Research, Development, and Technology

Turner-Fairbank Highway Research Center

6300 Georgetown Pike

McLean, VA 22101-2296

Geosynthetic Reinforced Soil

Integrated Bridge System 

Interim Implementation Guide

Publication No. FHWA-HRT-11-026 January 2011
FOREWORD

Geosynthetic Reinforced Soil (GRS) technology consists of closely spaced layers of geosynthetic 

reinforcement and compacted granular fill material. GRS has been used for a variety of earthwork

applications since the U.S. Forest Service first used it to build walls for roads in steep mountain 

terrain in the 1970s. Since then, the technology has evolved into the GRS Integrated Bridge

System (IBS), a fast, cost-effective method of bridge support that blends the roadway into the 

superstructure. GRS-IBS includes a reinforced soil foundation, a GRS abutment, and a GRS

integrated approach. The application of IBS has several advantages. The system is easy to design and

economically construct. It can be built in variable weather conditions with readily available labor,

materials, and equipment and can easily be modified in the field. This method has significant value

when employed for small, single-span structures meeting the criteria described in this manual.

As a result of the demonstrated performance of GRS-IBS, the technology was selected for the 

Federal Highway Administration’s (FHWA) Every Day Counts initiative, aimed at accelerating

implementation of proven, market-ready technologies. This manual is the first in a two-part series

and outlines the design and construction of GRS-IBS. The second document is a synthesis report 

to substantiate the design method. Both documents are a collaboration between many disciplines 

within FHWA: geotechnical, structural, hydraulic, maintenance, and pavement engineering. 

Jorge Pagán-Ortiz

Director, Office of Infrastructure

Research and Development

Notice

This document is disseminated under the sponsorship of the U.S. Department of Transportation 

in the interest of information exchange. The U.S. Government assumes no liability for the use 

of the information contained in this document. This report does not constitute a standard, 

specification, or regulation.

The U.S. Government does not endorse products or manufacturers. Trademarks or 

manufacturers’ names appear in this report only because they are considered essential to the 

objective of the document.

Quality Assurance Statement

The Federal Highway Administration (FHWA) provides high-quality information to serve 

Government, industry, and the public in a manner that promotes public understanding. Standards 

and policies are used to ensure and maximize the quality, objectivity, utility, and integrity of its 

information. FHWA periodically reviews quality issues and adjusts its programs and processes to 

ensure continuous quality improvement.
TECHNICAL REPORT DOCUMENTATION PAGE

1. Report No.

FHWA-HRT-11-026

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

4. Title and Subtitle

Geosynthetic Reinforced Soil Integrated Bridge System, Interim 

Implementation Guide

5. Report Date

January 2011

6. Performing Organization Code: 

7. Author(s)

Michael Adams, Jennifer Nicks, Tom Stabile, Jonathan Wu, 

Warren Schlatter, and Joseph Hartmann

8. Performing Organization Report No. 

9. Performing Organization Name and Address

Office of Infrastructure Research and Development

Federal Highway Administration

6300 Georgetown Pike

McLean, VA 22181

10. Work Unit No.

11. Contract or Grant No.

12. Sponsoring Agency Name and Address

Federal Highway Administration

U.S. Department of Transportation

1200 New Jersey Avenue, SE

Washington, DC 20590

13. Type of Report and Period Covered

14. Sponsoring Agency Code

15. Supplementary Notes

The FHWA Contracting Officer’s Technical Representative (COTR) was Mike Adams, HRDS-40.

16. Abstract

This manual outlines the state-of-the-art and recommended practice for designing and constructing Geosynthetic 

Reinforced Soil (GRS) technology for the application of the Integrated Bridge System (IBS). The procedures 

presented in this manual are based on 40 years of State and Federal research focused on GRS technology as 

applied to abutments and walls.

This manual was developed to serve as the first in a two-part series aimed at providing engineers with the 

necessary background knowledge of GRS technology and its fundamental characteristics as an alternative to 

other construction methods. The manual presents step-by-step guidance on the design of GRS-IBS. Analytical 

and empirical design methodologies in both the Allowable Stress Design (ASD) and Load and Resistance Factor 

Design (LRFD) formats are provided. Material specifications for standard GRS-IBS are also provided. Detailed 

construction guidance is presented along with methods for the inspection, performance monitoring, maintenance,

and repair of GRS-IBS. Quality assurance and quality control procedures are also covered in this manual.

The second part of this series (FHWA-HRT-11-027) is a synthesis report that covers the background of GRS-IBS 

and provides other supporting information to substantiate the design method.

17. Key Words

Geosynthetic Reinforced Soil (GRS), Integrated Bridge System 

(IBS), Design, Construction, Performance test, Geosynthetic, 

material specifications, Quality assurance, Quality control

18. Distribution Statement

19. Security Classif. (of this report)

Unclassified

20. Security Classif. (of this page)

Unclassified

21. No. of Pages

169

22. Price

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

SI* (MODERN METRIC) CONVERSION FACTORS 

APPROXIMATE CONVERSIONS TO SI UNITS

Symbol When You Know Multiply By To Find Symbol 

LENGTH

in inches 25.4 millimeters mm 

ft feet 0.305 meters m 

yd yards 0.914 meters m 

mi miles 1.61 kilometers km 

AREA 

in2 square inches 645.2 square millimeters mm2

ft2 square feet 0.093 square meters m2

yd2 square yard 0.836 square meters m2

ac acres 0.405 hectares ha

mi2 square miles 2.59 square kilometers km2

VOLUME 

fl oz fluid ounces 29.57 milliliters mL 

gal gallons 3.785 liters L 

ft3 cubic feet 0.028 cubic meters m3 

yd3 cubic yards 0.765 cubic meters m3 

NOTE: volumes greater than 1000 L shall be shown in m3

MASS oz ounces 28.35 grams g

lb pounds 0.454 kilograms kg

T short tons (2000 lb) 0.907 megagrams (or "metric ton") Mg (or "t") 

TEMPERATURE (exact degrees) o

F Fahrenheit 5 (F-32)/9 Celsius oC 

or (F-32)/1.8 

ILLUMINATION 

fc foot-candles 10.76 lux lx

fl foot-Lamberts 3.426 candela/m2 cd/m2

FORCE and PRESSURE or STRESS 

lbf poundforce 4.45 newtons N 

lbf/in2 poundforce per square inch 6.89 kilopascals kPa 

APPROXIMATE CONVERSIONS FROM SI UNITS 

Symbol When You Know Multiply By To Find Symbol 

LENGTH mm millimeters 0.039 inches in 

m meters 3.28 feet ft 

m meters 1.09 yards yd 

km kilometers 0.621 miles mi 

AREA mm2 square millimeters 0.0016 square inches in2 

m2 square meters 10.764 square feet ft2 

m2 square meters 1.195 square yards yd2 

ha hectares 2.47 acres ac

km2 square kilometers 0.386 square miles mi2 

VOLUME 

mL milliliters 0.034 fluid ounces fl oz 

L liters 0.264 gallons gal 

m3 cubic meters 35.314 cubic feet ft3 

m3 cubic meters 1.307 cubic yards yd3 

MASS 

g grams 0.035 ounces oz

kg kilograms 2.202 pounds lb

Mg (or "t") megagrams (or "metric ton") 1.103 short tons (2000 lb) T 

TEMPERATURE (exact degrees) o

C Celsius 1.8C+32 Fahrenheit oF 

ILLUMINATION 

lx lux 0.0929 foot-candles fc 

cd/m2 candela/m2 0.2919 foot-Lamberts fl

FORCE and PRESSURE or STRESS 

N newtons 0.225 poundforce lbf 

kPa kilopascals 0.145 poundforce per square inch lbf/in2

*SI is the symbol for the International System of Units. Appropriate rounding should be made to comply with Section 4 of ASTM E380. 

(Revised March 2003)
iii

TABLE OF CONTENTS

CHAPTER 1. GEOSYNTHETIC REINFORCED SOIL INTEGRATED BRIDGE 

SYSTEM .........................................................................................................................................1

1.1. INTRODUCTION.............................................................................................................1

1.2 BENEFITS OF GRS-IBS ..................................................................................................3

CHAPTER 2. NOTATION, ABBREVIATIONS, AND TERMINOLOGY.............................5

2.1 NOTATION........................................................................................................................5

2.2 ABBREVIATIONS..........................................................................................................11

2.3 TERMINOLOGY ............................................................................................................12

CHAPTER 3. MATERIALS.......................................................................................................15

3.1 INTRODUCTION............................................................................................................15

3.2 FACING ELEMENTS.....................................................................................................15

3.3 BACKFILL MATERIAL................................................................................................17

3.3.1 GRS Abutment Backfill............................................................................................17

3.3.2 RSF Backfill..............................................................................................................20

3.3.3 Integrated Approach Backfill....................................................................................20

3.4 GEOSYNTHETIC...........................................................................................................20

3.5 MISCELLANEOUS MATERIALS ...............................................................................23

CHAPTER 4. DESIGN METHODOLOGY FOR GRS-IBS...................................................25

4.1 OVERVIEW OF GRS-IBS DESIGN METHOD..........................................................25

4.2 BASIC DESIGN STEPS FOR GRS-IBS .......................................................................27

4.3 GRS-IBS DESIGN GUIDELINES.................................................................................28

4.3.1 Step 1—Establish Project Requirements..................................................................28

4.3.2 Step 2—Perform a Site Evaluation...........................................................................29

4.3.3 Step 3—Evaluate Project Feasibility ........................................................................30

4.3.4 Step 4—Determine Layout of GRS-IBS...................................................................30

4.3.5 Step 5—Calculate Applicable Loads........................................................................35

4.3.6 Step 6—Conduct an External Stability Analysis......................................................39

4.3.7 Step 7—Conduct Internal Stability Analysis............................................................44

4.3.8 Step 8—Implement Design Details ..........................................................................50

4.3.9 Step 9—Finalize Material Quantities and Layout ....................................................53

4.4 DESIGN EXAMPLE: BOWMAN ROAD BRIDGE, DEFIANCE COUNTY, OH ....54

4.4.1 Step 1—Establish Project Requirements..................................................................54

4.4.2 Step 2—Site Evaluation............................................................................................56

4.4.3 Step 3—Evaluate Project Feasibility ........................................................................58

4.4.4 Step 4—Determine Layout of GRS-IBS...................................................................58

4.4.5 Step 5—Calculate Loads...........................................................................................59

4.4.6 Step 6—Conduct an External Stability Analysis......................................................60

4.4.7 Step 7—Conduct Internal Stability Analysis............................................................63

4.4.8 Step 8—Implement Design Details ..........................................................................70

4.4.9 Step 9—Finalize Material Quantities and Layout ....................................................70
iv

CHAPTER 5. SPECIAL REQUIREMENTS FOR HYDRAULIC AND SEISMIC 

CONDITIONS..............................................................................................................................71

5.1 INTRODUCTION............................................................................................................71

5.2 HYDRAULIC DESIGN...................................................................................................71

5.3 HYDRAULIC DESIGN CONSIDERATIONS.............................................................71

5.4 SEISMIC DESIGN ..........................................................................................................73

5.5 IMPACT EVENTS ..........................................................................................................73

CHAPTER 6. IN-SERVICE PERFORMANCE.......................................................................75

6.1 INTRODUCTION............................................................................................................75

6.2 DEFORMATIONS ..........................................................................................................75

6.3 THERMAL CYCLES......................................................................................................78

6.4 MONITORING FOR SCOUR........................................................................................78

CHAPTER 7. CONSTRUCTION ..............................................................................................79

7.1 INTRODUCTION............................................................................................................79

7.2 LABOR AND EQUIPMENT..........................................................................................80

7.2.1 Labor Requirements..................................................................................................80

7.2.2 Tool and Equipment Requirements ..........................................................................80

7.3 SITE PREPARATION....................................................................................................82

7.3.1 Site Layout................................................................................................................82

7.3.2 Excavation.................................................................................................................82

7.3.3 Placement of Abutment Behind Existing Substructure ............................................83

7.4 RSF....................................................................................................................................85

7.5 COMPACTION ...............................................................................................................87

7.5.1 Compaction Procedure..............................................................................................88

7.6 REINFORCEMENT........................................................................................................88

7.6.1 Operating Equipment on Geosynthetic Reinforcement ............................................90

7.6.2 Bearing Reinforcement Bed......................................................................................90

7.6.3 Superelevation...........................................................................................................91

7.7 WALL FACE ...................................................................................................................91

7.7.1 Leveling Course ........................................................................................................92

7.7.2 Setting the CMU Block.............................................................................................92

7.7.3 Wall Face Alignment ................................................................................................92

7.7.4 Block Alignment for Battered Walls ........................................................................93

7.7.5 Superelevation...........................................................................................................94

7.7.6 Wall Corners.............................................................................................................95

7.7.7 Top of Facing Wall ...................................................................................................96

7.8 BEAM SEAT....................................................................................................................97

7.8.1 Beam Seat Procedure ................................................................................................99

7.8.2 Setback....................................................................................................................100

7.8.3 Aluminum Flashing ................................................................................................101

7.8.4 CIP or Precast Footing............................................................................................101

7.9 PLACEMENT OF SUPERSTRUCTURE...................................................................101

7.9.1 Crane Position on GRS Mass..................................................................................102

7.9.2 Reinforcement of Beam Seat ..................................................................................102

7.9.3 Setting Superstructure on Beam Seat (Without CIP Footing) ................................102

7.9.4 Wing Walls and Parapets........................................................................................103
v

7.10 APPROACH INTEGRATION...................................................................................103

7.10.1 Wrapped Reinforcement Layers on Sides.............................................................106

7.10.2 Preloading .............................................................................................................106

7.10.3 Paving ...................................................................................................................106

7.10.4 Guardrail Post .......................................................................................................107

7.11 SITE DRAINAGE........................................................................................................107

CHAPTER 8. IN-SERVICE INSPECTION, MAINTENANCE, AND REPAIR................109

8.1 INTRODUCTION..........................................................................................................109

8.2 IN-SERVICE INSPECTION........................................................................................109

8.3 MAINTENANCE...........................................................................................................110

8.4 REPAIR ..........................................................................................................................111

CHAPTER 9. QUALITY CONTROL AND QUALITY ASSURANCE ..............................115

9.1 INTRODUCTION..........................................................................................................115

9.2 ROLE OF THE CONTRACTOR ................................................................................115

9.3 TESTING........................................................................................................................115

9.3.1 Laboratory Testing..................................................................................................115

9.3.2 Field Testing ...........................................................................................................116

9.4 CONSTRUCTION INSPECTION...............................................................................116

9.4.1 Materials .................................................................................................................116

9.4.2 Equipment...............................................................................................................117

9.4.3 Project Layout.........................................................................................................117

9.4.4 Construction Activities ...........................................................................................117

9.5 DOCUMENTATION.....................................................................................................119

9.5.1 Compliance Documentation....................................................................................119

9.5.2 Record Drawings ....................................................................................................119

9.6 CONTRACTING METHODS......................................................................................119

9.6.1 Performance Method...............................................................................................120

9.6.2 Procedural Method..................................................................................................120

9.6.3 Contractor Submittals .............................................................................................122

9.7 PROJECT DRAWINGS AND DOCUMENTS...........................................................122

APPENDIX A. IN-SERVICE GRADATIONS .......................................................................123

APPENDIX B. PERFORMANCE TEST PROCEDURE......................................................125

B.1 PERFORMANCE TEST OVERVIEW ......................................................................125

B.2 PERFORMANCE TEST EXAMPLE: VEGAS MINI-PIER ...................................125

B.3 PERFORMANCE TEST EXAMPLE: DEFIANCE COUNTY 

PERFORMANCE TEST.....................................................................................................130

APPENDIX C. LRFD DESIGN PROCEDURE .....................................................................135

C.1 INTRODUCTION.........................................................................................................135

C.2 GRS-IBS DESIGN GUIDELINES ..............................................................................136

C.2.1 Step 6—Conduct an External Stability Analysis ...................................................137

C.2.2 Step 7—Conduct Internal Stability Analysis.........................................................139
vi

C.3 DESIGN EXAMPLE (LRFD): BOWMAN ROAD BRIDGE, 

DEFIANCE COUNTY, OH.................................................................................................141

C.3.6 Step 6—Conduct an External Stability Analysis ...................................................141

C.3.7 Step 7—Conduct Internal Stability Analysis.........................................................143

C.3.8 Step 8—Implement Design Details........................................................................147

APPENDIX D. TYPICAL GRS-IBS WORKING DRAWINGS...........................................149

D.1 SINGLE-PAGE PLAN SHEET...................................................................................149

D.2 ESTIMATED QUANTITIES AND GENERAL NOTES .........................................150

D.3 PROJECT PLAN AND PROFILE..............................................................................151

D.4 GRS ABUTMENT DETAILS......................................................................................152

D.5 SITE PLAN....................................................................................................................153

ACKNOWLEDGEMENTS ......................................................................................................155

REFERENCES...........................................................................................................................157
vii

LIST OF FIGURES

Figure 1. Illustration. Typical GRS-IBS cross section ....................................................................2

Figure 2. Illustration. GRS walls with different facings................................................................15

Figure 3. Photo. Split face CMU blocks........................................................................................16

Figure 4. Photo. Detail view of split face CMU blocks.................................................................16

Figure 5. Photo. Sample of VDOT 21-A gravel ............................................................................19

Figure 6. Photo. Sample of AASHTO No. 89 gravel ....................................................................20

Figure 7. Illustration. Geosynthetic roll direction..........................................................................21

Figure 8. Photo. Bitumen coating on concrete beam ends.............................................................23

Figure 9. Chart. Steps for GRS-IBS design ...................................................................................28

Figure 10. Photo. Bridge seat and setback distances.....................................................................31

Figure 11. Illustration. Clear space distance..................................................................................32

Figure 12. Illustration. RSF dimensions........................................................................................33

Figure 13. Illustration. Reinforcement schedule for a GRS abutment...........................................34

Figure 14. Illustration. Vertical and lateral pressures on a GRS abutment....................................35

Figure 15. Illustration. Boussinesq load distribution with depth for a strip load...........................37

Figure 16. Illustration. Internal lateral stress in GRS abutment wall due to bridge loading .........38

Figure 17. Illustration. External stability: direct sliding................................................................40

Figure 18. Illustration. External stability: bearing capacity...........................................................40

Figure 19. Illustration. External stability: global stability .............................................................40

Figure 20. Graph. Design envelope for vertical capacity and strain at 8-inch reinforcement 

spacing ...........................................................................................................................................45

Figure 21. Illustration. Lateral deformation of a GRS structure....................................................48

Figure 22. Illustration. Typical cross section of a GRS wing wall................................................50

Figure 23. Illustration. Typical cross section of a GRS abutment face wall .................................51

Figure 24. Photo. Guardrail lay-down distance .............................................................................52

Figure 25. Photo. GRS abutment and wing walls built around core of native soil........................52

Figure 26. Illustration. Top view of Bowman Road Bridge showing the bridge, abutments, 

and wing walls...............................................................................................................................54

Figure 27. Illustration. Aerial view of the existing site with the planned Bowman Road 

Bridge superimposed .....................................................................................................................55

Figure 28. Illustration. Schematic of the west abutment for Bowman Road Bridge .....................56

Figure 29. Illustration. Schematic of the east abutment for Bowman Road Bridge ......................56

Figure 30. Illustration. Reinforcement schedule and RSF dimensions for Bowman Road Bridge ....59

Figure 31. Screenshot. ReSSA results for global stability for Bowman Road Bridge ..................63

Figure 32. Graph. Stress-strain curve for Bowman Road Bridge showing ultimate capacity.......64

Figure 33. Graph. Vertical strain for Bowman Road Bridge.........................................................65

Figure 34. Illustration. Lateral pressure due to the bridge load.....................................................68

Figure 35. Illustration. Secondary reinforcement for superelevation at Bowman Road Bridge ......70

Figure 36. Illustration. Typical cross section for sloping rock (adapted)......................................72

Figure 37. Illustration. Survey level method for superstructure and wall settlement....................76

Figure 38. Photo. Location of survey targets on Tiffin River Bridge, Defiance County, OH.......77

Figure 39. Photo. Location of total station reference pole.............................................................77

Figure 40. Photo. Typical labor crew with centrally located track hoe .........................................80

Figure 41. Photo. Cut slope of retained soil...................................................................................82
viii

Figure 42. Photo. Horseshoe-shaped excavation with native soil still intact in middle ................83

Figure 43. Photo. GRS-IBS built behind an existing concrete abutment ......................................84

Figure 44. Illustration. Cross section of GRS-IBS built behind an existing concrete abutment ......84

Figure 45. Photo. Building the RSF behind an existing abutment.................................................85

Figure 46. Photo. RSF excavation below stream level ..................................................................85

Figure 47. Photo. RSF cut preparation...........................................................................................86

Figure 48. Photo. Encapsulation of fill in RSF..............................................................................86

Figure 49. Photo. Placement of wall block on wrapped RSF ........................................................87

Figure 50. Illustration. Typical reinforcement zones.....................................................................89

Figure 51. Reinforcement rolled out parallel to wall face .............................................................89

Figure 52. Illustration. Superelevation reinforcement schedule ....................................................91

Figure 53. Photo. Superelevation reinforcement layers.................................................................91

Figure 54. Photo. Checking block alignment with string line reference from back of block........93

Figure 55. Photo. Negative batter wall face...................................................................................94

Figure 56. Photo. Blocks trimmed to match superelevation..........................................................94

Figure 57. Photo. Right-angle wall corner.....................................................................................95

Figure 58. Photo. Vertical seam in wing wall................................................................................95

Figure 59. Photo. Rebar installed in vertical seam prior to grout..................................................96

Figure 60. Photo. Connecting the top courses of blocks................................................................96

Figure 61. Photo. Rounded coping cap..........................................................................................97

Figure 62. Photo. Square coping cap .............................................................................................97

Figure 63. Photo. Box beam placed on beam seat .........................................................................98

Figure 64. Photo. Detail view of box beam placed on beam seat..................................................98

Figure 65. Photo. Bearing area block grouted prior to beam placement .......................................98

Figure 66. Photo. Foam board and 4-inch block assembly to form beam seat..............................99

Figure 67. Photo. 4-inch concrete block on top of foam board against top CMU face block .......99

Figure 68. Photo. First 4-inch wrap butted against foam board...................................................100

Figure 69. Photo. Top 4-inch wrap butted against 4-inch solid block.........................................100

Figure 70. Photo. Aluminum flashing (drip edge) between beams and top of CMU block ........101

Figure 71. Photo. Steel girder on CIP footing .............................................................................101

Figure 72. Photo. Outrigger pads near wall face .........................................................................102

Figure 73. Photo. Additional reinforcement under beam ............................................................102

Figure 74. Photo. Parapet and wing wall construction, view 1....................................................103

Figure 75. Photo. Parapet and wing wall construction, view 2....................................................103

Figure 76. Photo. Placing reinforcement .....................................................................................104

Figure 77. Photo. First 6-inch fill lift...........................................................................................104

Figure 78. Photo. Secondary reinforcement sheet .......................................................................105

Figure 79. Photo. Completed wrapped approach layer................................................................105

Figure 80. Photo. Second 6-inch fill lift ......................................................................................105

Figure 81. Photo. Completed approach fill..................................................................................106

Figure 82. Photo. Use of GRS wall to repair damaged MSE wall...............................................111

Figure 83. Photo. CMU GRS wall with a shotcrete face .............................................................112

Figure 84. Photo. GRS wall damaged by large sandstone boulder..............................................112

Figure 85. Photo. Repair of a GRS wall after damage caused by rock fall .................................113

Figure 86. Illustration. Plan view of Vegas Mini-Pier Experiment .............................................126

Figure 87. Illustration. Elevation view of Vegas Mini-Pier Experiment.....................................126
ix

Figure 88. Photo. Vegas Mini-Pier Experiment reaction base.....................................................127

Figure 89. Illustration. Side view of Vegas Mini-Pier Experiment .............................................128

Figure 90. Illustration. Face view of Vegas Mini-Pier Experiment.............................................128

Figure 91. Photo. Hollow-core hydraulic ram bolted to the top channel beams .........................129

Figure 92. Graph. Vegas Mini-Pier stress-strain curve................................................................130

Figure 93. Illustration. Plan view of Defiance County experiment .............................................130

Figure 94. Illustration. Elevation view of Defiance County experiment.....................................131

Figure 95. Photo. Defiance County experiment before testing....................................................132

Figure 96. Graph. Defiance County stress-strain curve...............................................................132

Figure 97. Photo. Defiance County experiment after testing.......................................................133

Figure 98. Graph. Defiance County stress-strain curve (weaker fabric) .....................................134

Figure 99. Graph. Defiance County experiment comparison ......................................................134

Figure 100. Illustration. Single-page plan sheet ..........................................................................149

Figure 101. Illustration. Estimated quantities and general notes.................................................150

Figure 102. Illustration. Project plan and profile.........................................................................151

Figure 103. Illustration. Abutment details...................................................................................152

Figure 104. Illustration. Site plan ................................................................................................153
x

LIST OF TABLES

Table 1. GRS abutment well-graded backfill (VDOT 21-A) ........................................................18

Table 2. GRS abutment open-graded backfill (AASHTO No. 89)................................................19

Table 3. Conformance criteria ............................................................................................................22

Table 4. Bearing capacity factors...................................................................................................43

Table 5. Foundation and retained backfill soil properties..............................................................57

Table 6. Road base soil properties.................................................................................................57

Table 7. Reinforced fill properties.................................................................................................58

Table 8. Loads and surcharges for Bowman Road Bridge ............................................................60

Table 9. Depth of bearing bed reinforcement calculations............................................................67

Table 10. Required reinforcement along height of wall ................................................................69

Table 11. Defiance County, OH, AASHTO No. 89, clean, crushed limestone...........................123

Table 12. Warren County, OH, AASHTO No. 67, clean, crushed rock......................................123

Table 13. King County, WA, WSDOT 11/4-inch minus gravel, clean round rock with 

sand mixture-pit run.....................................................................................................................123

Table 14. St. Lawrence County, NY, NYSDOT No. 1, clean crushed rock................................123

Table 15. Urbana, IL, Seismic Test Abutment, IDOT CA6 road base, subrounded gravel 

with sand mix...............................................................................................................................124

Table 16. Typical load combinations and load factors................................................................135

Table 17. Load factors for permanent loads ................................................................................136

Table 18. Depth of bearing bed reinforcement calculations (LRFD)..........................................147
1

CHAPTER 1. GEOSYNTHETIC REINFORCED SOIL INTEGRATED BRIDGE SYSTEM

1.1. INTRODUCTION

The Geosynthetic Reinforced Soil (GRS) Integrated Bridge System (IBS) provides an economical 

solution to accelerated bridge construction. Employing this technology will help agencies save both

time and money in planning and executing projects. This interim implementation manual and its 

companion document were developed to assist deployment of this promising technology as part 

of the Federal Highway Administration’s (FHWA) Every Day Counts initiative.(1) The purpose 

of this manual is to provide a framework for GRS-IBS design and construction that is safe and 

consistent with the policies and procedures of FHWA and the American Association of State 

Highway and Transportation Officials (AASHTO), except where the behavior of this technology

relieves itself of those requirements. 

GRS-IBS was initially developed by FHWA during the Bridge of the Future initiative to help meet

the demand for the next generation of small, single span bridges in the United States. GRS-IBS 

can be built with lower cost, faster construction, and potential improved durability and can be 

used to build bridges on all types of roads, on or off the National Highway System.

GRS-IBS is a fast, cost-effective method of bridge support that blends the roadway into the 

superstructure to create a jointless interface between the bridge and the approach (see figure 1). 

It consists of three main components: the reinforced soil foundation (RSF), the abutment, and 

the integrated approach. The RSF is composed of granular fill material that is compacted and 

encapsulated with a geotextile fabric. It provides embedment and increases the bearing width 

and capacity of the GRS abutment. It also prevents water from infiltrating underneath and into the 

GRS mass from a river or stream crossing. This method of using geosynthetic fabrics to reinforce 

foundations is a proven alternative to deep foundations on loose granular soils, soft fine-grained 

soils, and soft organic soils.(2) The abutment uses alternating layers of compacted fill and closely 

spaced geosynthetic reinforcement to provide support for the bridge, which is placed directly on 

the GRS abutment without a joint and without cast-in-place (CIP) concrete. GRS is also used to 

construct an integrated approach to transition to the superstructure. This bridge system therefore 

alleviates the “bump at the bridge” problem caused by differential settlement between bridge 

abutments and approach roadways. 
2

Figure 1. Illustration. Typical GRS-IBS cross section.

The riding surface of GRS-IBS can be maintained as if it is part of the roadway pavement. No 

special attention to joints or the bridge deck is required. Unlike a traditional integral abutment,

IBS is unique in its use of GRS to support the superstructure. This method of accelerated bridge 

construction is as easy as 1-2-3: (1) a row of facing blocks, (2) a layer of compacted granular fill, and 

(3) a layer of geosynthetic reinforcement. The 1-2-3 process is repeated until the required abutment 

height is reached.

GRS-IBS has many other distinct and innovative qualities. GRS technology is extremely durable 

and can perform well in earthquakes if constructed as outlined in this manual. GRS abutments can

be built with readily available material using common construction equipment without the need 

for highly skilled labor. Construction of the abutment is contained within its footprint for a reduction

of environmental impact as well as a reduced work zone. Additional benefits are convenience 

and design flexibility, as GRS-IBS can be built in variable weather conditions and can be adapted

easily in the case of unforeseen site conditions.

This manual addresses the design and construction of GRS-IBS. In-service performance, inspection,

maintenance, and repair are also described, along with special requirements for hydraulic and 

seismic conditions. Finally, procedures for quality assurance (QA) and quality control (QC) 

(including necessary construction documents) are provided. The ultimate purpose of this manual 

is to allow designers and contractors to effectively design and construct a durable GRS-IBS.
3

1.2 BENEFITS OF GRS-IBS

Based on constructed demonstration projects, GRS-IBS is more cost-effective than traditional

bridge construction, utilizes common materials and construction techniques, and provides a safer 

work environment for personnel in work zones.(3) GRS-IBS bridges can be built in less time (in 

weeks, rather than months), which translates into less congestion; fewer road closures, disruptions,

and shutdowns around work zones; and lower materials and labor costs. The method of construction

is such that the abutments are built from the inside out, reducing the exposure of personnel to

potential roadside hazards. In addition, the technology is environmentally sensitive and results in 

minimal environmental impacts. The technology produces a reduced construction and carbon 

footprint, eliminates the need for installation of a deep foundation or CIP concrete, and can be 

adapted to fit the site-specific environmental needs.

