Technical Report Documentation Page

1. Report No.

FHWA-SA-99-015

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

4. Title and Subtitle

GEOTECHNICAL ENGINEERING CIRCULAR NO. 4

Ground Anchors and Anchored Systems

4. Report Date

June 1999

6. Performing Organization Code:

7. Author(s)

P.J. Sabatini, D.G. Pass, R.C. Bachus

8. Performing Organization Report No.

9. Performing Organization Name and Address

GeoSyntec Consultants

1100 Lake Hearn Drive

Atlanta, Georgia

10. Work Unit No.(TRAIS)

11. Contract or Grant No.

DTFH61-94-C-00099

12. Sponsoring Agency Name and Address

Office of Bridge Technology

Federal Highway Administration

HIBT, Room 3203

400 Seventh Street, S.W. 

Washington D.C. 20590

13 Type of Report and Period Covered

Technical Manual

14. Sponsoring Agency Code

15. Supplementary Notes

Contracting Officer’s Technical Representative: Chien-Tan Chang (HIBT)

FHWA Technical Consultants: Jerry DiMaggio (HIBT), Richard Cheney (HIBT)

16. Abstract:

This document presents state-of-the-practice information on the design and installation of cement-grouted

ground anchors and anchored systems for highway applications. The anchored systems discussed include

flexible anchored walls, slopes supported using ground anchors, landslide stabilization systems, and

structures that incorporate tiedown anchors.

This document draws extensively from the FHWA-DP-68-IR (1988) design manual in describing issues

such as subsurface investigation and laboratory testing, basic anchoring principles, ground anchor load

testing, and inspection of construction materials and methods used for anchored systems. This document

provides detailed information on design analyses for ground anchored systems. Topics discussed include

selection of design earth pressures, ground anchor design, design of corrosion protection system for ground

anchors, design of wall components to resist lateral and vertical loads, evaluation of overall anchored

system stability, and seismic design of anchored systems. Also included in the document are two detailed

design examples and technical specifications for ground anchors and for anchored walls.

17. Key Words

Ground anchors, soldier beam and lagging walls,

limit equilibrium, earth pressures, axial capacity,

tiedowns, seismic design, contracting,

specifications

18. Distribution Statement

No Restrictions. This document is available to the

public from the National Technical Information

Service, Springfield, Virginia 22161

19. Security Classif. (of this report)

Unclassified

20. Security Classification (of this page)

Unclassified

21. No. of

Pages

281

22. Price

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

ACKNOWLEDGEMENTS

The authors would like to express their appreciation to Mr. Richard S. Cheney, P.E., of the U.S.

Department of Transportation Federal Highway Administration (FHWA) for providing significant technical

assistance and review during preparation of the document. The authors would also like to thank Dr. Donald

A. Bruce, C.Eng of ECO GeoSystems Inc. for providing technical assistance. Dr. Richard Jewell, C.Eng,

formerly of GeoSyntec Consultants, assisted in preparing sections of the document and provided technical

assistance. The authors would also like to thank the following individuals who reviewed the document and

served on the Technical Working Group for this project:

• James J. Brennan, P.E. – Kansas Department of Transportation;

• Chien-Tan Chang, P.E. – FHWA;

• Joel Moskowitz, P.E., of Mueser Rutledge Consulting Engineers – Deep Foundations Institute;

• Heinz Nierlich of Dywidag-Systems International – Post Tensioning Institute;

• Monti Singla - FHWA;

• John P. Tiernan – Georgia Department of Transportation;

• David E. Weatherby of Schnabel Foundation Company – International Association of Foundation

Drilling; and

• Shan-Tai Yeh – Colorado Department of Transportation.

The authors would also like to acknowledge the following firms and agencies that provided photographs and

technical information:

• Dywidag-Systems International;

• Hayward Baker Inc.;

• Lang Tendons Inc.;

• Schnabel Foundation Company; and

• Williams Form Engineering Corp.

Finally, the authors would like to thank Mrs. Ann Taylor and Mr. Michael Harris of GeoSyntec Consultants

who drafted the figures and assisted in the layout of the document.
ii

PREFACE

This document presents state-of-the-practice information on the design and installation of cementgrouted ground anchors and anchored systems for highway applications. Anchored systems

discussed include flexible anchored walls, slopes supported using ground anchors, landslide

stabilization systems, and structures that incorporate tiedown anchors.

This document has been written, in part, to update the design manual titled "Permanent Ground

Anchors" (FHWA-DP-68-1R, 1988). This document draws extensively from the FHWA (1988)

design manual in describing issues such as subsurface investigation and laboratory testing, basic

anchoring principles, ground anchor load testing, and inspection of construction materials and

methods used for anchored systems. Since 1988, advances have been made in design methods and

from new construction materials, methods, and equipment.

Results of anchored system performance monitoring and research activities conducted since 1989 are

also included in this document. Most recently, research was conducted under a FHWA research

contract on the design and performance of ground anchors and anchored soldier beam and timber

lagging walls. As part of that research project, performance data on model- and full-scale anchored

walls were collected and analyzed. Several of the analysis methods and design procedures that were

recommended based on the results of the research are adopted herein. This research is described in

FHWA-RD-98-065 (1998), FHWA-RD-98-066 (1998), FHWA-RD-98-067 (1998), and FHWA-RD97-130 (1998).

This document provides detailed information on basic principles and design analyses for ground

anchors and anchored systems. Topics discussed include selection of design earth pressures, design

of corrosion protection systems for ground anchors, design of wall components to resist lateral and

vertical loads, evaluation of overall anchored system stability, and seismic design of anchored

systems. Also included in the document are two detailed design examples and technical

specifications for ground anchors and for anchored walls.
iii

TABLE OF CONTENTS

CHAPTER 1 INTRODUCTION......................................................................................................1

1.1 Purpose...................................................................................................................................1

1.2 Anchored System Service Life ..............................................................................................1

1.3 Background ............................................................................................................................2

CHAPTER 2 GROUND ANCHORS AND ANCHORED SYSTEMS..........................................4

2.1 INTRODUCTION ....................................................................................................................4

2.2 GROUND ANCHORS.............................................................................................................4

2.2.1 General.........................................................................................................................4

2.2.2 Types of Ground Anchors.............................................................................................6

2.2.2.1 General.............................................................................................................6

2.2.2.2 Straight Shaft Gravity-Grouted Ground Anchors................................................7

2.2.2.3 Straight Shaft Pressure-Grouted Ground Anchors..............................................8

2.2.2.4 Post-grouted Ground Anchors...........................................................................8

2.2.2.5 Underreamed Anchors......................................................................................8

2.2.3 Tendon Materials..........................................................................................................8

2.2.3.1 Steel Bar and Strand Tendons...........................................................................8

2.2.3.2 Spacers and Centralizers...................................................................................9

2.2.3.3 Epoxy-Coated Bar and Epoxy-Coated Filled Strand .......................................10

2.2.3.4 Other Anchor Types and Tendon Materials .....................................................10

2.2.4 Cement Grout .............................................................................................................11

2.3 ANCHORED WALLS............................................................................................................11

2.3.1 General.......................................................................................................................11

2.3.2 Soldier Beam and Lagging Wall...................................................................................12

2.3.2.1 General...........................................................................................................12

2.3.2.2 Soldier Beam..................................................................................................13

2.3.2.3 Lagging...........................................................................................................14

2.3.2.4 Construction Sequence....................................................................................15

2.3.3 Continuous Walls........................................................................................................15
TABLE OF CONTENTS (Continued)

iv

2.4 APPLICATIONS OF GROUND ANCHORS.......................................................................16

2.4.1 Highway Retaining Walls.............................................................................................16

2.4.2 Slope and Landslide Stabilization.................................................................................17

2.4.3 Tiedown Structures.....................................................................................................17

CHAPTER 3 SITE INVESTIGATION AND TESTING..............................................................19

3.1 INTRODUCTION ..................................................................................................................19

3.2 FIELD RECONNAISSANCE................................................................................................19

3.3 SUBSURFACE INVESTIGATION.......................................................................................20

3.3.1 General.......................................................................................................................20

3.3.2 Soil and Rock Stratigraphy..........................................................................................21

3.3.3 Groundwater...............................................................................................................22

3.4 LABORATORY SOIL AND ROCK TESTING...................................................................22

3.4.1 General.......................................................................................................................22

3.4.2 Classification and Index Properties..............................................................................23

3.4.3 Shear Strength............................................................................................................23

3.4.4 Consolidation..............................................................................................................23

3.4.5 Electrochemical Criteria...............................................................................................24

3.5 IN SITU SOIL AND ROCK TESTING................................................................................24

CHAPTER 4 BASIC PRINCIPLES OF ANCHORED SYSTEM DESIGN ...............................26

4.1 GENERAL DESIGN CONCEPTS FOR ANCHORED WALLS .....................................26

4.2 FAILURE MECHANISMS OF ANCHORED SYSTEMS...............................................28

4.2.1 General.......................................................................................................................28

4.2.2 Failure Mechanisms of the Ground Anchor..................................................................28

4.2.3 Failure of Soldier Beams .............................................................................................31

4.2.4. Failure of Lagging........................................................................................................32

4.3 SELECTION OF SOIL SHEAR STRENGTH PARAMETERS FOR DESIGN ............33

4.3.1 General.......................................................................................................................33

4.3.2 Drained Shear Strength of Granular Soils.....................................................................33

4.3.3 Undrained Shear Strength of Normally Consolidated Clay............................................33
TABLE OF CONTENTS (Continued)

v

4.3.4 Undrained Shear Strength of Overconsolidated Clay....................................................34

4.3.5 Drained Shear Strength of Overconsolidated Clay........................................................34

4.4 EARTH PRESSURES .........................................................................................................36

4.4.1 General.......................................................................................................................36

4.4.2 Active and Passive Earth Pressure...............................................................................36

4.4.3 Earth Pressure at Rest.................................................................................................41

4.4.4 Influence of Movement on Earth Pressure....................................................................41

CHAPTER 5 DESIGN OF ANCHORED SYSTEMS..................................................................46

5.1 INTRODUCTION ..................................................................................................................46

5.2 EVALUATION OF EARTH PRESSURES FOR WALL DESIGN.....................................47

5.2.1 Introduction................................................................................................................47

5.2.2 Background ................................................................................................................48

5.2.3 Terzaghi and Peck Apparent Earth Pressure Diagrams.................................................49

5.2.4 Recommended Apparent Earth Pressure Diagram for Sands........................................50

5.2.5 Recommended Apparent Earth Pressure Diagram for Stiff to Hard Fissured

Clays..........................................................................................................................52

5.2.6 Recommended Apparent Earth Pressure Diagram for Soft to Medium Clays ................57

5.2.7 Loading Diagrams for Stratified Soil Profiles................................................................60

5.2.8 Sliding Wedge Analysis Method..................................................................................60

5.2.9 Water Pressures..........................................................................................................62

5.2.10 Earth Pressures Due To Surface Loads........................................................................64

5.2.10.1 Uniform Surcharge Loads................................................................................64

5.2.10.2 Point Loads, Line Loads, and Strip Loads.......................................................64

5.3 GROUND ANCHOR DESIGN .............................................................................................65

5.3.1 Introduction................................................................................................................65

5.3.2 Location of Critical Potential Failure Surface................................................................65

5.3.3 Calculation of Ground Anchor Loads from Apparent Earth Pressure Diagrams.............65

5.3.4 Design of the Unbonded Length...................................................................................67

5.3.5 Compression Anchors.................................................................................................68

5.3.6 Design of the Anchor Bond Length..............................................................................69
TABLE OF CONTENTS (Continued)

vi

5.3.7 Spacing Requirements for Ground Anchors..................................................................75

5.3.8 Selection of Prestressing Steel Element ........................................................................77

5.4 WALL DESIGN BASED ON LATERAL PRESSURES .....................................................78

5.4.1 Design of Soldier Beams and Sheet-Piles.....................................................................78

5.4.2 Design of Lagging for Temporary Support ...................................................................81

5.4.3 Design of Wales and Permanent Facing .......................................................................83

5.5 LATERAL CAPACITY OF EMBEDDED PORTION OF WALL......................................84

5.5.1 General.......................................................................................................................84

5.5.2 Evaluation of Ultimate Passive Resistance ....................................................................84

5.5.2.1 Soldier Beam and Lagging Walls .....................................................................84

5.5.2.2 Continuous Walls............................................................................................86

5.5.3 Depth of Penetration below Excavation........................................................................86

5.5.4 Comparison of Wang-Reese and Broms Method for Competent Soils..........................87

5.6 AXIAL CAPACITY OF WALL..............................................................................................88

5.6.1 Introduction................................................................................................................88

