameter or larger are likely to be 

encountered frequently. Augers were used to drill the test borings in the top 4.5 meters. Below 

4.5 meters coring was required to drill through the very dense bouldery soil. Ground water 

throughout the site is near the river level and will follow river level fluctuations. 

Potential Impact of Site Conditions on Foundation Construction 

It is anticipated that the bottom of the footing excavation will be located at or above the 

groundwater level, provided that the footing is constructed during the summer or fall when the 

river level is relatively low. Due to the coarse, clean nature of the soils at the footing 

foundation level, if groundwater is encountered (as would be the case during periods of high 

water in the river), it will be difficult to keep the footing excavation dewatered, as water flow 

rates through the soil will be high. 

The boulder-y conditions at the site will have a impact on the construction of the micropiles at 

Abutments 1 and 2. The contractor should be aware that these conditions will reduce the rate at 

which the contractor can construct the micropiles. Cobbles and boulders should be expected 

throughout the soil mass starting about 4 meter depth below the abutment bottom of footing 

elevations, therefore rock drilling equipment will be required. Considering the presence of 

ground water and the lack of soil cohesion, the soil will quickly cave without support. Drilling 

FHWA-SA-97-070 (~00-06) B-3 
slurry is not likely to be effective due to the gravelly nature of the soil, and therefore, full depth 

casing is required during drilling. Permanent casing is to be left in place into the top of the 

grout bond zone, as shown on the plans. 

Due to the highly permeable nature of the gravel, grout overrun beyond the theoretical drill 

hole quantity should be expected. The contractor is responsible for estimating grout quantity. 

There will be no extra payment for grout overruns. 

Available Geotechnical Reports 

The following geotechnical report contains design and construction information relevant to the 

project and is available at the Project Engineer’s office: 

Cook, Kerry B., Chief Geotech Engineer (and Ace Fisherman), “Sample ProbZem No. I Bridge 

Replacement Project “, FHWA-WFLHD Geotechnical Report No. 00, January 1996. 

B-4 FHWA-SA-97-070 (~00-06)