The cost to build a GRS-IBS bridge is potentially 25–60 percent less than traditional methods, 

depending on the standard of construction. The savings is attributable to the simplicity and 

flexibility of the design, speed of construction (which is less dependent on weather conditions

than CIP abutments), use of readily available materials and equipment, and elimination of the 

deep foundation and other construction details associated with the approach way to the bridge. 

Furthermore, this method has the potential for reduced maintenance costs because it eliminates 

the bump at the end of the bridge, creating a smoother and safer transition. Also, the application 

of GRS technology in other facets of earthwork (e.g., walls, culverts, foundations, slope stability, 

rock fall barriers, etc.) has the potential to result in significant cost savings and more effective 

use of transportation funding.

In summary, the benefits of GRS-IBS include the following: 

Reduced construction time.

25–30 percent lower cost than standard pile cap abutments on deep foundations with 

2:1 slopes for off-system bridges.

50–60 percent lower cost than standard department of transportation bridges.

Construction that is less dependent on weather conditions.

Flexible design that is easily field-modified for unforeseen site conditions.

Easier maintenance due to fewer parts.

Construction with common equipment and materials.

Better quality control.

5

CHAPTER 2. NOTATION, ABBREVIATIONS, AND TERMINOLOGY

2.1 NOTATION

Angle between wall face and projection of the midline of the surcharge to the wall 

face [rad]

b Angle between wall face and projection of the midline of the bridge surcharge to 

the wall face [rad]

Angle between the projections of the inner and outer edge lines of the surcharge 

to the wall face [rad]

b Angle between the projections of the inner and outer edge lines of the bridge 

surcharge to the wall face [rad]

Unit weight of soil [F/L3]

b Unit weight of retained backfill [F/L3

]

DC MAX Maximum load factor for dead load (DL)

DC MIN Minimum load factor for DL 

EH MAX Maximum load factor for horizontal earth pressure

EH MIN Minimum load factor for horizontal earth pressure

ES MAX Maximum load factor for earth surcharge

ES MIN Minimum load factor for earth surcharge

EV MAX Maximum load factor for vertical earth pressure

EV MIN Minimum load factor for vertical earth pressure

f Unit weight of foundation soil [F/L3

]

LS Load factor for live load (LL) surcharge

r Unit weight of reinforced backfill [F/L3

]

rb Unit weight of road base material [F/L3

]

L Lateral strain

V Vertical strain

Friction factor between the wall base and the foundation

h Lateral pressure [F/L2

]

α

α

β

β

γ

γ

γ

γ

γ

γ

γ

γ

γ

γ

γ

γ

γ

γ

ε

ε

μ

σ
6

h,f Equivalent lateral stress distribution due to the retained soil behind the GRS 

abutment [F/L2

]

h,f Factored lateral pressure [F/L2

]

h,bridge Lateral pressure due to bridge DL surcharge within GRS [F/L2

]

h,bridge,eq Lateral pressure due to the equivalent bridge load [F/L2

]

h,bridge,f Factored lateral pressure due to the equivalent bridge load [F/L2

]

h,LL Lateral stress distribution due to the equivalent superstructure LL pressure [F/L2

]

h,q Lateral pressure due to surcharge loading [F/L2

]

h,rb Lateral pressure due to road base surcharge within GRS [F/L2

]

h,rb,f Factored lateral pressure due to road base surcharge within GRS [F/L2

]

h,t Lateral pressure due to traffic surcharge within GRS [F/L2

]

h,t,f Factored lateral pressure due to traffic surcharge within GRS [F/L2

]

h,total Total lateral pressure due to loads on GRS mass [F/L2

]

h,W Lateral stress due to weight of GRS [F/L2

]

v,base,n Nominal vertical pressure at the base of the GRS mass [F/L2

]

v,base,R Factored vertical pressure at the base of the GRS mass [F/L2

]

MD Total driving moment [L-F/L]

MD,R Total factored driving moment [L-F/L]

MR Total resisting moment [L-F/L]

MR,R Total factored resisting moment [L-F/L]

V Total vertical load [F/L]

VR Total factored vertical load [F/L]

Soil friction angle [deg]

b Friction angle of retained backfill [deg]

crit Critical friction angle [deg]

design Friction angle of reinforced fill used in design [deg]

f Friction angle of foundation soil [deg]

r Friction angle of reinforced backfill [deg]

σ

σ

σ

σ

σ

σ

σ

σ

σ

σ

σ

σ

σ

σ

σ

Σ

Σ

Σ

Σ

Σ

Σ
7

rb Friction angle of road base material [deg]

rep Repose angle [deg]

test Friction angle of reinforced fill found from standard direct shear test [deg]

Resistance factor for shear resistance

bc Resistance factor for bearing capacity

cap Resistance factor for ultimate capacity 

reinf Resistance factor of the required reinforcement strength

Batter angle [deg]

a Distance between the back of the wall face and a surcharge (setback) [L]

ab Setback distance between the back of the face and the beam seat [L]

arb Setback distance for the road base surcharge over the GRS mass [L]

at Setback distance for the traffic surcharge over the GRS mass [L]

b Bearing width for bridge; beam seat [L]

bblock Width of the facing element [L]

bq Width of surcharge loading [L]

bq,vol Width of the load along the top of the wall (including the setback) [L]

brb,t Distance over which the road base DL and roadway LL surcharges act over the 

GRS mass [L]

B Base length of reinforcement not including the wall face [L]

B’ Effective foundation width [L]

Bb Width of the bridge [L]

BRSF Width of the RSF [L]

Btotal Total base width of the GRS abutment including the block face

c Cohesion [F/L2]

Cb Cohesion of retained backfill [F/L2]

Cf Cohesion of foundation soil [F/L2]

Cr Cohesion of reinforced backfill [F/L2]

Cu Undrained shear strength of foundation soil [F/L2]

de Clear space distance [L]

Φτ

Φ

Φ

Φ

ω
8

dmax Maximum grain size [L]

D50riprap Mean grain size for riprap

Df Depth of embedment [L]

DL Maximum lateral displacement [L]

DV Vertical settlement in the GRS mass [L]

DRSF RSF depth [L]

eB,n Nominal eccentricity for bearing capacity calculations [L]

eB,R Factored eccentricity for bearing capacity calculations [L]

Fb Lateral force due to the retained backfill [F/L]

Fn Nominal driving force for direct sliding calculations [F/L]

Frb Lateral force due to the road base surcharge [F/L]

FR Factored driving force for direct sliding calculations [F/L]

Ft Lateral force due to LL on the roadway [F/L]

FS Factor of safety

FSbearing Factor of safety against bearing failure

FScapacity Factor of safety for vertical capacity using the empirical method

FSreinf Factor of safety for required reinforcement strength

FSslide Factor of safety against direct sliding

G Grade [L/L]

heq Equivalent height of overburden for traffic surcharge [L]

hrb Height of road base (equals height of bridge beam) [L]

H Height of the GRS abutment including the clear space distance [L]

Habut Height of the GRS abutment [L]

Ka Coefficient of active earth pressure

Kab Coefficient of active earth pressure for the retained backfill

Kar Coefficient of active earth pressure for the reinforced backfill

Kpr Coefficient of passive earth pressure for the reinforced backfill

Labut Abutment length [L]
9

Lblock Length of a facing block [L]

Lspan Span length of the bridge [L]

(LL + IM)total Governing abutment reaction for the HL-93 LL model for one lane

Dimensionless bearing capacity coefficient

Nblock Number of facing blocks in a column

Nc Dimensionless bearing capacity coefficient

Nlanes Number of lanes

Nq Dimensionless bearing capacity coefficient

q Surcharge load [F/L2

]

qb Equivalent superstructure DL pressure [F/L2

]

qLL Equivalent superstructure LL pressure [F/L2]

qn Bearing capacity of the foundation soil [F/L2]

qn,an Nominal ultimate load-carrying capacity of the foundation using the analytical 

method [F/L2]

qn,emp Nominal ultimate load-carrying capacity of the foundation using the empirical 

method [F/L2]

qR Factored bearing resistance [F/L2

]

qrb Surcharge due to the structural backfill (road base) DL [F/L2]

qt Equivalent roadway LL surcharge [F/L2]

qult,an Ultimate load-carrying capacity of GRS using the analytical method [F/L2

]

qult,emp Ultimate load-carrying capacity of GRS using the empirical method [F/L2]

QLL LL reaction load [F]

Rn Nominal resisting force for direct sliding calculations [F/L]

RR Factored resisting force for direct sliding calculations [F/L]

Se Superelevation angle [deg]

Sk Skew angle [deg]

Sv Reinforcement spacing [L]

Reinforcement strength at 2 percent reinforcement strain [F/L]

Tallow Allowable reinforcement strength [F/L]

Nγ

T@ε = 2%
10

Tf Ultimate reinforcement strength [F/L]

Tf,f Factored reinforcement strength [F/L]

Treq Required reinforcement strength [F/L]

Treq,f Factored required reinforcement strength [F/L]

Vallow,an Factored applied stress on top of GRS mass using the analytical method [F/L2]

Vallow,emp Factored applied stress on top of GRS mass using the empirical method [F/L2]

Vapplied Applied stress on top of GRS mass [F/L2

]

Vapplied,f Factored applied stress on top of GRS mass [F/L2]

W Weight of the GRS abutment backfill [F/L]

WB Total width of riprap [L]

Wblock Weight of an individual facing block [F]

Wface Weight of the facing elements [F/L]

WL Distance between abutment faces [L]

WRSF Weight of the RSF [F/L]

Wt Total weight (weight of GRS plus weight of bridge beam plus weight of the road 

base over the GRS mass only) [F/L]

Wt,R Factored total resisting weight (weight of GRS plus weight of bridge beam plus 

weight of the road base over the GRS mass only) [F/L]

WT Width of level riprap along the top [L] 

x Distance from the edge of the load to the point of interest for lateral pressure [L]

Ysc Contraction scour plus long-term degradation scour referenced to the thalweg [L]

YTot Distance from the top of riprip to the bottom of riprap [L]

xRSF Length of the RSF in front of the abutment wall face [L]

z Location along height of wall (measured from the top of the wall) [L]
11

2.2 ABBREVIATIONS

AASHTO American Association of State Highway and Transportation Officials

ASD Allowable Stress Design

ASTM American Society for Testing and Materials (known as ASTM International)

CIP Cast-in-place

CMU Concrete masonry unit

DL Dead load

EDM Electronic distance measurement

FHWA Federal Highway Administration 

GRS Geosynthetic Reinforced Soil

HEC Hydraulic Engineering Circular

IBS Integrated Bridge System

IDOT Illinois Department of Transportation

LL Live load

LRFD Load and Resistance Factor Design

MSE Mechanically stabilized earth

NCHRP National Cooperative Highway Research Program

NYSDOT New York State Department of Transportation

PET Polyethylene terephtalate (polyester)

PP Polypropylene

HDPE High density polyethylene

QA Quality assurance

QC Quality control

RSF Reinforced soil foundation

SPT Standard penetration test

SRW Segmental retaining wall

VDOT Virginia Department of Transportation

WSDOT Washington State Department of Transportation
12

2.3 TERMINOLOGY

Biaxial: Reinforcement strength is approximately equal in both the machine and the cross 

machine directions.

Clear space: The vertical distance between the top of the wall face (block) and base superstructure. 

Typically, this distance is about 3 inches or at least 2 percent of the wall height.

GRS: Alternating layers of compacted granular fill reinforced with geosynthetic reinforcement 

(e.g., geotextiles, geogrids). The primary reinforcement spacing in GRS is less than or equal to

12 inches. Facing elements can be frictionally connected to the reinforcement layers to form the 

outer wall. The facing elements do not need mechanical connections to each other or the layers 

of reinforcement. The outer wall facing can be built with natural rock, concrete modular block, 

gabions, timber, or geosynthetic wrapped face. GRS is generic and can be built with any combination 

of geosynthetic reinforcement, compacted granular fill, and facing system, although some 

combinations of the three components are more compatible than others.

GRS abutment: A GRS system designed and built to support a bridge. Usually, GRS abutments

have three sides: the abutment face wall and two wing walls. All GRS abutments must have the 

abutment face wall. In some circumstances, depending on the layout, a GRS abutment can be 

built with one or none of the wing walls. 

GRS abutment face wall: The vertical or near vertical wall parallel to the center of bearing and 

designed to support the bridge. The length of a GRS abutment face wall is typically the total width

of the bridge structure plus any additional width necessary to accommodate the structure (e.g.,

guardrail deflection distance). 

GRS-IBS: A unique application of GRS technology in the specific context of bridge abutments.

GRS-IBS is different from other, more general GRS abutments that use many common elements 

associated with traditional bridge abutments. GRS-IBS bridge abutments are built to economically 

support a bridge on the granular fill directly behind the block face. GRS-IBS can be used to 

integrate the bridge structure with the bridge approach to create a jointless bridge system. One 

version of GRS-IBS uses adjacent concrete box beams or void slabs supported directly on the 

GRS abutments without a concrete footing or elastomeric pads. The bridge has no CIP concrete 

or approach slab. A typical cross section of IBS shows a GRS mass compacted directly behind 

the bridge beams to form the approach way and to create a smooth transition from the roadway to 

the bridge. Another version of GRS-IBS uses steel girders with either a CIP footing or a precast 

sill. The footing or sill is placed directly on the GRS abutment. The reinforcement layers behind 

the beam ends are wrapped to confine the compacted approach fill against the beam ends and the 

adjacent side slopes to prevent lateral spreading. Since the wrapped-face GRS mass behind the 

beam ends is free standing, the active lateral pressure against the beam ends is considered negligible.

The wrapped-face fill also prevents migration of fill during thermal bridge cycles and vehicle LL.

GRS mass or GRS structure: A composite mass built with GRS that creates a freestanding, 

internally supported structure with reduced lateral earth pressures with considerable strength. 

This design permits the use of lightweight modular blocks and the elimination of mechanical 
13

connections between blocks and the reinforcement. A GRS mass is not rigid and is therefore 

tolerant to differential foundation settlement. 

GRS wall: Any wall built with GRS.

GRS wing wall: A wall attached and adjacent to the abutment face wall. The wing walls are built

at the same time as the abutment face wall and at a right or other angle to the abutment face wall. 

The wing walls are built to support the roadway and the approach embankment. The wing walls 

must be designed to retain the soil fill in the core of the approach embankment and to protect the 

abutment from erosion. 

Setback: The lateral distance from the back of the wall face to the front of the bearing area. This 

distance must be a minimum of 8 inches.

Uniaxial: Reinforcement strength is larger is one direction than the other.

15

CHAPTER 3. MATERIALS

3.1 INTRODUCTION 

Building GRS is as easy as 1-2-3: (1) a row of blocks (the facing elements), (2) a layer of compacted

granular fill to the height of the facing blocks, and (3) a layer of geosynthetic reinforcement. The 

materials used for each step of the process need not be proprietary and are readily available.

Recommendations are made to optimize the design based on numerous case histories and field 

experiments. There are also several miscellaneous materials needed for the details of GRS-IBS. 

3.2 FACING ELEMENTS

The facing element is not a structural member of GRS-IBS. Its purpose is to provide a form for 

compaction, serve as a façade, and protect the granular fill from outside weathering. Since the 

facing is not a structural element of a GRS mass, it is up to the user to define the type of facing 

used. It may be made of various materials, including concrete, timber, natural rock, metal, 

automobile tires, shotcrete, and gabion baskets. While some of the facing elements shown would

not be appropriate for use in GRS-IBS bridges, figure 2 shows various facing elements that have 

been used in the construction of GRS walls.

Figure 2. Illustration. GRS walls with different facings.

(4)
16

The most commonly used facing element for GRS walls and abutments is the split face concrete 

masonry unit (CMU) with nominal dimensions of 8 inches by 8 inches by 16 inches and actual 

dimensions of 75/8 inches by 75/8 inches by 155/8 inches (see figure 3 and figure 4). It is important to 

use the actual dimensions in designing and detailing GRS-IBS. CMU blocks are lightweight, easy to 

place, and ensure compaction at every 8-inch lift before placement of the next geosynthetic layer. 

As seen in figure 3, the reinforcement extends directly beneath each layer of CMU blocks as a 

frictional connection. 

Figure 3. Photo. Split face CMU blocks.

Figure 4. Photo. Detail view of split face CMU blocks.

The CMU should have a minimum compressive strength of 4,000 psi and a water absorption limit

of 5 percent. In colder climates, a freeze-thaw test (ASTM C1262-10) should be conducted to

assess the durability of the CMU and ensure it follows the standard specification (ASTM C1372).

One method to ensure the overall quality of the CMU is to review the QA/QC process of a

particular producer.

There are several types of CMU that are commonly used in GRS-IBS construction: solid face, 

hollow core, and corner block. All of these blocks come in the standard dimensions previously 

described. In addition to the 75

/8-inch height, there are a 35/8-inch solid CMU blocks that can be 
17

used as spacers to form the beam seat (see chapter 7). CMU blocks have been used for GRS

construction because they are readily available and inexpensive. They are also compatible with the 

frictional connection to the recommended reinforcement. Since the facing element is not structural 

in a GRS wall or abutment, any facing element can be used. With other facing elements, however,

special design considerations may apply, and such considerations are beyond the scope of this guide.

3.3 BACKFILL MATERIAL

Backfill selection for GRS-IBS is important because it is a major structural component for the

abutment. The backfill must be properly compacted to a minimum of 95 percent of maximum dry 

density according to AASHTO T-99. Other procedures to determine the degree of compaction can 

also be used (e.g., modulus-based test methods), as discussed in chapter 7. In GRS-IBS construction,

other areas to consider for backfill selections are the RSF and the integrated approach. 

Locally sourced aggregates, as long as they meet the material qualifications, are the most economical

choice for GRS construction. Most State specifications for aggregate, which are usually met by 

local quarries and aggregate suppliers, will satisfy the material requirements. Recommendations 

are provided in this section for GRS abutment, RSF, and approach-way backfills.

It should be noted that some backfill materials are easier to work with than others. Certain backfills

are more suitable for compacting behind a given facing element than others. These factors need 

to be considered when selecting the backfill for a given project. 

It has been observed that some fine-grained sands and open-graded coarse aggregates with a

maximum grain size greater than 2 inches are difficult to compact directly behind the face of a 

frictionally connected split face CMU block. The selection of a compatible fill and facing 

element is therefore necessary for the following purposes:

Ensure adequate compaction directly behind the face.

Control face alignment.

Limit post construction lateral deformation.

3.3.1 GRS Abutment Backfill

Because a GRS abutment is designed to support load, the backfill is considered a structural

component. Abutment backfill should consist of crushed, hard, durable particles or fragments of 

stone or gravel. These materials should be free from organic matter or deleterious material such 

as shale or other soft particles that have poor durability. The backfill should follow the size and 

quality requirements for crushed aggregate material normally used locally in the construction and 

maintenance of highways by Federal or State agencies.

Abutment backfill typically consists of either well-graded or open-graded aggregates (example 

gradations are shown in sections 3.3.2.1 and 3.3.2.2, respectively). It is recommended that either 

one of these gradations or a blend in between the two be used as backfill behind GRS abutments. 

At the time of this report, open-graded aggregates had been selected on all GRS-IBS projects due to 

the relative ease of construction and favorable drainage characteristics (see appendix A). If the 
18

abutment will be submerged at any point in time, open-graded gravel should be used because it is 

free-draining. The friction angle of the backfill should be no less than 38 degrees. 

Lower quality granular or natural fill materials can be used if the amount of fines is limited to less

than 12 percent for drainage. However, a performance test must be conducted (see appendix B)

to quantify the deformation and composite behavior of the mass. The engineer should be cautious 

when using fills of a lower quality than specified, as the allowable load may be significantly 

reduced. Safety factors for reinforcement strength and ultimate capacity will also deviate from 

what is specified in design (see chapter 4). It is therefore recommended to follow the abutment 

backfill specifications outlined in this chapter.

In addition to the gradation requirement, the backfill selection is dependent on the following factors:

Ability to ensure compaction.

Drainage (open-graded backfill is recommended for an abutment located in a flood 

zone to facilitate the flow of water out of the abutment).

Workability (open-graded fine aggregates (about 0.5 inches) are easier to spread, 

level, and compact than well-graded fill).

Angular particles are recommended to maximize the shear strength of the GRS mass.

3.3.1.1 Well-Graded Backfill

Most State transportation department subbase aggregates have a specification for well-graded 

backfill. A maximum grain size of 2 inches is recommended for efficient compaction behind the 

abutment wall facing. An example of this type of aggregate is shown in table 1 and in figure 5. 

The exact gradation is not required. As long as the maximum aggregate size is not exceeded, 

the amount of fines passing the No. 200 sieve is not greater than 12 percent, and the friction angle 

is at least 38 degrees, the backfill material will be adequate for GRS-IBS.

Table 1. GRS abutment well-graded backfill (VDOT 21-A).

Gradation

(VDOT 21-A)

U.S. Sieve Size Percent Passing

2 inch 100

1 inch 94–100

3

/8 inch 63–72

No. 10 32–41

No. 40 14–24

No. 200 6–12

Plasticity Index (PI)

(AASSHTO T-90) PI ≤ 6

Soundness

(AASHTO T-104)

The backfill shall be substantially free of shale or other poor 

durability particles. The material shall have a magnesium 

sulfate loss of less than 30 percent after four cycles (or a 

sodium value less than 15 percent after five cycles).
19

Figure 5. Photo. Sample of VDOT 21-A gravel.

3.3.1.2 Open-Graded Backfill

Recommended open-graded backfill material consists of clean, crushed angular (not rounded) 

stone. The minimum maximum grain size to efficiently achieve compaction behind the 

abutment wall face is 0.5 inches. An example of a typical open-graded abutment backfill is 

shown in table 2 and in figure 6. The amount of fines passing the No. 200 sieve should be as

close to 0 percent as possible and no more than 5 percent.

Table 2. GRS abutment open-graded backfill (AASHTO No. 89).

Gradation

(AASHTO M-43)

U.S. Sieve Size Percent Passing

1

/2 inch 100

3

/8 inch 90–100

No. 4 20–55

No. 8 5–30

No. 16 0–10

No. 50 0–5

Plasticity Index (PI)

(AASHTO T-90) PI ≤ 6

Soundness

(AASHTO T-104)

The backfill shall be substantially free of shale or other poor

durability particles. The material shall have a magnesium 

sulfate loss of less than 30 percent after four cycles (or a 

sodium value less than 15 percent after five cycles).
20

Figure 6. Photo. Sample of AASHTO No. 89 gravel.

3.3.2 RSF Backfill

The backfill for the RSF should be well-graded so a dense packing can occur during compaction. 

The recommended backfill is the same as that used in abutment construction (see table 1). 

3.3.2.1 Riprap Protection

Riprap protection should be sized appropriately for the class of stone specified. The stone used 

should be hard, durable, angular, free of organic and spoil material, and resistant to weathering 

and water action. It should be free of clay or soft shale seams that can slake when exposed to 

water. Hydraulic Engineering Circular 23 (HEC-23) should be used to adequately size riprap or 

other scour countermeasures.(5)

3.3.3 Integrated Approach Backfill

The GRS located directly behind the beam end is necessary to provide a smooth, integrated

transition from the approach way to the bridge deck. This area of GRS-IBS is called the approachway transition. The fill material used for this transition should be a well-graded gravel similar to 

that used for the RSF backfill (see table 1). 

3.4 GEOSYNTHETIC

Since GRS is generic, there are many types of geosynthetic materials of various strengths available

for abutment construction. At the time of this report, all in-service GRS-IBSs have used a biaxial,

woven polypropylene (PP) geotextile in the abutment. This geotextile was used for several reasons,

including cost, ease of placement, and compatibility with the friction connection that is used

between the block facing and the GRS mass. While any geosynthetic meeting the requirements 

outlined in this section can be used in the abutment, a geotextile must be used for the RSF and 

the integrated approach to encapsulate the material.
21

An ultimate strength of at least 4,800 lb/ft is used for GRS load-bearing applications. In some 

cases, it might be appropriate to specify stronger reinforcement strength depending on the design 

requirements. Chapter 4 provides design guidance on the required reinforcement strength for a 

particular application, which is a function of the lateral stress, reinforcement spacing, and 

backfill properties.

The reinforcement strength at 2 percent strain is also an important consideration in the performance

of GRS-IBS (see chapter 4). Limiting the required reinforcement strength to less than the

reinforcement strength at 2 percent strain will ensure long-term performance and serviceability.

In some situations, the permittivity and apparent opening size of a geosynthetic need to be considered

to ensure adequate long-term drainage, particularly when the abutment may be submerged at any 

point. Since the use of a free-draining backfill is recommended in this situation, a rapid release of 

water from the reinforced soil fill can occur. Nevertheless, the impact of water on wall design needs

to be considered, particularly in situations where rapid drawdown can occur as the result of receding

floodwaters. It is also important to ensure that the geosynthetic material is capable within its

specific environment.

Geosynthetics can be either uniaxial or biaxial, meaning the reinforcement either has more strength

in one direction or it has equal strength in both directions along its length. The term machine

direction (or warp direction) refers to the strength along the length of the roll, and the term crossmachine direction (or fill or weft direction) refers to the strength along the width of the roll (see 

figure 7). If a uniaxial reinforcement is used, having greater strength in the cross-machine 

direction allows for easy placement, as the geosynthetic can be rolled out parallel to the wall. 

When using geosynthetics that are uniaxial in the machine direction, the placement must be 

perpendicular to the wall, adding to construction time. It is recommended, however, that biaxial

reinforcement be used to eliminate construction placement errors and ensure approximately 

equal strength in both directions.

Figure 7. Illustration. Geosynthetic roll direction.
22

It is important to properly select the geosynthetic for the specific site conditions. The following 

should be specified for geosynthetic reinforcement:

Laboratory test results documenting ultimate strength in accordance with ASTM D4595

for geotextiles or ASTM D6637 for geogrids. Tests should be conducted at a strain 

rate of 10 percent per minute.

Follow industry standards on the hydrolysis resistance of polyester (PET), oxidative 

resistance of PP and high density polyethylene (HDPE), and stress cracking resistance 

of HDPE for all components of the geosynthetic

Laboratory tests documenting direct sliding coefficients for various soil types or 

project specific soils in accordance with ASTM D5321.

Manufacturing QC program and data indicating minimum test requirements, test 

methods, test frequency, and lot size for each product. Further minimum conformance 

requirements as prescribed by the manufacturer shall be indicated. Table 3 shows the 

minimum conformance criteria required for approval. 

Table 3. Conformance criteria.

Test Test Procedure

Wide Width Tensile (geotextiles) ASTM D4595

Wide Width Tensile (geogrids) ASTM D6637

Specific Gravity (HDPE only) ASTM D1505

Melt Flow Index (PP and HDPE) ASTM D1238

Inherent Viscosity (PET only) ASTM D4603

Carboxyl End Group (PET only) ASTM D2455

Single Rib Tensile (geogrids) ASTM D6637

The primary resin used in manufacturing shall be identified as to its ASTM type, 

class, grade, and category.

o For HDPE resin, type, class, grade, and category in accordance with

ASTM D1248 shall be identified. For example: Type III, Class A, 

Grade E5, Category 5.

o For PP resins, group, class, and grade in accordance with ASTM D4101 shall 

be identified. For example: Group 1, Class 1, Grade 4.

o For PET resins, minimum production inherent viscosity (ASTM D4603) and 

maximum carboxyl end groups (ASTM D2455) shall be identified.

For all products, the minimum UV resistance as measured by ASTM D4355 shall be 

identified.
23

3.5 MISCELLANEOUS MATERIALS

The three main materials involved in GRS construction are the facing element, the backfill, and 

the geosynthetic reinforcement. Other miscellaneous materials are also necessary during 

construction, including the following:

Concrete block wall fill: Concrete block wall fill, along with rebar, is used to fill in 

and bind together the top three courses of facing blocks (see chapter 7). It is also used 

for runoff coping and, if necessary, to connect the wing wall to the abutment face 

when a vertical seam is located at the corners. The concrete used should be ASTM 

Class A concrete with 4,000 psi compressive strength.

Rebar: No. 4 rebar (0.5-inch diameter), preferably epoxy-coated, is used in the concrete

block wall fill to pin the top three courses of facing blocks. If necessary, it can also be 

used to connect the wing wall to the abutment face at the corners (see chapter 7).

Flashing: Flashing (e.g., aluminum flashing) can be used for two main purposes: (1) to 

serve as a drip edge under the superstructure within the clear space to shed potentially 

corrosive fluids off of the dry cast block as a precaution and (2) to prevent animals from

burrowing into the abutment (see chapter 7). Typical dimensions of the aluminum

fascia are 4 inches by 1.5 inches. This may not be necessary and is a decision left to 

the engineer.

Foam board: A rigid foam insulation board is used to provide setback and to create a 

bearing buffer between the superstructure and the wall face (see chapter 7). The foam 

board is 2-inches thick by 12-inches wide.

Bitumen coating: A bitumen coating is often shop-installed on a concrete beam

where it will be embedded within the GRS abutment and wing walls to prevent 

corrosion of the embedded concrete (see figure 8). 

Figure 8. Photo. Bitumen coating on concrete beam ends.

25

CHAPTER 4. DESIGN METHODOLOGY FOR GRS-IBS

4.1 OVERVIEW OF GRS-IBS DESIGN METHOD

During the past 30 years, GRS technology has been used to build walls, shallow foundations, 

culverts, bridge abutments, and rock fall barriers. The technology also has been used to stabilize

slopes and repair roadways. This chapter focuses on the GRS design method used for GRS-IBS

including an abutment and wing walls. While GRS technology can provide solutions in a 

variety of applications and under certain extreme conditions, the design method described in this 

manual provides a recipe for design of GRS-IBS with limitations on abutment heights, bridge 

spans, and design loads. 