5.6.2 Axial Load Evaluation.................................................................................................89

5.6.3 Axial Capacity Design of Driven Soldier Beams...........................................................90

5.6.3.1 General...........................................................................................................90

5.6.3.2 Effective Stress Analysis for Driven Soldier Beams...........................................90

5.6.3.3 Total Stress Analysis for Driven Soldier Beams in Clays...................................92

5.6.4 Axial Capacity Design Of Drilled-in Soldier Beams......................................................93

5.6.4.1 General...........................................................................................................93

5.6.4.2 Cohesionless Soils...........................................................................................93

5.6.4.3 Cohesive Soils ................................................................................................94

5.6.4.4 Design Issues for Concrete Backfill of Predrilled Soldier Beam Holes...............95

5.7 ANCHORED SLOPES AND LANDSLIDE STABILIZATION SYSTEMS......................96

5.7.1 General.......................................................................................................................96

5.7.2 Design Concepts.........................................................................................................96

5.7.3 Limit Equilibrium Calculations......................................................................................97

5.7.3.1 Overall Approach...........................................................................................97
TABLE OF CONTENTS (Continued)

vii

5.7.3.2 Method 1 Analysis..........................................................................................98

5.7.3.3 Method 2 Analysis........................................................................................100

5.7.4 Modeling Lateral Wall Resistance in Limit Equilibrium Analyses..................................101

5.7.5 Comparison of Methods to Evaluate Required Earth Load in Homogeneous

Soils .........................................................................................................................102

5.8 GROUND MASS STABILITY............................................................................................105

5.8.1 Introduction..............................................................................................................105

5.8.2 Basal Stability...........................................................................................................105

5.8.2.1 General.........................................................................................................105

5.8.2.2 Evaluation of Bottom Heave Potential in Soft to Medium Clays ......................105

5.8.3 External Stability.......................................................................................................107

5.8.3.1 Introduction..................................................................................................107

5.8.3.2 Evaluation of External Stability Using Limit Equilibrium...................................108

5.9 TIEDOWN DESIGN ............................................................................................................109

5.9.1 Introduction..............................................................................................................109

5.9.2 Uplift Capacity of Rock Tiedown Anchors.................................................................109

5.9.3 Uplift Capacity of Soil Tiedown Anchors...................................................................110

5.9.4 Design of Tiedown Anchors to Resist Hydrostatic Uplift.............................................112

5.10 SEISMIC DESIGN............................................................................................................113

5.10.1 Introduction..............................................................................................................113

5.10.2 Internal Stability Using Pseudo-Static Theory.............................................................113

5.10.2.1 Lateral Earth Pressure...................................................................................113

5.10.2.2 Wall Design Considerations...........................................................................116

5.10.2.3 Liquefaction..................................................................................................117

5.10.3 External Stability.......................................................................................................117

5.10.3.1 Pseudo-Static Analysis..................................................................................117

5.10.3.2 Seismic Deformation Analysis........................................................................118

5.11 OTHER DESIGN ISSUES................................................................................................119

5.11.1 Wall and Ground Movements....................................................................................119

5.11.2 Drainage Systems for Anchored Walls and Slopes.....................................................120
TABLE OF CONTENTS (Continued)

viii

5.11.3 Wall System Appurtenances......................................................................................121

5.11.4 Resisting the Upper Anchor Test Load ......................................................................122

5.11.5 Anchored Walls for Fill Applications..........................................................................122

CHAPTER 6 CORROSION CONSIDERATIONS IN DESIGN..............................................124

6.1 INTRODUCTION .............................................................................................................124

6.2 CORROSION AND EFFECTS ON GROUND ANCHORS..........................................124

6.2.1 Mechanism of Metallic Corrosion..............................................................................124

6.2.2 Types of Corrosion for Prestressing Steel..................................................................124

6.3 CORROSION PROTECTION OF GROUND ANCHORS ...........................................126

6.3.1 Requirements of Corrosion Protection Systems..........................................................126

6.3.2 Design of Corrosion Protection Systems....................................................................126

6.3.2.1 General.........................................................................................................126

6.3.2.2 Anchorage Protection....................................................................................131

6.3.2.3 Unbonded Tendon Length Protection............................................................132

6.3.2.4 Tendon Bond Length Protection....................................................................132

6.3.2.5 Protection Against Stray Currents..................................................................132

6.3.2.6 Corrosion Protection of Anchors for Structures Subject to Hydrostatic

Uplift ............................................................................................................133

6.4 SELECTION OF CORROSION PROTECTION LEVEL..............................................133

6.4.1 General.....................................................................................................................133

6.4.2 Service Life of the Anchored Structure ......................................................................133

6.4.3 Aggressivity of the Ground Environment.....................................................................133

6.4.4 Consequences of Failure of the Anchored System......................................................135

6.4.5 Cost for a Higher Level of Protection.........................................................................135

6.5 CORROSION OF STRUCTURAL STEEL, CEMENT GROUT, AND CONCRETE.135

6.5.1 Corrosion and Protection of Steel Soldier Beams and Sheet Piles...............................135

6.5.2 Degradation and Protection of Cement Grout and Concrete.......................................136
TABLE OF CONTENTS (Continued)

ix

CHAPTER 7 LOAD TESTING AND TRANSFER OF LOAD TO THE ANCHORED

SYSTEM.................................................................................................................137

7.1 INTRODUCTION...............................................................................................................137

7.2 CONCEPTS FOR MONITORING ANCHOR BOND ZONE CAPACITY....................137

7.3 TESTING AND STRESSING EQUIPMENT...................................................................139

7.3.1 General.....................................................................................................................139

7.3.2 Equipment Used in Load Testing ...............................................................................140

7.3.2.1 Hydraulic Jack and Pump..............................................................................140

7.3.2.2 Stressing Anchorage......................................................................................141

7.3.2.3 Pressure Gauges and Load Cells ...................................................................142

7.3.2.4 Dial Gauge to Measure Movement ................................................................142

7.3.2.5 Jack Chair ....................................................................................................142

7.4 ANCHOR LOAD TESTING..............................................................................................142

7.4.1 Introduction..............................................................................................................142

7.4.2 Performance Tests....................................................................................................143

7.4.2.1 General.........................................................................................................143

7.4.2.2 Procedures for Performance Test ..................................................................143

7.4.2.3 Recording of Performance Test Data.............................................................145

7.4.2.4 Analysis of Performance Test Data................................................................146

7.4.3 Proof Tests...............................................................................................................147

7.4.3.1 General.........................................................................................................147

7.4.3.2 Proof Test Procedures and Recording and Analysis of Proof Test Data..........147

7.4.4 Extended Creep Testing............................................................................................148

7.4.4.1 General.........................................................................................................148

7.4.4.2 Procedures for Extended Creep Test.............................................................148

7.4.4.3 Recording and Analysis of Extended Creep Test Data....................................149

7.4.5 Acceptance Criteria ..................................................................................................150

7.4.5.1 General.........................................................................................................150

7.4.5.2 Creep...........................................................................................................150

7.4.5.3 Apparent Free Length...................................................................................150
TABLE OF CONTENTS (Continued)

x

7.4.5.4 Ground Anchor Acceptance Decision Tree....................................................152

7.4.5.5 Modification of Design or Installation Procedures...........................................154

7.5 ANCHOR LOCK-OFF LOAD ...........................................................................................154

7.6 LIFT-OFF TESTING..........................................................................................................155

CHAPTER 8 CONTRACTING APPROACHES .......................................................................156

8.1 INTRODUCTION..................................................................................................................156

8.2 METHOD CONTRACTING APPROACH............................................................................157

8.2.1 Introduction..............................................................................................................157

8.2.2 Contract Documents for Method Approach...............................................................158

8.3 PERFORMANCE CONTRACTING APPROACH...............................................................159

8.3.1 Introduction..............................................................................................................159

8.3.2 Implementing Performance Contracting Approach......................................................159

8.3.2.1 Pre-bid Wall Design......................................................................................159

8.3.2.2 Pre-bid Typical Section Design......................................................................160

8.3.2.3 Post-bid Wall Design ....................................................................................160

8.3.3 Contract Documents for Performance Approach........................................................161

8.3.4 Review and Approval................................................................................................162

8.4 CONTRACTOR DESIGN/BUILD APPROACH...................................................................163

8.5 RECOMMENDATIONS........................................................................................................163

CHAPTER 9 CONSTRUCTION INSPECTION AND PERFORMANCE

MONITORING.....................................................................................................164

9.1 INTRODUCTION ................................................................................................................164

9.2 INSPECTION ROLES UNDER CERTAIN CONTRACT APPROACHES ....................164

9.3 PRE-PROJECT PREPARATION .......................................................................................165

9.4 INSPECTION OF CONSTRUCTION MATERIALS .......................................................165

9.4.1 Introduction..............................................................................................................165

9.4.2 Inspection of Wall Materials......................................................................................166

9.4.3 Inspection of Ground Anchor Materials .....................................................................166

9.4.4 Storage and Handling of Construction Materials.........................................................167
TABLE OF CONTENTS (Continued)

xi

9.5 INSPECTION OF CONSTRUCTION ACTIVITIES.........................................................167

9.5.1 Surface-Water Control..............................................................................................167

9.5.2 Vertical Wall Element Installation...............................................................................168

9.5.2.1 Drilled-in Soldier Beams................................................................................168

9.5.2.2 Driven Soldier Beams....................................................................................168

9.5.2.3 Sheet-Piles....................................................................................................169

9.5.3 Excavation................................................................................................................169

9.5.4 Anchor Construction.................................................................................................169

9.5.4.1 Introduction..................................................................................................169

9.5.4.2 Anchor Hole Drilling......................................................................................170

9.5.4.3 Tendon Insertion...........................................................................................170

9.5.4.4 Anchor Grouting ...........................................................................................171

9.5.4.5 Anchorage Installation...................................................................................172

9.5.5 Ancillary Wall Element Installation.............................................................................173

9.5.5.1 Timber Lagging Installation............................................................................173

9.5.5.2 Wall Drainage System Installation..................................................................173

9.5.5.3 Horizontal Drains ..........................................................................................173

9.5.5.4 Permanent Facing Installation.........................................................................174

9.6 SHORT-TERM AND LONG-TERM MONITORING.....................................................174

9.6.1 Monitoring of Anchor Load Tests..............................................................................174

9.6.2 Short-Term Monitoring of Wall Performance.............................................................175

9.6.3 Long-Term Monitoring..............................................................................................176

REFERENCES

BIBLIOGRAPHY
TABLE OF CONTENTS (Continued)

xii

APPENDIX

APPENDIX A Design Examples

APPENDIX B Development of Wang - Reese Equations

APPENDIX C Example Calculation of Bending Moment for Wall in Weak Cohesive Soil

APPENDIX D Predesign Load Testing Procedures to Evaluate Ultimate Ground Anchor Load

APPENDIX E Specification for Ground Anchors

APPENDIX F Specifications for Anchored Sheet-pile or Soldier Beam and Lagging Wall
xiii

LIST OF TABLES

Table Page

1 Soil density/consistency description based on SPT blowcount values (after AASHTO,

1988) .................................................................................................................................24

2 Summary of common in situ tests for soils........................................................................25

3 Typical factors influencing bond stress transfer for small diameter ground anchors........30

4 Typical design steps for an anchored wall (modified after FHWA-RD-81-150, 1982)....47

5 Summary of trapezoidal apparent pressure envelopes for temporary excavations in stiff to

hard clays............................................................................................................................53

6 Presumptive ultimate values of load transfer for preliminary design of small diameter

straight shaft gravity-grouted ground anchors in soil.........................................................71

7 Presumptive average ultimate bond stress for ground/grout interface along anchor bond

zone (after PTI, 1996) ........................................................................................................73

8 Presumptive ultimate values of load transfer for preliminary design of ground anchors in

rock.....................................................................................................................................74

9 Properties of prestressing steel bars (ASTM A722)...........................................................77

10 Properties of 15-mm diameter prestressing steel strands (ASTM A 416, Grade 270

(metric 1860)).....................................................................................................................78

11 Guidance relationship between tendon size and trumpet opening size..............................78

12 Recommended thickness of temporary timber lagging (after FHWA-RD-75-130, 1976)82

13 Maximum design bending moments for wales and permanent facing (after AASHTO,

1996) ..................................................................................................................................83

14 Recommended factors of safety for axial capacity

of driven and drilled-in soldier beams................................................................................90

15 Bearing capacity factors for evaluation of end bearing in drilled shafts in clays ..............95

16 Procedure to evaluate total lateral earth load using slope stability computer

programs.............................................................................................................................98
xiv

17 Procedure to evaluate total lateral earth load for anchored systems constructed in weak

cohesive soils....................................................................................................................101

18 Values of KREQ in cohesionless soil using various methods to evaluate earth

pressures...........................................................................................................................103

19 Horizontal stress coefficient, K, for pressure grouted anchors (after Kulhawy et al.,

1983) ................................................................................................................................111

20 Corrosion protection requirements (modified after PTI, 1996) .......................................131

21 Steps for the performance test..........................................................................................144

22 Test procedure for ground anchor proof test....................................................................147

23 Load schedule and observation periods for extended creep test for permanent

anchor...............................................................................................................................149
xv

LIST OF FIGURES

Figure Page

1 Components of a ground anchor.............................................................................................4

2 Anchorage components for a bar tendon.................................................................................5

3 Anchorage components for a strand tendon............................................................................6

4 Main types of grouted ground anchors (modified after Littlejohn, 1990)...................................7

5 Cut away view of bar tendon..................................................................................................9

6 Cut away view of strand tendon............................................................................................10

7 Construction sequence for permanent soldier beam and lagging wall ......................................12

8 Comparison of concrete gravity wall and anchored wall for a depressed roadway..................16

9 Applications of ground anchors and anchored systems ..........................................................18

10 Geotechnical boring layout for permanent anchored wall........................................................20

11 Potential failure conditions to be considered in design of anchored walls.................................27

12 Contribution of ground anchors to wall stability.....................................................................28

13 Simplified drained stress-displacement relationship for a stiff clay (modified after CIRIA, 1984) 35

14 Mobilization of Rankine active and passive horizontal pressures for a smooth retaining wall ....37

15 Limiting active and passive horizontal pressures.....................................................................37

16 Active and passive earth pressure coefficients (effect of wall inclination) ................................39

17 Active and passive earth pressure coefficients (effect of backslope inclination) ......................40

18 Cross section of model wall (modified after FHWA-RD-98-067, 1998)...............................42

19 Lateral wall movements and earth pressures with excavation at first anchor level (cantilever

stage) (modified after FHWA-RD-98-067, 1998)................................................................42

20 Lateral wall movements and earth pressures during anchor stressing (modified after FHWARD-98-067, 1998)..............................................................................................................43
LIST OF FIGURES (Continued)

Figure Page

xvi

21 Lateral wall movements and earth pressures with excavation at lower anchor level (modified

after FHWA-RD-98-067, 1998) .........................................................................................44

22 Lateral wall movements and earth pressures with excavation at design grade (modified after

FHWA-RD-98-067, 1998).................................................................................................45

23 Terzaghi and Peck apparent pressure envelopes (after Terzaghi and Peck, 1967)..................50

24 Recommended apparent earth pressure diagram for sands.....................................................51

25 Measured anchor loads for seven projects (after Ulrich, 1989)..............................................54

26 Wall pressure envelopes (after Winter, 1990).......................................................................55

27 Recommended apparent earth pressure envelope for stiff to hard clays..................................56

28 Henkel’s mechanism of base failure ......................................................................................58

29 Values of KA based on Terzaghi and Peck envelope and Henkel's method.............................59

30 Force equilibrium method for anchored walls (after FHWA-RD-98-065, 1998)....................61

31 Flow net for a retaining wall (after CIRIA, 1984)..................................................................62

32 Gross and net water pressures across a retaining wall (modified after CIRIA, 1984)..............63

33 Calculation of anchor loads for one-level wall........................................................................66

34 Calculation of anchor loads for multi-level wall......................................................................67

35 Types of compression anchors..............................................................................................69

36 Mobilization of bond stress for a tension anchor....................................................................72

37 Vertical and horizontal spacing requirements for ground anchors............................................76

38 Calculation of wall bending moments using hinge method.......................................................79

39 Calculation of wall bending moments using tributary area method...........................................80

40 Relationship between lateral earth pressure, wall deflection, and depth of wall embedment .....85

41 Broms method for evaluating ultimate passive resistance........................................................86

42 Comparison of Broms and Wang-Reese method for wall in sand...........................................88
LIST OF FIGURES (Continued)

Figure Page

xvii

43 Comparison of Broms and Wang-Reese method for wall in clay............................................88

44 Chart for estimating β coefficient versus soil type friction angle (after Fellenius,

1991) ..................................................................................................................................91

45 Chart for estimating Nt coefficients versus soil type friction angle (after Fellenius,

1991) ..................................................................................................................................92

46 Adhesion values for piles in cohesive soils (after Tomlinson, 1980)........................................93

47 Modeling the ground anchor force in limit equilibrium analysis (after FHWA-RD-97-130, 1998)

............................................................................................................................................99

48 Limit equilibrium analyses used to evaluate total lateral earth load for anchored systems

constructed in weak cohesive soils analysis (after FHWA-RD-97-130, 1998).....................100

49 Total passive force for example wall in cohesionless soil......................................................102

50 Comparison of limit equilibrium methods for cohesive soils (after FHWA-RD-98-065, 1998)104

51 Analysis of basal stability (modified after Terzaghi et al., 1996)............................................106

52 Failure surfaces for external stability evaluations ..................................................................108

53 Inverted cone mechanisms for overall rock mass stability.....................................................110

54 Stability of structure subjected to hydrostatic uplift ..............................................................112

55 Forces behind a gravity wall ...............................................................................................115

56 Effect of seismic coefficients and friction angle on seismic active pressure coefficient (after Lam

and Martin, 1986)..............................................................................................................116

57 Variation of failure surface inclination with horizontal acceleration coefficient ........................118

58 Permanent seismic deformation chart (after Hynes and Franklin, 1984)................................119

59 Settlement profile behind braced and anchored walls...........................................................120

60 Examples of corrosion protection for anchorages................................................................128

61 Examples of corrosion protection classes I and II for strand tendons....................................129

62 Examples of corrosion protection classes I and II for bar tendons........................................130
LIST OF FIGURES (Continued)

Figure Page

xviii

63 Decision tree for selection of corrosion protection level (modified after PTI, 1996)..............134

64 Skin friction versus strain diagrams for ground anchors........................................................137

65 Stress propagation in bond length of ground anchor.............................................................138

66 Evaluation of critical creep tension......................................................................................139

67 Typical equipment for load testing of strand ground anchor..................................................140

68 Typical equipment for load testing of bar ground anchor......................................................141

69 Plotting of performance test data (after PTI, 1996)..............................................................145

70 Plotting of elastic and residual movement for a performance test (after PTI, 1996)...............146

71 Plotting of proof test data (after PTI, 1996)........................................................................148

72 Plotting of extended creep test data (after PTI, 1996).........................................................149

73 Ground anchor acceptance decision tree (after PTI, 1996)..................................................153

A-1 Subsurface stratigraphy and design cross section................................................................ A-2

A-2 Apparent earth pressure diagram and surcharge pressure diagram...................................... A-4

A-3 Location of unbonded and bond lengths for ground anchors............................................... A-8

A-4 Embedment depth calculations (Wang-Reese method)..................................................... A-12

A-5 Subsurface stratigraphy and design cross section.............................................................. A-18

A-6 Secant residual friction angle (after Stark and Eid, 1994) ................................................. A-20

A-7 Slope stability analysis of existing site conditions............................................................... A-22

A-8 Apparent earth pressure diagram..................................................................................... A-23

A-9 Calculation of TH3 and M4 ............................................................................................... A-25

A-10 Location of unbonded and bond lengths for ground anchors............................................. A-27

B-1 Passive wedge failure for a soldier beam in sand (after Reese, . al., 1974)............................B-2

B-2 Intersecting failure wedges for soldier beams in sand (after Wang and Reese, 1986)............B-3

B-3 Plastic flow around a soldier beam toe (after Wang and Reese, 1986).................................B-4
LIST OF FIGURES (Continued)

Figure Page

xix

B-4 Passive wedge failure for a soldier beam in clay (after Reese, 1958)....................................B-6

B-5 Failure wedges for soldier beams in clay (after Wang and Reese, 1986)..............................B-7

D-1 Determination of critical creep tension................................................................................ D-2

D-2 Extrapolation of creep curves for determining working tension............................................ D-4
1

CHAPTER 1

INTRODUCTION

1.1 PURPOSE

The purpose of this document is to provide state-of-the-practice information on ground anchors and

anchored systems for highway applications. Ground anchors discussed in this document are cement

grouted, prestressed tendons that are installed in soil or rock. Anchored systems discussed include

flexible anchored walls, slopes supported using ground anchors, slope and landslide stabilization

systems, and structures that incorporate tiedown anchors. The intended audience includes

geotechnical, structural, and highway design and construction specialists involved with the design,

construction, contracting, and inspection of these systems.

Ground anchors and anchored systems have become increasingly more cost-effective through

improvements in design methods, construction techniques, anchor component materials, and on-site

acceptance testing. This has resulted in an increase in the use of both temporary and permanent

anchors. The reader should recognize that, as a result of the evolving nature of anchoring practice,

the information presented herein is not intended to be prescriptive. Design, construction, and load

testing methods are described that are currently used in U.S. practice.

1.2 ANCHORED SYSTEM SERVICE LIFE

The focus of this document is on design methods and procedures for permanent ground anchors and

anchored systems. Permanent anchored systems are generally considered to have a service life of 75

to 100 years. However, anchored systems are also commonly used for temporary applications. The

service life of temporary earth support systems is based on the time required to support the ground

while the permanent systems are installed. This document has adopted the American Association of

State Highway and Transportation Officials (AASHTO) guidance which considers temporary

systems to be those that are removed or become inoperative upon completion of the permanent

systems. The time period for temporary systems is commonly stated to be 18 to 36 months but may

be shorter or longer based on actual project conditions.

Furthermore this document has subdivided temporary systems into “support of excavation” (SOE)

temporary systems and “critical” temporary systems. In general the owner will determine which

temporary systems are to be designated as critical. Often that decision is based on the owner’s need

to restrict lateral movement of the support system to minimize ground movements behind the support

system. In this document, it is recommended that critical temporary systems be designed to the same

criteria used for permanent anchored systems. Conversely, SOE anchored systems are commonly

designed to less restrictive criteria than permanent anchored systems. The owner commonly assigns

the responsibility for design and performance of SOE anchored systems to the contractor. The

design of these SOE anchored systems is often based more on system stability than on minimizing

ground movements.
2

In this document, the basic design recommendations pertain to both permanent anchored systems and

critical temporary systems. In this document, the term “permanent anchored systems” or “permanent

applications” include critical temporary systems. Whenever appropriate in this document, discussion

is provided concerning the differences in design requirements for SOE systems and permanent

systems. The following components of an anchored system design are generally less restrictive for

temporary SOE systems as compared to permanent systems: (1) selection of timber lagging; (2)

allowable stresses in structural components; (3) factors of safety; (4) design for axial load; (5)

surcharge loads used to evaluate wall loadings; (6) seismic design criteria; and (7) anchor load

testing.

1.3 BACKGROUND

The first use of ground anchors in the U.S. was for temporary support of excavation systems. These

systems were typically designed and constructed by specialty contractors. The use of permanent

ground anchors for public sector projects in the U.S. did not become common until the late 1970s

and today, represent a common technique for earth retention and slope stabilization for highway

applications. In certain design and construction conditions, anchored systems offer several

advantages over more conventional systems that have resulted in economic and technical benefits.

For example, benefits of anchored walls over concrete gravity retaining walls for support of a

highway cut include:

• unobstructed workspace for excavations;

• ability to withstand relatively large horizontal wall pressures without requiring a significant

increase in wall cross section;

• elimination of the need to provide temporary excavation support since an anchored wall can

be incorporated into the permanent structure;

• elimination of need for select backfill;

• elimination of need for deep foundation support;

• reduced construction time; and

• reduced right-of-way (ROW) acquisition.

In 1979, the U.S. Department of Transportation Federal Highway Administration (FHWA) Office of

Technology Applications authorized a permanent ground anchor demonstration project. The

objective of the project was to provide highway agencies with adequate information to promote

routine use of permanent ground anchors and anchored walls. The purpose of the demonstration

project was to: (1) study existing ground anchor technology and installation procedures; (2)

determine areas where additional work was required; (3) update existing technology; (4) develop a

basic design manual; and (5) solicit installations on highway projects. Between 1979 and 1982, two

FHWA research reports were completed (“Permanent Ground Anchors” FHWA Report Nos.

FHWA-RD- 81-150, 151, and 152 and “Tiebacks” FHWA Report No. FHWA-RD-82-047) and pilot

test projects were begun by highway agencies. A design manual was developed by FHWA in 1984,
3

which was updated in 1988 (FHWA-DP-68-1R, 1988), as part of the demonstration project. During

the demonstration project, five U.S. highway projects with permanent anchored systems were

instrumented and performance data were gathered (see FHWA-DP-90-068-003, 1990). Today,

ground anchors and anchored systems have become an integral component of highway design in the

U.S.