The design methods described in this chapter are appropriate for GRS structures (an abutment and 

wing walls) with a vertical or near vertical face and at a height that does not exceed 30 ft. Although 

the majority of bridges built with GRS-IBS have spans of less than 100 ft, spans of up to 140 ft 

have been constructed. While larger spans are possible, the bearing stress on the GRS abutment 

is limited to 4,000 lb/ft2

. The demands of longer spans on GRS-IBS are not fully understood at 

this time, and it is recommended that engineers limit bridge spans to approximately 140 ft until 

further research has been completed.

GRS-IBS abutment capacities are dependent on a combination of the strength of the fill material and

the strength of the reinforcement when built in accordance with the two rules of GRS construction:

(1) good compaction (95 percent of maximum dry unit weight, according to AASHTO T99) of 

high-quality granular fill and (2) closely spaced layers of reinforcement (12 inches or less). It is 

recommended that design or allowable bearing pressure be limited to 4,000 lb/ft2.

For design pressures larger than 4,000 lb/ft2, the performance criteria must be checked against 

the applicable stress-strain curve resulting from a performance test (discussed later in this chapter

and in appendix B). The performance criteria for GRS-IBS consist of a tolerable vertical strain of 

0.5 percent and lateral strain of 1 percent. A significant amount of research and practical experience

has shown that GRS-IBS designed and constructed within the limits defined in this manual will 

produce safe, durable systems.

The design process starts with establishing the project requirements from which the preliminary 

geometry of GRS-IBS is determined. Once the geometry is defined, it is then evaluated against 

external and internal modes of failure. An iterative process is used to assess the geometry and make

adjustments as necessary to facilitate construction and assure long-term performance. Economy 

should also be a consideration when evaluating each design alternative (e.g., deeper embedment 

versus larger footing).

A general and identifying feature of the GRS-IBS design is a mass built with alternating layers of 

compacted granular fill material and closely spaced reinforcement (less than or equal to 12 inches).

In nearly all of the GRS masses built in the United States as full-scale experiments or as in-service

structures, however, the design has been based on an 8-inch layered system. There are other features

and principles common to a GRS mass. Most GRS walls have been built with dry-stacked concrete

facing blocks and are flexible (in terms of global bending stiffness).
26

A GRS abutment is a type of gravity structure. Therefore, external stability should be evaluated for

the direct sliding, bearing capacity, global stability, and overturning failure modes limiting this type

of construction. However, because a GRS mass is relatively ductile and free of tensile strength, 

overturning about the toe, in a strict sense, is not a possible response to earth pressures at the back

of the mass or loading on its top. Other attributes of GRS-IBS also tend to preclude overturning as a 

mode of failure. GRS-IBS consists of two abutments supporting an integrated superstructure that 

would function as a strut to resist overturning, and each GRS mass has a reinforced integration

zone above its heel, also resisting the overturning mode of failure. Consequently, while direct 

sliding, bearing capacity, and global stability are evaluated in conventional ways, overturning is 

sometimes addressed by inspection and comparison to observations of past performance.

Observations of past performance show that the flexible, internally stabilized soil mass of GRS-IBS

construction, in combination with an RSF, results in more uniform stress distribution, resisting 

any applied vertical and lateral loads. Observations also show that, in addition to lack of overturning, 

the combination of vertical and lateral loads, as limited by analysis of direct sliding, bearing 

capacity, and global stability, does not cause excessive deformation at the face of the GRS mass 

or other undesirable performance.

While this combination of unique features and behavior eliminates the need to analyze overturning

as a failure mode for completed GRS-IBS, the engineer may choose to analyze for overturning 

during an intermediate phase of construction with consideration for the time needed for an overturning

mechanism to develop and the concurrent level of loading or for project configurations different 

from those described herein. For example, overturning may still be a viable failure mode for 

abutment wing walls constructed with GRS technology if they retain soil other than reinforced 

soil from the abutment or opposite wing wall (i.e., if they retain natural soil).

GRS is inherently internally stable because of the interaction between the soil and the reinforcement

layers. The strength and stiffness of a GRS mass depends on the unique combination of compacted

soil and reinforcement. The vertical capacity of the GRS abutment can be determined either 

empirically or analytically. 

Empirically, the capacity is found using a stress-strain curve specific to the combination of the 

reinforcement type and granular fill material. If the designer uses a combination of the materials 

previously tested, then the appropriate stress-strain curve can be used for design. If the designer 

decides to change the materials from those already tested, then a performance test can be performed

to obtain an applicable stress-strain curve for the empirical method. Guidelines on how to conduct 

a performance test are given in appendix B. 

Alternatively, the designer can predict the ultimate vertical capacity of the GRS abutment by using

an analytical equation. The equation is a function of reinforcement spacing, soil strength, and soil

grain size. Note that the analytical method does not predict vertical deformation. A performance 

test is needed to adequately predict the deformation behavior of the GRS abutment. This design 

method is based on the results of many full-scale experiments and verified using case history 

performance data collected on several in-service GRS structures more than 20 years old.
27

The design of GRS-IBS is based on the following assumptions: 

The spacing of the reinforcement (12 inches or less) is a principal factor in the 

performance of GRS-IBS.

A GRS mass is a composite material that is stabilized internally.

Both the compacted granular fill and the reinforcement layers strain laterally together 

in response to vertical stress until the system approaches a failure condition.

A GRS mass is not supported externally, and therefore, the facing system is not 

considered a structural element in design.

Lateral earth pressure at the face of a GRS mass (i.e., thrust) is not significant,

eliminating connection failure as a possible limit state.

The facing elements of a GRS mass are frictionally connected to the geosynthetic 

reinforcement.

Under the prescribed granular fill and reinforcement conditions, reinforcement creep 

is not a concern for the sustained loads. Therefore, individual reduction factors for 

reinforcement creep are not necessary. Creep can be accommodated safely within the 

factor of safety used for design. 

As described in greater detail in subsequent sections of this manual, GRS-IBS design and 

construction processes follow from these basic assumptions and principles.

4.2 BASIC DESIGN STEPS FOR GRS-IBS 

There are nine basic steps in the design of GRS-IBS (see figure 9). Note that the design philosophy

illustrated in this section is Allowable Stress Design (ASD). It is FHWA policy that design for 

all Federal-aid funded projects be conducted using the AASHTO Load and Resistance Factor 

Design (LRFD) methodology. Guidelines to design GRS-IBS in an LRFD format are presented in 

appendix C. The LRFD format presented was normalized to produce the same results as the ASD

method and does not represent a statistically based calibration that would be consistent with other 

AASHTO LRFD methods. After sufficient data is produced and collected as a result of this

technology deployment and other efforts, a thorough statistical analysis will be performed to 

produce LRFD specifications for the design of GRS-IBS.
28

CONDUCT EXTERNAL STABILITY ANALYSIS

Direct Slide, Bearing Capacity, Global Stability

IMPLEMENT DESIGN DETAILS

Reinforced Soil Foundation, Guardrails, Drainage, Utilities

FINALIZE GRS-IBS

Reinforcement and facing block layout, fill

CONDUCT INTERNAL STABILITY ANALYSIS

Capacity, Deformations, Reinforcement Strength

CALCULATE LOADS

Live, dead, impact and earthquake loads

DETERMINE LAYOUT OF GRS-IBS

Geometry, excavation

EVALUTE PROJECT FEASIBILITY

Logistics, Technical Requirements, Performance Objectives

PERFORM A SITE EVALUATION

Topography, Soil Conditions (Foundation and Retained Backfill), Groundwater

Drainage, Hydrological Conditions, Existing Structures

ESTABLISH PROJECT REQUIREMENTS

Geometry, Loading Conditions, Performance Criteria

Figure 9. Chart. Steps for GRS-IBS design.

4.3 GRS-IBS DESIGN GUIDELINES 

4.3.1 Step 1—Establish Project Requirements

The following parameters must be defined:

Geometry of abutment and wing walls.

o Height.

No

No
29

o Length.

o Batter (vertical or near vertical).

o Wall placement with respect to ground conditions: back slope, toe slope.

o Skew.

o Grade.

o Superelevation.

Loading conditions.

o Soil surcharge.

o DL.

o LL.

o Seismic load.

o Impact loads.

o Loads from adjacent structures.

Performance criteria.

o Design format (e.g., ASD, LRFD).

o Tolerable movements.

§ Vertical settlement.

§ Lateral displacements.

§ Differential settlement.

§ Angular distortion between abutments.

o Design life.

o Constraints.

§ Environmental.

§ Construction.

4.3.2 Step 2—Perform a Site Evaluation

To properly assess conditions at the site, a site visit must be conducted. During this visit, the 

following must be performed by the agency and/or its designer:

Study the existing topography with respect to the proposed GRS-IBS.

Check any existing structures/roads for problems to aid in the assessment and design.
30

Conduct a subsurface investigation. Refer to AASHTO’s Standard Practice for 

Conducting Geotechnical Subsurface Investigations for more information. 

Alternatively, refer to FHWA’s Soils and Foundations Manual.

(6,7)

o Foundation soil properties ( f, ’f, C’f, Cu).

o Groundwater conditions.

Evaluate soil properties for the retained earth ( b, b, C’b, Cb).

Evaluate soil properties for the reinforced backfill ( r, r, Cr, dmax). In addition to 

the basic soil properties, the maximum diameter of the granular backfill (dmax) is 

necessary to determine the ultimate capacity and required reinforcement strength. The 

gradation of the reinforced backfill is also important. 

Evaluate hydraulic conditions. This can be accomplished through consultation with a 

qualified hydrologist.

4.3.3 Step 3—Evaluate Project Feasibility

The feasibility of the project should be evaluated in terms of cost, logistics, technical requirements, 

and performance objectives. In particular, in the case of abutments for bridges constructed over

water, the potential for scour, sedimentation, and/or channel instability must be evaluated in 

accordance with the policy and procedures of both FHWA and AASHTO. It is necessary to 

determine the potential for scour at all bridges constructed over water. If the abutment will be

impacted by scour, additional design requirements are necessary (see chapter 6). These additional

design requirements can be determined and implemented through a hydraulic and scour analysis 

of the site. Once the scour potential is determined, a countermeasure can be designed to protect 

the abutment against failure during a flood due to the scour that will occur at the toe of the abutment.

A designed countermeasure will also protect the abutment from lateral channel migration that 

could undermine the foundation.

4.3.4 Step 4—Determine Layout of GRS-IBS

The layout of GRS-IBS is ultimately based on site conditions (e.g., desired road alignment, right 

of way, geotechnical issues, and hydraulic considerations). A survey should be conducted to 

determine the location of the GRS abutment and the layout. The layout of the abutment face wall 

needs to coincide with the wing walls because the system is built from the bottom up one course 

at a time. Both walls are built at the same time. Use the following steps to design the abutment:

1. Define the geometry of the abutment face wall and wing walls. 

2. Layout the abutment with respect to the superstructure (skew, superelevation, grade).

o The recommended minimum bearing width (b) for the superstructure is 2.5 ft 

for span lengths (Lspan) greater than or equal to 25 ft, as shown in figure 10.

For span lengths (Lspan) less than 25 ft, the minimum bearing width is 2.0 ft.

γ

γ

γ
31

3. Account for setback and clear space to calculate the elevation of the abutment face wall 

and the span length of the bridge.

o The setback distance (ab) between the back of the face and the beam seat 

should be the height of a standard CMU (nominally 8 inches) or more, as 

shown in figure 10.

o The minimum clear space (de), defined as the distance from the top of the 

uppermost facing block to the bottom of the superstructure, should be 3 inches or 

2 percent of the abutment height, whichever is greater (see figure 11). The gap 

is to ensure that the superstructure does not bear on the facing block due to an 

unforeseen event.

Figure 10. Photo. Bridge seat and setback distances.
32

Figure 11. Illustration. Clear space distance.

4. Determine the depth and volume of excavation necessary for construction. A GRS 

abutment is inherently stable and therefore can be built with a truncated base to reduce

the excavation. Truncation also reduces the requirements for backfill and reinforcement.

o For span lengths (Lspan) greater than or equal to 25 ft, a minimum base width 

of the wall including the block face (Btotal) of 6 ft should initially be chosen. 

For span lengths (Lspan) less than 25 ft, a minimum base width of the wall 

including the block face (Btotal) of 5 ft should initially be chosen. Whether a 

cut or fill situation, there should be a minimum base-to-height (Btotal/H) ratio 

of 0.3. If GRS-IBS is to cross water, the base of the abutment should be 

placed at the calculated scour depth (see chapter 5). 

o Excavation of one-quarter the total width of the base of the abutment including

the block face should be made at the base in front of the face of the wall to 

accommodate for construction of the RSF. The total width of the RSF should 

extend beyond the base of the GRS abutment by one-fourth the width of the 

base (see figure 12).

o The depth of the excavation for the RSF (DRSF) should equal one-quarter the 

total width of the base of the GRS abutment including the block face (see 

figure 12). Additional excavation may be necessary depending on the soil 

conditions (e.g., compressible soils) and should be determined by the engineer. 
33

In some situations, it may be beneficial to improve the ground beneath the 

RSF to reduce settlement of the bridge system.

o Before designing and constructing an RSF, it is prudent to conduct a soil 

investigation of the existing foundation soil including applicable lab tests to 

determine the soil’s properties, as discussed in section 4.3.2.

Figure 12. Illustration. RSF dimensions.

5. Select the length of reinforcement for the abutment. The minimum reinforcement length 

at the lowest level should extend the width of the base (Btotal) and have a minimum 

base-to-height ratio (B/H) (not including the facing block) of 0.3. The minimum 

reinforcement length at the lowest level should extend the width of the base (Btotal), 

with a minimum of 5–6 ft or a base-to-height ratio (B/H) of 0.3 (see previous step). 

Once the base length of the reinforcement is chosen, the reinforcement schedule should 

follow the cut slope, if applicable, up to a B/H ratio of 0.7. From there, the reinforcement

length can get progressively longer in reinforcement zones (see figure 13). Not every 

layer will need to extend fully to the cut slope. The progressively longer lengths of 

reinforcement serve to improve the quality of construction and overall stability of the 

GRS abutment. The reinforcement zones also serve to provide a transition from the 

substructure to the superstructure. The exact details of the reinforcement zones, such 

as number of layers and length, are left to the designer. For cut slopes flatter than 1:1, 

reinforcement zones with lengths larger than 1H may not be necessary. The backfill 

between the reinforced zone and the cut slope or retained soil must be the same structural

backfill as the reinforced fill and compacted to the same effort (see chapter 3). The 

reinforcement spacing should be no more than 12 inches at the wall face, in 

accordance with the two rules of GRS construction.
34

6. Add a bearing reinforcement zone underneath the bridge seat to support the increased 

loads due to the bridge (see figure 13). This bearing bed reinforcement serves as an 

embedded footing in the reinforced soil mass. The bearing bed reinforcement spacing 

directly underneath the beam seat should be, at a minimum, half the primary spacing 

(e.g., for an 8-inch primary spacing, the bearing bed reinforcement spacing will equal 

4 inches). In general, the minimum length of the bearing bed reinforcement should be 

twice the setback plus the width of the bridge seat. The depth of the bearing reinforcement

zone is determined based on internal stability design for required reinforcement strength

(see section 4.4.7.3.1). At a minimum, there should be five bearing bed reinforcement 

layers (see figure 13). 

Figure 13. Illustration. Reinforcement schedule for a GRS abutment.

7. Blend the reinforcement layers in the integration zone to create a smooth transition. 

The layers should extend to the cut slope, if applicable, with the exception of the top 

reinforcement layer, depending on the site. This top layer should extend beyond the 

cut slope to prevent moisture infiltration. The integration zone is part of the integrated 

approach of GRS-IBS (see figure 13). It is added behind the bridge superstructure to 

limit the development of a tension crack at the cut slope and reinforced soil interface 

and to blend the approach way on to the roadway to create a smooth transition. The 

number of reinforcement layers in the integration zone depends on the height of the 

superstructure, but each wrapped layer should be no more than 12 inches in height. 

Additional work is needed to integrate the substructure with the superstructure within

the integration zone. This is described in chapter 7.
35

4.3.5 Step 5—Calculate Applicable Loads

The applicable external pressures and loads (permanent and transient) on the reinforced zone of 

the GRS abutment should be calculated. The most common pressures (which may be resolved 

into forces) on GRS-IBS for stability computations are depicted in figure 14.

Figure 14. Illustration. Vertical and lateral pressures on a GRS abutment.

The applicable pressures on a GRS abutment are as follows:

qt = equivalent roadway LL surcharge

h,t = lateral stress distribution due to the equivalent roadway LL surcharge

qrb = surcharge due to the structural backfill of the integrated approach (road base)

h,rb = lateral stress distribution due to the structural backfill of the integrated approach 

qb = equivalent superstructure DL pressure

h,bridge = lateral stress distribution due to the equivalent superstructure DL pressure

h,b = equivalent lateral stress distribution due to retained soil behind the GRS abutment

qLL = equivalent superstructure LL pressure

σ

σ

σ

σ
36

h,LL = lateral stress distribution due to the equivalent superstructure LL pressure

h,W = lateral stress distribution due to the weight of the GRS fill

4.3.5.1 Lateral Pressures and Stresses

The lateral earth pressure can be calculated according to classical soil mechanics for active earth 

pressure. The active earth pressure coefficient (Ka) is calculated according to equation 1.

(1)

Where is the friction angle of interest (for example, substitute b when calculating Kab for the 

retained soil). The lateral stress distribution due to the weight of the GRS fill ( h,W) is found 

using Rankine’s active stress condition, shown in equation 2.

(2)

Where r is the unit weight of the reinforced fill, z is the depth from the top of the wall, and Kar is 

the coefficient of active earth pressure (equation 1) using the friction angle of the reinforced fill ( r).

The lateral stress distributions due to the equivalent roadway LL surcharge ( h,t) and structural 

backfill of the integrated approach ( h,rb) are found according to equation 3 and equation 4, 

respectively.

(3)

(4)

Where qt is the equivalent roadway LL surcharge, qrb is the surcharge due to structural backfill 

(road base), and Kab is the coefficient of active earth pressure (equation 1) using the friction 

angle of the retained backfill ( b). Note that equation 3 and equation 4 assume that the loading is 

continuous across the retained soil. 

Where the loads are not continuous across the GRS abutment or retained soil, the lateral pressure is

based on Boussinesq theory for load distribution through a soil mass for an area transmitting a 

uniform stress a distance x from the edge of the load (see figure 15).(8) The actual pressure using 

this theory depends on the location of interest. For required reinforcement strength calculations, the

location of interest is directly underneath the beam seat centerline (e.g., x = bq/2 for the bridge DL).

σ

σ

KKaa = 1−ssssss

1+ssssss = tt 2 �45oo − 2

�

σ

σσh,WW = γγrrzzKKaa

γ

σ

σ

σσh,tt = qqttKKaaaa

σσh,rr = qqrr KKaa
37

Figure 15. Illustration. Boussinesq load distribution with depth for a strip load.

The lateral pressure due to surcharge loading ( h,q) is calculated according to equation 5.

(5)

Where q is the surcharge pressure (e.g., qb for the bridge surcharge), Ka is the coefficient of active

earth pressure (equation 1), and and are the angles shown in figure 15, found using equation 6

and equation 7, respectively. Note that and must be input in radians in equation 5.

(6)

(7)

The lateral pressure in the GRS abutment due to the superstructure DL and LL will have a trend 

similar to that shown in figure 16, where the stress is highest at the top of the GRS abutment and 

lowest at the base. Note that the bearing bed reinforcement underneath the beam seat helps to 

mitigate the increased vertical (and thus lateral) pressures in this location. In fact, the bearing bed 

reinforcement is recommended in the design of GRS abutments for this reason.

σ

σσh,qq = qq

ππ [αα + sin(αα) cos⁡(αα + 2ββ)]KKaa

α β

α β

αα = tt −1 �

xx

zz

� − ββ

ββ = tt −1 �

xx− qq

zz �
38

Figure 16. Illustration. Internal lateral stress in GRS abutment wall due to bridge loading.

Note that other load distributions are available besides Boussinesq. For example Westergaard is 

more applicable to a GRS mass than Boussinesq. However, it gives lower stresses than Boussinesq,

and therefore, using Boussinesq will provide a more conservative estimate of stresses. 

4.3.5.2 Dead Loads

4.3.5.2.1 Bridge: In a GRS-IBS design with adjacent concrete box beams, the bridge superstructure

bears directly upon the GRS abutment. For superstructures with spread girders, a footing (which 

bears directly upon the GRS abutment) is necessary to ensure even load distribution on the GRS 

abutment. The equivalent DL design pressure on the abutment seat includes the dead loads due to 

the bridge beams, asphalt, overlay, guardrail, and any other applicable permanent loads related to 

the superstructure. 

4.3.5.2.2 Road Base: Behind the bridge beams, road base is wrapped in geotextile (called the 

integrated approach). The wrapped face controls lateral load from the road base on the beam or 

abutment sill.

4.3.5.3 Live Loads

There are two applications of LL that affect the design of GRS-IBS: LL on the approach 

pavement and LL on the superstructure. Both of these live loads are defined by AASHTO and 

should be appropriately quantified by the design engineer.

(9)
39

4.3.5.3.1 LL on the Approach Pavement: An LL surcharge (qt) is used to account for the vehicular 

load on the approach pavement leading up to the superstructure. This load consists of a uniform 

height (heq) of earth that produces an equivalent lateral effect on the abutment as the application 

of the vehicular LL specified for the superstructure. The equivalent height of earth is dependent 

on the abutment height and the orientation of the abutment with respect to the roadway (e.g.,

perpendicular). This load is used for both internal and external stability analyses.

4.3.5.3.2 LL on the Superstructure: The vehicular LL used for designing GRS-IBS is determined

by applying the HL-93 LL model to the superstructure. This model consists of appropriately locating

a design truck or design tandem in combination with a design lane load in each design lane of the 

bridge to create the maximum force effect at each abutment. The vehicular portion of the LL model

is amplified for dynamic load allowance (impact). The governing LL is distributed to the abutment

by multiplying by the number of design lanes and dividing by the bridge seat bearing area. This

equivalent distributed LL pressure on the abutment seat (qLL) can be determined using equation 8.

(8)

Where Nlanes is the number of design lanes on the bridge, b is the bridge seat bearing width (see

figure 14), Bb is the width of the bridge, and (LL+IM)total is the governing abutment reaction for 

the HL-93 LL model for one lane.

If the bridge seat bearing width is unknown and needs sizing, the LL from the superstructure 

should be quantified as a reaction (QLL) rather than a pressure (see equation 9).

(9)

4.3.5.4 Design Pressure

Adding LL on the superstructure and bridge DL per abutment will give the total load that the bridge

seat must support. Dividing this total load by the area of the bridge seat will give the bearing 

pressure. For abutment applications, the bearing pressure should be targeted to around 4,000 lbs/ft2.

If this is exceeded, the width of the bridge seat should be increased. Although higher design 

pressures have been successfully applied to in-service GRS-IBS, this is not encouraged.(1)

4.3.6 Step 6—Conduct an External Stability Analysis

The external stability of GRS-IBS is evaluated by looking at the following potential external 

failure mechanisms:

Direct sliding (shown in figure 17).

Bearing capacity (shown in figure 18)

Global stability (shown in figure 19).

qLL = (LL + IM)total (Nlanes )

b(Bb)

QLL = (LL + IM)total (Nlanes )
40

Figure 17. Illustration. External stability: direct sliding.

Figure 18. Illustration. External stability: bearing capacity.

Figure 19. Illustration. External stability: global stability.
41

4.3.6.1 Direct Sliding

The GRS abutment must resist translation, or direct sliding. The LL on the approach pavement (qt)

is assumed to act only over the retained backfill and not the reinforced soil mass. While the 

contribution of qt (and qLL) is ignored for both a wall and an abutment, the bridge load (qb) has a 

stabilization effect against direct sliding when considering an abutment. Since the road base extends 

over the GRS abutment and the retained backfill, it acts to both stabilize and drive direct sliding. 

Contributions to both the driving force and to the resisting force from the road base must be 

taken into account because it is a permanent load.

The thrust forces behind the GRS abutment from the retained backfill (Fb), the road base (Frb), and

the roadway LL surcharge (Ft) are calculated using equation 10, equation 11, and equation 12. 

(10)

(11)

(12)

Where b is the unit weight of the retained backfill, Kab is the active earth pressure coefficient for

the retained backfill (equation 1), H is the height of the wall including the clear space distance, 

qrb is the road base DL, and qt is the roadway LL. 

The total driving force (Fn) is calculated by summing each thrust force previously calculated, as 

shown in equation 13. 

(13)

The resisting force (Rn) is calculated according to equation 14. 

(14)

Where Wt is the total resisting weight (calculated in equation 15), is the friction factor between 

the wall base and the foundation (taken as tan crit), and crit is the critical friction angle. Since 

the RSF is encapsulated with geotextile, sliding at the base of the GRS abutment will occur

between soil and the geotextile reinforcement. The critical friction angle will therefore be the

interface friction angle between the soil and reinforcement. The interface friction angle should be 

determined with an interface direct shear test for the particular combination of geosynthetic and 

reinforced fill material (ASTM D5321). If this information is not available for geotextiles and 

geogrids, assume that the friction factor is equal to 2/3 times the tangent of the reinforced granular

fill friction angle ( = 

2

/3tan( r)). 

(15)

Where W is the weight of the GRS abutment (calculated in equation 16), qb is the bridge DL, b is 

the width of the bridge load (measured along the direction of the roadway), qrb is the road base 

FF = 1

2 γγ KKaaaa HH2

FFrr = qqrr KKaaaa HH

FFtt = qqttKKaaaa HH

γ

FF = FF + FFrr + FFtt

RR = WWttμμ

μ

μ

WWtt = WW + qq + qqrr rr ,tt
42

DL, and brb,t is the width over the GRS abutment where the road base DL acts (see figure 14). 

The LL on the approach pavement and the superstructure are not included as resisting forces 

because they are transient loads.

(16)

Where r is the unit weight of the reinforced fill, H is the height of the GRS abutment including the

clear space distance, and B is the base width of the GRS abutment not including the wall facing.

The factor of safety against direct sliding (FSslide) is computed according to equation 17. The 

factor of safety must be greater than or equal to 1.5. If not, consider lengthening the reinforcement

at the base.

(17)

4.3.6.2 Bearing Capacity

To prevent bearing failure, the vertical pressure at the base of the RSF must not exceed the allowable 

bearing capacity of the underlying soil foundation. The vertical pressure is a result of the weight 

of the GRS abutment, the weight of the RSF, the bridge seat load, the LL on the superstructure,

and the LL on the approach pavement. The pressure at the base ( v,base,n) is calculated according 

to a Meyerhof-type distribution, shown in equation 18.

(10)

(18)

Where V is the total vertical load on the GRS abutment (calculated in equation 19), BRSF is the 

width of the RSF, and eB,n is the eccentricity of the resulting force at the base of the wall 

(calculated in equation 20).

(19)

Where W is the weight of the GRS abutment (equation 16), WRSF is the weight of the RSF, Wface

is the weight of the facing elements, qt is the roadway LL, brb,t is the width of the traffic and road 

base load over the GRS abutment, qrb is the road base surcharge, qb is the bridge DL, b is the 

width of the bridge seat, and qLL is the LL on the superstructure.

(20)

Where MD is the total driving moment, MR is the total resisting moment, and V is the total 

vertical load (equation 19). The moments should be calculated about the bottom and center of the 

RSF for the specific layout of the GRS abutment. If eB,n is negative, take eB,n equal to zero for the 

term BRSF-2eB,n.

WW = γγrrHHHH

γ

FFFFssssssssss = RR

FF ≥ 1.5

σ

σσvv, , = ∑ VV

RR − 2 ,

Σ

∑ VV = WW + WWRR + WWff ff + rr ,tt(qqtt + qqrr ) + (qq + qqLLLL)

, = ∑ MMDD − ∑ MMRR

∑ VV

Σ Σ Σ
43

The bearing capacity of the foundation (qn) can be found using equation 21.

(9)

(21)

Where cf is the coehsion of the foundation soil, Nc, , and Nq are dimensionless bearing 

capacity coefficients as shown in table 4, f is the unit weight of the foundation soil, B’ is the 

effective foundation width (equal to BRSF-2eB,n), and Df is the depth of embedment. The friction

angle in table 4 should be taken as the foundation’s friction angle ( f). If groundwater is present, 

modifications to equation 21 may be necessary and are provided by AASHTO.(9)

Table 4. Bearing capacity factors.

(9)

Nc Nq Nc Nq

0 5.14 1.0 0.0 23 18.1 8.7 8.2

1 5.4 1.1 0.1 24 19.3 9.6 9.4

2 5.6 1.2 0.2 25 20.7 10.7 10.9

3 5.9 1.3 0.2 26 22.3 11.9 12.5

4 6.2 1.4 0.3 27 23.9 13.2 14.5

5 6.5 1.6 0.5 28 25.8 14.7 16.7

6 6.8 1.7 0.6 29 27.9 16.4 19.3

7 7.2 1.9 0.7 30 30.1 18.4 22.4

8 7.5 2.1 0.9 31 32.7 20.6 26.0

9 7.9 2.3 1.0 32 35.5 23.2 30.2

10 8.4 2.5 1.2 33 38.6 26.1 35.2

11 8.8 2.7 1.4 34 42.2 29.4 41.1

12 9.3 3.0 1.7 35 46.1 33.3 48.0

13 9.8 3.3 2.0 36 50.6 37.8 56.3

14 10.4 3.6 2.3 37 55.6 42.9 66.2

15 11.0 3.9 2.7 38 61.4 48.9 78.0

16 11.6 4.3 3.1 39 67.9 56.0 92.3

17 12.3 4.8 3.5 40 75.3 64.2 109.4

18 13.1 5.3 4.1 41 83.9 73.9 130.2

19 13.9 5.8 4.7 42 93.7 85.4 155.6

20 14.8 6.4 5.4 43 105.1 99.0 186.5

21 15.8 7.1 6.2 44 118.4 115.3 224.6

22 16.9 7.8 7.1 45 133.9 134.9 271.8

The factor of safety against bearing failure (FSbearing) is computed according to equation 22. The 

factor of safety must be greater than or equal to 2.5. If not, increase the width of the GRS abutment

and RSF (by increasing the length of the reinforcements), replace the foundation soil with a more 

competent soil, or add embedment depth.