This document has been written, in part, to update the FHWA (1988) design manual titled

"Permanent Ground Anchors". That document provides an introduction to basic ground anchor

concepts and provides the practicing highway engineer with sufficient information to contract for

permanent ground anchors and anchored systems. This document draws extensively from FHWA

(1988) in describing issues such as subsurface investigation and laboratory testing, basic anchoring

principles, ground anchor load testing, and inspection of construction materials and methods used for

anchored systems. Since 1988, advances have been made in design methods resulting from anchored

system performance data and from new construction materials, methods, and equipment.

Results of research activities conducted since 1989 are also included in this document. Most

recently, research was conducted under a FHWA research contract on the design and performance of

ground anchors and anchored soldier beam and timber lagging walls. As part of that research

project, performance data on two full-scale anchored walls and four large-scale model anchored

walls were collected and analyzed. The settlement, axial load, and downdrag force on soldier beams,

and lateral wall movements of the wall systems were evaluated (see FHWA-RD-98-066, 1998 and

FHWA-RD-98-067, 1998). Several of the analysis methods and design procedures that were

recommended based on the results of the research (see FHWA-RD-97-130, 1998) are adopted herein.

Procedures used for ground anchor acceptance testing have also been improved since FHWA (1988)

was published. The AASHTO Task Force 27 report "In-Situ Soil Improvement Techniques” (1990)

included both a generic construction specification for permanent ground anchors and a ground

anchor inspection manual. Those documents form the basis for the construction standards developed

by many highway agencies. The Post-Tensioning Institute (PTI) document titled “Recommendations

for Prestressed Rock and Soil Anchors" (PTI, 1996) is a document commonly referenced that was

developed collectively by owners, design consultants, specialty contractors, and material suppliers.

AASHTO Task Force 27 (1990) and PTI (1996) were used as the basis for the chapters of this

document on ground anchor acceptance testing and ground anchor corrosion protection. Information

from those documents was also used to develop the generic ground anchor specification provided in

appendix E.
4

CHAPTER 2

GROUND ANCHORS AND ANCHORED SYSTEMS

2.1 INTRODUCTION

The previously referenced AASHTO Task Force 27 (1990) and PTI (1996) documents introduced

standardized terminology and definitions of ground anchor components. The terminology presented

in those documents is adopted and used throughout this document. Ground anchor materials,

anchored system construction, and anchored system applications are presented in this chapter.

2.2 GROUND ANCHORS

2.2.1 General

A prestressed grouted ground anchor is a structural element installed in soil or rock that is used to

transmit an applied tensile load into the ground. Grouted ground anchors, referenced simply as

ground anchors, are installed in grout filled drill holes. Grouted ground anchors are also referred to

as “tiebacks”. The basic components of a grouted ground anchor include the: (1) anchorage; (2) free

stressing (unbonded) length; and (3) bond length. These and other components of a ground anchor

are shown schematically in figure 1. The anchorage is the combined system of anchor head, bearing

plate, and trumpet that is capable of transmitting the prestressing force from the prestressing

Wall

Unbonded Length

Tendon Bond Length

 Bonded Tendon

Unbonded Tendon

Anchor Bond Length

Anchor

Diameter

Anchor Head

Bearing Plate

Anchor Grout

Trumpet

Sheath

Figure 1. Components of a ground anchor.
5

steel (bar or strand) to the ground surface or the supported structure. Anchorage components for a

bar tendon and a strand tendon are shown in figure 2 and figure 3, respectively. The unbonded

length is that portion of the prestressing steel that is free to elongate elastically and transfer the

resisting force from the bond length to the structure. A bondbreaker is a smooth plastic sleeve that is

placed over the tendon in the unbonded length to prevent the prestressing steel from bonding to the

surrounding grout. It enables the prestressing steel in the unbonded length to elongate without

obstruction during testing and stressing and leaves the prestressing steel unbonded after lock-off.

The tendon bond length is that length of the prestressing steel that is bonded to the grout and is

capable of transmitting the applied tensile load into the ground. The anchor bond length should be

located behind the critical failure surface.

A portion of the complete ground anchor assembly is referred to as the tendon. The tendon includes

the prestressing steel element (strands or bars), corrosion protection, sheaths (also referred to as

sheathings), centralizers, and spacers, but specifically excludes the grout. The definition of a tendon,

as described in PTI (1996), also includes the anchorage; however, it is assumed herein that the

tendon does not include the anchorage. The sheath is a smooth or corrugated pipe or tube that

protects the prestressing steel in the unbonded length from corrosion. Centralizers position the

tendon in the drill hole such that the specified minimum grout cover is achieved around the tendon.

For multiple element tendons, spacers are used to separate the strands or bars of the tendons so that

each element is adequately bonded to the anchor grout. The grout is a Portland cement based

mixture that provides load transfer from the tendon to the ground and provides corrosion protection

for the tendon.

BAR

ANCHOR

NUT

B

P

E

L

ARING

ATE

Figure 2. Anchorage components for a bar tendon.
6

Figure 3. Anchorage components for a strand tendon.

2.2.2 Types of Ground Anchors

2.2.2.1 General

There are three main ground anchor types that are currently used in U.S. practice: (1) straight shaft

gravity-grouted ground anchors (Type A); (2) straight shaft pressure-grouted ground anchors (Type

B); and (3) post-grouted ground anchors (Type C). Although not commonly used today in U.S.

practice, another type of anchor is the underreamed anchor (Type D). These ground anchor types are

illustrated schematically in figure 4 and are briefly described in the following sections.

Drilling methods for each of the three main soil and rock ground anchors include rotary, percussion,

rotary/percussive, or auger drilling. Detailed information on these drilling techniques may be found

in Bruce (1989). The procedures and methods used to drill holes for ground anchors are usually

selected by the contractor. The choice of a particular drilling method must also consider the overall

site conditions and it is for this reason that the engineer may place limitations on the drilling method.

The drilling method must not adversely affect the integrity of structures near the ground anchor

locations or on the ground surface. With respect to drilling, excessive ground loss into the drill hole

and ground surface heave are the primary causes of damage to these structures. For example, the use

of large diameter hollow stem augered anchors should be discouraged in sands and gravels since the

auger will tend to remove larger quantities of soil from the drill hole as compared to the net volume

of the auger. This may result in loss of support of the drill hole. In unstable soil or rock, drill casing

is used. Water or air is used to flush the drill cuttings out of the cased hole. Caution should be

exercised when using air flushing to clean the hole. Excess air pressures may result in unwanted
7

removal of groundwater and fines from the drill hole leading to potential hole collapse or these

excess pressures may result in ground heave.

Figure 4. Main types of grouted ground anchors (modified after Littlejohn, 1990, “Ground

Anchorage Practice”, Design and Performance of Earth Retaining Structures, Geotechnical Special

Publication No. 25, Reprinted by permission of ASCE).

2.2.2.2 Straight Shaft Gravity-Grouted Ground Anchors

Straight shaft gravity-grouted ground anchors are typically installed in rock and very stiff to hard

cohesive soil deposits using either rotary drilling or hollow-stem auger methods. Tremie (gravity

displacement) methods are used to grout the anchor in a straight shaft borehole. The borehole may

be cased or uncased depending on the stability of the borehole. Anchor resistance to pullout of the

grouted anchor depends on the shear resistance that is mobilized at the grout/ground interface.
8

2.2.2.3 Straight Shaft Pressure-Grouted Ground Anchors

Straight shaft pressure-grouted ground anchors are most suitable for coarse granular soils and weak

fissured rock. This anchor type is also used in fine grained cohesionless soils. With this type of

anchor, grout is injected into the bond zone under pressures greater than 0.35 MPa. The borehole is

typically drilled using a hollow stem auger or using rotary techniques with drill casings. As the

auger or casing is withdrawn, the grout is injected into the hole under pressure until the entire anchor

bond length is grouted. This grouting procedure increases resistance to pullout relative to tremie

grouting methods by: (1) increasing the normal stress (i.e., confining pressure) on the grout bulb

resulting from compaction of the surrounding material locally around the grout bulb; and (2)

increasing the effective diameter of the grout bulb.

2.2.2.4 Post-grouted Ground Anchors

Post-grouted ground anchors use delayed multiple grout injections to enlarge the grout body of

straight shafted gravity grouted ground anchors. Each injection is separated by one or two days.

Postgrouting is accomplished through a sealed grout tube installed with the tendon. The tube is

equipped with check valves in the bond zone. The check valves allow additional grout to be injected

under high pressure into the initial grout which has set. The high pressure grout fractures the initial

grout and wedges it outward into the soil enlarging the grout body. Two fundamental types of postgrouted anchors are used. One system uses a packer to isolate each valve. The other system pumps

the grout down the post-grout tube without controlling which valves are opened.

2.2.2.5 Underreamed Anchors

Underreamed anchors consist of tremie grouted boreholes that include a series of enlargement bells

or underreams. This type of anchor may be used in firm to hard cohesive deposits. In addition to

resistance through side shear, as is the principal load transfer mechanism for other anchors,

resistance may also be mobilized through end bearing. Care must be taken to form and clean the

underreams.

2.2.3 Tendon Materials

2.2.3.1 Steel Bar and Strand Tendons

Both bar and strand tendons are commonly used for soil and rock anchors for highway applications

in the U.S. Material specifications for bar and strand tendons are codified in American Society for

Testing and Materials (ASTM) A722 and ASTM A416, respectively. Indented strand is codified in

ASTM A886. Bar tendons are commonly available in 26 mm, 32 mm, 36 mm, 45 mm, and 64 mm

diameters in uncoupled lengths up to approximately 18 m. Anchor design loads up to approximately

2,077 kN can be resisted by a single 64-mm diameter bar tendon. For lengths greater than 18 m and

where space constraints limit bar tendon lengths, couplers may be used to extend the tendon length.

As compared to strand tendons, bars are easier to stress and their load can be adjusted after lock-off.
9

Strand tendons comprise multiple seven-wire strands. The common strand in U.S. practice is 15 mm

in diameter. Anchors using multiple strands have no practical load or anchor length limitations.

Tendon steels have sufficiently low relaxation properties to minimize long-term anchor load losses.

Couplers are available for individual seven-wire strands but are rarely used since strand tendons can

be manufactured in any length. Strand couplers are not recommended for routine anchor projects as

the diameter of the coupler is much larger than the strand diameter, but strand couplers may be used

to repair damaged tendons. Where couplers are used, corrosion protection of the tendon at the

location of the coupler must be verified.

2.2.3.2 Spacers and Centralizers

Spacer/centralizer units are placed at regular intervals (e.g., typically 3 m) along the anchor bond

zone. For strand tendons, spacers usually provide a minimum interstrand spacing of 6 to 13 mm and

a minimum outer grout cover of 13 mm. Both spacers and centralizers should be made of noncorrosive materials and be designed to permit free flow of grout. Figure 5 and figure 6 show a cut

away section of a bar and a strand tendon, respectively.

Figure 5. Cut away view of bar tendon.
10

2.2.3.3 Epoxy-Coated Bar and Epoxy-Coated Filled Strand

Epoxy-coated bar (AASHTO M284) and epoxy-coated filled strand (supplement to ASTM A882),

while not used extensively for highway applications, are becoming more widely used for dam

tiedown projects. The epoxy coating provides an additional layer of corrosion protection in the

unbonded and bond length as compared to bare prestressing steel.

For epoxy-coated filled strand, in addition to the epoxy around the outside of the strand, the center

wire of the seven-wire strand is coated with epoxy. Unfilled epoxy-coated strand is not

recommended because water may enter the gaps around the center wire and lead to corrosion.

Unlike bare strand, creep deformations of epoxy-coated filled strands themselves are relatively

significant during anchor testing. When evaluating anchor acceptance with respect to creep, the

creep of the epoxy-coated filled strands themselves must be deducted from the total creep

movements to obtain a reliable measurement of the movements in the bond zone. Estimates of

intrinsic creep movements of epoxy-coated filled strand are provided in PTI (1996).

CENTRALIZER

SHEATH

SPACER

GROUT

TUBE

STRAND

Figure 6. Cut away view of strand tendon.