(22)

qq = ffNN +

1

2

′

γγffNNγγ + γγffDDffNNqq

Nγ

γ

f Nγ f Nγ

FFFF bb = qq

σσvv, ,

≥ 2.5
44

Beyond bearing capacity, consolidation settlement should be evaluated to ensure excessive 

deformations will not occur over the life of the bridge. Design considerations such as excavation 

and the RSF reduce the pressure on the foundation soil. Nevertheless, settlement of the foundation

soil should be assessed as with any other spread footing according to FHWA guidance.(7)

Determining the criterion for tolerable foundation settlement is left up to the engineer.

A stress history analysis should be conducted to ascertain settlement and stability prediction. 

Answers to the following questions will provide insight on the stress history for an efficient design:

Is the site bridge a replacement project built in the same location?

What was the performance of the existing bridge?

Were there any chronic maintenance issues associated with the existing structure?

What was the combined weight of the abutment and superstructure within the footprint 

of the new bridge foundation? How does that stress compare with the stress of the 

new structure?

Does the site involve an excavation equivalent to the weight of the new GRS-IBS? 

Can the new bridge be built behind the existing foundation?

4.3.6.3 Global Stability

Global stability is evaluated according to classical slope stability theory using either rotational or 

wedge analysis. To facilitate the global stability check, it is prudent to collect accurate soil property

information. Standard slope stability computer programs can then be used to assess the global and

compound stability of a GRS structure. The factor of safety for global stability should equal at 

least 1.5.

4.3.7 Step 7—Conduct Internal Stability Analysis

The internal stability analysis will vary slightly depending on the whether ASD or LRFD is the 

chosen design method. ASD is presented in this chapter. For guidance on LRFD, refer to appendix B.

4.3.7.1 Ultimate Capacity 

The ultimate vertical capacity of a GRS abutment is found either empirically or analytically. It is 

recommended that the ultimate capacity be found empirically if possible. A performance test

should be conducted to determine the ultimate capacity if the reinforced fill is different from those

used in the performance tests reported in this guide (see appendix A). Testing will provide the 

most accurate results for the design. If a performance test cannot be performed, the analytical 

method can be used to determine the ultimate capacity.

4.3.7.1.1 Empirical Method: Empirically, the results of an applicable performance test using the 

same geosynthetic reinforcement and compacted granular backfill as planned for the site should 

be used. The ultimate vertical capacity in this case is defined as the stress at which the performance 
45

test mass strains 5 percent vertically. The ultimate vertical capacity is found in figure 20. For this 

performance test, the nominal capacity (qult,emp) is equal to 26 ksf for a vertical strain of 5 percent.

Figure 20. Graph. Design envelope for vertical capacity and strain 

at 8-inch reinforcement spacing.

Note that figure 20 represents the load-settlement performance of a GRS structure with reinforcement

spaced at 8 inches, well-compacted AASHTO No. 89 fill material (having a friction angle of 

48 degrees and no cohesion), and 4,800 lb/ft woven PP geosynthetic reinforcement. Other

materials have also been tested and are shown in the synthesis report.

(1)

If the materials used are outside the recommendations provided in chapter 3, then a performance 

test must be performed to obtain the applicable stress-strain curve similar to figure 20. Guidance 

on setting up a performance experiment is given in appendix B. The total allowable pressure on 

the GRS abutment (Vallow, emp) is the ultimate capacity (qult,emp) divided by a factor of safety for 

capacity (FScapacity) of 3.5, as shown in equation 23.

(23)

The applied vertical stress (Vapplied), which is equal to the unfactored sum of the vertical pressures 

on the bridge bearing area, must be less than Vallow,emp (see equation 24). This includes the DL from

the bridge (qb) and the LL on the superstructure (qLL). The DL due to the road base (qrb) and the 

LL due to the approach pavement (qt) are located behind the bearing area and are therefore not 

included in vertical capacity calculations related to the bridge superstructure. 

(24)

VVaa aa , eeee = qquu , eeee

FFFF cc

= qquu , eeee

3.5

VVaa = qq + qqLLLL ≤ VVaa aa , eeee
46

4.3.7.1.2 Analytical Method: As an alternative, the load-carrying capacity of a GRS wall and 

abutment can also be evaluated using an analytical formula called the soil-geosynthetic composite

capacity.

(11) The analytical formula was originally developed for GRS walls, but it is applicable 

to GRS abutments as well. Note that the analytical method assumes that the backfill satisfies the 

criteria outlined in chapter 3.

The ultimate load-carrying capacity (qult,an) of a GRS wall constructed with a granular backfill 

can be determined by the soil-geosynthetic composite capacity equation shown in equation 25.

(11)

(25)

Where Sv is the reinforcement spacing, dmax is the maximum grain size of the reinforced backfill, 

Tf is the ultimate strength of the reinforcement, and Kpr is the coefficient of passive earth pressure

for the reinforced fill (calculated in equation 26).

(26)

Where r is the friction angle of the reinforced backfill. The friction angle should be determined 

from a large-scale direct shear device (ASTM D3080).

The total allowable pressure on the GRS abutment (Vallow,an) is the ultimate capacity found 

analytically (qult,an) divided by a factor of safety for capacity (FScapacity) of 3.5 (see equation 27).

(27)

The applied vertical stress (Vapplied), which is equal to the unfactored sum of the vertical pressures 

on the bridge bearing area, must be less than Vallow,an (see equation 28). This includes the DL from 

the bridge (qb) and the equivalent LL on the bridge (qLL). The DL due to the road base (qrb) and 

the LL due to the approach pavement (qt) are located behind the bearing area and are therefore 

not included in capacity calculations related to the bridge superstructure.

(28)

4.3.7.2 Deformations

The approach for determining vertical deformation involves empirically finding the strain from 

an applicable performance test curve. If the materials used are within the specifications given in 

chapter 3, then the curve shown in figure 20 can be used. Otherwise, a performance test must be 

conducted (see appendix B). The lateral strain is then determined analytically assuming the 

theory of zero volume change.(12)

4.3.7.2.1 Vertical: The vertical strain of the GRS abutment is found from the intersection of the 

applied vertical stress due to the DL (qb) and the performance test design envelope for vertical strain

(see figure 20). The vertical strain should be limited to 0.5 percent unless the engineer decides to 

permit additional deformation. The vertical deformation, or settlement, of the GRS abutment is the

vertical strain multiplied by the height of the wall or abutment. Because the GRS abutment is built 

qquu ,aa = �0.7

� vv

6 � TTff

VV

�KK

KK = 1+ssssssфrr

1−ssssssфrr

= tt 2 �45oo + rr

2 �

VVaa aa ,aa = qquu ,aa

FFFF cc

= qquu ,aa

3.5

VVaa = qq + qqLLLL ≤ VVaa aa ,aa
47

with a granular fill, the majority of settlement within the GRS abutment will occur immediately 

after the placement of DL (qb) and before the bridge is opened to traffic. 

The settlement of the underlying foundation soils is determined separately using classic soil 

mechanics theory for immediate (elastic) and consolidation settlement. Factors such as excavation

and the RSF should be taken into account, as the removal of overburden relieves stress on the 

foundation soil. Settlement of the foundation soil can be calculated using the FHWA Soils and 

Foundations Reference Manual.

(7)

4.3.7.2.2 Lateral: In response to a vertical load, the composite behavior of a properly constructed

GRS mass is such that both the reinforcement and soil strain laterally together. This fact can be 

used to predict both the maximum lateral reinforcement strain and the maximum face deformation

at a given load. The method conservatively assumes a zero volume change in the GRS abutment,

which represents a worst-case scenario. The maximum lateral displacement of the abutment face 

wall can be estimated using equation 29.

(12) The lateral strain ( L) is then found using equation 30

and should be limited to 1 percent.

(29)

(30)

Where bq,vol is the width of the load along the top of the wall (including the setback), Dv is the vertical

settlement in the GRS abutment, H is the wall height including the clear space distance, and V

is the vertical strain at the top of the wall. Note that equation 29 and equation 30 come from the 

assumptions of a triangular lateral deformation and a uniform vertical deformation (see figure 21).

This assumption is based on observed deformation behavior of GRS. Also note that the location 

of the maximum lateral deformation depends on the loading and fill conditions, but the volume

gained will still equal the volume lost. The maximum deformation of a GRS abutment often 

occurs in the top third of the abutment/wall.

(11–13)

ε

DDLL = 2 qq,vv DDvv

HH

εεLL = DDLL

qq,vv

= 2DDvv

HH = 2εεvv

ε
48

Figure 21. Illustration. Lateral deformation of a GRS structure.

4.3.7.3 Required Reinforcement Strength

The required reinforcement strength in the direction perpendicular to the wall face (Treq) can be 

determined analytically by equation 31.

(11) The required reinforcement strength should be calculated

at each layer of reinforcement to ensure adequate strength throughout the GRS abutment.

(31)

Where Sv is the reinforcement spacing, dmax is the maximum grain size of backfill, and h is the total

lateral stress within the GRS abutment at a given depth and location (calculated in equation 32).

(32)

Where h,W is the lateral earth pressure using Rankine’s active stress condition (equation 2), 

h,bridge,eq is the lateral pressure due to the equivalent bridge load (calculated in equation 33), 

h,rb is the lateral pressure due to the road base (calculated in equation 34), and h,t is the lateral

pressure due to the roadway LL (calculated in equation 35). To simplify calculations, the approach

LL and road base DL are extended across the abutment. The vertical components of these loads are

then subtracted from the bridge DL and LL, giving an equivalent bridge load. The lateral stress 

due to the equivalent bridge load is then calculated according to Boussinesq theory. The location 

of interest to determine the maximum lateral pressure is directly underneath the centerline of the 

bridge bearing width.

(33)

(34)

TTrr = � σσh

0.7

� vv 6 �

� vv

σ

σσh = σσh,WW + σσh, rr , qq + σσh,rr + σσh,tt

σ

σ

σ σ

σσh, rr , qq = (qq + qqLLLL) − (qqrr + qqtt)

ππ

[αα + sin⁡(αα )cos⁡(αα + 2ββ )]KKaa

σσh,rr = qqrr KKaa
49

(35)

Where qb, qrb, qt, and qLL are the bridge DL, road base DL, roadway LL, and bridge LL surcharges,

respectively, and b and b are the angles shown in figure 15, found using equation 36 and

equation 37, respectively. 

(36)

(37)

The required reinforcement strength (Treq) must satisfy two criteria: (1) it must be less than the 

allowable reinforcement strength (Tallow), and (2) it must be less than the strength at 2 percent 

reinforcement strain ( ) 

In design, a minimum value of the ultimate reinforcement strength (Tallow) is needed to ensure 

adequate ductility and satisfactory long-term performance. In addition, it is prudent to specify 

the resistance required at the working load ( ) to ensure satisfactory performance under 

the in-service condition. 

For abutments, a minimum ultimate tensile strength (Tf) of 4,800 lb/ft is required. The allowable

reinforcement strength (Tallow) is found by applying a factor of safety for reinforcement 

strength (FSreinf) of 3.5 to the ultimate strength (see equation 38). The required reinforcement 

strength (Treq) must be less than Tallow.

(38)

Since geosynthetic reinforcements of similar strength can have rather different load-deformation 

relationships depending on the manufacturing process and the polymer used, it is important that 

Treq be less than the strength at 2 percent reinforcement strain. The strength of the reinforcement 

at 2 percent ( ) is often given by the geosynthetic manufacturer. If Treq is greater than 

, a different geosynthetic must be chosen, the ultimate strength must be increased, or the 

reinforcement spacing must be decreased. 

While the strength of the reinforcement can theoretically vary along the height of the GRS abutment,

it is recommended that only one strength of reinforcement be used throughout the entire abutment.

This simplifies the construction process and avoids placement errors for the reinforcement.

4.3.7.3.1 Depth of Bearing Bed Reinforcement: The required reinforcement strength (Treq) is 

found at each 8-inch primary spacing layer. If Treq is greater than the allowable reinforcement 

strength (Tallow) or the strength at 2 percent strain ( ), then the reinforcement spacing must 

be reduced to 4 inches to the depth at which Treq is less than Tallow or .This depth is 

termed the bearing reinforcement bed. The minimum required depth is five courses of block. 

σσh,tt = qqttKKaa

α β

αα = tt −1 �

2zz

� − ββ

ββ = tt −1 �

−

2zz

�

T@ε=2%

T@ε=2%

TTaa aa = TTff

FFFFrr

= TTff

3.5

T@ε=2%

T@ε=2%

T@ε=2%

T@ε=2%
50

To check that 4-inch spacing for the bearing reinforcement bed is adequate, calculate the required

reinforcement strength again for this new spacing in the top layers to ensure that Treq is less than 

Tallow and for all layers throughout the GRS abutment.

4.3.8 Step 8—Implement Design Details

Figure 22 and figure 23 are typical cross sections of a GRS wall (or wing wall) and an abutment 

face wall and illustrate design details that will be discussed in this section.

Courtesy of Defiance County, OH

Figure 22. Illustration. Typical cross section of a GRS wing wall.

T@ε=2%
51

Courtesy of Defiance County, OH

Figure 23. Illustration. Typical cross section of a GRS abutment face wall.

In the case of an abutment, finalize the design layout for ease of construction, drainage, and other 

considerations that might affect the performance, serviceability, or efficiency of design. The 

following are some GRS design implications and related details for consideration: 

Conduct a hydraulic analysis in accordance with all appropriate regulatory and policy 

guidance (see chapter 5). Consult a licensed hydraulic engineer if necessary.

Ensure that the face of the abutment (which includes the parapet) is wide enough to 

accommodate the installation of guardrails (see figure 24). The additional width should

be enough to allow the guardrail to lay down. This lay-down length is approximately 

4 ft. Steel rail posts should be used because wooden posts are nearly impossible to 

drive into the GRS mass. 
52

Figure 24. Photo. Guardrail lay-down distance.

Consider a core of native soil in the center of the abutment face and two adjacent wing 

walls to minimize excavation (see figure 25). The wing walls can be truncated like the 

abutment. Extend the wing walls sufficiently into the cut slope to prevent erosion caused

by undermining or piping. This should be a minimum of two facing-block lengths.

Figure 25. Photo. GRS abutment and wing walls built around core of native soil

Determine whether to build wing walls with either a full face or a stepped face that 

leads into the cut slope. The decision depends on several factors related to the height 

of the abutment, grade of fill slope (which is usually at 2:1), and time and materials. 

For abutments less than 12 ft in height, a full face is probably most efficient as it is 

the easiest to construct. However, for abutments greater than 12 ft, it might be more 
53

efficient to design a stepped-face wall (i.e., tiered wall) that leads into the cut slope.

Stepped walls use less material but require additional labor in building the second 

foundation to support the extended stepped wall. In either case, all facing blocks 

should be supported on well-compacted structural fill.

Include channel drains along the wings walls to facilitate runoff. The drain path should

not be located directly against the wall face. Armor the drain path with a strip of 

geotextile beneath a layer of channel rock. Grade in compacted native soil against the 

wing walls with a slope leading to the drainage path.

GRS-IBS has been used for bridges with skew, superelevation, and grade without 

problems or serviceability issues. 

o For a skewed bridge, it is important to maintain the minimum bearing area of 

2.5 ft along the length of the abutment face wall. 

o For a bridge with superelevation, it is important to ensure that the minimum 

number of bearing bed reinforcement layers beneath the beam seat (calculated 

in step 7) are installed across the length of the abutment face. 

o At this time, there are no special considerations for opposing GRS abutments 

that support a bridge on a grade. 

Contain the GRS integrated approach fill by wrapping the geotextile layers adjacent 

to the beam ends to prevent lateral spreading (see chapter 7). Extend the reinforcement

layers at the approach back onto the road, as indicated in figure 23.

Avoid any abrupt transition of soil type from the roadway to the bridge. While the RSF

and abutment should use a reinforcement with a minimum ultimate strength of 4,800 lb/ft, 

a lighter geosynthetic of about 2,400 lb/ft could be used for the integrated approach.

However, it is recommended that only one strength of reinforcement be used on site

to simplify the construction process and avoid placement errors.

Plan ahead to avoid trenching, and account for the possible installation of utilities.

Locate and plan to accommodate existing and potential future utilities.

4.3.9 Step 9—Finalize Material Quantities and Layout

To develop the reinforcement schedule, choose a reinforcement length that makes use of the entire

roll of the reinforcement material. Reinforcement material is usually 12- to 18-ft wide. For example,

the width of a PP geosynthetic roll is 12 ft, and the base of a GRS wall is 6 ft including the width 

of the wall face. The roll can be cut in half by a chainsaw, and a 6-ft-wide roll can be used to build

the base of the wall. The remaining 6-ft-wide rolls can be used for secondary or intermediate layers

of reinforcement in the walls.
54

Draw the layout to scale to avoid errors in the calculation of quantities. Add 10 percent to the 

estimate of all materials. When using CMU, use the exact dimensions of 75

/8 inches by 75/8 inches 

by 155/8 inches and buy both corner and face blocks.

Building GRS abutments vertically without a batter eliminates the need to trim blocks. This will 

make it more difficult to hide lateral movement and may give an illusion of instability when the 

structure is, in fact, stable. Use only high–quality, well-graded gravel, as specified in chapter 3.

4.4 DESIGN EXAMPLE: BOWMAN ROAD BRIDGE, DEFIANCE COUNTY, OH

Construction of Bowman Road Bridge was completed in October 2005 by a Defiance County, OH,

construction crew. This project represents the initial deployment of GRS-IBS. The structure 

was chosen for a design example because it demonstrates many of the variables that can be

accommodated by GRS-IBS technology and illustrates the versatility of the construction method.

4.4.1 Step 1—Establish Project Requirements

GRS-IBS was used for the Bowman Road Bridge project. The project included an abutment and 

a wing wall on each side of the bridge. A top view of the proposed project is shown in figure 26.

Figure 27 is an aerial view of the site with the proposed bridge superimposed. 

Figure 26. Illustration. Top view of Bowman Road Bridge showing the bridge, 

abutments, and wing walls.
55

Figure 27. Illustration. Aerial view of the existing site with the planned 

Bowman Road Bridge superimposed.

Schematics of the proposed abutments are shown in figure 28 and figure 29. The project 

requirements are as follows:

Geometry.

o Wall height (Habut): 15.25 ft.

o Abutment length (Labut): 43.6 ft.

o Bridge width (Bb): 34 ft.

o Batter ( ): 2 degrees.

o Wall placement with respect to ground conditions (back slope, toe slope): None.

o Skew (Sk): 24 degrees.

o Grade (G): 0.006 ft/ft.

o Superelevation (Se): 7.6 degrees.

Loading Conditions.

o Soil surcharge: Road base will be placed behind the bridge beam to create a 

smooth transition.

o DL: DL includes the weight of the bridge beam along with any corresponding 

components.

o LL: LL includes traffic and truck loads which are simulated as a surcharge.

o Seismic load: Seismic effects are negligible in this area.

o Impact loads: No impact loads are considered.

o Loads from adjacent structures: Not applicable.

ω
56

Performance Criteria.

o Design code: ASD.

o Tolerable movements.

§ Vertical settlement: Vertical strain ( V) is limited to 0.5 percent.

§ Lateral displacements: Lateral strain ( L) is limited to 1 percent.

o Design life: 100 years.

o Constraints.

§ Environmental: None.

§ Construction: Sheet piling from the existing bridge remains in place, 

reducing the need for two wing walls in the IBS. Only one wing wall 

is required.

Figure 28. Illustration. Schematic of the west abutment for Bowman Road Bridge.

Figure 29. Illustration. Schematic of the east abutment for Bowman Road Bridge.

4.4.2 Step 2—Site Evaluation

The previous bridge at the site was replaced because it was functionally obsolete and structurally 

deficient. The previous bridge did not experience any problems related to settlement or excessive 

deformations due to the site conditions. However, a sheet pile wall was installed to protect the 

stone wall abutments from erosion. The site evaluation determined that the existing sheet piling 

ε

ε
57

should remain in place to support the stream bank. This eliminated the need for wing walls on 

one side adjacent to the old bridge. 

The replacement structure required realignment to meet current road design standards for roadway

safety because the location had been prone to accidents. The new Bowman Road Bridge crosses 

Powell Creek. The proposed location of the new abutments adjacent to the old bridge was not 

expected to cause any problems with the stream flow.

A hydraulic analysis confirmed that the existing bridge did not have any appreciable potential 

scour. Therefore, an RSF with appropriate scour countermeasures (in this case, riprap) was used.

A subsurface evaluation was conducted by performing standard penetration tests (SPTs) near the 

site. The physical characteristics of the soil were determined through index tests taken on split 

spoon samples. The foundation soil at the site was an overconsolidated clay (with intermediate 

layers of sandy silt and gravels) with N-values greater than 50 blows per ft at the elevation of the 

bottom of the abutment (determined from figure 28 and figure 29). Local experts indicated the 

clay had historically been preloaded with a nearly 1-mi-thick sheet of ice. The clay in this region 

is also known to be fat and sticky when wet. The bearing capacity of the stiff clay had not been a 

problem in past projects in the area. 

The N-value of the foundation soil can be correlated into an undrained shear strength using published

guidance.(8) For blow counts greater than 30 blows per ft, the unconfined compressive strength is

greater than 8,000 lb/ft2. The undrained shear strength is therefore estimated as at least 4,000 lb/ft2. 

The design properties for the foundation soil are shown in table 5.The retained backfill is composed

of the same material as the foundation soil.

Table 5. Foundation and retained backfill soil properties.

Property Notation Measurement

Foundation and backfill soil unit weight f, b 120 lb/ft3

Foundation and backfill soil undrained shear strength cu, cb 4,000 lb/ft2

Foundation and backfill soil effective cohesion c’f, c’b 400 lb/ft2

Foundation and backfill soil effective friction angle ’f, ’b 28 degrees

The road base was a granular fill material that was brought to the site. For the Bowman Road 

Bridge project, the properties of the road base are given in table 6.

Table 6. Road base soil properties.

Property Notation Measurement

Road base unit weight rb 140 lb/ft3

Road base cohesion crb 0 lb/ft2

Road base friction angle rb 40 degrees

γ γ

γ
58

The reinforced fill for the GRS abutment was a select granular fill (AASHTO No. 89 stone). 

Testing was performed on this fill to determine the c and properties. The properties of this fill 

are provided in table 7.

Table 7. Reinforced fill properties.

Property Notation Measurement

Reinforced fill unit weight r 110 lb/ft3

Maximum diameter of reinforced fill dmax 0.5 inches

Reinforced fill cohesion cr 0 lb/ft2

Reinforced fill friction angle r 48 degrees

4.4.3 Step 3—Evaluate Project Feasibility

As mentioned in step 2, scour was not a significant concern for this bridge. The project was therefore

considered feasible for this site. Scour protection was added as a precaution. The riprap was sized

for 8.8–10.2 ft/s to create a scour protection apron adjacent to and in front of the abutment face 

and wing walls. Prior to placement, a 5- to 8-ft-wide strip of geotextile reinforcement between 

the face of the RSF and the riprap was pinned under the first course of facing blocks to secure it 

in place. The purpose of the geotextile reinforcement was to create a barrier to mitigate loss of 

soil beneath the riprap.

4.4.4 Step 4—Determine Layout of GRS-IBS

1. Define the geometry of the abutment face wall and wing walls: See step 1. 

2. Layout the abutment with respect to the superstructure: See figure 29. The distance 

between the abutment faces was 72 ft. Therefore, since the length of the bridge was 

greater than 25 ft, the minimum bearing width (b) for the superstructure was 2.5 ft. A

bearing width of 4 ft, however, had been chosen for this bridge. 

3. Account for setback and clear space: The bridge seat had a setback of 8 inches from 

the edge of the wall. The clear space was 4 inches, which was greater than 2 percent 

of the wall height.

4. Determine the depth and volume of excavation necessary for construction:

o A base width of the wall including the block face of 6 ft was chosen for this 

abutment since the span length was greater than 25 ft and 0.3H was less than 

the 6-ft minimum. Subtracting the wall face width (7.625 inches), the 

reinforcement length at the base of the wall was 5.4 ft. This equates to a B/H 

ratio of 0.35, which is greater than the minimum B/H ratio of 0.3.

o Excavation of 1.5 ft (one-quarter the width, including the block face) was 

made at the base in front of the face of the wall to accommodate for 

construction of the RSF. The total width of the RSF was therefore 7.5 ft.

γ
59

o The depth of the excavation for the RSF was equal to one-quarter the width of 

the base including the block face—1.5 ft (see figure 12). 

5. Select the length of reinforcement for the abutment: The reinforcement length at the 

base of the wall was equal to 6 ft (or 5.4 ft not including the reinforcement necessary 

for the frictional connection). The reinforcement lengths up the wall were chosen 

based on the cut slope angle and an optimization of the width of the reinforcement 

rolls. The reinforcement schedule is shown in figure 30.

Figure 30. Illustration. Reinforcement schedule and RSF dimensions 

for Bowman Road Bridge.

6. Add a bearing reinforcement zone underneath the bridge seat: The primary reinforcement

spacing was 8 inches at the wall face. The spacing of the bearing reinforcement bed 

was 4 inches, half of the primary spacing. The length of the bearing reinforcement

bed was 5 ft. The depth of the bearing reinforcement bed would be determined when 

the internal stability analysis was conducted (step 7). At a minimum, however, there 

would be five intermediate layers between the primary reinforcement layers (at 8-inch 

spacing) in the bearing reinforcement zone (see figure 12).

7. Blend the reinforcement layers in the integration zone to create a smooth transition: 

Additional work was needed to integrate the substructure with the superstructure

within the integration zone at the approach (see figure 12). There were three layers 

of wrapped geotextile reinforcement spaced at 0.9 ft. This is described in chapter 7.

4.4.5 Step 5—Calculate Loads

The applicable surcharges and loads associated with the structure were a combination of vertical and

lateral components. The vertical components include the surcharges due to the DL (superstructure 
60

and road base from the integrated approach) and the LL (superstructure and roadway), along with

the weight of the GRS abutment. The lateral earth pressure due to the retained backfill, shown in 

table 8, were also considered. Lateral loads resulting from the DL and LL were calculated separately

during the external and internal stability calculations performed in step 6 and step 7.

Table 8. Loads and surcharges for Bowman Road Bridge.

Property Notation Measurment Equation

Bridge DL qb 2,600 lb/ft2 Given

Bridge LL qLL 1,400 lb/ft2 Given

Roadway LL qt 298 lb/ft2

Road base DL qrb 385 lb/ft2

Weight of GRS abutment W 9,257 lb/ft

Weight of RSF WRSF 1,575 lb/ft

Weight of facing blocks Wface 768 lb/ft

Lateral load (retained backfill) Fb 5,258 lb/ft

Note that the weight of the GRS abutment was calculated with B equal to the shortest reinforcement

layer not including the width of the wall face. This is a conservative assumption to simplify hand 

calculations. Several software programs are available that can account for the varying shape due 

to different reinforcement lengths along the height of the abutment. The weight of the facing 

blocks (Wface) is the weight of an individual CMU block (42 lb) divided by the length of the

block (15.625 inches), multiplied by the total number of blocks in a single column (24 in this case).

4.4.6 Step 6—Conduct an External Stability Analysis

4.4.6.1 Direct Sliding

The driving forces on the GRS abutment include the lateral forces due to the retained backfill, 

the road base, and the traffic surcharge. 

The force due to the backfill is calculated in equation 39.

(39)

qqtt = h γγ

h = 2.48 ft

qqrr = hrr γγrr

hrr = 2.75 ft

WW = γγrr

WWRR FF = RR FFDDRR FFγγrr

WWffaa = NN oo kk

WW oo kk

LL oo kk

FF = 1

2 γγ HH2KKaaaa

KKaaaa = 1 − ssssssф

1 + ssssssф = 0.361

FF = 1

2 γγ KKaaaa HH2 = 12 (120)(0.361)(15.58)2 = 5258 lb/ft
61

The lateral force due to the road base and traffic surcharges are calculated in equation 40 and 

equation 41, respectively.

(40)

(41)

The total driving force (Fn) is then calculated in equation 42.

(42)

The resisting force (Rn) is calculated according to equation 14. The total resisting weight (Wt) 

includes the weight of GRS plus the weight of the bridge beam plus the weight of the road base 

over the GRS abutment. Since the live loads are not permanent, they cannot be counted as a 

resisting force. Total resisting weight (Wt) is calculated in equation 43.

(43)

The friction force ( ) is equal to tan crit. The interface friction angle between the reinforced fill 

and the geotextile was measured at 39 degrees by conducting an interface direct shear test. The 

resisting force (Rn) calculation is shown in equation 44.

(44)

The factor of safety against direct sliding (FSslide) is calculated in equation 45 to make sure it is 

greater than 1.5.

(45)

4.4.6.2 Bearing Capacity

Before calculating the applied vertical bearing pressure, the eccentricity of the resulting force at 

the base of the wall must first be calculated using equation 20. 

The moments are calculated around the center of the base of the RSF. The driving moments 

(calculated as a counterclockwise moment) include the lateral force due to the retained backfill, 

the road base DL, and the roadway LL. The calculation is shown in equation 46.

(46)

FFrr = qqrr KKaaaa HH = 385(0.361)(15.58) = 2165 lb/ft

FFtt = qqttKKaaaa HH = 298(0.361)(15.58) = 1676 lb/ft

FF = FF + FFrr + FFtt = 5258 + 2165 + 1676 = 9099 lb/ft

WWtt = WW + qq + qqrr rr = 9257 + 2600(4) + 385(0.7) = 19927 lb/ft

μ

RR = WWttμμ = 19927tan(39) = 16137 lb/ft

FFFFssssssssss = RR

FF = 161379099 = 1.8 ≥ 1.5

�MMDD = FF �

HH

3

� + FFrr �

HH

2

� + FFtt �

HH

2

�

= 5258 �

15.58

3 � + 2165 �

15.58

2 � + 1676 �

15.58

2 �

= 57228 ft lb/ft
62

The resisting moments (calculated as a clockwise moment) include the vertical force due to the 

bridge and road base DLs and the bridge and roadway LLs. The weight of the GRS abutment is 

also included as a resisting moment. This calculation is shown in equation 47.