2.2.3.4 Other Anchor Types and Tendon Materials

In addition to cement grouted anchors incorporating high strength prestressing steels, alternative

anchor types and tendon materials are used in the U.S. Examples include Grade 60 and Grade 75

grouted steel bars, helical anchors, plate anchors, and mechanical rock anchors. The design and

testing methods described in this document are used for cement grouted anchors that use high

strength prestressing steels. These methods may not be appropriate for use with the alternative

anchor types mentioned above.
11

Research on the use of fiber reinforced plastic (FRP) prestressing tendons is currently being

performed (e.g., Schmidt et al., 1994). FRP tendons have high tensile strength, are corrosion

resistant, and are lightweight. These products, however, are not used in current U.S. construction

practice. Other materials such as fiberglass and stainless steel have been used experimentally but

cost and/or construction concerns have restricted widespread use.

2.2.4 Cement Grout

Anchor grout for soil and rock anchors is typically a neat cement grout (i.e., grout containing no

aggregate) conforming to ASTM C150 although sand-cement grout may also be used for large

diameter drill holes. Pea gravel-sand-cement grout may be used for anchor grout outside the tendon

encapsulation. High speed cement grout mixers are commonly used which can reasonably ensure

uniform mixing between grout and water. A water/cement (w/c) ratio of 0.4 to 0.55 by weight and

Type I cement will normally provide a minimum compressive strength of 21 MPa at the time of

anchor stressing. For some projects, special additives may be required to improve the fluid flow

characteristics of the grout. Admixtures are not typically required for most applications, but

plasticizers may be beneficial for applications in high temperature and for long grout pumping

distances.

2.3 ANCHORED WALLS

2.3.1 General

A common application of ground anchors for highway projects is for the construction of anchored

walls used to stabilize excavations and slopes. These anchored walls consist of nongravity

cantilevered walls with one or more levels of ground anchors. Nongravity cantilevered walls employ

either discrete (e.g., soldier beam) or continuous (e.g., sheet-pile) vertical elements that are either

driven or drilled to depths below the finished excavation grade. For nongravity cantilevered walls,

support is provided through the shear and bending stiffness of the vertical wall elements and passive

resistance from the soil below the finished excavation grade. Anchored wall support relies on these

components as well as lateral resistance provided by the ground anchors to resist horizontal pressures

(e.g., earth, water, seismic, etc.) acting on the wall.

Various construction materials and methods are used for the wall elements of an anchored wall.

Discrete vertical wall elements often consist of steel piles or drilled shafts that are spanned by a

structural facing. Permanent facings are usually cast-in-place (CIP) concrete although timber lagging

or precast concrete panels have been used. Continuous wall elements do not require separate

structural facing and include steel sheet-piles, CIP or precast concrete wall panels constructed in

slurry trenches (i.e., slurry (diaphragm) walls), tangent/secant piles, soil-cement columns, and jet

grouted columns.
12

2.3.2 Soldier Beam and Lagging Wall

2.3.2.1 General

Soldier beam and lagging walls are the most commonly used type of anchored wall system in the

U.S. This wall system uses discrete vertical wall elements spanned by lagging which is typically

timber, but which may also be reinforced shotcrete. These wall systems can be constructed in most

ground types, however, care must be exercised in grounds such as cohesionless soils and soft clays

that may have limited “stand-up” time for lagging installation. These wall systems are also highly

pervious. The construction sequence for a permanent soldier beam and lagging wall is illustrated in

figure 7 and is described below.

Figure 7. Construction sequence for permanent soldier beam and lagging wall.
13

2.3.2.2 Soldier Beam

The initial step of construction for a soldier beam and lagging wall consists of installing the soldier

beams from the ground surface to their final design elevation. Horizontal spacing of the soldier

beams typically varies from 1.5 to 3 m. The soldier beams may be steel beams or drilled shafts,

although drilled shafts are seldom used in combination with timber lagging.

Drilled-in Soldier Beams

Steel beams such as wide flange (WF) sections or double channel sections may be placed in

excavated holes that are subsequently backfilled with concrete. It is recommended that the

excavated hole be backfilled with either structural or lean-mix concrete from the bottom of the hole

to the level of the excavation subgrade. The selection of lean-mix or structural concrete is based on

lateral and vertical capacity requirements of the embedded portion of the wall and is discussed in

chapter 5. From the excavation subgrade to the ground surface, the hole should be backfilled with

lean-mix concrete that is subsequently scraped off during lagging and anchor installation. Structural

concrete is not recommended to be placed in this zone because structural concrete is extremely

difficult to scrape off for lagging installation. Lean-mix concrete typically consists of one 94 lb bag

of Portland cement per cubic yard of concrete and has a compressive strength that does not typically

exceed approximately 1 MPa. As an alternative to lean-mix concrete backfill, controlled low

strength material (CLSM) or “flowable fill” may be used. This material, in addition to cement,

contains fine aggregate and fly ash. When allowing lean-mix concrete or CLSM for backfilling

soldier beam holes, contract specifications should require a minimum compressive strength of 0.35

MPa. Like lean-mix concrete, CLSM should be weak enough to enable it to be easily removed for

lagging installation.

Ground anchors are installed between the structural steel sections and the distance between the

sections depends upon the type of ground anchor used. Drill hole diameters for the soldier beams

depend upon the structural shape and the diameter of the anchor. Replacement anchors can be

installed between the structural sections at any location along the soldier beam. The ground anchor

to soldier beam connection for drilled-in soldier beams can be installed on the front face of the

structural sections or between the sections. For small diameter ground anchors, the connection may

be prefabricated before the soldier beams are installed. The connections for large-diameter anchors

are made after the anchors have been installed.

Driven Soldier Beams

Steel beams such as HP shapes or steel sheet piles are used for driven soldier beams. Driven soldier

beams must penetrate to the desired final embedment depth without significant damage. Drive shoes

or “points” may be used to improve the ability of the soldier beams to penetrate a hard stratum. High

strength steels also improve the ability of the soldier beams to withstand hard driving. If the soldier

beams cannot penetrate to the desired depth, then the beams should be drilled-in. Thru-beam

connections or horizontal wales are used to connect ground anchors to driven soldier beams.
14

A thru-beam connection is a connection cut in the beam for a small diameter ground anchor. Thrubeam connections are usually fabricated before the beam is driven. This type of connection is

designed so the ground anchor load is applied at the center of the soldier beam in line with the web

of the soldier beam. Large-diameter (i.e., greater than approximately 150 mm) ground anchors

cannot be used with thru-beam connections. Thru-beam connections are used when few ground

anchor failures are anticipated because when a ground anchor fails, the failed anchor has to be

removed from the connection or a new connection has to be fabricated. A “sidewinder connection”

may be used with a replacement anchor for a temporary support of excavation wall, but it is not

recommended for a permanent wall. A sidewinder connection is offset from the center of the soldier

beam, and the ground anchor load is applied to the flange some distance from the web. Sidewinder

connections subject the soldier beams to bending and torsion.

Horizontal wales may be used to connect the ground anchors to the driven soldier beams. Horizontal

wales can be installed on the face of the soldier beams, or they can be recessed behind the front

flange. When the wales are placed on the front flange, they can be exposed or embedded in the

concrete facing. If the wales remain exposed, then the ground anchor tendon corrosion protection

may be exposed to the atmosphere and it is therefore necessary that the corrosion protection for the

anchorage be well designed and constructed. However, since exposed wales are unattractive and

must be protected from corrosion, they are not recommended for permanent anchored walls. Wales

placed on the front face of the soldier beams require a thick cast-in-place concrete facing. Wales can

be recessed to allow a normal thickness concrete facing to be poured. Recessed wales must be

individually fabricated and the welding required to install them is difficult and expensive. If a wale

is added during construction, the horizontal clear distance to the travel lanes should be checked

before approval of the change.

2.3.2.3 Lagging

After installation of the soldier beams, the soil in front of the wall is excavated in lifts, followed by

installation of lagging. Excavation for lagging installation is commonly performed in 1.2 to 1.5 m

lifts, however, smaller lift thicknesses may be required in ground that has limited “stand-up” time.

Lagging should be placed from the top-down as soon as possible after excavation to minimize

erosion of materials into the excavation. Prior to lagging installation, the soil face should be

excavated to create a reasonably smooth contact surface for the lagging. Lagging may be placed

either behind the front flange of the soldier beam or on the soldier beam. Lagging placed behind

soldier beam flanges is cut to approximate length, placed in-between the flanges of adjacent soldier

beams, and secured against the soldier beam webs by driving wood wedges or shims. Lagging can

also be attached to the front flange of soldier beams with clips or welded studs. In rare

circumstances, lagging can be placed behind the back flange of the soldier beam. With either

lagging installation method, gaps between the lagging and the retained ground must be backpacked

to ensure good contact. Prior to placing subsequent lagging a spacer, termed a “louver”, is nailed to

the top of the lagging board at each end of the lagging. This louver creates a gap for drainage

between vertically adjacent lagging boards. The size of the gap must be sufficiently wide to permit

drainage, while at the same time disallowing the retained soil to fall out from behind the boards.

Typically, placing vertically adjacent lagging boards in close contact is considered unacceptable,

however, some waterproofing methods may require that the gap between the lagging boards be

eliminated. In this case, the contractor must provide an alternate means to provide drainage.
15

Concrete lagging has been used, but its use may be problematic due to difficulties in handling and

very tight tolerances on the horizontal and vertical positioning of the soldier beam to ensure easy

installation of standard length concrete lagging. Trimming of concrete lagging is very difficult and

field splicing is not possible. Also, the concrete lagging near the anchor location may crack during

anchor testing or stressing.

2.3.2.4 Construction Sequence

Top-down installation of lagging continues until the excavation reaches a level of approximately 0.6

m below the design elevation of a ground anchor. At this point, the excavation is halted and the

ground anchor is installed. Deeper excavation (i.e., greater than 0.6 m) below the level of a ground

anchor may be required to allow the anchor connection to be fabricated or to provide equipment

access. The wall must be designed to withstand stresses associated with a deeper excavation. The

anchor is installed using appropriate drilling and grouting procedures, as previously described.

When the grout has reached an appropriate minimum strength, the anchor is load tested and then

locked-off at an appropriate load. Excavation and lagging installation then continues until the

elevation of the next anchor is reached and the next anchor is installed. This cycle of excavation,

lagging installation, and ground anchor installation is continued until the final excavation depth is

reached.

When the excavation and lagging reach the final depth, prefabricated drainage elements may be

placed at designed spacings and connected to a collector at the base of the wall. The use of shotcrete

in lieu of timber lagging can be effective in certain situations. However, since the shotcrete is of low

permeability, drainage must be installed behind the shotcrete. Drainage systems for anchored walls

are discussed further in chapter 5. For permanent walls, a concrete facing is typically installed. The

facing is either precast or CIP concrete.

2.3.3 Continuous Walls

Ground anchors are also used in continuous wall systems such as sheet-pile walls, tangent or secant

pile walls, slurry walls, or soil mixed walls. Continuous walls are commonly used for temporary

excavation support systems. Sheet-pile walls are constructed in one phase in which interlocking

sheet-piles are driven to the final design elevation. Where difficult driving conditions are

encountered, a template is often utilized to achieve proper alignment of the sheet-piles, however, it

should be recognized that these wall systems may not be feasible for construction in hard ground

conditions or where obstructions exist. Interlocking sheet-piles may be either steel or precast

concrete, however, steel sheet-piles are normally used due to availability and higher strength than

precast concrete sheet-piles. Additional information on wall construction procedures, materials, and

equipment for other continuous wall systems is presented in FHWA-HI-99-007 (1999).

Unlike soldier beam and lagging walls, continuous walls act as both vertical and horizontal wall

elements. Cycles of excavation and anchor installation proceed from the top of the excavation and

then between the level of each anchor. Because of the relative continuity of these wall systems,

water pressure behind continuous walls must be considered in design. In cases where the continuous

wall must resist permanent hydrostatic forces, a watertight connection must be provided at the

ground anchor/wall connection.
16

2.4 APPLICATIONS OF GROUND ANCHORS

2.4.1 Highway Retaining Walls

Anchored walls are commonly used for grade separations to construct depressed roadways, roadway

widenings, and roadway realignments. The advantages of anchored walls over conventional concrete

gravity walls have been described in section 1.2. Figure 8 provides a comparative illustration of a

conventional concrete gravity wall and a permanent anchored wall for the construction of a

depressed roadway. The conventional gravity wall is more expensive than a permanent anchored

wall because it requires temporary excavation support, select backfill, and possibly deep foundation

support. Anchored walls may also be used for new bridge abutment construction and end slope

removal for existing bridge abutments (see FHWA-RD-97-130, 1998).