(47)

The total vertical load is equal to the sum of the weight of the GRS abutment, the weight of the 

RSF, and the load due to the DLs (bridge and road base) and LLs (bridge and roadway). This 

calculation is shown in equation 48.

(48)

Thus, the eccentricity of the resulting force at the base of the RSF is calculated in equation 49. 

(49)

The applied vertical pressure is then calculated in equation 50.

(50)

The bearing capacity is calculated in equation 51. The bearing capacity factors Nc, , and Nq

were found using table 4 for the foundation friction angle of 0 degrees.

(51)

The factor of safety against bearing capacity failure is calculated in equation 52 to make sure it is 

greater than 2.5.

(52)

�MMRR = (qq + qqLLLL ) ��

2 + aa� − � RR

2 − xxRR − ��

+ �qqtt rr ,tt + qqrr rr ,tt� � RR

2 − rr

2 � + WW � RR2 − 2

�

= (2600 ∗ 4 + 1400 ∗ 4) ��

4

2 + 0.67� − �

7.5

2 − 1.5 − 0.64��

+ (298 ∗ 0.7 + 385 ∗ 0.7) �

7.5

2 − 0.7

2 � + 9059 �

7.5

2 − 5.42 � = 28098 ff /ff

�VV = WW + WWRR + WWffaa + qqtt tt + qqrr rr + qq + qqLLLL

= 9257 + 1575 + 768 + 298(0.7) + 385(0.7) + 2600(4) + 1400(4)

= 28078 lb/ft

, = ∑ MMDD − ∑ MMRR

∑ VV = 57228 − 28098

28078 = 1.04

σσvv, , = ∑ VV

RR − 2 ,

= 28078

7.5 − 2(1.04) = 5180

lb

ft2

Nγ

qq = ffNN +

1

2

′

γγffNNγγ + γγffDDffNNqq

= 4000(5.14) +

1

2 (7.5 − 2 ∗ 0.94)(120)(0) + 120(1.5)(1.0) = 20740 psf

FFFF bb = qq

σσvv, ,

= 20740

5180 = 4.0 ≥ 2.5
63

4.4.6.3 Global Stability

Global and compound stability was checked using the software program ReSSA. Figure 31 is a 

screenshot of the global stability failure mode. The factor of safety was found to equal 6.6, much 

greater than the minimum requirement of 1.5. Global and compound stability were satisfied.

Figure 31. Screenshot. ReSSA results for global stability for Bowman Road Bridge.

4.4.7 Step 7—Conduct Internal Stability Analysis

4.4.7.1 Ultimate Capacity

The ultimate capacity of a GRS abutment can be determined using two different methods: 

empirical or analytical. 

4.4.6.1.1 Empirical Method: The empirical method uses the load test results of a performance 

test on a GRS composite material identical (or very similar) to that used in the field. The ultimate 

capacity is found empirically as the stress at 5 percent vertical strain from the stress-strain curve 

shown in figure 32. For this curve, the ultimate capacity (qult,emp) is 26 ksf.
64

Figure 32. Graph. Stress-strain curve for Bowman Road Bridge showing ultimate capacity.

The total allowable pressure on the GRS abutment (Vallow,emp) is the ultimate capacity (qult) 

divided by a factor of safety for capacity (FScapacity) of 3.5, as shown in equation 53.

(53)

The applied vertical stress (Vapplied), which is equal to the unfactored sum of the vertical pressures 

on the bridge bearing area, must be less than Vallow,emp. This includes the DL from the bridge (qb) 

and the LL due to the notional HL-93 load model (qLL), as shown in equation 54.

(54)

4.4.7.1.2 Analytical Method: Alternatively, the ultimate capacity can be found analytically for a 

granular backfill. Sv is equal to 8 inches, dmax is equal to 0.5 inches, Tf is equal to 4800 lb/ft, and r

is equal to 48 degrees (see table 7). Although the spacing under the bridge bearing area was

4 inches, 8 inches was chosen in equation 55 to be conservative.

(55)

The passive earth pressure for the reinforced fill was determined with equation 56.

(56)

VVaa aa , eeee = qquu , eeee

FFFF cc

= 26000

3.5 = 7429 ff 2

VVaa = qq + qqLLLL = 2600 + 1400 = 4000 ff 2 ≤ VVaa aa , eeee

qquu ,aa = �0.7

� vv

6 � TTff

VV

�KK = �0.7

� 8

6(0.5)

� 4800

0.67 � 6.786 = 18781 lb/ft2

KK = 1 + ssssssφrr

1 − ssssssφrr

= 1 + ssssss48

1 − ssssss48 = 6.786
65

The total allowable pressure on the GRS abutment (Vallow,an) is the ultimate capacity (qult,an) 

divided by a factor of safety for capacity (FScapacity) of 3.5, as shown in equation 57.

(57)

The applied vertical stress (Vapplied), which is equal to the unfactored sum of the vertical pressures 

on the bridge bearing area, must be less than Vallow. This includes the DL from the bridge (qb) and 

the LL due to trucks (qLL). Applied vertical stress is calculated in equation 58.

(58)

4.4.7.2 Deformations

4.4.7.2.1 Vertical: The vertical strain is estimated by using figure 32, as illustrated in figure 33

for the bridge DL (qb) of 2,600 psf. The vertical strain is therefore about 0.3 percent—under the 

tolerable limit of 0.5 percent. The road base surcharge is not included since it does not act over 

the same location.

Figure 33. Graph. Vertical strain for Bowman Road Bridge.

The vertical deformation is the product of the vertical strain and the height of the GRS abutment 

(including the clear space distance), as shown in equation 59.

(59)

VVaa aa ,aa = qquu

FFFF cc

= 18781

3.5 = 5366 ff 2

VVaa = qq + qqLLLL = 2600 + 1400 = 4000 ff 2 ≤ VVaa aa ,aa

DDvv = εεvvHH = 0.003(15.58) = 0.047 ft
66

4.4.7.2.2 Lateral: The lateral strain and deformation are found in equation 60 and equation 61.

(60)

(61)

4.4.7.3 Required Reinforcement Strength 

The strength of the reinforcement used at Bowman Road Bridge was 4,800 lb/ft. Applying a 

factor of safety of 3.5, the allowable reinforcement strength is 1,371 lb/ft. According to the

manufacturer, is equal to 1,370 lb/ft. 

The maximum required reinforcement strength is found as a function of depth, as shown in 

equation 62.

(62)

The lateral stress ( h) is a combination of the lateral stresses due to the road base DL ( h,rb), the 

roadway LL ( h,t), the GRS reinforced soil ( h,W), and an equivalent bridge load ( h,bridge,eq). To 

simplify calculations, the roadway LL and road base DL can be extended across the abutment.

The vertical components of these loads are then subtracted from the bridge DL and LL, giving an 

equivalent bridge load. The lateral stresses due to the equivalent bridge load are then calculated 

according to Boussinesq theory. The lateral stress is calculated for each depth of interest (each 

layer of reinforcement). All lateral stresses are calculated and shown in table 9.

εεLL = 2εεvv = 2(0.3%) = 0.6%

DDLL = 2DDvv

HH ( + aa ) = 2(0.047)15.85 (4 + 0.67) = 0.028 ft

T@ε=2%

TTrr = � σσh − σσ

0.7

� vv

6 �

� vv

σ σ

σ σ σ
67

Table 9. Depth of bearing bed reinforcement calculations.

Distance 

from top 

of wall

Equivalent 

Bridge Load

Road Base 

DL and 

Roadway LL

GRS 

Fill Total

Required

Strength

Ultimate 

Check

2 Percent

Check

z (ft)

Treq

(lb/ft)

Treq > 

Tallow

Treq >

0.7 2.50 -1.25 482 57 44 11 593 1024 NO NO

1.3 1.97 -0.98 449 57 44 22 572 987 NO NO

2.0 1.57 -0.79 400 57 44 32 533 920 NO NO

2.7 1.29 -0.64 350 57 44 43 493 852 NO NO

3.3 1.08 -0.54 305 57 44 54 460 794 NO NO

4.0 0.93 -0.46 269 57 44 65 434 749 NO NO

4.7 0.81 -0.40 239 57 44 76 415 716 NO NO

5.3 0.72 -0.36 214 57 44 86 401 692 NO NO

6.0 0.64 -0.32 193 57 44 97 391 675 NO NO

6.7 0.58 -0.29 176 57 44 108 385 664 NO NO

7.3 0.53 -0.27 162 57 44 119 381 658 NO NO

8.0 0.49 -0.24 149 57 44 130 380 655 NO NO

8.7 0.45 -0.23 139 57 44 140 380 655 NO NO

9.3 0.42 -0.21 129 57 44 151 381 658 NO NO

10.0 0.39 -0.20 121 57 44 162 384 663 NO NO

10.7 0.37 -0.19 114 57 44 173 388 669 NO NO

11.3 0.35 -0.17 108 57 44 184 392 676 NO NO

12.0 0.33 -0.17 102 57 44 195 397 685 NO NO

12.7 0.31 -0.16 97 57 44 205 403 695 NO NO

13.3 0.30 -0.15 92 57 44 216 409 705 NO NO

14.0 0.28 -0.14 88 57 44 227 415 717 NO NO

14.7 0.27 -0.14 84 57 44 238 422 729 NO NO

An example calculation for the required reinforcement strength at a depth (z) of 5.3 ft, or the 

eighth reinforcement layer from the top (see figure 34), is presented here. First, the lateral 

pressure is found in equation 63. Remember, the location of interest is directly under the 

centerline of the bridge load (where x = 0.5b = 0.5(4ft) = 2 ft).

(63)

The calculation of each aspect of the lateral pressure is shown in equation 64 through equation 67.

(64)

(65)

α β

σh,bridge,eq

(psf)

σh,rb

(psf)

σh,t

(psf)

σh,W

(psf)

σh,total

(psf) T@ε=2%

σσh = σσh,WW + σσh, rr , qq + σσh,rr + σσh,tt = 86 + 214 + 57 + 44 = 401

lb

ft2

σσh,WW = γγrrzzKKaa = 110(5.3) �

1 − sin⁡(48deg)

1 + sin⁡(48deg)

� = 110(5.3)(0.147) = 86

lb

ft2

σσh, bb , = (qq + qqLLLL) − (qqrr + qqtt)

ππ

[αα + sin⁡(αα )cos⁡(αα + 2ββ )]KKaa

= (2600 + 1400) − (385 + 298)

ππ

[0.72rr

+ sin(0.72rr ) cos(0.72rad + 2 ∗ −0.36rr )]0.147 = 214

lb

ft2
68

(66)

(67)

The values for and are found in equation 68 and equation 69.

(68)

(69)

Figure 34. Illustration. Lateral pressure due to the bridge load.

σσh,rr = qqrr KKaa = 385(0.147) = 57

lb

ft2

σσh,tt = qqttKKaa = 298(0.147) = 44

lb

ft2

α β

αα = tt −1 �

2zz

� − ββ = tt −1 � 4

2(5.3)

�— (−20.7 ) = 41.3 = 0.72rr

ββ = tt −1 �

−

2zz

� = tt −1 � −4

2(5.3)

� = −20.7 = −0.36 rr
69

Based on table 9, the required reinforcement strength does not exceed the allowable strength or 

the strength at 2 percent at any reinforcement layer. Therefore, no bearing bed reinforcement is 

needed; however, the minimum requirement is that the bearing bed reinforcement should extend 

through five courses of blocks. In actuality, six courses of block were chosen to extend the bearing

reinforcement bed in this case (to a depth of 4 ft below the top of the wall). This was chosen to 

be conservative since this was the first bridge built with GRS technology.

Applying 4-inch spacing to the top six courses of blocks and 8-inch spacing for the remaining 

height of the wall, the required reinforcement strength was found (see table 10). The maximum 

required reinforcement is 716 lb/ft, which is less than the factored reinforcement strength of 

1,371 lb/ft and the reinforcement strength at 2 percent. There should, therefore, be no issues with 

reinforcement strength in the abutment.

Table 10. Required reinforcement along height of wall.

z (ft) Treq (lb/ft)

0.3 594 319

0.7 593 318

1.0 586 314

1.3 572 307

1.7 553 297

2.0 533 286

2.3 513 275

2.7 493 265

3.0 476 255

3.3 460 247

3.7 446 239

4.0 434 233

4.7 415 716

5.3 401 692

6.0 391 675

6.7 385 664

7.3 381 658

8.0 380 655

8.7 380 655

9.3 381 658

10.0 384 663

10.7 388 669

11.3 392 676

12.0 397 685

12.7 403 695

13.3 409 705

14.0 415 717

14.7 422 729

σh,total (psf)
70

4.4.8 Step 8—Implement Design Details

All design details were considered. Since it was a skewed bridge, the bearing area of 3 ft was 

maintained along the length of the face wall. The bearing bed reinforcement schedule was also 

maintained across the abutment face due to the superelevation, as shown in figure 35.

Figure 35. Illustration. Secondary reinforcement for superelevation at Bowman Road Bridge.

4.4.9 Step 9—Finalize Material Quantities and Layout

The amount of reinforcement necessary is based on the reinforcement schedule. Reinforcement 

material came in 12- to 18-ft-wide rolls. The number of facing blocks was determined from the 

height and length of the abutment and wing walls. The amount of backfill required was determined

in a similar fashion. Once final quantities are established, it is a good rule of thumb to order at 

least 10 percent more to account for unforeseen conditions.
71

CHAPTER 5. SPECIAL REQUIREMENTS FOR HYDRAULIC AND 

SEISMIC CONDITIONS

5.1 INTRODUCTION 

This chapter describes how extreme events such as scour, seismicity, or impact may alter the 

design of GRS-IBS. 

5.2 HYDRAULIC DESIGN

When bridges are constructed to span a waterway, their foundations must be designed, detailed,

and constructed in compliance with section 2.6 (Hydrology and Hydraulics) of the AASHTO 

LRFD Bridge Design Specifications or an FHWA Division Office-approved drainage or bridge 

manual.

(9) These provisions apply equally to both shallow and deep foundations.

GRS-IBS has been successfully used to build abutments near rivers and streams. However, assessing

the potential impact of stream instability, scour, and adverse flow conditions is a vital consideration

in the decision to use this technology. The potential for issues with stream instability, scour, and

adverse flow conditions can lead to deep foundation bottom elevations or expensive countermeasures

that could reduce the cost-effectiveness of GRS-IBS abutments. If the potential for abutment 

scour, contraction scour, long-term degradation, or channel migration is high, costly design

considerations or countermeasures could be required. Other factors, such as channel instability 

and adverse flow conditions (skewed approach flow, highly contracted flow, high velocity flow 

through the bridge opening, etc.) at the bridge, could also result in costly design considerations 

or countermeasures to stabilize the channel against further instability. Any of these conditions 

might make it advisable to select an alternative bridge abutment technology.

A thorough hydraulic analysis, scour evaluation, and assessment of channel stability of a bridge 

design will include an appropriate estimate of the design flow, development of water surface 

profiles through the proposed opening, assessment of scour (abutment, contraction, and longterm degradation), and if necessary, the design of countermeasures to protect the bridge or 

stabilize the channel. FHWA and others have developed procedures to assist the engineer in 

performing these analyses, and these procedures should be followed for GRS-IBS design.(5,14,15)

5.3 HYDRAULIC DESIGN CONSIDERATIONS

There are a number of important factors to consider when completing a thorough hydraulic design

and scour evaluation of a bridge. The determination of a scour elevation based on the computed 

scour depth; the selection, design and installation of a scour countermeasure; and postconstruction

inspection are important factors that must be adequately addressed. The following factors should 

be considered:

Scour depth: The scour depth at an abutment is to be calculated as the sum of the 

depth of contraction scour and long-term degradation. The elevation of the design 

scour depth is to be calculated by projecting the elevation of the depth of scour from 

the lowest point in the channel to each of the abutments. 
72

Scour countermeasures: When scour depth is calculated as described in this section, 

a designed scour countermeasure is included. Design scour countermeasures include 

riprap aprons, gabion mattresses, and articulating concrete blocks (see section 4.5.3).

The purpose of installing a designed scour countermeasure is to prevent loss of soil 

from underneath a GRS abutment from scour that occurs at or near the abutment. Soil 

loss can reduce bearing capacity or lead to settlement, which can cause structural 

failure (see section 4.5.3). Figure 36 shows a cross section of a typical abutment 

riprap countermeasure recommended for smaller, more culvertlike structures (flow 

length through structure is longer than structure width). See HEC-23 for additional 

details regarding the specific requirements for the design and configuration of this 

countermeasure.

(5) Larger, more bridgelike structures (opening length is greater 

than the flow distance through the structure) must be evaluated for scour using the 

procedures outlined in HEC-18 and HEC-20 and use a designed countermeasure as 

outlined in HEC-23.

(14,15,5)

Source: HEC-23 Figure 18-10

Figure 36. Illustration. Typical cross section for sloping rock (adapted).(5)

Inspection: After construction, scour countermeasure condition and channel instability

should be assessed during each regular bridge inspection and after extreme flood events.

Any countermeasure failure or significant change in channel condition should be noted

and scheduled for repair or stabilization. Without proper inspection and maintenance, 

a scour countermeasure may fail or a channel may become unstable, which can lead to

undermining of an abutment. The FHWA’s HEC-20 discusses approaches for evaluating 

channel instability, and HEC-23 discusses approaches for inspection and monitoring 

the effects of scour.(15,5)
73

Another hydraulic consideration is drainage. The potential for unbalanced water pressure exists 

when a wall can become partially submerged by a flood or when surface drainage is not controlled.

All GRS structures should include consideration for surface and subsurface drainage. Critical areas

are behind the wall at the interface between the GRS mass and the retained fill, at the base of the 

wall, and any location where a fill slope meets the wall face. For example, the design needs to

include provisions for surface drainage along the fill slope adjacent to the wing walls. Section 6.5

discusses drainage details.

5.4 SEISMIC DESIGN

External stability for seismic design will need to be checked for GRS-IBS just like with any other 

gravity structure. Design considerations for external stability and seismicity include increasing 

the base width of the wall and increasing the length of the reinforcement at the top of the wall. 

Additional bearing capacity and overall external stability is generally improved by increasing the 

base width of the wall. Additional stability is created by increasing the length of the reinforcement

at the top of the wall or abutment. This integrated approach has also been shown to be beneficial 

because it keys the structure into the existing terrain, preventing the development of a failure plane

along the cut slope, which can lead to progressive failure. 

No seismic design requirements are necessary for the internal stability of GRS-IBS. Reinforced 

soil walls have been known to perform better than conventional retaining walls under seismic 

loading, as evidenced by observations of actual performance in strong earthquake events. (See 

references 16–19.) A National Cooperative Highway Research Program (NCHRP) study is being 

conducted to establish guidelines for the design and construction of GRS abutments under seismic 

loading.(20) As part of the NCHRP study, a 12-ft-high GRS abutment supporting a bridge load of 

about 1,000 kips was subject to sinusoidal motions on a shake table. No significant damage or 

movement was recorded until the acceleration was increased to 1.0 g, at which time base sliding 

between the GRS abutment and foundation soil became apparent. The superstructure would not 

have failed due to the deformation of the GRS mass at the 1.0-g acceleration. This experiment 

suggests that a GRS abutment is capable of withstanding at least low to medium earthquakes

without any special provisions. 

5.5 IMPACT EVENTS 

There is limited information on vehicle impact against GRS-IBS. Typically, GRS walls along 

roadways are built behind a crash barrier. A niche function of GRS technology, however, is 

rock-fall protection. That application is not covered in this manual, but it serves to show that 

GRS is capable of withstanding considerable lateral and vertical impacts without failure or 

loss of serviceability. 

75

CHAPTER 6. IN-SERVICE PERFORMANCE

6.1 INTRODUCTION

This chapter presents methods of evaluating the performance of GRS-IBS, including deformations, 

thermal movements, and scour monitoring. The performance of in-service GRS-IBS structures can

be found in the synthesis report.

(1) A distinctive feature in the design of GRS-IBS is that it works 

with settlement instead of resisting it to create a compatible connection between the approach and

the road, providing a long-term solution to the bump at the end of the bridge. Reducing the bump 

at the end of bridge will improve the overall performance and serviceability of the bridge. This 

bump not only creates a chronic maintenance issue but also induces an amplification of LL on 

the superstructure, creating fatigue on bridge elements. 

Recently, some owners have shifted their focus on the combined effects of corrosion and fatigue in

the evaluation of a bridge’s health. The design objective of IBS addresses these two critical durability

issues by attempting to create a smooth, jointless, affordable bridge system. GRS-IBS was developed

to meet the demand for the next generation of small, single span bridges in the United States as part

of the FHWA’s Bridge of the Future initiative and is built without many of the standard abutment 

components associated with a traditional bridge (e.g., approach slab, sleeper slab, traditional bridge

bearings, joint details). The performance of the first series of GRS-IBS indicates that this method 

has considerable potential to advance the state of the practice.

6.2 DEFORMATIONS

The performance of nearly 20 bridges built with GRS-IBS has been an improvement on similar 

bridges built with conventional construction techniques. The GRS-IBS bridges have performed 

as well as the conventional bridges structurally and functionally in addition to eliminating the bump

at the end of the bridge that often results from conventional construction. The suppression of the 

bump at the end of the bridge has been maintained for all GRS-IBS bridges that are in service. 

The first bridge constructed with the IBS method, the Bowman Road Bridge, has been in service 

since 2005. As of 2010, there has been no development of a crack in the asphalt layer from the 

road to the bridge.

The total settlement and deformation (and thus vertical strain) of the GRS abutment due to bridge

load is recorded using either a standard survey level and rod system, as shown in figure 37, or an 

electronic distance measurement (EDM) survey referenced off a permanent survey pole and

benchmarks. The precision of all survey measurements (both the survey level method and the 

EDM system) is ±0.005 ft.
76

Figure 37. Illustration. Survey level method for superstructure and wall settlement.

Using either settlement measurement technique, the settlement is recorded for both the abutment 

face wall and the superstructure. The difference between the settlement measured on the abutment

face wall and the superstructure is the vertical deformation within the GRS mass alone due to the 

bridge load. To determine secondary settlement (i.e., creep), plot settlement versus log-time.

For surveys where a survey level is used, bridge settlement should be measured at four locations 

(each corner of the bridge) to check for angular distortion and differential settlement. Wall settlement

is recorded by the rod off the top of the CMU facing block adjacent to the superstructure, and 

superstructure settlement is measured with the rod off a guardrail hanger bolt (see figure 37).

For surveys with an EDM system, the total station is referenced off of a permanent pole embedded

beneath the frost line and within accessible sight to both abutment face walls. Targets placed on 

the abutment wall face and the bridge beam or footing are then used to measure movement 

relative to the permanent pole. Figure 38 shows an example of this for the Tiffin River Bridge in 

Defiance County, OH. Lateral and vertical movement of the abutment wall face was measured 

using custom reflective survey targets, which are shown in the black circles in figure 38. Movement

of the GRS abutment was measured using targets placed on the concrete footing itself, which are 

shown in the red circles in figure 38. The difference between the two readings (movement of the 

abutment and movement of the wall face) provides the compression of the GRS abutment alone, 

not including foundation settlement. Note that the permanent pole should be installed prior to 

placement of the steel girders on the CIP footings (see figure 39). 
77

Figure 38. Photo. Location of survey targets on Tiffin River Bridge, Defiance County, OH.

Figure 39. Photo. Location of total station reference pole.
78

6.3 THERMAL CYCLES

Thermal cycles occur on every bridge structure due to sustained temperature variations. The severity 

of the expansion and contraction depends on the coefficient of thermal expansion of the bridge. 

However, observations of bridges built in moderate climates indicate considerable compatibility

between the superstructure and the integrated approach, resulting in a smooth transition.

GRS-IBS accommodates movement through the integrated transition zone behind the beam ends. 

The road base is wrapped with geotextile and then compacted directly against the beam end. This 

process is described in detail in chapter 7. The wrapped face confines the soil and allows the beam

to contract without the fill behind the beam ends sloughing off to fill the void. Because of this, 

excess pressures behind the beam during expansion are also avoided. The road base is not only 

wrapped vertically but also laterally to prevent lateral spread. 

6.4 MONITORING FOR SCOUR

The riprap protection should be monitored during each bridge inspection or after an extreme flood.

Any movement of rock should be noted and replaced to prevent scour from progressing and

undermining the RSF or the abutment. In all current installations of GRS abutments, no problems

have been reported. It should be noted that these installations have been built in non-scour-critical

environments and with appropriate countermeasures, as recommend in current practice.(5)

An indicator of scour on an abutment face or wing wall can be achieved by using colored blocks

on the bottom five to eight rows of the abutment. Solid blocks are recommended in the bottom 

rows as they are more likely to resist any impact of moving riprap, ice, or other abrasion associated

with the normal water elevation. The solid colored blocks are also covered from view by the riprap.

Any exposure of colored blocks could indicate movement or undermining of the riprap, requiring 

inspection and possibly remediation or repair to protect the RSF and abutment from scour.
79

CHAPTER 7. CONSTRUCTION

7.1 INTRODUCTION

GRS construction uses basic earthwork methods, primarily for excavation and compaction, along 

with sound general construction practices. The materials are readily available, which is a benefit 

of the generic nature of the system. This chapter provides guidance on most field-related scenarios.

All methods that are presented have been field-tested and applied during the construction of

GRS-IBS. The techniques outlined can be applied to efficiently construct the layered system and 

have been proven to quickly construct the GRS-IBS. The contractor will ultimately choose the 

methods most efficient for the site, the crew, and the equipment on hand.

The guidance outlined here applies to GRS structures, specifically abutments built with CMU blocks.

This guidance can also be adapted to other GRS structures built with different facing systems. 

GRS construction has two principal components: (1) logistics and (2) aspects associated with 

actual construction. Logistics occur after the final design and before construction, outlining a 

plan for implementation and control of the construction process. Even though building a GRS

abutment is as simple as a row of facing block, a layer of well-compacted granular fill, and a 

sheet of reinforcement, the process will be hampered without adequate planning to ensure 

optimum flow and placement of material during the course of the project.

As a result, the single-sheet plan was devised to provide information on the reinforcement 

schedule and the facing block schedule. The single-sheet plan also contains information on the 

limits of excavation and details about assembly of the GRS structure. A second sheet may be 

necessary to detail quantities and construction notes.

This chapter conveys the importance of the following details as a means to rapid GRS construction:

Careful attention to the first row of blocks. Since all other courses of block are built off 

the first row, it is essential to ensure that the bottom row is level and even for fast 

construction.

Optimization of crew size and equipment for enhanced productivity. Too many laborers

or excess onsite equipment can cause confusion and slow down the construction process.

Allowing time for a labor crew to adjust to the construction of the GRS-IBS. Having 

each crew member do their part in one of three basic steps of GRS construction (laying

a course of facing block, compacting a layer of granular backfill, and placing a layer 

of reinforcement) dramatically improves productivity.

Establishing the central position of the excavator. Typically, it is best to limit 

movement of the excavator by locating it toward the back of the abutment, where it 

can both reach and place material without moving.
80

7.2 LABOR AND EQUIPMENT 

7.2.1 Labor Requirements 

A typical labor crew on GRS-IBS projects has consisted of about five workers: four laborers and

an equipment operator (see figure 40). The equipment operator is central to the project and provides

support to the labor crew. The equipment operator is responsible for shaping the excavation to 

facilitate construction of the RSF and the GRS abutment in addition to placing fill material and 

moving facing units into the work area. Typically, one member of the labor crew has the role of 

foreman and is responsible for layout of excavation limits, grades, alignment of wall face, placement

of facing blocks, compaction of fill, placement of geosynthetic reinforcement, and other activities

to streamline production and the flow of material to the job site. 

Figure 40. Photo. Typical labor crew with centrally located track hoe.

7.2.2 Tool and Equipment Requirements 

Specialized equipment is not required to construct GRS-IBS. Simple tools that are readily available

and relatively inexpensive can be used. These include hand tools, measuring devices, and heavy 

equipment. The contractor may modify the included lists depending on the site, the crew, and the 

size of the IBS.

Typical hand tools include the following:

Gravel rake (concrete spreader).

Shovels (flat blade and spade).

Heavy rakes.
81

Broom to sweep top of blocks.

Wisk broom.

2- to 3-lb sledgehammer and wood two-by-fours to align blocks.

Heavy rubber mallet.

Spade trowel.

Razor knives or utility knives to cut reinforcement.

Hand tamper with metal base plate.

Chainsaw to cut reinforcement roll.

Concrete saw.

5-gallon bucket.

Block lifter.

Standard concrete mixing and finishing tools.

Typical measuring devices include the following:

Survey equipment.

Laser level.

String line to align blocks.

4-ft carpenter’s level.

Plum bob to check wall batter.

Measuring tapes.

Chalk line.

Typical heavy equipment includes the following:

Walk-behind vibratory plate tampers (200 lb and 18 inches wide or larger).

Track hoe excavator.

Riding smooth drum vibratory roller (compacting 3.28 ft from wall face).
82

Pallet forks for excavator (for moving CMU block in and out of work area).

Trash pump and hose for dewatering foundation excavation.

Backhoe (as needed for material staging).

7.3 SITE PREPARATION 

GRS is built from the bottom up and generally from within the footprint of the structure. Staging 

and delivery of materials to the site should allow for continuous GRS construction and effective 

use of the space. Delivered material should be easily accessible to the excavator, which is the 

central piece of equipment. As shown in figure 41, the excavator is positioned inside the wall

area for easy placement of fill, block, and other materials. Labor should be organized to assemble

construction materials as needed on the work platform.

Figure 41. Photo. Cut slope of retained soil.

7.3.1 Site Layout

Site preparation begins with a survey of the bridge site to stake limits for the excavation. 

Reference stakes should be located in an area where they will remain undisturbed during 

construction of the base of the wall, usually about 5 ft from the excavation. 

The base of the GRS abutment and wing walls should be constructed to within 1 inch of the 

staked elevations. The external GRS abutment and wing walls should be constructed to within 

±0.5 inches of the surveyed staked dimensions. 