Figure 8. Comparison of concrete gravity wall and anchored wall for a depressed roadway.
17

2.4.2 Slope and Landslide Stabilization

Ground anchors are often used in combination with walls, horizontal beams, or concrete blocks to

stabilize slopes and landslides. Soil and rock anchors permit relatively deep cuts to be made for the

construction of new highways (figure 9a). Ground anchors can be used to provide a sufficiently

large force to stabilize the mass of ground above the landslide or slip surface (figure 9b). This force

may be considerably greater than that required to stabilize a vertical excavation for a typical highway

retaining wall. Horizontal beams or concrete blocks may be used to transfer the ground anchor loads

to the ground at the slope surface provided the ground does not “run” or compress and is able to

resist the anchor reaction forces at the excavated face. Cost, aesthetics, and long-term maintenance

of the exposed face will affect the selection of horizontal beams or blocks.

2.4.3 Tiedown Structures

Permanent ground anchors may be used to provide resistance to vertical uplift forces. Vertical uplift

forces may be generated by hydrostatic or overturning forces. The method is used in underwater

applications where the structure has insufficient dead weight to counteract the hydrostatic uplift

forces. An example application of ground anchors to resist uplift forces is shown in figure 9c. The

advantage of ground anchors for tiedown structures include: (1) the volume of concrete in the slab is

reduced compared to a dead weight slab; and (2) excavation and/or dewatering is reduced.

Disadvantages of ground anchors for tiedowns include: (1) potentially large variations in ground

anchor load resulting from settlement and heave of the structure; and (2) difficulty in constructing

watertight connections at the anchor-structural slab interface, which is particularly important for

hydrostatic applications; and (3) variations in stresses in the slab. A major uplift slab that

incorporated tiedowns was constructed for the Central Artery Project in Boston, Massachusetts (see

Druss, 1994).

Although not a highway application, permanent rock anchor tiedowns may be used to stabilize

concrete dams (figure 9d). Existing dams may require additional stabilization to meet current safety

standards with respect to maximum flood and earthquake requirements. Anchors provide additional

resistance to overturning, sliding, and earthquake loadings.
18

Figure 9. Applications of ground anchors and anchored systems.
19

CHAPTER 3

SITE INVESTIGATION AND TESTING

3.1 INTRODUCTION

The purpose of this chapter is to describe basic site characterization and soil and rock property

evaluation for ground anchor and anchored system design. These activities generally include field

reconnaissance, subsurface investigation, in situ testing, and laboratory testing. The engineering

properties and behavior of soil and rock material must be evaluated because these materials provide

both loading and support for an anchored system.

Site investigation and testing programs are necessary to evaluate the technical and economical

feasibility of an anchored system for a project application. The extent of the site investigation and

testing components for a project should be consistent with the project scope (i.e., location, size,

critical nature of the structure, and budget), the project objectives (i.e., temporary or permanent

structures), and the project constraints (i.e., geometry, constructability, performance, and

environmental impact). Typical elements of a site investigation and testing program are described

herein.

3.2 FIELD RECONNAISSANCE

Field reconnaissance involves visual inspection of the site and examination of available documents

regarding site conditions. Information collected during field reconnaissance should include the

following:

• surface topography and adjacent land use;

• surface drainage patterns, and surface geologic patterns including rock outcrops, landforms,

existing excavations, and evidence of surface settlement;

• site access conditions and traffic control requirements for both investigation and

construction activities;

• areas of potential instability such as deposits of organic or weak soils, steep terrain slide

debris, unfavorably jointed or dipping rock, and areas with a high ground-water table;

• extent and condition (e.g., visible damage, corrosion) of existing above and below ground

utilities and structures; and

• available right-of-way (ROW) and easements required for the installation of ground anchors

and anchored systems.
20

3.3 SUBSURFACE INVESTIGATION

3.3.1 General

Subsurface investigation activities for anchored systems typically involve soil borings and rock

coring. Figure 10 illustrates guideline recommendations for locations of subsurface borings for a

permanent anchored wall or slope. Information on the subsurface soil and rock stratigraphy and

ground-water conditions are typically obtained from subsurface investigation activities. Subsurface

investigation may also involve conducting in situ soil or rock tests and obtaining disturbed and

undisturbed samples for laboratory testing. Detailed information and guidance on subsurface

investigation are provided in AASHTO (1988) and FHWA-HI-97-021 (1997).

Figure 10. Geotechnical boring layout for permanent anchored wall.
21

3.3.2 Soil and Rock Stratigraphy

The soil and rock stratigraphy at the project site, including the thickness, elevation, and lateral extent

of various layers, should be evaluated through implementation of a project-specific subsurface

investigation. The following potentially problematic soils and rock should also be identified during

the subsurface investigation which may significantly affect the design and construction of the

anchored system:

• cohesionless sands and silts which tend to ravel (i.e., cave-in) when exposed, particularly

when water is encountered, and which may be susceptible to liquefaction or vibrationinduced densification;

• weak soil or rock layers which are susceptible to sliding instability;

• highly compressible materials such as high plasticity clays and organic soils which are

susceptible to long-term (i.e., creep) deformations; and

• obstructions, boulders, and cemented layers which adversely affect anchor hole drilling,

grouting, and wall element installation.

As shown in figure 10, subsurface borings should be advanced at regular intervals along, behind, and

in front of the wall alignment or slope face. Borings should be located at the site extremities along

the wall alignment so that stratigraphy information can be interpolated from the boring information.

Typical boring spacing is 15 to 30 m for soil anchors and 30 to 60 m for rock anchors. The back

borings are located such that the borings are advanced within the anchor bond zone so that

potentially weak or unsuitable soil or rock layers can be identified. Back and wall boring depths

should be controlled by the general subsurface conditions, but should penetrate to a depth below the

ground surface of at least twice the wall or slope height. Front borings may be terminated at a depth

below the proposed wall base equal to the wall height. Borings should be advanced deeper if there is

a potential for soft, weak, collapsible, or liquefiable soils at depth. For very steeply inclined ground

anchors or for vertical anchors, borings should also, at a minimum, penetrate through to the depth of

the anchor bond zone. Additional borings may be required to characterize the geometry of a

landslide slip surface.

As a general recommendation, soil samples should be obtained at regular, approximately 1.5 m deep,

intervals and at all changes in the underlying soil strata for visual identification and laboratory

testing. Methods of soil sampling include the Standard Penetration Test (SPT) (ASTM D1586) and,

for cohesive soils, the use of thin-wall tubes (ASTM D1587). The cone penetration test (CPT)

(ASTM D3441) may be used, if necessary, to develop a continuous subsurface soil profile.

A minimum rock core of 3 m should be recovered for subsurface conditions in which bedrock is

encountered within the previously recommended investigation depths and for all designs that include

rock anchors. A description of rock type, mineral composition, texture (i.e., stratification, foliation),

degree of weathering, and discontinuities is generally obtained. An estimate of intact rock strength

can be evaluated using percentage of core recovery and rock quality designation (RQD). The

orientations (i.e., strike and dip) of discontinuities and fractures should be included whenever

possible in the rock description so that the potential for sliding instability can be evaluated. This

latter information may be available from rock outcrop exposures at or near the site. For jointed rock

which has been infilled with soil, the joint fill material should be sampled for laboratory shear
22

strength testing. Soil samples and rock cores collected during the site investigation should be

preserved and made available to the designer and the contractor during the design and bidding phase

of a project, respectively.

3.3.3 Groundwater

The groundwater table and any perched groundwater zones must be evaluated as part of a subsurface

investigation program. The presence of ground water affects overall stability of the system, lateral

pressures applied to the wall facing, vertical uplift forces on structures, drainage system design,

watertight requirements at anchor connections, corrosion protection requirements, and construction

procedures. At a minimum, the following items need to be considered for anchored systems that will

be constructed within or near the groundwater table:

• average high and low groundwater levels;

• corrosion potential of ground anchors based on the aggressivity of the ground water;

• soil and/or rock slope instability resulting from seepage forces;

• necessity for excavation dewatering and specialized drilling and grouting procedures; and

• liquefaction potential of cohesionless soils.

For tiedown structures designed to resist uplift forces, unanticipated changes in groundwater levels

can result in excessive consolidation settlement and a resulting decrease in ground anchor loads for

cases in which the groundwater level decreases. For cases in which the groundwater level increases,

ground anchor loads may increase above design loads.

Groundwater level information is often obtained by observation of the depth to which water

accumulates in an open borehole at the time of, or shortly after, exploration. It is important to allow

sufficient time to pass after borehole excavation so that water levels can reach equilibrium. Water

levels in the subsurface may be measured more accurately using piezometers or observation wells.

Water level measurements can be made over a duration of time to obtain an indication of potential

water level fluctuations.

3.4 LABORATORY SOIL AND ROCK TESTING

3.4.1 General

Laboratory testing of soil and rock samples recovered during subsurface exploration is often

performed to evaluate specific properties necessary for the design of an anchored system. In this

section, laboratory tests typically performed to evaluate properties of soil and rock materials are

presented along with appropriate ASTM and AASHTO testing specifications.
23

3.4.2 Classification and Index Properties

All soil samples taken from borings and rock core samples should be visually identified in the

laboratory and classified according to ASTM D2488 and ASTM D2487 or the Unified Rock

Classification System (URCS). Index soil properties used in the analysis and design of anchored

systems include unit weight, moisture content, gradation, and Atterberg limits. Unit weights of

foundation material and retained soil are used in evaluating earth pressures and in evaluating the

external stability of the anchored system. Moisture content (ASTM D2216) information and

Atterberg limits (ASTM D4318; AASHTO T89, T90) may be used with existing correlations to

estimate compressibility and shear strength of in situ clayey soils and to evaluate the suitability of

ground anchors in cohesive soils. In addition, the presence of organic materials should be

determined by either visual description or according to ASTM D2974. The results of soil grain size

distribution testing (ASTM D422; AASHTO T88) can be used to develop appropriate drilling and

grouting procedures for ground anchors and to identify potentially liquefiable soils.

3.4.3 Shear Strength

Unconfined compression (ASTM D2166; AASHTO T208), direct shear (ASTM D3080; AASHTO

T236), or triaxial compression (ASTM D4767; AASHTO T234) testing are typically performed to

evaluate soil shear strength. Total stress and effective stress strength parameters of cohesive soils

are typically evaluated from the results of undrained triaxial tests with pore pressure measurements.

For permanent anchor applications involving cohesive soils, both undrained and drained strength

parameters should be obtained, and the design of the anchored system should consider both shortterm and long-term conditions. For critical applications involving cohesionless soils, direct shear or

triaxial compression testing can be used to evaluate drained shear strength. Typically, however,

drained shear strength of cohesionless soil is usually evaluated based on correlations with in situ test

results (e.g., SPT and CPT). The selection of design soil shear strengths for anchored systems is

described in chapter 4.

Laboratory strength testing of intact rock samples is not often performed for anchored system

applications. For the actual field conditions, the strength of the rock mass is typically controlled by

discontinuities. If, however, no adverse planes of weakness exist, the compressive strength of the

intact rock, evaluated using unconfined compression (ASTM D2938), direct shear (ASTM D5607),

or triaxial compression (ASTM D2664; AASHTO T226) testing, may be used to estimate ultimate

bond stress (see PTI 1996).

3.4.4 Consolidation

Settlement analyses are not commonly performed for anchored systems constructed in stiff soils and

cohesionless soils, but should be performed for structures subjected to groundwater drawdown (both

during construction and for long-term conditions) that are constructed in compressible soils.

Excessive settlement in these applications may be detrimental to nearby structures and these

settlements may result in long-term lateral movements of anchored systems that exceed tolerable

limits. The results of index tests including moisture content and Atterberg limits can be used for

initial evaluation of settlement parameters. Results of one-dimensional consolidation (ASTM

D2435; AASHTO T216) tests are used to evaluate the parameters necessary for a settlement

analysis.
24

3.4.5 Electrochemical Criteria

For permanent anchored systems, the aggressiveness of the ground must be evaluated. Aggressive

ground conditions usually do not preclude use of anchored systems if proper corrosion protection for

the anchored system is provided. Corrosion potential is of primary concern in aggressive soil

applications and is evaluated based on results of tests to measure the following properties: (1) pH

(ASTM G51; AASHTO T289); (2) electrical resistivity (ASTM G57; AASHTO T288); (3) chloride

content (ASTM D512; AASHTO T291); and (4) sulfate content (ASTM D516; AASHTO T290).

Detailed information on ground anchor corrosion and corrosion protection measures is described in

chapter 6.

3.5 IN SITU SOIL AND ROCK TESTING

In situ testing techniques are often used to estimate several of the soil properties previously

introduced in section 3.4. There are in situ testing techniques which can be used to estimate rock

properties, although the use of in situ testing in rocks is not as widespread as the use in soils.

The SPT is the most common in situ geotechnical test used in evaluating the suitability of ground

anchors in cohesionless soils. The SPT blowcount value N can be used to estimate the relative

density (see table 1) and shear strength of sandy soils. The advantage of the SPT over other in situ

tests is that its use is widespread throughout the U.S. and a disturbed sample can be obtained for

visual identification and laboratory index testing. For cohesionless soils, SPT N<10 may indicate

that the ground is not suitable for ground anchors. SPT blowcounts may be used to evaluate the

consistency of cohesive soil strata (see table 1), but not as a reliable indication of shear strength.

Table 1. Soil density/ consistency description based on SPT blowcount values

(after AASHTO, 1988).

Cohesionless Soils Cohesive Soils

Relative Density SPT N (blows/300 mm) Consistency SPT N (blows/300 mm)

Very loose 0-4 Very soft 0-1

Loose 5-10 Soft 2-4

Medium dense 11-24 Medium stiff 5-8

Dense 25-50 Stiff 9-15

Very dense > 51 Very stiff 16-30

Hard 31-60

Very hard >61

Other in situ testing procedures may be used to evaluate the suitability of ground anchors for a

particular type of ground. These include: (1) CPT; (2) vane shear test (FVT) (ASTM D2573); (3)

pressuremeter test (PMT) (ASTM D4719); and (4) flat plate dilatometer test (DMT). The following

studies and reports by FHWA have been devoted to the use of in situ testing techniques in soil:

• Cone Penetration Test (FHWA-SA-91-043, 1992);

• Pressuremeter Test (FHWA-IP-89-008, 1989); and
25

• Flat Plate Dilatometer Test (FHWA-SA-91-044, 1992).

Basic information on these tests is summarized in table 2. Empirical correlations have been

developed and may be used to obtain a preliminary estimate of property values. These correlations

are published elsewhere (e.g., Kulhawy and Mayne, 1990). In many parts of the country,

correlations have been developed for these tests in recognition of local soils and local conditions.

Table 2. Summary of common in situ tests for soils.

Type of Test Suitable for Not suitable for Properties that can be estimated

SPT sand soft to firm clays,

gravels

Stratigraphy, strength, relative density

CPT sand, silt, and clay gravel Continuous evaluation of stratigraphy,

strength of sand, undrained shear

strength of clay, relative density, in situ

stress, pore pressures

FVT soft to medium clay sand and gravel undrained shear strength

PMT soft rock, dense

sand, nonsensitive

clay, gravel, and till

soft, sensitive

clays, loose silts

and sands

strength, Ko, OCR, in situ stress,

compressibility, hydraulic conductivity,

elastic shear modulus

DMT sand and clay gravel soil type, Ko, OCR, undrained shear

strength, and elastic modulus

27

(a) Tensile failure of

tendon

(b) Pullout failure of

grout/ground bond

(c) Pullout failure of

tendon/grout bond

(d) Failure of wall in bending (e) Failure of wall due to

insufficient passive capacity

(g) Failure due to insufficient (h) Failure by overturning

 axial capacity

(f) Failure by forward rotation

(cantilever before first anchor installed)

(i) Failure by sliding (j) Rotational failure of

ground mass

Figure 11. Potential failure conditions to be considered in design of anchored walls.
28

Figure 12. Contribution of ground anchors to wall stability.

4.2 FAILURE MECHANISMS OF ANCHORED SYSTEMS

4.2.1 General

Many different types of anchored systems can usually fulfill the needs of a particular project. To

achieve maximum economy, the objective of the designer is to specify only those parameters that are

necessary for long-term stability of the anchored system and to leave the final selection of the anchor

details to the contractor. Anchor system performance is evaluated by testing each installed anchor at

loads that exceed the design load. To determine the parameters that should be specified, the designer

must consider various possible failure mechanisms.

4.2.2 Failure Mechanisms of the Ground Anchor

There are several possible failure mechanisms of ground anchors. These are usually caused by

excessive static loading of an anchor. Excessive loads can be related to: (1) tension placed in the

anchor during load testing or at lock-off; (2) excavation sequence; (3) surcharge by construction

materials or equipment; (4) construction of adjacent structures; or (5) a combination of these causes.

Ground anchor failure mechanisms may involve the steel tendon, the ground mass, the ground-grout

zone, and the grout-tendon zone, as described subsequently.

Failure of the Steel Tendon

As the anchor is loaded, the steel tendon component of the anchor is stressed in tension. If the

applied load is greater than the structural capacity of the tendon, failure is inevitable. Therefore, a

factor of safety must be used with respect to structural failure of the steel. It is recommended that the

tendon load not exceed 60 percent of the specified minimum tensile strength (SMTS) for final design

and 80 percent of SMTS for temporary loading conditions (e.g., loading during testing).
29

Failure of the Ground Mass

Failure of the soil mass, as referred to herein, involves failure resulting from anchor loads, not

external forces such as landslides which potentially introduce excessive static loading to the anchor.

For shallow soil anchors, failure of the ground mass is characterized by uplift of a mass of soil in

front of the anchor bond zone followed by pullout of the bond zone. A shear surface develops in the

soil mass ahead of the anchor as increasing stresses cause complete mobilization of resistance in the

anchor bond zone. The failure surface simulates a passive earth pressure failure. Practically, failure

of the soil mass is not a factor for anchors embedded more than 4.5 m below the ground surface.

For rock anchors, the likely plane of failure for shallow installations in sound bedrock is along a cone

generated at approximately a 45 degree angle from the anchorage. In fractured or bedded rock, the

cone shape and size varies with the distribution of bedding and cleavage planes and the grout take in

fissures. Even in fractured rock, rock mass failure seldom occurs in anchors embedded more than

4.5 m below ground because the bond strength between the rock and grout or the grout and tendon is

much less than the rock strength.

Failure of the Ground-Grout Bond

Ground anchors mobilize skin friction between the anchor bond zone and the ground. In general,

this bond is dependent on the normal stress acting on the bond zone grout and the adhesion and

friction mobilized between the ground and the grout. Anchors which are underreamed may also

develop the base resistance of the increased annular area.

In general, the ground-grout bond is mobilized progressively in uniform soil or rock as the stress is

transferred along the bond length. Initially, as the anchor is stressed, the portion of the bond length

nearest the load application elongates and transfers load to the ground. As the resistance in this

portion of the bond length is mobilized, stress is transferred farther down. During this process, the

anchor continues to elongate to mobilize deeper bond zones. Once the stress is transferred to the end

of the bond zone and the ultimate ground-grout bond is exceeded, anchor failure by pullout occurs.

Anchors which have been improperly grouted such that a column of grout exists between the bearing

plate or wall and the top of the bond zone will show no load transfer into the bond length when the

load is increased. Factors influencing stress transfer for small diameter ground anchors with bond

lengths in a uniform soil are summarized in table 3.

Experience has shown that increasing the bond length for typical soil anchors beyond 9 to 12 m does

not result in significant increases in resistance. A possible reason for this observation is that after the

load has been transferred that distance down the bond zone, sufficient movement at the ground-grout

interface has occurred in the upper bond length to decrease the upper ground-grout interface

resistance to residual strength levels. Bond lengths greater than 12 m may be used effectively

provided special procedures are used to bond the tendon to the grout such that capacity can be

mobilized along the longer length.
30

Table 3. Typical factors influencing bond stress transfer for small diameter ground anchors.

Soil Type

Factor Cohesionless Cohesive

Soil Properties Friction angle and grain size

distribution.

Adhesion and plasticity index.

Drilling Method Driven casing increases

normal stress and friction.

Drilling without casing or with fluids

decreases capacity.

Bond Length Steady increase in anchor

capacity to 6 m with

moderating increases to 12 m.

Steady increase in anchor capacity for

soils with undrained strength less than

96 kPa.

Hole Diameter Slight increase in anchor

capacity to 100 mm.

Anchor capacity increases to 300 mm.

Grout Pressure Anchor capacity increases

with increasing pressure.

Anchor capacity increases only with

stage grouting. High initial pressures

should be avoided.

Note: To ensure ground-grout bond, the drill hole should be cleaned and the grout should be placed

as quickly as possible after the hole has been drilled.

Failure at the ground-grout interface may also be characterized by excessive deformations under

sustained loading (i.e., creep). Soil deposits that are potentially susceptible to excessive creep

deformations include: (1) organic soils; (2) clay soils with an average liquidity index (LI) greater

than 0.2; (3) clay soils with an average liquid limit (LL) greater than 50; and (4) clay soils with an

average plasticity index (PI) greater than 20. Conservative anchor design loads and working bond

stress values are recommended for design involving permanent anchor installations in such soils,

unless based on results from a predesign or preproduction test program. Predesign and

preproduction test programs are described in section 5.3.6.

The LL, plastic limit (PL) and moisture content (wn) of a clay soil are commonly measured clayey

soil index properties. The LI indicates where the moisture content of the clay falls within the range

between the plastic and liquid limits. Liquidity index for a soil is defined as:

PI

w PL

LI n − = (Equation 1)

A low LI indicates that the moisture content is relatively close to the PL of the soil, indicating a

potentially overconsolidated or stiff soil. A LI close to 1.0 indicates that the moisture content is

relatively close to the LL for the soil, indicating a potentially normally consolidated or soft soil.
31

Failure of Grout-Tendon Bond

The bond between the grout and steel tendon must not be exceeded if the full strength of the

supporting ground is to be mobilized. The failure mechanism of the grout-tendon bond involves

three components: (1) adhesion; (2) friction; and (3) mechanical interlock. Adhesion is the physical

coalescence of the microscopically rough steel and the surrounding grout. This initial bond is

replaced by friction after movement occurs. The friction depends on the roughness of the steel

surface, the normal stress, and the magnitude of the slip. Mechanical interlock consists of the grout

mobilizing its shear strength against major tendon irregularities such as ribs or twists. This interlock

is the dominant bond mechanism for threadbars where the ultimate strength of the bar may be

developed in a short embedment in the grout. The grout-tendon bond on smooth steel tendons is

mobilized progressively in a fashion similar to the ground-grout bond. “Slip” occurs only after the

maximum intensity of grout-tendon bond resistance has been mobilized over nearly the total bond

length. After this slip, the tendon will only offer frictional resistance (amounting to about half the

maximum total resistance obtained) to further elongation. Experience has shown that:

• Bond resistance of the grout to the tendon is not linearly proportional to the compressive

strength of the grout. Although the bond strength usually increases as the compressive

strength of the grout increases, the ratio of bond to ultimate strength decreases with

increasing grout strengths. For example, a 17.2 MPa bond strength for 27.6 MPa grout may

only increase by 12 percent to 19.3 MPa when the grout strength is increased by 25 percent

to 34.5 MPa.

• Bond resistance developed by added embedment increases as the tendon length increases,

but at reduced unit values.

• Flaky rust on bars lowers the bond, but wiping off the loosest rust produces a rougher

surface which develops a bond equal to or greater than an unrusted bar. Obviously pitted

bars cannot be accepted even though the grout tendon bond may be adequate.

• The loose powdery rust appearing on bars after short exposures does not have a significant

effect on grout-tendon bond.

Mill test reports should be requested by the owner for each lot used to fabricate the tendons. Test

reports should include the results of bond capacity tests performed in accordance with the

prestressing strand bond capacity test described in ASTM A981. ASTM A981 provides a standard

test method to evaluate the bond strength between prestressing strand and cement grout. This

specification was developed in 1997 in response to an industry initiative concerning the effects of

certain residues from the manufacturing process that appeared to reduce the bond between the strand

and the cement grout.

4.2.3 Failure of Soldier Beams

Soldier beams are subject to both lateral and vertical loads from the retained soil mass and the forces

imparted from prestressing the anchors. The lateral resistance of the soldier beam is most critical

during stressing and testing of the first anchor level, and for the final excavation condition when all

wall loads have been applied. In the former case, stressing of the upper anchor to the test load is
32

often done at shallow depths where the available passive resistance behind the soldier beam is low.

Soldier beam deflections can be minimized in design by applying a safety factor of 1.5 to the passive

resistance and in construction by ensuring that the upper lagging is tight against the soil and that the

soil behind the soldier beam has not been removed. For the final excavation condition, the passive

resistance in front of the wall must be adequate to restrain the toe of the soldier beam for long term

wall loadings and for any future undercuts of the area in front of the wall.

Load transfer of the vertical loads on the soldier beams is more complex than for simple deep

foundation elements. As the excavation for the wall deepens, vertical load is transferred above grade

to the soil behind the back face of the soldier beam, but the magnitude of the load that is transferred

is difficult to estimate. Theoretically, if adequate downward movement of the soldier beam (relative

to the soil) occurs, load will be transferred to the soil mass behind the wall. However, this load

transfer also results in the development of a negative interface wall friction angle for the active block

of soil behind the wall resulting in an increase in the earth pressures behind the wall. In this

document, it is assumed that no load transfer (i.e., interface wall friction angle = 0°) occurs above the

excavation base since: (1) relative movements between the soldier beam and soil are small; (2)

removal of soil from the excavation face may reduce the “bond” between the soldier beam and soil;

and (3) the actual amount of load transferred is usually small. Other design procedures which utilize

load transfer above the excavation base can be used if appropriate documentation can be provided on

relative movements required to develop load transfer.