7.3.2 Excavation

All excavations should comply with Occupational Safety and Health Administration requirements.(21)

Excavation of the site involves shaping the slope for temporary slope stability, safety, and

constructability. The temporary cut in the retained soil should be designed to accommodate 

movement of labor. The design of a temporary excavation needs to consider the loading imposed 
83

by heavy equipment and the reach limits of the excavator. Figure 41 shows a typical cut slope in 

stiff clay. The excavation should include provisions for drainage with a sloped cut to facilitate 

the movement of water. Any open excavations that form a pit should be backfilled with crushed 

aggregate and compacted. Excavation also includes the clearing and grubbing of vegetation. In 

situations where the retained fill is stable, the volume of excavation can be limited to reduce the 

size of the GRS mass. In the case of an abutment application, this would form a horseshoe 

shaped excavation, as shown in figure 40 and figure 42.

Figure 42. Photo. Horseshoe-shaped excavation with native soil still intact in middle.

7.3.3 Placement of Abutment Behind Existing Substructure

In some situations, it may be beneficial to build GRS-IBS behind an existing substructure. Project

feasibility, environmental considerations, and other factors need to be assessed before selecting 

this type of project layout. Building the bridge behind an existing substructure often requires the 

removal of the top part of the abutment walls to provide additional space for the width of the new 

GRS-IBS. Figure 43 through figure 45 illustrate this technique. Note that the design of the 

GRS-IBS will be the same whether it is built behind an existing abutment or not.
84

Courtesy of St. Lawrence County, NY

Figure 43. Photo. GRS-IBS built behind an existing concrete abutment.

Courtesy of St. Lawrence County, NY

Figure 44. Illustration. Cross section of GRS-IBS built behind an existing concrete abutment.
85

Figure 45. Photo. Building the RSF behind an existing abutment.

7.4 RSF

The depth and footprint of the excavation for the RSF should be based on external stability, as 

described in chapter 4. The base of the RSF should be cut smooth. It should be excavated to 

uniform depth, and all loose, unstable material should be removed from the site (see figure 46). 

If the base of the excavation is left open, it should be graded to one end to facilitate the removal 

of any intrusion of water with a pump. If flooded, all water should be removed along with soft, 

saturated soils. The excavation should be backfilled as soon as possible to provide a suitable 

foundation and avoid adverse weather delays. The construction of the RSF can typically be 

completed in less than one day but is dependent on the size and depth of excavation, type of 

materials, equipment, and experience.

Figure 46. Photo. RSF excavation below stream level.
86

The base of the excavation should be compacted before construction of the RSF. This may 

require proof rolling, and any soft spots or voids should be backfilled with compacted fill 

material. Figure 47 shows the preparation of the RSF cut.

Figure 47. Photo. RSF cut preparation.

The RSF should be encapsulated in geotextile reinforcement placed perpendicular to the abutment

face to protect it from possible erosion (see figure 48). The reinforcement sheets should be measured

and sized to fully enclose the RSF on three sides: the face and the two wing wall sides. If the GRS

abutment is adjacent to water, the reinforcement sheets should overlap, starting with the first layer

on the upstream side of the RSF. All overlapped sections of reinforcement in the area of the RSF 

should be oriented to prevent running water from penetrating the layers of reinforcement. The first

layer of reinforcement should be placed on the upstream side of the abutment with subsequent

layers, if needed, overlapped a minimum of 3 ft on the downstream side. This prevents water from

infiltrating the RSF. The wrapped corners of the RSF need to be tight and without exposed soil 

within the RSF to complete the encapsulation.

Figure 48. Photo. Encapsulation of fill in RSF.
87

Typical reinforcement spacing in the RSF is 12 inches. The reinforcement should be pulled taught

to remove all wrinkles prior to placing and compacting the structural backfill. Fill should be placed

from the face to the back to roll folds or wrinkles to the free end of the reinforcement layer.

The RSF should be constructed with structural fill, as specified in chapter 3. The structural fill is 

to be compacted in accordance with section 7.5 in compacted lifts not to exceed 6 inches (with 

two compacted lifts per each 12-inch layer). The first course of wall block sits directly on the 

RSF, as shown in figure 49, so it is important that the fill material is graded and level before 

encapsulating the RSF.

Figure 49. Photo. Placement of wall block on wrapped RSF.

While the base of a typical GRS abutment is built with solid CMU, damage can occur during the

placement of channel rock protection or from other large pieces of concrete rubble that extend 

above the solid block zone. Riprap protection should be placed in a manner to prevent damage to 

the CMU wall face. Impact of large rock or concrete fragments during placement can crack the 

CMU block. Larger rocks should be uniformly distributed and placed firmly in contact with each 

other, with smaller rocks and fragments filling the voids between the larger rocks. This procedure

often requires hand placement of smaller rocks to fill the voids. If any CMU block is damaged, 

refer to chapter 8 for repair procedures.

7.5 COMPACTION

Compaction of the backfill should be to at least 95 percent of maximum dry density according to 

AASHTO T-99. Backfill material containing fines should be compacted at a moisture content close

to optimum (±2 percent). Lifts of 8 inches should be compacted using vibratory roller compaction

equipment. The facing blocks provide a form for each lift of fill. Other stiffness-based compaction

control methods can be used. For open-graded fills, compact to non-movement or no appreciable 

displacement and assess with visual inspection.
88

Since the facing elements are not rigidly connected to the reinforcement, hand-operated compaction

equipment (e.g., a lightweight mechanical tamper, plate, or roller) is required within 1.5 ft of the 

front of the wall face. It is very important for adequate GRS performance that the backfill is 

properly compacted. The top 5 ft of the abutment should be compacted to 100 percent of the

maximum density according to AASHTO T-99.

Onsite compaction equipment should be selected to achieve the required density of the fill materials.

Considering that compaction is critical to the success of the project, compaction equipment should

be in good operating order for efficient use. In addition, backup equipment should be available to 

provide quality construction throughout the project and to avoid construction delays. 

7.5.1 Compaction Procedure 

Once fill is placed at the required thickness and graded, all areas behind the CMU block should 

be compacted to the required density. Any depression behind the facing block should be filled 

level to the top of the CMU block prior to compaction.

Compaction directly behind the CMU block should be performed in a manner that maintains 

wall alignment while improving the density of fill behind the block. This can be achieved in the

following ways:

Placing a fill lift directly behind the CMU block face and rodding or foot tamping along

the row of CMU block while exerting downward pressure on the block to prevent lateral

movement. For multiple lifts, the top lift height is slightly higher than the block to 

compensate for compression of the fill during compaction.

Using a lightweight vibratory plate compactor directly behind the CMU block while 

exerting downward pressure on the block to prevent lateral movement.

Using larger vibratory compactors for the remainder of the fill area 3 ft from the face 

of the GRS wall. Check for outward block movement and adjust accordingly. 

The most common compaction QC tool is the nuclear density gauge. Other instruments are also

available for compaction control such as the Clegg hammer, the soil stiffness gauge, or the falling

weight deflectometer. These devices are typically used by correlating their measurements to soil

density and moisture content. Method-based compaction specifications can also be used. For 

open-graded fills, compact to non-movement or no appreciable displacement and assess with 

visual inspection.

7.6 REINFORCEMENT

Generally, the length of the reinforcement layers will follow the cut slope, as shown in figure 50. 

While the reinforcement layers in the GRS abutment can be any geosynthetic, the RSF and 

integrated approach should be constructed and encapsulated with a geotextile to confine the 

compacted granular fill. The geosynthetic should be placed so that the strongest direction is

perpendicular to the abutment face, as shown in figure 51. Where the roll ends, the next roll should

begin. Overlapping between sheets is required. The geosynthetic reinforcement should extend 

between layers of CMU block to provide a frictional connection. The geosynthetic reinforcement 
89

should cover a minimum of 85 percent of the top surface of the CMU block; any excess can be 

removed by either burning with a propane torch or cutting with a razor knife.

Figure 50. Illustration. Typical reinforcement zones.

Figure 51. Reinforcement rolled out parallel to wall face.
90

After the geosynthetic is rolled out, it should be laid so that it is taut, free of wrinkles, and flat. 

The geosynthetic can be held in place with the fill. Placement of fill should be from the wall face 

backward to remove and prevent the formation of wrinkles in the geosynthetic. A conscious effort

should be taken during placement of fill to prevent the development of wrinkles.

Splices of reinforcement can occur without overlap. Splice seams should be staggered to avoid a 

continuous break in the reinforcement throughout the GRS structure. All splice seams should run 

perpendicular to the wall face.

Overlaps of adjacent geosynthetic should be trimmed where they are in contact with the surface 

of the CMU block to avoid varying geosynthetic thicknesses between the CMU block. Any seams

in the geosynthetic should be staggered with each successive layer of the GRS abutment. All seams

between adjacent sheets of geosynthetic located in the area beneath the footprint of the bridge seat

should be perpendicular to the abutment wall face.

7.6.1 Operating Equipment on Geosynthetic Reinforcement

Driving should not be allowed directly on the geosynthetic reinforcement. Place a minimum 

6-inch layer of granular fill prior to operating any vehicles or equipment over the geosynthetic 

reinforcement. Rubber-tired equipment may pass over the geosynthetic reinforcement at speeds 

less than 5 mi/h. Skid steers and tracked vehicles can cause considerable damage to the geosynthetic.

On one occasion, a track hoe operating on a GRS structure turned and pulled the fabric causing 

deformation to the wall face. For this reason, it is recommended to restrict the use of these vehicles 

on GRS structures. If absolutely necessary, use may be permitted provided no sudden braking or 

sharp turning occur and a minimum 6-inch cover is placed. 

7.6.2 Bearing Reinforcement Bed 

The bearing reinforcement bed provides additional strength in the upper GRS wall layers directly 

beneath the bearing area of the superstructure. These reinforcement layers are not sandwiched 

between two consecutive rows of block but are placed behind the CMU block at 4-inch spacing. 

This 4-inch reinforcement spacing is generally placed in the top five layers of the GRS abutment 

or as determined by design (see chapter 4). 

Bearing bed reinforcement spacing in superelevated abutment walls requires additional planning.

The 4-inch reinforcement spacing needs to be in place for the top five courses of block at the lowest

elevation across the abutment wall (see figure 50 and figure 52). The reinforcement schedule will 

guide field personnel in the proper placement of the geosynthetic along a wall block course.
91

Figure 52. Illustration. Superelevation reinforcement schedule.

7.6.3 Superelevation

The reinforcement layers become stair-stepped in the upper wall layers as the superelevation of 

the abutment is constructed (see figure 53). The reinforcement terminates along the angle surface 

of the superelevation. The GRS wall reinforcement schedule should show the termination of each 

layer of reinforcement across the abutment wall from low to high elevation (see figure 52).

Figure 53. Photo. Superelevation reinforcement layers.

7.7 WALL FACE

This manual focuses on the use of CMU for the wall facing; however since GRS is internally stable,

any facing elements can be used in construction. For flexible facings other than the recommended 

CMU block (including wrapped, timber, natural rock, or welded wire basket formed facing),

other construction guidelines may need to be followed. These are outlined by Wu et al.(22) The 

design guidelines for GRS-IBS, however, remain the same as those in this manual.
92

7.7.1 Leveling Course

Setting the first course of facing block level and to grade is critical in maintaining wall alignment 

for the entire height of the abutment. Typically, the first course is placed on top of the RSF,

directly on the geotextile. However, due to the large aggregate size of the RSF fill material, a 

thin leveling layer of fine aggregate can help set the CMU blocks to grade and prevent them from

rocking. The leveling layer should be kept to a minimum thickness, no more than 0.5 inches. If 

the leveling layer exceeds this thickness and there is the potential for water to erode and undermine

the aggregate, mortar or grout should be placed in the gap between the RSF and the first course. 

7.7.2 Setting the CMU Block

CMU block wall construction should begin at the lowest portion of the excavation with each layer

placed horizontally, as shown on the plans. Each layer should be constructed entirely before 

beginning the next layer. A stretcher or running bond should be maintained between courses of 

block so that the joints between the blocks are offset with each row.

If a designed scour countermeasure such as a riprap apron is used, a geotextile filter fabric should

be placed under the apron and anchored between the first and second courses of CMU block.

Since the CMU blocks are dry stacked without mortar, it is important to avoid cracking the block 

and to maintain a horizontal uniform elevation by sweeping the top surface of the block clean of 

debris and fill material prior to the placement of the next layer of geosynthetic and CMU block. 

Gravel material between layers of block creates point loads that can cause cracks. Also, gravel 

material between the blocks causes them to rock, making it difficult to secure a good fit. 

When setting a course of block, each block should be placed tightly against the adjoining block, 

preventing gaps from which fill material can escape. Before proceeding to the next layer, it is 

often useful to walk along the top of the blocks to easily identify a poorly seated block. 

In order to avoid cutting a block when the CMU block schedule shows the wall terminating with 

half a block, a full CMU block can be turned 90 degrees, placing the 8-inch width toward the 

face. This typically occurs at the termination of a wing wall. The end block that forms the 

termination does not have to be a corner CMU (with two finished sides) because the ends of 

most wing walls are embedded into the fill slope.

7.7.3 Wall Face Alignment

When placing and compacting fill behind the CMU block, it is sometimes necessary to set the block

back about 0.5 inches to allow for lateral outward movement of the CMU block during compaction.

It should be noted that each combination of wall facing and backfill reacts differently during the 

compaction process, and adjustment of the setback distance between block courses should be 

performed as needed to maintain the necessary batter. 

The vertical GRS wall should be checked for plumbness at least every other layer, and any deviations

greater than 0.25 inches should be corrected. Before placement of the backfill, every other row of 

block alignment should be checked with a string line referenced off the back of the facing block 

from wall corner to corner (see figure 54). 
93

Figure 54. Photo. Checking block alignment with string line reference from back of block.

If CMU blocks become displaced during construction, they can often be hammered back into 

position using a 3-lb sledgehammer and a block of wood as protection. If the CMU block is

excessively out of alignment, the fill material needs to be excavated, the CMU block repositioned,

and the fill material replaced and recompacted. 

7.7.4 Block Alignment for Battered Walls 

Block alignment for battered walls is similar to that for vertical walls. In abutment situations where

the face wall turns to form the wing wall, however, it is necessary to trim blocks on either end to 

account for the reduced wall length. All cuts should be performed to maintain the standard running

or stretcher bond between the rows of dry-stacked blocks, with the vertical joints of each course 

midway between those of adjoining courses.

In special situations, negative battered walls have been constructed when the top area needs to be 

greater than the bottom, as in the case of road widening shown in figure 55. The negative batter 

can be created by offsetting the CMU block by a measured amount in consecutive wall layers 

then filling and compacting as specified. 
94

Figure 55. Photo. Negative batter wall face.

7.7.5 Superelevation

When the plan shows a superelevation for the bridge, the top courses of CMU beneath the

superstructure should be trimmed to match the elevation difference and clear space across the 

abutment (see figure 56). This will produce a sloped face wall and aid in construction of the 

beam seat. One method is to snap a chalk line along the back of the block at the superelevation 

slope. A carpenter’s angle finder can also be used to mark the cut.

Figure 56. Photo. Blocks trimmed to match superelevation.
95

7.7.6 Wall Corners

Right-angle wall corners, as shown in figure 57, are constructed with CMU corner blocks that 

have architectural detail on two sides, providing an aesthetic finish. Facing wall and wing wall 

courses should be staggered to form a tight, interlocking, stable corner.

Figure 57. Photo. Right-angle wall corner.

Walls with angles larger or smaller than 90 degrees require additional effort. The corner blocks 

need to be cut to form the angled face. As a result, a vertical seam or joint is formed at the corner 

(see figure 58). Corners with vertical seams may have open block joints, making it prudent to 

fill the corner blocks with a concrete mix and install bent rebar to close and connect the seam at 

each course of block, as shown in figure 59. This procedure secures the two faces and prevents 

compaction-induced separation during construction of subsequent GRS layers. It may also be 

used wherever added strength at the wall corner is desired. 

Figure 58. Photo. Vertical seam in wing wall.
96

Figure 59. Photo. Rebar installed in vertical seam prior to grout.

7.7.7 Top of Facing Wall

The top three courses of CMU block in the abutment are susceptible to movement simply from 

not having the weight of successive layers holding them in place. To prevent displacement, the 

hollow cores of the top three courses of CMU blocks are filled with a concrete wall fill and pinned

together with No. 4 rebar, preferably epoxy-coated and embedded with a minimum 2-inch cover

(see figure 60).

Figure 60. Photo. Connecting the top courses of blocks.

To grout and pin the top of the wall, the reinforcement between the top two courses of CMU block

needs to be removed to open the core for placement of concrete wall fill and a 20-inch-long No. 4

rebar dowel, preferably epoxy-coated with 2-inch cover (see chapter 3). This can be accomplished

either by cutting the reinforcement with a razor knife or by burning the reinforcement. 
97

The concrete wall fill is placed in two steps. After the block void is filled with concrete to the top 

of the block and the steel rebar is inserted, a thin layer of the same concrete mix is placed on top of

the block to form the coping cap, as shown in figure 61 and figure 62. The coping is then handtroweled either square or round and sloped to drain. A wet-cast cap is more durable than a dry-cast

cap and eliminates the need to furnish and install a separate cap unit.

Figure 61. Photo. Rounded coping cap.

Figure 62. Photo. Square coping cap.

Once the top of wall has been grouted and pinned, care should be taken to avoid any construction 

activity that may pull on the top layer of reinforcement. The frictional connection between the

block is strong, and when courses are pinned together, the entire grouted wall face can be pulled 

out of alignment. 

7.8 BEAM SEAT

The beam seat is constructed directly above the bearing bed reinforcement zone. The superstructure

is then positioned on top of the beam seat, as shown in figure 63 and figure 64. The purpose of 

the beam seat is to ensure that the superstructure bears on the GRS abutment and not the wall 

facing block and to provide the necessary clear space between the superstructure and the wall 

face. Typically, the clear space is 3 inches, or 2 percent of the abutment height, depending on the 

required design (see chapter 4).
98

Figure 63. Photo. Box beam placed on beam seat.

Figure 64. Photo. Detail view of box beam placed on beam seat.

In general, the thickness of the beam seat is approximately 8 inches and consists of two 4-inch lifts

of wrapped-face GRS. Remember, before construction of the beam seat, the cores of the CMU 

blocks on the abutment wall face must be pinned with No. 4 rebar and filled with concrete wall 

mix (see figure 65). 

Figure 65. Photo. Bearing area block grouted prior to beam placement.
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7.8.1 Beam Seat Procedure 

Once the block elevation beneath the bearing area is established and the hollow cores are filled 

with grout, the beam seat is ready for construction. The following steps should be used:

1. Place precut 4-inch-thick foam board on the top of the bearing bed reinforcement. 

Sometimes, a thin layer of backfill may be necessary beneath the foam board for

grading purposes and to ensure the proper clear space height and drainage (crown in 

bridge) (see figure 66). The foam board should butt against the back face of the CMU 

block. The exposed edge of the foam board helps form the nose of the reinforcement 

wrap across the length of the bearing area. 

Figure 66. Photo. Foam board and 4-inch block assembly to form beam seat.

2. Set 4-inch solid concrete blocks on top of the foam board across the entire length of 

the bearing area (see figure 67). The back edge of the top CMU face block holds the 

4-inch concrete block in place during compaction. Note that the distance between the 

top of the grouted CMU block and the top of the beam seat (the clear space) is the 

distance the beams can settle before bearing on the facing blocks.

Figure 67. Photo. 4-inch concrete block on top of foam board against top CMU face block.
100

3. Use the first 4-inch wrapped layer of compacted fill as the thickness to the top of the 

foam board (see figure 68).

Figure 68. Photo. First 4-inch wrap butted against foam board.

4. Place the second 4-inch wrapped layer of compacted fill to the top of the 4-inch solid 

block, creating the clear space as shown in figure 69. The top of this layer controls the 

beam elevation, and should therefore be carefully compacted and graded. 

Figure 69. Photo. Top 4-inch wrap butted against 4-inch solid block.

5. Before folding the final wrap, it may be necessary to grade the surface aggregate of 

the beam seat slightly high, to about 0.5 inches, to aid in seating the superstructure 

and to maximize contact with the bearing area.

7.8.2 Setback

The setback is the distance between the back of the facing block and the front of the beam seat. This

distance can be established during construction of the beam seat and placement of the block and foam

board used to form the beam seat wrap. The setback distance is usually 8 inches but can be greater.
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7.8.3 Aluminum Flashing

The aluminum flashing drip edge is installed prior to setting the bridge beams and is placed in 

between the bottom of the beams and the foam board. The flashing is held in place by the pressure

of the beams on the compressible foam board (see figure 70). The length of the flashing should 

extend beyond the outside edge of the bridge beams and be trimmed to fit against the parapets. 

Figure 70. Photo. Aluminum flashing (drip edge) between beams and top of CMU block.

7.8.4 CIP or Precast Footing

For GRS-IBS built without adjacent concrete beams, a CIP or precast footing may be necessary, 

as with steel beams or spread girders (see figure 71).

Figure 71. Photo. Steel girder on CIP footing.

7.9 PLACEMENT OF SUPERSTRUCTURE

Prepare the beam seat as described in section 7.8.1. The grade of the beam seat will control the 

final elevation of the bridge.
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7.9.1 Crane Position on GRS Mass

The crane used for placement of the superstructure can be positioned on the GRS abutment 

provided the outrigger pads are sized within the capacity of the GRS mass. The outrigger pads 

should be sized for 4,000 psf near the face of the abutment wall with greater loads able to be 

supported with increasing distance from the abutment face (see figure 72).

Figure 72. Photo. Outrigger pads near wall face.

7.9.2 Reinforcement of Beam Seat

An additional layer of reinforcement should be placed between the beam seat and concrete or steel

beams to provide additional protection of the beam seat (see figure 73). The additional layer of 

reinforcement may decrease the sliding resistance between the superstructure and the beam seat.

Figure 73. Photo. Additional reinforcement under beam.

7.9.3 Setting Superstructure on Beam Seat (Without CIP Footing)

Since the bearing surface is aggregate under a layer of geosynthetic reinforcement, it is important 

to set beams square and level. They should never be dragged over the beam seat surface, which 
103

could create the potential for an uneven bearing area or a void under the beam, producing uneven 

bearing stresses between bridge elements.

7.9.4 Wing Walls and Parapets

Wing walls and parapets are constructed after the superstructure is set. The CMU block in the 

parapet wall should be trimmed or saw cut for a custom fit against the beam edge to prevent the 

loss of fill material. Figure 74 and figure 75 show the construction of the parapet against the

superstructure. If the gap between the superstructure and the facing block is difficult to fill using 

thin slices of cut block, a mortar mix should be used to close the space.

Figure 74. Photo. Parapet and wing wall construction, view 1.

Figure 75. Photo. Parapet and wing wall construction, view 2.

7.10 APPROACH INTEGRATION

Proper approach construction at the road and superstructure interface is essential to minimizing 

settlement in front of the bridge beams and eliminating the bump at the end of the bridge. This is 

accomplished by compacting and reinforcing the approach fill in wrapped geotextile layers and 

blending the integration zone with the approach road base course. The material for the integration

zone should be well-graded, as outlined in chapter 3.
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Once the superstructure is in place, the approach to the bridge can be constructed using the 

following steps:

1. Trim a geotextile reinforcement sheet to provide the planned length after it is 

wrapped, and place it behind the beam end (see figure 76). The width of the sheet 

should allow for wrapping of the sides after the fill layer is placed and compacted. 

Wrapping of the sides prevents lateral migration of the fill. 

Figure 76. Photo. Placing reinforcement.

2. Place a 6-inch lift of fill and compact, per compaction specifications for road base (see

figure 77). Add a secondary layer of reinforcement on top of the 6-inch lift, and then 

place another 6-inch lift of fill and compact (see figure 78). Fold back the reinforcement

sheet to wrap the compacted fill layer and smooth wrinkles (see figure 79).

Figure 77. Photo. First 6-inch fill lift.
105

Figure 78. Photo. Secondary reinforcement sheet.

Figure 79. Photo. Completed wrapped approach layer.

3. Repeat these steps until approximately 2 inches from top of beam grade, as shown in 

figure 80.

Figure 80. Photo. Second 6-inch fill lift.
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Multiple sheets can be used along the width of the approach, as long as all seams are kept 

perpendicular to the beam ends. The typical wrap reinforcement spacing is 12 inches, with

intermediate layers spaced at 6 inches and compacted in 6-inch lifts. However, in the case of 

beams with a reduced depth, the spacing of the wrapped layers may need to be reduced and the 

intermediate layers eliminated. The top wrap fold should increase in length with each successive 

wrapped layer until the fill is 2 inches below the bridge grade.

7.10.1 Wrapped Reinforcement Layers on Sides

If lateral spreading of the fill in the integrated approach will be an issue (e.g., wing walls are not 

sufficient to confine the fill at the sides), the reinforcement sheets comprising the wrapped layers 

should be folded over along the sides and perpendicular to the bridge (see figure 81).

Figure 81. Photo. Completed approach fill.

7.10.2 Preloading

In some situations, it might be beneficial to preload the abutment before paving to minimize

postconstruction deformation or settlement within the GRS mass. A simple method of preloading

can be achieved be parking fully loaded trucks on the bridge for several days before placement of 

the asphalt pavement. 

7.10.3 Paving

The top layer of reinforcement should be kept approximately 2 inches below the beam grade. 

This will allow a layer of aggregate cover to be placed to protect the reinforcement from contact 

with hot mix asphalt.

When GRS-IBS is built with adjacent precast concrete beams, a layer of paving fabric is extended

over the beams onto the approach way. Extending the paving fabric 3 ft over the beam approach

interface is recommended. This is necessary to bridge the gap and provide an interface to 

accommodate thermal movement, minimize surface water infiltration, and prevent cracks in the road.

Note that paving fabric is already used on top of the beams as a barrier to water infiltration and 

to absorb stresses to minimize reflective and fatigue cracking of the new asphalt surface layer.
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7.10.4 Guardrail Post 

Steel H posts are recommended for any railing that is driven through the reinforcement. It is also 

possible to drill through the GRS mass with an auger to set other types of posts.

7.11 SITE DRAINAGE

The GRS-IBS construction area should be protected from surface runoff during the project. Critical

areas are behind the abutment wall at the interface between the GRS abutment and the retained 

fill, at the base of the abutment, and at any location where a fill slope meets the wall face. Design 

needs to include provisions for surface drainage along the fill slope adjacent to the wing walls.

Provisions for drainage should also be included at the boundary of the wing walls and the fill 

slope. Long walls built along variable elevation or abutment wing walls are often stepped to 

reduce excavation. In these situations, the termination of wall steps should be sufficiently

embedded to prevent problems with erosion. The drainage swell or channel should be separated 

from the wall to avoid flow directly against the wall face.

Site preparation for drainage should include the following: 

Grading: The site should be graded every night in anticipation of precipitation to 

avoid saturation of soil.

Diversion trenches: An alternative to grading is placing diversion trenches around 

the perimeter to divert water.

Compaction of loose soil: Any loose soil placed to construct GRS should be graded 

and compacted before stoppage of work for the day. Also, onsite stockpiles of fill 

material containing fines should be protected from excess precipitation. 

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CHAPTER 8. IN-SERVICE INSPECTION, MAINTENANCE, AND REPAIR

8.1 INTRODUCTION 

A key feature of GRS-IBS is that it has fewer parts than conventional bridges and abutments and 

should therefore need less maintenance. Like other bridges, the main components are the

superstructure and the substructure. The superstructure is the same as a conventional bridge and 

should have the same protocol for inspection, rating, maintenance, and repair. IBS is also 

somewhat similar to an integral abutment in how the ends are embedded in the approach. A 

difference is that the IBS is embedded in compacted gravel, whereas an integral abutment is 

encased in concrete. Both bridges are designed without a joint to limit the effect of water at the

beam ends for improved durability.

At the time of this report, nearly 30 GRS-IBS bridges had been built for local or off-system service, 

with the oldest built in 2005. None of the bridges show any signs of distress. All indications are 

that GRS-IBS works well in the local road environment, suggesting that the long-term performance

of this system is adequate. In addition, IBS has fewer components and is designed for a smooth 

transition, thereby reducing impact loads (a major contributor to fatigue of the superstructure).

This promise of improved performance, however, does not mean that the bridge system is immune

to the common problems of conventional systems. This chapter focuses on potential requirements 

unique to this IBS and other components associated with the integrated approach. 

8.2 IN-SERVICE INSPECTION

Both superstructure and substructure elements should be included as part of the visual inspection

process. As previously indicated, the superstructure is similar to a conventional bridge and therefore

has a similar procedure for inspection. The following elements should be included as part of the 

inspection of the IBS substructure: 

Pavement: If the bridge has asphalt pavement, check for a transverse crack, shoving, 

or separation at the approach end wall interface. 

Approach: Check the approaches for vehicle rideability and smoothness.

Parapet walls: Check the interface between the beams and the parapet wall for 

separation or shifting.

Beam ends: Check embedded beam ends for corrosion (i.e., rust stains) at the beam bases. 

Scour: Monitor GRS walls built adjacent to a water channel for scour. Riprap or other

appropriate countermeasures should be monitored at each bridge inspection or after an

extreme flood event. Any movement of rock should be noted and repaired to prevent 

scour from progressing and endangering the RSF or the abutment. No problems have 

been noted in installations of GRS abutments even after sequential flooding events. An

indicator of scour on an abutment face or wing wall can be achieved by using colored 

blocks on the bottom five to eight rows. Solid blocks are recommended at the bottom, 

as they are more likely to resist any impact of moving riprap, ice, or other abrasion 
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associated with the normal water elevation. The colored solid blocks are also covered

from view by the initial riprap, and any exposure of colored block during inspection 

serves as a visual check for movement or undermining of the riprap, indicating a need 

for remediation or repair to protect the RSF and abutment from scour.

Drainage: All GRS structures should include consideration for surface drainage. Check

the critical drainage paths where the fill slope meets the wing walls leading to the base

of the wall. It is imperative that the wing walls have sufficient embedment to prevent 

erosion due to roadway runoff.

Wall face cap: Inspect the coping for cracks.

Modular blocks: For GRS walls built with modular facing blocks, check for the 

following: 

o Cracked blocks.

o Separated blocks.

o Block durability problems. Refer to Chan et al. for more information about 

block durability due to freeze thaw, spalling, and efflorescence.

(23)

Guardrail: Inspect traffic barriers for damage.