Vertical load capacity below the excavation base is calculated using common procedures for deep

foundations (i.e., driven piles or drilled shafts). Two issues, however, are unique to evaluating axial

capacity for soldier beam walls and must be considered. These issues are described below.

• Stress relief in front of the wall caused by excavation will reduce the effective stresses acting

on the embedded portion of the soldier beam. This reduction in stress may vary with depth

based on the width of the excavation. Common practice is to assume the effective stress is

equal to the average of the effective stress imparted by the retained soil height behind the

wall and by the depth of the soil in front of the wall.

• Structural sections are commonly placed in predrilled holes which are filled with concrete.

In the case of a structural concrete filling, it is usually assumed that axial and lateral load are

shared by the steel and the concrete and lateral capacity computations may be performed on

the basis of the hole diameter. However, in the case of nonstructural (i.e., “lean-mix”)

concrete, the shear capacity between the structural section and the lean-mix concrete fill may

not be adequate to provide load sharing between the steel and the concrete. This shear

capacity should therefore be checked as part of the determination of axial and lateral soldier

beam capacity.

4.2.4. Failure of Lagging

In general, the timber lagging is only used for support of temporary loads applied during excavation,

however, pressure-treated timber lagging has been used to support permanent loads. The

contribution of the temporary lagging is not included in the structural design of the final wall face.

Temporary timber lagging is not designed by traditional methods, rather lagging is sized from charts
33

developed based on previous project experience which accounts for soil arching between adjacent

soldier beams (FHWA-RD-75-130, 1976).

4.3 SELECTION OF SOIL SHEAR STRENGTH PARAMETERS FOR DESIGN

4.3.1 General

The purpose of this section is to provide guidance on selection of soil shear strength parameters for

anchored system design. Shear strength parameters of the retained soil are required to evaluate earth

pressures acting on a wall, axial and lateral capacity of the embedded portion of a wall, and external

stability of an anchored system. The evaluation of shear strength parameters for temporary walls

constructed in normally to lightly overconsolidated soft to medium clays and for temporary and

permanent walls constructed in heavily overconsolidated stiff to hard clays is emphasized herein.

4.3.2 Drained Shear Strength of Granular Soils

The drained shear strength of granular soil is conventionally represented by a drained effective stress

friction angle, φ’. Because the undisturbed sampling of granular soil deposits is difficult, the

representative friction angle to be used for wall design may be estimated using the results of in situ

penetration tests such as the SPT and the CPT.

4.3.3 Undrained Shear Strength of Normally Consolidated Clay

Instability under undrained conditions develops mainly under the condition of contractive shear, i.e.,

the mechanism of deformation which attempts to mobilize frictional shearing resistance also causes

the soil to want to contract under the prevailing confining stresses. This tendency to contract during

shear is typical for normally to lightly overconsolidated soft to medium clay soils. Since this

tendency cannot be realized, due to the clay soil permeability in relation to the rate of shearing,

positive porewater pressures are generated in the soil which reduce the effective stress and hence the

mobilized frictional shearing resistance. In such cases the short term undrained shearing resistance

in the soil is less than would have been the case if drainage (contraction of the soil volume) could

have occurred. The short term condition is critical for temporary anchored walls constructed in

normally to lightly overconsolidated clay soils.

The undrained shear strength, Su, may be determined by in situ (e.g., CPT, FVT) and laboratory

testing methods. A detailed discussion of the methods used to evaluate Su is beyond the scope of this

document, but this information may be found elsewhere (e.g., Kulhawy and Mayne, 1990). Typically

Su is evaluated using laboratory triaxial tests on nominally undisturbed cohesive soil samples at the

natural water content of the soil. The preferred method to evaluate the undrained strength in the

laboratory is through consolidated undrained triaxial testing with pore pressure measurements. The

use of unconfined compression tests and/or unconsolidated undrained triaxial tests may lead to

erroneous measured strengths due to sampling disturbance and the omission of a reconsolidation

phase.
34

The undrained shear strength is not a fundamental property of a soil and is affected by the mode of

testing, boundary conditions, rate of loading, initial stress state, and other variables. Consequently,

the measured undrained shear strength should be different depending on the type of test performed.

The designer should consider how the actual undrained shear strength mobilized under field loading

conditions differs from that measured using laboratory or in situ testing methods. For example, for a

temporary anchored wall in soft to medium clay, the undrained shear strength used to evaluate the

earth pressures acting on the wall may be determined from a triaxial compression test. The lateral

capacity of the wall toe, however, is more appropriately evaluated using the undrained strength from

a triaxial extension test. The extension loading path more accurately approximates the unloading

caused by soil excavation as compared to a compression loading path and, more importantly,

experience has shown that strength in the passive zone (inside the excavation) can be less than that in

the active zone (in the retained ground) for certain clay soils. Alternatively, correlations may be used

to “convert” the undrained strength measured in a conventional triaxial compression test into an

undrained strength for a different loading path (see Kulhawy and Mayne, 1990).

4.3.4 Undrained Shear Strength of Overconsolidated Clay

In clay soils subjected to unloading conditions that result from excavation to form an anchored wall,

the soil attempts to expand as it mobilizes frictional shearing resistance. This is resisted causing

negative porewater pressure to be developed which increases the effective stress in the soil and hence

increases the mobilized frictional shearing resistance. Thus, in an overconsolidated clay subject to

excavation, the short-term (undrained) strength and stability potentially exceeds that which would

apply once drainage has occurred.

Temporary and permanent anchored walls are commonly constructed in stiff to hard

overconsolidated clays. Heavily overconsolidated clay soils are often fissured. Due to the fissured

nature of overconsolidated clays, which can permit relatively rapid local drainage at the level of the

discontinuities in the clay, it is generally difficult to define with any certainty the period of time

during which the enhanced undrained shear strength of the clay may reliably be assumed to apply.

Therefore, in overconsolidated clays, design analyses should be performed in terms of drained,

effective stress parameters. Drained strength parameters for overconsolidated clays are discussed

subsequently.

4.3.5 Drained Shear Strength of Overconsolidated Clay

The behavior of an overconsolidated stiff clay can be illustrated as shown in figure 13. As the

sample is sheared under drained conditions, the displacement of the soil sample is relatively uniform

until the peak stress, τp, is reached. After the peak, displacements begin to concentrate on the newly

formed failure plane or discontinuity, and the shear stress reduces to τd. The shear strength, τd, of the

newly formed discontinuity is approximately equal to the shear strength of the same clay constituents

in a normally consolidated state (i.e., the fully softened strength), such as that produced by laboratory

consolidation from a slurry. For relatively high plasticity clays, further displacement beyond that

corresponding to the fully softened strength results in a continued reduction in shear stress, and,

eventually, at very large displacements along a major discontinuity, the residual strength of the clay

soil, τr, is reached.
35

Figure 13. Simplified drained stress-displacement relationship for a stiff clay (modified after

Construction Industry Research and Information Association (CIRIA), 1984).

For anchored systems in stiff to hard overconsolidated clays, the designer must decide as to which

strength, i.e., peak, fully softened, or residual, should be used for design. Since the additional

strength at peak resulting from cohesion (c’ in figure 13) tends to reduce relatively rapidly with

increasing strain beyond peak, soil deformations associated with flexible anchored walls may be

sufficient to appreciably reduce this cohesion. Therefore, unless local experience indicates that a

particular value of cohesion can be reliably accounted for, zero cohesion should be used in the

analyses of anchored walls in stiff to hard fissured clays for long-term (drained) conditions.

Conservative drained shear strength for analysis of anchored walls is therefore the fully softened

strength. This strength may be evaluated using triaxial compression testing with pore pressure

measurements.

Residual strengths should be used for anchored systems that are designed for a location in which

there is evidence of an existing failure surface within the clay (e.g., an anchored system used to

stabilize an active landslide). For these conditions, assume that sufficiently large deformations have

occurred to reduce the strength to a residual value. A study by Stark and Eid (1994) presents a

correlation between residual friction angle and the clay size fraction and liquid limit for clay soils.
36

4.4 EARTH PRESSURES

4.4.1 General

A wall system is designed to resist the lateral earth pressures and water pressures that develop behind

the wall. Earth pressures develop primarily as a result of loads induced by weight of the retained

soil, earthquake ground motions, and various surcharge loads. For purposes of anchored wall system

design, three different lateral earth pressure conditions are considered: (1) active earth pressure; (2)

passive earth pressure; and (3) at-rest earth pressure.

The distinction between actual ground behavior and conventional design assumptions is particularly

important when considering earth pressures. The simple linear assumptions about active and passive

pressures based on theoretical analyses are a considerable simplification of some very complex

processes which depend on the following factors: (1) the mode of wall movement (rotation,

translation); (2) wall flexibility; (3) soil stiffness and strength properties; (4) horizontal prestress in

the ground; and (5) and wall/soil interface friction. For anchored wall systems with flexible wall

elements, semi-empirical “apparent earth pressure envelopes” are commonly used.

4.4.2 Active and Passive Earth Pressure

Active and passive horizontal earth pressures may be considered in terms of limiting horizontal

stresses within the soil mass, and, for purposes of this discussion, a smooth (i.e., zero wall friction)

wall retaining ground with a horizontal backslope is considered (figure 14); this case defines

Rankine conditions. Consider an element of soil in the ground under a vertical effective stress, σv′

(figure 15). In considering the potential movements of a retaining wall, the element may be brought

to failure in two distinct ways that are fundamentally important in the context of retaining wall

design. The horizontal soil stress may be increased until the soil element fails at B, when the stress

reaches its maximum value c’h (max). This scenario will occur when significant outward movement of

the wall increases the lateral earth pressure in the soil at the base of the wall (see figure 14).

Similarly, the horizontal stress may be reduced until failure at A, when the stress reaches its

minimum value c’h (min). This scenario models the outward movement which reduces the lateral

earth pressures behind the wall (see figure 14).
37

Figure 14. Mobilization of Rankine active and passive horizontal pressures

 for a smooth retaining wall.

Figure 15. Limiting active and passive horizontal pressures.
38

The geometry of figure 15 gives the following two relationships:

tan (45 2) 1 sin

1 sin K ' 2 '

'

'

A

v

h (min) = − φ

+ φ

− φ = = σ′

σ

(Equation 2)

tan (45 2) 1 sin

1 sin K ' 2 '

'

'

P

v

h (max) = +φ − φ

+ φ = = σ′

σ

(Equation 3)

where KA is the active earth pressure coefficient and KP is the passive earth pressure coefficient. The

definitions of KA and KP, based on equations 2 and 3, are consistent with a Rankine analysis for a

cohesionless (i.e., c=0) retained soil.

For a cohesive soil defined by effective stress strength parameters φ′ and c′, the active and passive

earth pressure coefficients are:

tan (45 / 2) 2c K tan (45 / 2) '

'

v

'

2 '

A − φ

σ = − φ − (Equation 4)

tan (45 / 2) 2c K tan (45 / 2) '

'

v

'

2 '

P + φ

σ = + φ + (Equation 5)

For the undrained case with φ = 0 and c = Su, the total stress active and passive earth pressure

coefficients are:

v

u

AT

2S K 1

σ = − (Equation 6)

v

u

PT

2S K 1

σ = + (Equation 7)

where σv is the total vertical stress.

For most anchored wall applications, the effect of wall friction on active earth pressures is relatively

small and is often ignored. The active earth pressure coefficient, KA, may be evaluated using the

appropriate equations from above or, for more general cases, from the lower part of figure 16 or

figure 17. The earth pressure coefficients depicted in figure 16 and figure 17 are based on the

assumption of log-spiral shaped failure surfaces for the active and passive sides of the wall. To

evaluate the passive earth pressure coefficient, KP, the upper part of figure 16 or 17 should be used.

It is acknowledged that in addition to the Rankine equations and the log-spiral method, a third

closed-form technique, herein referred to as the Coulomb method, is often used to calculate lateral

earth pressures. For this method, equations are available to calculate KA and KP (NAVFAC, 1982).

While calculations of KA are considered to be reasonable, the Coulomb method is unreliable for

evaluating passive earth pressures since the planar shape of the assumed Coulomb failure surface is
39

Figure 16. Active and passive earth pressure coefficients (effect of wall inclination).
40

Figure 17. Active and passive earth pressure coefficients (effect of backslope inclination).