Wall face: Inspect wall faces for excessive lateral movement or settlement.

o Lateral deformation can be checked visually or with a plumb bob referenced 

from known points from the top of the wall to the bottom. 

o Visual inspection for wall settlement can be achieved by checking for 

distortion between the horizontal courses of block.

Clear space: Inspect, measure, and record the distance from the top of the wall face to 

the base of the superstructure beam for any settlement within the GRS abutment mass.

A clear space must be maintained throughout the life of the bridge to prevent loading 

on the facing elements.

Drip edge: Inspect any drip edge detail at the top of the wall beneath the beam for 

water diversion.

Burrows: Inspect and remove any animal burrows adjacent to the walls. 

8.3 MAINTENANCE

If properly designed and constructed, GRS-IBS should need minimal maintenance because it has 

fewer parts (e.g., no approach slab, sleeper slab, CIP parapet walls, bridge bearings, or joint details).

Since the bridge superstructure is built with common materials, general maintenance should be 

similar to that of a conventional bridge system. Maintenance duties might include the following:

Sealing of a pavement crack, particularly one forming at the beam approach interface.
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Stabilization of drainage ditches to prevent erosion along the wing wall.

Removal of vegetation growth from the wall face unless it is part of the design.

Sealing of any gaps in the facing large enough to allow for a loss of fill.

8.4 REPAIR

This section includes tips and suggested methods of repair in the event of damage to the GRS 

abutment wall face. Damage can occur as result of impact, unforeseen scour, or poor wall face 

durability. Since a GRS abutment is internally supported, the face is not considered a structural 

element. However, its integrity is important to ensuring long-term performance of the GRS abutment.

The following are repair procedures for potential problems:

Damage to a few hollow-core blocks within the face of the wall: Chip out the face 

of the damaged block and replace it with the face of another block. The face piece 

should be cut slightly smaller and be secured with mortar. 

Repair of deteriorating facing blocks or scour damage: Although there is no case 

history for this, shotcrete can be used to repair the face of a modular block wall. 

Figure 82 shows a GRS wall built with CMU blocks being used to repair a failed

MSE wall. Figure 83 shows the same CMU covered with shotcrete. Note that drains 

were installed at the base of the wall to facilitate the flow of water from the GRS 

abutment. In some situations, it might be necessary to install vertical strip drains in 

the face of the GRS wall before applying the shotcrete.

Figure 82. Photo. Use of GRS wall to repair damaged MSE wall.
112

Figure 83. Photo. CMU GRS wall with a shotcrete face.

Damage to the top rows of CMU block: Figure 84 and figure 85 show a GRS wall 

before and after the repair of a rock-fall impact. The repaired section is set slightly 

back from the original wall alignment. To repair this wall, the boulder was removed,

and each soil layer within the damaged zone was excavated. To access the fill, the 

fabric layers were cut perpendicular to the face and peeled back enough to access all

the reinforcement layers within the damaged zone. This process was repeated until the 

damaged zone was exposed. The exposed zone was rebuilt using the 1-2-3 method 

explained in chapter 7, one layer at a time, from the bottom up. In areas where the

reinforcement was excessively damaged, new reinforcement was spliced in to reestablish 

the frictional connection. The top courses were then pinned and grouted.

Figure 84. Photo. GRS wall damaged by large sandstone boulder.
113

Figure 85. Photo. Repair of a GRS wall after damage caused by rock fall.

Excessive settlement of the beam seat: While this has not been observed, it is

possible that the superstructure could experience excessive movement either due to 

compression of the GRS abutment or external instability. If the clear space is lost and

the superstructure is causing distress to the wall, it is possible to saw a new gap to 

relieve the pressure. An alternative method would be to pressure grout and elevate 

the superstructure back to its original grade, which may also require repair to the 

approach pavement. 

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CHAPTER 9. QUALITY CONTROL AND QUALITY ASSURANCE

9.1 INTRODUCTION

Quality is everyone’s responsibility. The quality of GRS-IBS begins with an understanding of 

the concept of wall strength and the interrelation of components of a GRS system and with sound 

construction practices. 

QC consists of implementation, measurement, and enforcement of sound construction practices 

and field inspection procedures to ensure construction quality as outlined in this manual. QC also 

involves the selection of quality materials. The successful completion of a project is dependent 

on a proper monitoring program with necessary adjustments at each stage of construction. QC is

the responsibility of the builder.

QA is necessary to ensure the finished product meets specifications through inspection, testing,

and final acceptance. The process involves constant evaluation of the project activities related to 

planning, design, development of plans and specifications, construction, and all interactions 

associated with these fundamental activities. QA can either be the responsibility of the owner 

agency or a third-party agency.

9.2 ROLE OF THE CONTRACTOR 

Since GRS is a nonproprietary generic wall system, the contractor building the wall can be 

responsible for developing and maintaining a QA/QC plan for project quality. Prequalification 

based on the procedures outlined in this manual should be a necessary requirement for this type 

of construction. 

9.3 TESTING

QC testing performed during construction mainly applies to onsite field testing of backfill 

material and associated laboratory tests. 

9.3.1 Laboratory Testing 

Gradation and moisture-density tests (e.g., Proctor compaction test) will be required for field 

monitoring of the backfill material. The classification tests and moisture-density tests should 

follow AASHTO standards for aggregate sampling and testing.

Large-scale direct shear tests or triaxial tests are the most effective methods for determining the 

friction angle for coarse-grained backfill aggregates. These methods of testing are preferred over 

the standard direct shear test (AASHTO T236) or smaller diameter triaxial tests that are

performed on the minus No. 10 material.
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9.3.2 Field Testing

Fill placement and compaction is the predominant construction activity that needs to be monitored

in a GRS-IBS project. Field density tests should be performed on each layer. The field test 

method should be applicable to the aggregate type that is used for the backfill material.

Dense-graded backfill, which consists of State transportation department crushed base course,

can be tested with a nuclear gauge. State transportation department density testing procedures 

can be used. Moisture content should be monitored and controlled prior to fill placement for an 

effective compaction process.

A procedural (method-based) specification is preferable for the compaction of open-graded fill 

material, which exhibits a high percentage of void space. Open-graded gravels are not conducive to 

in-place nuclear density testing procedures as the direct transmission nuclear gauge procedure is 

difficult to perform (the transmission hole will typically not stay open), and nuclear backscatter

testing is not effective due to poor soil/gauge contact.

In lieu of density testing for open-graded gravels, maximum density can be achieved with a

recommended procedural specification. The procedure can specify three to five passes with a 

walk-behind vibratory plate compactor near the wall face. Larger ride-on vibratory rollers with 

greater frequency and efficiency can be used in the core of the GRS mass with fewer passes 3 ft 

from the wall face.

9.4 CONSTRUCTION INSPECTION 

Thorough inspection before and during construction will ensure the GRS structure is built in 

accordance to the plans and guidelines. Inspection requires an understanding of GRS design and 

methodology. Familiarity and understanding of the drawings is necessary. It is important to have 

firsthand knowledge of the GRS construction processes. A properly implemented field inspection 

program provides an opportunity to take corrective action during the construction process. 

A critical component of construction is compaction behind the facing element followed by placement

of the geosynthetic reinforcement. Those responsible for performing these construction activities 

are best suited for maintaining the quality of each GRS wall layer. Note that in the RSF and the 

integrated approach, a geotextile must be used to prevent migration of fill material and erosion. 

9.4.1 Materials 

Once materials are delivered to the site, they should be inspected for compliance with the guidelines

and project specifications. Materials should be visually inspected for quality, damage, and defects. 

Backfill: In addition to the quarry material certificate showing the gradation of the aggregate, a 

visual inspection should be performed to verify maximum grain size, amount of fines and grain 

shape (angular or rounded), excess fines, moisture content, and durability.

Facing block: As outlined in chapter 3, the facing block should be inspected for integrity,

consistency, and dimension tolerances. Confirm that sufficient quantities and proper block 

type (e.g., solid block, corners, and face block) are present onsite and ready for use. 
117

Geosynthetic reinforcement: Verify that the specified type and strength of geosynthetic is correct 

along with the required roll dimensions. Chapter 3 provides detailed tests that should be

documented for each roll of reinforcement. 

9.4.2 Equipment

Compaction of the backfill in a GRS wall or abutment is a critical construction activity. It should 

be confirmed that the compaction equipment onsite is compatible with the selected backfill material.

Verify that the required hand tools are onsite for spreading and grading aggregate, maintaining 

the facing alignment, and sweeping the top of the CMU facing block.

9.4.3 Project Layout

Verify that all layout reference points are established, with particular emphasis on the location of 

the following areas:

Center line of superstructure.

RSF area within lines and grade of working drawings.

Bearing area of the bridge beams.

Wing wall width and length.

Clear space and setback.

Span length.

Center of bearing to center of bearing.

Elevations.

Grades.

9.4.4 Construction Activities

GRS is built from the bottom to the top. Those responsible for inspection need to make certain each

layer is constructed and tested in accordance with the contract drawings and specifications before 

proceeding with subsequent layers. Inspection and QC/QA activities are discussed in this section.

Working bench: Before excavation, the working bench/platform needs to be inspected for 

stability with consideration for drainage. Any movement should be controlled.

Foundation excavation: The foundation should be cut as outlined in the plan and inspected for 

any soft areas before compaction and proof rolling.
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Geotextile-wrapped RSF foundation: For encapsulation of the RSF, it should be confirmed that 

the open edge of any overlap is facing downstream and the three sides (two wing walls and one 

abutment face wall) are contained by a layer of geotextile to prevent erosion. 

Leveling course: In order to set the first course of facing block level and plumb, the top elevation 

of the RSF should be as close to grade as possible. Often, a thin (0.5-inch) leveling layer of 

aggregate is placed under the first course. Inspection of this leveling layer should be performed to

determine its thickness or the need to replace it with a low slump wet concrete/grout mix.

Compaction of backfill: Inspection of backfill operations should verify compliance with the 

construction guidelines outlined in chapter 7. Compaction behind the wall face and within the bearing

area is important. Inspection should confirm that each lift never exceeds the specified thickness. 

Compaction control should be maintained through field density tests or other soil stiffness-based 

methods. For backfill material containing fines (minus No. 200), the moisture content should be 

within the specified range (±2 percent). This improves the compaction process. Compaction of 

open-graded aggregate should be observed to ensure nonmovement of material under the compaction

equipment; this observation is an indication of compaction or stiffness and is dictated by the 

number of passes.

Reinforcement installation: Inspect the installation of each reinforcement layer to ensure it is 

properly placed, has adequate facing element coverage for the frictional connection, and is free 

of wrinkles. Anticipate the location and placement of the bearing bed reinforcement layers,

particularly in situations when the bridge is superelevated. 

Facing block placement: Prior to placement of the reinforcement layer over the facing block, the 

block should be inspected to verify a clean surface. This is essential in maintaining wall alignment

and avoiding block cracking due to point loads. The inspection process should ensure that there is

no rocking motion when setting the block, which can be indicative of point load bearing. 

Wall alignment: Visual inspection should be performed at regular intervals during construction. 

This will help ensure that the wall is within vertical and horizontal tolerances for alignment. QC 

should be performed on block alignment using a string line on at least every third course.

Vertical alignment can be checked with a plumb bob. 

Wall termination: Make sure that all wing wall terminations are sufficiently embedded to prevent 

undermining from erosion. A terminated course needs to be founded on a stable compacted layer 

of granular fill material as outlined in chapter 7 or on an excavated cut into native soils.

Fill slope side: The fill slope at the wing walls is usually built with native soil as the GRS wall

advances upward. The fill slope should be constructed with a drain path that leads away from the

wall face. Surface runoff should be diverted to prevent saturation of the soil fill slope. Temporary

drainage may need to be installed to preserve the integrity of the cut slope.

Site drainage: The working platform should be compacted and graded to drain surface water away

from the working area. Any pit excavation should be sloped to drain to a location that can be pumped.
119

Heavy equipment operation: It is beneficial to have construction equipment centrally located in 

the work area and to have materials strategically stocked near the equipment for efficient transfer 

to the labor crew. Equipment operators should take caution when working near large layers of 

exposed geosynthetic. 

Beam seat: Construction of the beam seat should be inspected to confirm the use of methods 

described in chapter 7. It is important to verify that the beam seat is constructed at the correct 

elevation and grade to provide the specified clear space and setback. 

CMU core grouting: The core of the top three courses of CMU blocks should be filled with concrete

wall mix. Rebar dowels should be cut to length (20 inches) and inserted into the core of the top 

three courses. The concrete mix should be rodded with a rebar dowel before insertion to eliminate 

voids. Sufficient concrete should be available to form the coping cap during the same pour.

Wrapped integrated approach: Prior to placement of the geotextile reinforcement, it should be 

verified that the length is adequate to wrap the fill and extend back towards the road as shown in 

the one-sheet plan. Sufficient reinforcement width should also be available to laterally confine 

the approach fill if necessary.

The lift thicknesses for each lift should be checked to ensure that they do not exceed the maximum 

thickness and that secondary reinforcement is placed within fill layers that are greater than 8 inches.

If a granular road base is used in the wrapped approach, verify that its compaction conforms to 

density requirements for the road as well as the GRS. 

At the top of the integrated approach, verify that a 1- to 2-inch layer of aggregate is placed on the 

top reinforcement layer for protection from hot mix asphalt. Verify that the paving fabric, if used,

bridges the interface from the deck to the approach as described in chapter 7.

9.5 DOCUMENTATION

9.5.1 Compliance Documentation

Field test results should be carefully measured and archived as a permanent part of the job record.

This information can also be used to modify field (construction and inspection) practices.

The main field measurement, moisture density tests, should be documented during construction.

Other documentation should include construction modifications, field changes, and daily 

construction reports.

9.5.2 Record Drawings

As-built plans should be prepared and provided to the owner upon completion of the project. 

9.6 CONTRACTING METHODS

Of the two types of contracting methods commonly used for specialty construction, the procedural

method and performance method, the preferred approach for the GRS-IBS is the procedural method.

GRS performance-based methods can be developed when the technique becomes more widespread. 
120

The generic nature of GRS walls and abutments fits well with the performance-based method, 

which can advance the technology by creating an opportunity to develop new techniques, details, 

and equipment.

9.6.1 Performance Method 

In a performance-based contract, the contractor can choose a GRS system based on its performance

and constructability. The contractor should verify that the GRS-IBS is constructible and performs

as outlined in the requirements. Careful attention should be placed on the compatibility between 

the backfill material, the wall facing, and the reinforcement to assure that the wall meets the 

necessary requirements.

Under this method, design and performance criteria should be based on the data provided in

chapter 4 and chapter 6. Material and construction specifications can be based on the information 

in chapter 3 and chapter 7. This contract method requires that the reviewers have considerable 

knowledge in GRS technology to accept design submittals.

9.6.2 Procedural Method 

In this contract method, the agency or owner provides a detailed set of design plans and construction

specifications in the bid document. QA begins with an initial plan, design, and review of construction

materials. Approval should be dependent on someone experienced in the design and construction 

of the GRS system. Also, the completed project should be in compliance with local agency 

building codes and regulations.

Fully detailed plans and items requiring review prior to initiating a GRS project should consist of 

the following:

Design calculations.

o Stability analysis.

o Bearing capacity.

o Hydraulic analysis.

o Loads.

Project drawings (examples of typical GRS-IBS working drawings are presented in 

appendix D).

o Plan drawings.

o Cross sectional drawings of all abutment wall faces and wing walls.

o Elevation drawings.

o Horizontal and vertical curve details.

o Construction details addressing guardrails, parapets, beam seat, wing wall 

configurations, etc. 

o General notes.
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o Fabric schedule.

o Block schedule.

o One-sheet plan for quick reference.

Geotechnical report.

o Plan view of testing.

o Subsurface profile (if necessary).

o Test boring logs (if necessary) .

o Laboratory test data.

o Engineering properties of foundation soil, retained soil, and settlement analysis.

o Allowable/ultimate bearing pressure of foundation soil.

o Ground water and free water conditions.

o Existing abutment conditions (if replacement bridge).

o Historical flood events.

Hydrology report.

o Annual Q50, Q100, and Q200 floodwater levels and velocities.

o Stable particle size analysis for scour potential.

o Land uses that could impact flood levels.

Verification of experience (prequalification): The contractor should be certified or 

prequalified in GRS construction methods in accordance with the procedures presented

in this guide. As an alternative, a contractor should be able to verify demonstrated 

knowledge in constructing GRS structures. For GRS-IBS projects, the contractor 

should provide information on the successful construction of many GRS walls and 

abutments. It is important that the contractor have an understanding of the compatible 

relationship between wall facings and backfill material.

QC/QA plan: A QC/QA plan should be developed by the agency or contractor

performing the work and should be followed by the contractor. The QC/QA plan 

should detail types of measurements and documentation that will be maintained 

during construction to ensure compliance with GRS guidelines and standards.
122

9.6.3 Contractor Submittals

Materials used to construct GRS-IBS are readily available from a number of sources. The only 

requirement is that they meet the standards provided in this manual. The main materials that 

should be reviewed prior to construction are as follows (see chapter 3 for more details):

CMU block specifications or other type of facing element.

Backfill gradation, type, and source. The backfill submittal should include aggregates 

used for the RSF, abutment wall, and integrated approach.

Geosynthetic reinforcement.

9.7 PROJECT DRAWINGS AND DOCUMENTS

Appendix D provides typical GRS-IBS working drawings that include a one-page plan sheet, 

estimated quantities and general notes, project plans and profile, GRS abutment details, and a 

site plan.
123

APPENDIX A. IN-SERVICE GRADATIONS

At the time of this report, all GRS-IBS projects had selected open-graded gravel due to its relative

ease of construction and drainage characteristics. The various counties and agencies that are 

building GRS-IBS have selected locally available materials for their projects. The gradations and 

general description are shown in table 11 through table 15.

Table 11. Defiance County, OH, AASHTO No. 89, clean, crushed limestone.

U.S. Sieve Size Percent Passing

1

/2 inch 100

3

/8 inch 90–100

No. 4 20–55

No. 8 5–30

No. 16 0–10

No. 50 0–5

Table 12. Warren County, OH, AASHTO No. 67, clean, crushed rock.

U.S. Sieve Size Percent Passing

1 inch 100

3

/4 inch 90–100

3

/8 inch 20–55

No. 4 0–10

No. 8 0–5

Table 13. King County, WA, WSDOT 1

1

/4-inch minus gravel, clean round rock 

with sand mixture-pit run.

U.S. Sieve Size Percent Passing

11/4 inch 100

1 inch 90–100

No. 4 50–80

No. 40 0–30

No. 200 0–7

Table 14. St. Lawrence County, NY, NYSDOT No. 1, clean crushed rock.

U.S. Sieve Size Percent Passing

1 inch 100

1

/2 inch 90–100

1

/4 inch 0–15

No. 200 0–1
124

Table 15. Urbana, IL, Seismic Test Abutment, IDOT CA6 road base, subrounded gravel 

with sand mix.

U.S. Sieve Size Percent Passing

1

1

/2 inch 100

1 inch 90–100

1

/2 inch 60–90

No. 4 30–56

No. 16 10–40

No. 200 4–12
125

APPENDIX B. PERFORMANCE TEST PROCEDURE

B.1 PERFORMANCE TEST OVERVIEW

A performance test (or mini-pier experiment) is recommended to design a GRS abutment if the 

selected reinforced fill is outside the gradations given in chapter 3. A mini-pier experiment is a 

large element GRS load test to provide the designer with material strength properties of a particular

GRS composite mass built with a unique combination of reinforcement, compacted fill, and facing

elements. The procedure involves axially loading the GRS mass while measuring vertical settlement

and lateral deformation to monitor performance. This information can then be used to aid in the 

design process to predict performance of a full-scale GRS composite mass.

The performance test concept has been applied to smaller-scale models from small triaxial-sized 

samples to 2-ft cubed specimens in small-capacity test frames.(24,25) However, due to the size of 

the aggregates that are often used in GRS fills, larger-scale models must be used to adequately 

predict the performance of a full-scale GRS abutment. Several large-scale tests have also been 

conducted.(11,25,26) The proposed performance test, outlined in this appendix, has been shown to 

accurately predict the actual settlement of in-service GRS abutments (see chapter 4).(12,27,28)

The reinforcement, fill (compaction effort), and facing materials in the performance test should be

the same as what is planned for the GRS abutment. The spacing of the reinforcement is dictated by

the abutment design, which is recommended to be 8 inches at most. The bearing reinforcement bed

should extend through the top two courses of blocks. Since the specifics of the performance test 

will be dictated by the actual abutment design and materials, set dimensions and materials will vary

between each mini-pier experiment. The base-to-height ratio should be 2:1 (with the base width 

measured inside the facing elements). It is advisable to have a contact area that is slightly less than 

the footprint of the GRS mass, inset 2–3 inches on all sides. Two examples are provide to guide 

the designer or user on how to construct and test a model. 

B.2 PERFORMANCE TEST EXAMPLE: VEGAS MINI-PIER

The following example is an actual performance test that was conducted, called the Vegas MiniPier Experiment. The reinforcement strength and spacing and the fill and facing elements are 

different from those outlined in the design of a GRS abutment presented in chapter 4. This

example shows how a performance test can facilitate design with a unique combination of 

reinforcement, soil, and facing elements.

In this test, the abutment was to be constructed with a reinforcement spacing of 6 inches and a 

bearing reinforcement bed of 3-inch spacing extended into the top two courses of blocks. The 

reinforcement was a woven PP geotextile with a wide width tensile strength of 2,400 lb/ft, per 

ASTM D4595. 

The gravel was classified as a GP-GM soil according to ASTM D2488 with a maximum diameter of 

1 inch. The cohesion and friction angle of this material were 580 lb/ft2 and 40 degrees. Compaction

of the fill for the mini-pier was performed with a hand tamper at the optimum moisture content. 

Considerable effort to compact each lift of soil was made, but soil density measurements were not
126

taken during the construction process. Note that compaction testing (or a method specification 

for open-graded gravels) should be performed for each lift in a standard performance test.

The facing elements were segmental retaining wall (SRW) blocks. The modular block was solid 

dry-cast concrete and had a split face. The height of the block was 6 inches and equal to the

spacing of the reinforcement. The unit weight of each block was 82 lb. The blocks were frictionally

connected to the GRS mass without the aid of pins or mechanical connection. To form the corners, 

the block was split, as shown in figure 86. The layout of the SRW blocks and the reinforcement 

schedule are shown in figure 86 and figure 87. 

Figure 86. Illustration. Plan view of Vegas Mini-Pier Experiment.

Figure 87. Illustration. Elevation view of Vegas Mini-Pier Experiment.

The scaled GRS mass was built on a concrete base pad (see figure 88). The base pad was elevated

on the cinderblock to make room for the two bolted channel beams. The load test setup is illustrated

in figure 89 and figure 90. The top set of bolted channels was supported on the top concrete pad,

which was centered on the GRS mass. The top pad was not supported on the SRW blocks (see 

figure 89). The upper and lower channel beams were coupled together with threaded bar. Four 

hollow-core hydraulic jacks were bolted to the top channel beams (see figure 91). All jacks were 

connected to a manifold and controlled with a servo-controlled hydraulic pump. 
127

Figure 88. Photo. Vegas Mini-Pier Experiment reaction base.
128

Figure 89. Illustration. Side view of Vegas Mini-Pier Experiment.

Figure 90. Illustration. Face view of Vegas Mini-Pier Experiment.
129

Figure 91. Photo. Hollow-core hydraulic ram bolted to the top channel beams.

During the load test, vertical stress was applied in increments of 5 psi. Load was measured with a 

hydraulic oil pressure gauge and a load cell. Vertical stress was calculated by dividing the sum of 

the force from each jack by the area of the GRS mass. The top area of the GRS mass was 3.5 ft by

3.5 ft. The area of the top pad was 3 ft by 3 ft, less than the area of the GRS mass (see figure 89). 

Each load increment was maintained between 5 to 7 min. Vertical and lateral deformations of the 

GRS mass were recorded during each load increment. Deformations were measured with dial gauges

referenced off scaffolding next to the mini-pier. Vertical settlements were measured on the four 

corners of the top pad while lateral deformations were measured at five points along one wall face 

of the GRS mass. Ceramic tiles were glued to the concrete pad and SRW block to create a smooth

surface for accurate measurement of deformation. 

The experiment was terminated at a vertical stress of 145 psi. The vertical strain at this stress was 

measured at 2.9 percent. At this point, the stroke on the dial gauges and linear variable differential

transformers ran out, so the test was ended. The resulting stress-strain curve is shown in figure 92.
130

Figure 92. Graph. Vegas Mini-Pier stress-strain curve.

B.3 PERFORMANCE TEST EXAMPLE: DEFIANCE COUNTY PERFORMANCE TEST

The following example is an actual performance test that was conducted, called the Defiance County

Performance Test. The reinforcement strength and spacing and the fill and facing elements were 

the same as those outlined in the design of a GRS abutment in chapter 4. The layout of the CMU 

blocks and the reinforcement schedule are shown in figure 93 and figure 94.

Figure 93. Illustration. Plan view of Defiance County experiment.
131

Figure 94. Illustration. Elevation view of Defiance County experiment.

In this test, the abutment was constructed with a reinforcement spacing of 8 inches and a bearing 

reinforcement bed of 4-inch spacing extended in the top two courses of block. The reinforcement 

was a woven PP geotextile, with a wide width tensile strength of 4,800 lb/ft, per ASTM D4595. 

The gravel was classified as AASHTO No. 89 stone, according to ASTM D2488, with a maximum

diameter of 0.5 inches. There was zero cohesion and a 48 degree friction angle (as determined using

a large shear box test). Compaction of the fill was performed with a hand tamper. Considerable 

effort to compact each lift of fill was made, but soil density measurements were not taken during 

the construction process. 

The facing elements were CMU blocks. The modular block was solid dry-cast concrete and had a 

split face. The height of the block was 8 inches and was equal to the spacing of the reinforcement. 

The unit weight of each block was 42 lb. The blocks were connected to the GRS without the aid 

of pins or mechanical connection. The layout of the CMU blocks and the reinforcement schedule 

are shown in figure 93 and figure 94.

The scaled GRS mass was built on a concrete base pad similar to that of the Vegas Mini-Pier

Experiment (see figure 95). The red lines on the concrete base and top reaction pads in figure 95

show the principle direction of the rebar reinforcement to transfer the force from the channels 

to the pads. The base pad was elevated with cinderblocks to make room for the two bottom

reaction channel beams. The top set of bolted channels was supported on the top concrete pad, 

which was centered on the GRS mass. The top pad was set inside the perimeter of the CMU 

block by 2.5 inches The upper and lower channel beams were coupled together with threaded 

bar, bearing plates, and nuts. 
132

Figure 95. Photo. Defiance County experiment before testing.

Load increments were applied to capture both the linear elastic and plastic portions of the stressstrain behavior of the composite mass (see figure 96). Load was measured with a hydraulic oil 

pressure gauge and load cell. Vertical stress was calculated by dividing the sum of the force from 

each jack by the area of the GRS mass. The top area of the GRS mass was 3.2 ft by 3.2 ft. The 

area of the top pad was 2.75 ft by 2.75 ft to create a uniform load distribution.

Figure 96. Graph. Defiance County stress-strain curve.
133

Vertical and lateral deformations of the GRS mass were recorded during each load increment. 

Deformations were measured with dial gauges referenced off scaffolding next to the mini-pier 

(see figure 95). Vertical settlements were measured on the four corners of the top pad while

lateral deformations were measured at five points along one wall face of the GRS mass. Ceramic

tiles were glued to the concrete pad and CMU block to create a smooth surface for accurate 

measurement of deformation. 

The experiment was terminated at a vertical stress of 180 psi. The vertical strain at this stress was 

measured at 4.8 percent. After this extreme loading, the mini-pier remained standing with a 

symmetrical bulge in the upper third of the wall (see figure 97). Note the slightly displaced 

blocks in this area. The resulting stress-strain curve is shown in figure 96.

Figure 97. Photo. Defiance County experiment after testing.

The Defiance County experiment was also conducted with a weaker fabric (2,400 lb/ft), using the 

same fill material and reinforcement spacing. The resulting stress-strain curve is shown in 

figure 98. Note that the strain upon completion of the test was 4.6 percent, about the same as the 

performance test with stronger fabric (4,800 lb/ft). The ultimate applied stress, however, is much 

different as a result of the decreased strength of the reinforcement that was used (see figure 99). 

At a working stress of 4 ksf, the strain using the weaker fabric is about 1.0 percent (see figure 98),

well above the tolerable limit of 0.5 percent. Using the results of these tests, the stronger fabric 

(4,800 lb/ft) was chosen for the GRS-IBS application in Defiance County, OH.
134

Figure 98. Graph. Defiance County stress-strain curve (weaker fabric).

Figure 99. Graph. Defiance County experiment comparison.
135

APPENDIX C. LRFD DESIGN PROCEDURE

C.1 INTRODUCTION

An AASHTO LRFD procedure is presented in this appendix to permit the designer a choice of 

design methodology formats for GRS-IBS.(9) This procedure is primarily a fit to the ASD solution

but does include some LRFD features. For example, checking the external stability of earthretaining structures is well defined by AASHTO LRFD. For internal stability of GRS structures, 

however, LRFD is not well defined yet. Since there is not enough statistical information available

to fully calibrate LRFD for internal stability, the LRFD method has been fit to ASD. This means 

that the designer will arrive at the same answer for internal stability whether using ASD or LRFD.

In the future, once more performance tests and case studies have been evaluated, the resistance 

factors for both vertical capacity and reinforcement strength may change from those presented in 

this appendix.

For LRFD, AASHTO defines the various load factors and load combinations that need to be

considered in the design of bridge and transportation structures.(9) Table 16 and table 17 reproduce

these load combinations and load factors. Resistance factors for external stability are also given 

by AASHTO.(9) For direct sliding, a resistance factor for shear resistance ( r) is included. For 

sliding of soil on soil, r is equal to 1. For bearing capacity, the resistance factor ( bc) is equal to 

0.65. Finally, for global stability, the resistance factor is 0.65. 

Table 16. Typical load combinations and load factors.(9)

Load 

Combination 

Limit State

DC

DD

DW

EH

EV

ES

EL

PS

CR

SH

LL

IM

CE

BR

PL

LS WA WS WL FR TU TG SE

Use one of these at a time

EQ IC CT CV

Strength I (unless 

noted) 1.75 1.00 — — 1.00 0.50/1.20 — — — —

Strength II 1.35 1.00 — — 1.00 0.50/1.20 — — — —

Strength III — 1.00 1.40 — 1.00 0.50/1.20 — — — —

Strength IV — 1.00 — — 1.00 0.50/1.20 — — — — — —

Strength V 1.35 1.00 0.40 1.00 1.00 0.50/1.20 — — — —

Extreme Event I EQ 1.00 — — 1.00 — — — 1.00 — — —

Extreme Event II 0.50 1.00 — — 1.00 — — — — 1.00 1.00 1.00

Service I 1.00 1.00 1.00 0.30 1.0 1.00 0.50/1.20 — — — —

Service II 1.00 1.30 1.00 — — 1.00 0.50/1.20 — — — — — —

Service III 1.00 0.80 1.00 — — 1.00 0.50/1.20 — — — —

Service IV 1.00 — 1.00 0.70 — 1.00 0.50/1.20 — 1.0 — — — —

Fatigue I—LL, 

IM and CE only — 1.50 — — — — — — — — — — —

Fatigue I II—LL, 

IM and CE only — 0.75 — — — — — — — — — — —

γp γTG γSE

γp γTG γSE

γp γTG γSE

γp

γp γTG γSE

γp γ

γp

γTG γSE

γTG γSE
136

Table 17. Load factors for permanent loads.(9)

Type of Load, Foundation Type, and 

Method Used to Calculate Downdrag

Load Factor

Maximum Minimum

DC: Component and Attachments

DC: Strength IV only

1.25

1.50

0.90

0.90

DD: Downdrag

Piles, Tomlinson Method

Piles, Method

Drilled shafts, O’Neill and Reese (1999) Method

1.40

1.05

1.25

0.25

0.30

0.35

DW: Wearing Surfaces and Utilities 1.50 0.65

EH: Horizontal Earth Pressure

Active

At-Rest

AEP for anchored walls

1.50

1.35

1.35

0.90

0.90

N/A

EL: Locked-in Construction Stresses 1.00 1.00

EV: Vertical Earth Pressure

Overall Stability

Retaining Wall and Abutments

Rigid Buried Structure

Rigid Frames

Flexible Buried Structures other than Metal Box Culverts

Flexible Metal Box Culverts and Structural Plate Culverts 

with Deep Corrugations

1.00

1.35

1.30

1.35

1.95

1.50

N/A

1.00

0.90

0.90

0.90

0.90

ES: Earth Surcharge 1.50 0.75

C.2 GRS-IBS DESIGN GUIDELINES

The following nine basic steps for the design of GRS-IBS are reproduced from section 4.4:

1. Establish project requirements.

2. Perform a site evaluation.

3. Evaluate project feasibility.

4. Determine layout of GRS-IBS.

5. Calculate loads.

6. Conduct external stability analysis.

7. Conduct internal stability analysis.

8. Implement design details.

9. Finalize GRS-IBS.

α

λ
137

Once these steps are accomplished, the GRS-IBS can be constructed. The basic design guidelines 

are the same whether using ASD or LRFD. However, the detailed equations within step 6 and 

step 7 will differ between the two design methods. In this appendix, only the differences in step 6 

and step 7 that result from conversion to the LRFD format are presented. Refer to section 4.4 for 

discussion on each of these design elements and the equivalent ASD equations and to section 4.5 

for the ASD calculation.

C.2.1 Step 6—Conduct an External Stability Analysis

The external stability of a GRS-IBS is evaluated by looking at the following potential external 

failure mechanisms:

Direct sliding.

Bearing capacity.

Global stability.

C.2.1.1 Direct Sliding

The total factored driving force for LRFD (FR) is calculated in much the same way as in ASD (Fn)

except load factors are applied to each component of thrust force. Equation 70 modifies equation 13

to include the load factors EH MAX, ES MAX, and LS, which are determined using table 16 and 

table 17.

(70)

The factored resisting force (RR) is calculated using equation 71. This equation is the LRFD 

modification of equation 14 that includes a shear resistance factor ( ). For sliding, is 

equal to 1.0.

(9)

(71)

Where Wt,R is determined using equation 72, which is equation 15 modified to include the 

appropriate load factors, EV MIN, DC MIN and DC MIN, from table 17.

(72)

For LRFD, the ratio of the factored resistance and the factored driving force must be greater than 

or equal to 1.0 (see equation 73). If not, consider lengthening the reinforcement at the base. 

(73)

C.2.1.2 Bearing Capacity

In this section, the ASD equations to evaluate bearing capacity have been modified to include the 

appropriate load and resistance factors of LRFD. Equation 74 is the LRFD version of equation 18. 

γ γ γ

FFRR = γγEE MMMMMM FF + γγEE MMMMMM FFrr + γγLL FFtt

Φτ Φτ

RRRR = Φτ(WWTT.RRμμ)

γ γ γ

WWtt,RR = γγEE MMMM WW + γγDDDD MMMM (qq ) + γγEE MMMM �qqrr rr ,tt�

RRRR

FFRR

≥ 1.0
138

(74)

Where VR is the total factored vertical load on the GRS abutment (see equation 75), BRSF is the 

width of the RSF, and eB,R is the eccentricity of the resulting force at the base of the wall (see 

equation 76).

(75)

(76)

Where EV MAX, LS, ES MAX and DC MAX, are load factors found from table 16 and table 17, W is 

the weight of the GRS abutment, WRSF is the weight of the RSF, Wface is the weight of the facing 

elements, qt is the roadway LL, qrb is the road base DL, brb,t is the width of the traffic and road 

base surcharges over the GRS abutment, qb is the bridge DL, b is the width of the bridge seat, qLL

is the bridge LL, MD,R is the total factored driving moment, MR,R is the total factored resisting 

moment, and VR is the total factored vertical load. The moments should be calculated about the 

bottom center of the RSF length for the specific layout of the GRS abutment. If eB,Rn is negative, 

take eB,R equal to zero for the term BRSF-2eB,R.

The factored bearing capacity of the foundation (qR) can be found using equation 77. Based on 

AASHTO, the resistance factor ( bc) is equal to 0.65.(9)

(77)

Where cf is the cohesion of the foundation soil, Nc, , and Nq are dimensionless bearing capacity

coefficients (see table 4), f is the unit weight of the foundation soil, B’ is the effective foundation

width (equal to BRSF-2eB,R), and Df is the depth of the embedment. 

The ratio of the factored bearing resistance and the factored applied pressure must be greater 

than or equal to 1.0 (see equation 78). If not, increase the width of the GRS abutment and RSF 

(by increasing the length of the reinforcement layers), replace the foundation soil with a more 

competent soil, or add embedment depth.

(78)

C.2.1.3 Global Stability

According to AASHTO, the Service I Load Combination should be used to evaluate global 

stability.(9) For the Service I limit state, the load factor is 1.0 for permanent loads. When the 

geotechnical parameters are not well defined or the slope does contain or support a structural 

element, the shear resistance factor is 0.65. This corresponds to a factor of safety of 1.5.

σσvv, ,RR = ∑ VVRR

RR −2 ,RR

Σ

�VVRR = γγEE MMMMMM (WW) + γγEE MMMMMM (WWRR ) + γγDDDD MM (WWffaa ) + γγLL (qqtt rr .tt)

+ γγEE MMMMMM �qqrr rr ,tt� + γγDDDD MMMMMM(qq ) + γγLL (qqLLLL )

,RR = ∑ MMDD,RR − ∑ MMRR,RR

∑ VVRR

γ γ γ γ

Σ Σ

Σ

qqRR = Φbc � ffNN +

1

2

′

γγffNNγγ + γγffDDffNNqq �

Nγ

γ

qqRR

σσvv, ,RR

≥ 1.0
139

C.2.2 Step 7—Conduct Internal Stability Analysis

C.2.2.1 Vertical Capacity

It is recommended that the ultimate capacity be found empirically, if possible. A resistance factor 

for capacity ( cap) of 0.45 should be applied to the nominal vertical capacity (qn) to account for 

uncertainty. This resistance factor value is based on fitting to the ASD method.

C.2.2.1.1 Empirical Method: The factored applied pressure on the GRS abutment (Vapplied,f,emp) 

is equal to the sum of the vertical pressures on the bridge bearing area multiplied by their respective

load factors (see equation 79). The vertical pressures include the bridge DL (qb) and LL (qLL).

(79)

Where DC MAX and LL are load factors found from table 16 and table 17. The factored applied 

pressure must be less than or equal to the factored vertical capacity (see equation 80). The 

resistance factor ( cap) is equal to 0.45. 

(80)

C.2.2.1.2 Analytical Method: As an alternative, the load-carrying capacity of a GRS abutment 

can be evaluated using an analytical formula, referred to as the soil-geosynthetic composite 

capacity equation.

(11)

The nominal ultimate load-carrying capacity (qn,an) of a GRS wall constructed with a granular backfill

can be determined by the soil-geosynthetic composite capacity equation, shown in equation 81.

(11)

(81)

Where Sv is the reinforcement spacing, dmax is the maximum grain size of the reinforced backfill, 

Tf is the ultimate strength of the reinforcement, and Kpr is the coefficient of passive earth pressure

determined using equation 26.

The factored applied pressure on the GRS mass (Vapplied,f) is equal to the sum of the vertical forces

multiplied by their respective load factors (see equation 82). This includes the bridge DL (qb) and 

LL (qLL). The DL due to the road base (qrb) and the LL due to the approach pavement (qt) are

located behind the bearing area and are therefore not included in capacity calculations related to 

the bridge superstructure.

(82)

Where DC MAX and LL are load factors found from table 16 and table 17. The factored applied 

pressure must be less than the factored ultimate capacity (see equation 83).The resistance factor 

( cap) is equal to 0.45.

(83)

Φ

VVaa ,ff = γγDDDD MMMMMM qq + γγLLLLqqLLLL

γ γ

Φ

Ф (qq , eeee )

VVaa ,ff

≥ 1.0

qq ,aa = �0.7

� vv

6 xx

� TTff

VV

�KK

VVaa ,ff = γγDDDD MMMMMM qq + γγLLLLqqLLLL

γ γ

Φ

Ф (qq ,aa )

VVaa ,ff

≥ 1.0
140

C.2.2.2 Deformations

The method to estimate both vertical and lateral deformations is not dependent on the design 

code chosen (ASD or LRFD). Therefore, refer to section 4.4.7.2 to estimate deformations.

C.2.2.3 Required Reinforcement Strength

The factored required reinforcement strength in the direction perpendicular to the wall face (Treq,f)

can be determined analytically by equation 84. The required factored reinforcement strength should

be calculated at each layer of reinforcement to ensure adequate strength throughout the GRS 

abutment. For the serviceability check (comparing the required strength to the reinforcement 

strength at 2 percent strain), the unfactored required reinforcement strength should be used (see 

equation 85). 

(84)

(85)

Where Sv is the reinforcement spacing, dmax is the maximum grain size of backfill, h,f is the total 

factored lateral stress within the GRS abutment at a given depth and location (see equation 86), 

and h is the total unfactored lateral stress within the GRS abutment at a given depth and location 

(equation 32).

(86)

Where h,W,f is the factored lateral earth pressure using Rankine’s active stress condition (see 

equation 87), h,bridge,f is the factored lateral pressure due to the equivalent bridge load (see 

eqautaion 88), h,rb,f is the factored lateral pressure due to the road base DL (see equation 89), 

and h,t,f is the factored lateral pressure due to the roadway LL (see equation 90).

(87)

(88)

(89)

(90)

Where DC MAX, ES MAX, LS, and LL are load factors found from table 16 and table 17; qb, qrb, 

qt, and qLL are the bridge DL, road base DL, roadway LL and bridge LL, respectively; Kar is the 

active earth pressure coefficient for the reinforced fill; and b and b are the angles shown in 

figure 15, found using equation 91 and equation 92, respectively.

TTrr ,ff = � σσh,ff

0.7

� vv

6 �

� vv

TTrr = � σσh

0.7

� vv

6 �

� vv

σ

σ

σσh,ff = σσh,WW,ff + σσh, bb ,ff + σσh,rr ,ff + σσh,tt,ff

σ

σ

σ

σ

σσh,WW,ff = γγEE MMMMMM (γγrrzzKKaa )

σσh, bb ,ff = (γγDDDD MMMMMM qq + γγLLLLqqLLLL) − (γγEE MMMMMM qqrr + γγLL qqtt)

ππ

[αα + sin⁡(αα )cos⁡(αα + 2ββ )]KKaa

σσh,rr ,ff = γγEE MMMMMM qqrr KKaa

σσh,tt,ff = γγLL qqttKKaa

γ γ γ γ

α β
141

(91)

(92)

For abutments, a minimum wide width tensile strength (Tf) of 4,800 lb/ft is required. A resistance 

factor for reinforcement strength ( reinf) of 0.4 should be applied to the ultimate strength (Tf) to 

determine the factored reinforcement strength (Tf,f). The factored required reinforcement strength 

(Treq,f) must be less than this factored reinforcement strength (Tf,f), as shown in equation 93.

(93)

Since geosynthetic reinforcements of similar strength can have rather different load-deformation 

relationships depending on their material, it is important that the nominal (unfactored) Treq be less

than the strength at 2 percent reinforcement strain. The strength of the reinforcement at 2 percent 

( ) is often given by the geosynthetic manufacturer. If the unfactored Treq is greater than

, either a different geosynthetic must be chosen or the ultimate strength must be increased. 

C.3 DESIGN EXAMPLE (LRFD): BOWMAN ROAD BRIDGE, DEFIANCE COUNTY, OH

In this section, the equations formatted for the LRFD method in section C.2 are demonstrated.

For additional details and discussion in support of these calculations, see the corresponding 

sections of the design example in the ASD format contained in section 4.5.

C.3.6 Step 6—Conduct an External Stability Analysis

C.3.6.1 Direct Sliding

The driving forces on the GRS abutment are comprised of the lateral forces due to the retained 

backfill, the road base and the traffic surcharge. The force due to the backfill is calculated in 

equation 94.

(94)

The lateral force due to the road base and traffic surcharges are calculated in equation 95 and 

equation 96.

(95)

(96)

αα = tt −1 �

2zz

� − ββ

ββ = tt −1 �

−

2zz

�

Φ

TTff,ff

TTrr ,ff

= Фrr (TTff)

TTrr ,ff

= 0.4(TTff)

TTrr ,ff

≥ 1.0

T@ε=2%

T@ε=2%

FF = 1

2 γγ KKaaaa HH2 = 1

2 (120)(0.361)(15.58)2 = 5258 lb/ft

FFrr = qqrr KKaaaa HH = 385(0.361)(15.58) = 2165 lb/ft

FFtt = qqttKKaaaa HH = 298(0.361)(15.58) = 1676 lb/ft
142

The total factored driving force (FR) is then calculated in equation 97. The load factors are 

determined using table 16 and table 17.

(97)

The factored resisting force (RR) is calculated according to equation 71, where T is the resistance

factor for shear resistance (equal to 1.0 for sliding).

(9) The total resisting weight (Wt,R) includes 

the weight of GRS plus the weight of bridge beam plus the weight of the road base over the GRS 

abutment, as shown in equation 72. Since the LLs are not permanent, they cannot be counted as a 

resisting force. The total resisting weight is calculated in equation 98.

(98)

The friction force ( ) is equal to tan crit. The interface friction angle between the reinforced fill 

and the geotextile was measured at 39 degrees by conducting an interface direct shear test. The 

factored resisting force is calculated in equation 99.

(99)

This resisting force (15,239 lb/ft) is greater than the driving force (14,068 lb/ft); therefore, direct 

sliding is not an issue.

C.3.6.2 Bearing Capacity

Before calculating the factored applied vertical bearing pressure, the factored eccentricity of the 

resulting force at the base of the wall must be calculated. 

The moments are calculated around the center of the base of the RSF. The driving moments 

(calculated as a counterclockwise moment) include the lateral force due to the retained backfill, 

the road base DL, and the roadway LL and are calculated in equation 100.

(100)

The resisting moments (calculated as a clockwise moment) include the vertical force due to the 

bridge and road base DLs and the bridge and roadway LLs. The resisting moments are calculated 

in equation 101. The weight of the GRS abutment is also included as a resisting moment.

(101)

FFRR = γγEE MMMMMM FF + γγEE MMMMMM FFrr + γγLL FFtt = 1.5(5258) + 1.5(2165) + 1.75(1676)

= 14068 lb/ft

WWtt,RR = 1(9257) + 0.9[2600(4)] + 0.75[385(0.7)] = 18819 lb/ft

μ

RRRR = Φττ �WWtt,RRμμ� = 1(18819)tan(39) = 15239 lb/ft

∑ MMDD = 1.5 �5258 �

15.58

3 �� + 1.5 �2165 �

15.58

2 �� + 1.75 �1676 �

15.58

2 �� = 89106 ff /ff

�MMRR = (1.25 ∗ 2600 ∗ 4 + 1.75 ∗ 1400 ∗ 4) ��

4

2 + 0.67� − �

7.5

2 − 1.5 − 0.64�� �� � � ��

+ (1.75 ∗ 298 ∗ 0.7 + 1.5 ∗ 385 ∗ 0.7) �

7.5

2 − 0.72 � + (1.35 ∗ 9059 �

7.5

2 − 5.42 �)

= 39625 ff /ff
143

The total vertical load is equal to the sum of the weight of the GRS abutment, the weight of the 

RSF, and the load due to the DLs (bridge and road base) and the LLs (bridge and roadway). The 

total vertical load is calculated in equation 102.

(102)

The eccentricity of the resulting force at the base of the RSF is then calculated in equation 103.

(103)

The vertical pressure is a result of the weight of the GRS mass, the bridge seat load, and the 

traffic surcharge and is calculated in equation 104.

(104)

The nominal bearing capacity is then calculated in equation 105. The degrees bearing capacity 

factors (Nc and ) were found using table 4 for the foundation friction angle of 0.

(105)

Applying the resistance factor of 0.65 to the nominal bearing capacity, the factored bearing

capacity (qR) is equal to 13,481 psf. The factored bearing capacity is greater than the factored 

vertical pressure (7,862 psf), so bearing capacity is not an issue in this case.

C.3.6.3 Global Stability

Global and compound stability was checked using the software program ReSSA. Global stability 

is not a problem.

C.3.7 Step 7—Conduct Internal Stability Analysis

C.3.7.1 Vertical Capacity

The ultimate capacity of a GRS abutment can be determined using two different methods: 

empirical or analytical. 

C.3.7.1.1 Empirical Method: The empirical method uses the results of a performance test on a 

GRS composite material identical (or very similar) to that used in the field. The ultimate vertical 

capacity is found by looking at the applicable stress-strain curve from the performance test for a 

�VVRR = 1.35(9257) + 1.35(1575) + 1.25(768) + 1.75 ∗ 298(0.7) + 1.5 ∗ 385(0.7)

+ 1.25 ∗ 2600(4) + 1.75 ∗ 1400(4) = 39153 /ff

,RR = ∑ MMDD − ∑ MMRR

∑ VVRR

= 89106 − 36296

39153 = 1.26 ft

σσvv, , = ∑ VVRR

− 2 ,RR

= 39153

7.5 − 2(1.26) = 7862 /ff 2

Nγ

qq = ffNN +

1

2

′

γγffNNγγ + γγffDDffNNqq

= 4000(5.14) +

1

2 (7.5 − 2 ∗ 1.16)(120)(0) + 120(1.5)(1.0)

= 20740 psf
144

vertical strain of 5 percent (see figure 32). The capacity (qn,emp) is equal to 26 ksf. Note that the 

linear line extension shown in figure 32 is to project the capacity to 5 percent.

The factored applied pressure on the GRS mass (Vapplied,f) is found in equation 106:

(106)

The ratio of the factored vertical capacity (with a resistance factor of 0.45) to the factored applied

pressure (Vapplied,f) must be greater than or equal to 1, as shown in equation 107.

(107)

C.3.7.1.2 Analytical Method: Alternatively, the nominal capacity is found analytically for a

granular backfill, where Sv is equal to 8 inches, dmax is equal to 0.5 inches, Tf is equal to 4,800 

lb/ft, and r is equal to 48 degrees. Note that although the spacing under the bridge bearing area 

is 4 inches, 8 inches was chosen to be conservative in the calculation for the entire mass.

(108)

Where the coefficient of passive earth pressure for the reinforced fill (Kpr) is found in equation 109.

(109)

The factored applied pressure on the GRS mass (Vapplied,f) is found in equation 110.

(110)

The ratio of the factored ultimate capacity (with a resistance factor of 0.45) to the factored 

applied pressure (Vapplied,f) must be less than 1, as shown in equation 111. 

(111)

C.3.7.2 Deformations

C.3.7.2.1 Vertical Deformation: The vertical strain is estimated by using figure 33 for the total 

bridge load (qb) of 2,600 lb/ft2. The vertical strain is, therefore, about 0.3 percent—under the

tolerable limit of 0.5 percent. Note that the road base surcharge is not included because it does 

not act over the same location.

VVaa ,ff = γγDDDD MMMMMM qq + γγLLLLqqLLLL = 1.25(2600) + 1.75(1400) = 5700

lb

ft2

Ф (qq , eeee )

VVaa ,ff

= 0.45(26000)

5700 = 2.1 ≥ 1.0

qq ,aa = �0.7

� vv

6 � TTff

VV

�KK = �0.7

� 8

6(0.5)

� 4800

0.67 � 6.786 = 18781 lb/ft2

KK = 1 + ssssssφrr

1 − ssssssφrr

= 1 + ssssss48

1 − ssssss48 = 6.786

VVaa ,ff = γγDDDD MMMMMM qq + γγLLLLqqLLLL = 1.25(2600) + 1.75(1400) = 5700

lb

ft2

Ф ,aa (qq ,aa )

VVaa ,ff

= 0.45(18781)

5700 = 1.5 ≥ 1.0
145

The vertical deformation is the product of the vertical strain and the height of the GRS mass and 

is calculated in equation 112.

(112)

C.3.7.2.2 Lateral Deformation: The lateral strain and deformation are found in equation 113

and equation 114.

(113)

(114)

C.3.7.3 Required Reinforcement Strength

The strength of the reinforcement used at Bowman Road Bridge is 4,800 lb/ft. Applying a resistance

factor ( reinf) of 0.4, the factored reinforcement strength (Tf,f) is 1,920 lb/ft. According to the 

manufacturer, is equal to 1,370 lb/ft. The maximum required reinforcement strength is 

found as a function of depth, as shown in equation 115. 

(115)

The factored lateral stress ( h,f) is a combination of the factored lateral stresses due to the road base

DL ( h,rb,f), the roadway LL ( h,t,f), the GRS reinforced soil ( h,W,f), and an equivalent bridge

load ( h,bridge,f). To simplify calculations, the roadway LL and road base DL can be extended across

the abutment. The vertical components of these loads are then subtracted from the bridge DL and 

LL, giving an equivalent bridge load. The lateral stresses due to the equivalent bridge load are then

calculated according to Boussinesq theory. The lateral stress is calculated for each depth of 

interest (each layer of reinforcement). All lateral stresses are calculated in table 18.

An example calculation for the required reinforcement strength at a depth (z) of 5.3 ft (the eighth 

reinforcement layer from the top) is shown in equation 116. First, the lateral pressure must be 

found. Remember, the location of interest is directly under the centerline of the bridge load 

(where x = 0.5b = 0.5(4ft) = 2 ft).

(116)

Where the lateral pressure is found using equation 117 through equation 120.

(117)

DDvv = εεvvHH = 0.003(15.58) = 0.047 ft

εεLL = 2εεvv = 2(0.3%) = 0.6%

DDLL = 2DDvv

HH ( + aa ) = 2(0.047)15.58 (4 + 0.67) = 0.028 ft

T@ε=2%

TTrr ,ff = �

σσh,ff − σσ

0.7

� vv

6 �

� vv

σ

σ σ σ

σ

σσh,ff = σσh,WW,ff + σσh, bb ,ff + σσh,rr ,ff + σσh,tt,ff = 117 + 297 + 85 + 77 = 576

lb

ft2

σσh,WW,ff = γγEE MMMMMM (γγrrzzKKaa ) = 1.35 �110(5) �

1 − sin⁡(48deg)

1 + sin⁡(48deg)

�� = 117

lb

ft2
146

(118)

(119)

(120)

The values for and are found in equation 121 and 122.

(121)

(122)

The lateral earth pressure and required reinforcement strength should be found along the entire depth

of the wall. The reinforcement spacing (Sv) for the required reinforcement strength calculation is 

4 inches where secondary reinforcement layers are present and 8 inches where there are no secondary

reinforcement layers. The depth at which secondary reinforcement layers are present is determined

by applying the 8-inch reinforcement spacing for the entire height of the GRS mass. The depth at 

which the required reinforcement spacing does not exceed the factored reinforcement capacity of 

1,920 lb/ft is the depth above which 4-inch spacing is required (see table 18). 

σσh, ,ff = (γγDDDD MMMMMM qq + γγLLLLqqLLLL) − (γγEE MMMMMM qqrr + γγLL qqtt)

ππ

[αα + sin⁡(αα )cos⁡(αα + 2ββ )]KKaa

= (1.25 ∗ 2600 + 1.75 ∗ 1400) − (1.5 ∗ 385 + 1.75 ∗ 298)

ππ

[0.72rr

+ sin(0.72rr ) cos(0.72rad + 2 ∗ −0.36rr )]0.147 = 297

lb

ft2

σσh,rr = γγEE MMMMMM qqrr KKaa = 1.5(385)(0.147) = 85

lb

ft2

σσh,tt = γγLL qqttKKaa = 1.75(298)(0.147) = 77

lb

ft2

α β

αα = tt −1 �

2zz

� − ββ = tt −1 � 4

2(5.3)

�—20.7 = 41.3 = 0.72rr

ββ = tt −1 �

−

2zz

� = tt −1 � −4

2(5.3)

� = −20.7 = −0.36 rr
147

Table 18. Depth of bearing bed reinforcement calculations (LRFD).

z 

(ft)

Treq

(lb/ft)

Treq

> Tf,f

Treq

(lb/ft)

0.7 2.5 -1.2 669 85 77 15 845 592 1458 NO 1022 NO

1.3 2.0 -1.0 623 85 77 29 814 566 1406 NO 976 NO

2.0 1.6 -0.8 555 85 77 44 760 519 1312 NO 895 NO

2.7 1.3 -0.6 485 85 77 58 705 470 1217 NO 811 NO

3.3 1.1 -0.5 424 85 77 73 659 428 1136 NO 739 NO

4.0 0.9 -0.5 373 85 77 88 622 395 1074 NO 682 NO

4.7 0.8 -0.4 331 85 77 102 595 370 1027 NO 638 NO

5.3 0.7 -0.4 297 85 77 117 575 350 993 NO 605 NO

6.0 0.6 -0.3 268 85 77 131 562 336 969 NO 580 NO

6.7 0.6 -0.3 245 85 77 146 552 326 953 NO 563 NO

7.3 0.5 -0.3 225 85 77 160 547 319 944 NO 551 NO

8.0 0.5 -0.2 207 85 77 175 544 315 939 NO 543 NO

8.7 0.5 -0.2 192 85 77 190 544 312 939 NO 539 NO

9.3 0.4 -0.2 180 85 77 204 546 312 942 NO 538 NO

10.0 0.4 -0.2 168 85 77 219 549 312 947 NO 539 NO

10.7 0.4 -0.2 158 85 77 233 553 314 955 NO 542 NO

11.3 0.3 -0.2 149 85 77 248 559 317 965 NO 547 NO

12.0 0.3 -0.2 141 85 77 263 566 321 976 NO 554 NO

12.7 0.3 -0.2 134 85 77 277 573 325 989 NO 561 NO

13.3 0.3 -0.1 128 85 77 292 581 330 1003 NO 570 NO

14.0 0.3 -0.1 122 85 77 306 590 336 1018 NO 579 NO

14.7 0.3 -0.1 116 85 77 321 599 342 1034 NO 590 NO

Based on table 18, the required reinforcement strength does not exceed the allowable strength or 

the strength at 2 percent at any reinforcement layer. Therefore, no bearing bed reinforcement is 

needed. However, the minimum requirement is that the bearing bed reinforcement should extend 

through five courses of blocks. In actuality, six courses of block were chosen to extend the 

bearing reinforcement bed in this case (to a depth of 4 ft below the top of the wall). This was 

chosen to be conservative since this was the first bridge built with GRS technology. There 

should, therefore, be no issues with reinforcement strength in the abutment.

C.3.8 Step 8—Implement Design Details

All design details were considered. Since it is a skewed bridge, a bearing area of 3 ft was 

maintained along the length of the face wall. The bearing bed reinforcement schedule was also 

maintained across the abutment face due to the superelevation (see figure 35).

α β

σh,bridge

(psf)

σh,rb

(psf)

σh,t

(psf)

σh,W

(psf)

σh,total,f

(psf)

σh,total

(psf)

Treq

> T@2%

149

APPENDIX D. TYPICAL GRS-IBS WORKING DRAWINGS

D.1 SINGLE-PAGE PLAN SHEET

Figure 100. Illustration. Single-page plan sheet.
150

D.2 ESTIMATED QUANTITIES AND GENERAL NOTES 

Figure 101. Illustration. Estimated quantities and general notes.
151

D.3 PROJECT PLAN AND PROFILE 

Figure 102. Illustration. Project plan and profile.
152

D.4 GRS ABUTMENT DETAILS 

Figure 103. Illustration. Abutment details.
153

D.5 SITE PLAN 

Figure 104. Illustration. Site plan.

155

ACKNOWLEDGEMENTS

The interim implementation manual and synthesis report have been reviewed by technical experts

in various disciplines: geotechnical, structural, hydraulic, maintenance, and pavement engineering.

The authors recognize the efforts and contributions of many members of the FHWA for this work,

including Daniel Alzamora, Scott Anderson, Silas Nichols, Larry Arneson, Kornel Kerenyi, 

Joseph Krolak, Rich Barrows, Brian Collins, and Jorge Pagan.

The authors also recognize the contributions of other technical reviewers on this manual, including 

Jim Collin, Erik Loehr, Paul Macklin, Christopher Meehan, Al Ruckman, Naresh Samtani, and 

Calvin VanBuskirk.

157

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