Research, Development, and Technology

Turner-Fairbank Highway Research Center

6300 Georgetown Pike

McLean, VA 22101-2296

Geosynthetic Reinforced Soil

Integrated Bridge System 

Synthesis Report

Publication No. FHWA-HRT-11-027 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 report.

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 report is the second in a two-part series

and provides the background and other supporting information to substantiate the design method 

of GRS-IBS. The first document is a manual covering the design and construction of GRS-IBS. 

This two-part document series designs GRS as a composite material with known and predictable 

performance and deformations. 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-11-027

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

4. Title and Subtitle

Geosynthetic Reinforced Soil Integrated Bridge System, 

Synthesis Report

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

Washington, DC

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 report is the second in a two-part series to provide engineers with the necessary background knowledge of 

Geosynthetic Reinforced Soil (GRS) technology and its fundamental characteristics as an alternative to other 

construction methods. It supplements the interim implementation manual (FHWA-HRT-11-026), which outlines 

the design and construction of the GRS Integrated Bridge System (IBS). The research behind the proposed design 

method is presented along with case histories to show the performance of in-service GRS-IBS and GRS walls.

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

64

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 BACKGROUND ................................................................................................................2

1.3 COMPOSITE BEHAVIOR ..............................................................................................5

1.3.1 Reinforcement Spacing...............................................................................................6

1.3.2 Reinforcement Strength ..............................................................................................7

1.3.3 Reinforcement Length ................................................................................................8

CHAPTER 2. NOTATION, ABBREVIATIONS, AND TERMINOLOGY...........................11

2.1 NOTATION......................................................................................................................11

2.2 ABBREVIATIONS..........................................................................................................12

2.3 TERMINOLOGY ............................................................................................................13

CHAPTER 3. DESIGN METHODOLOGY FOR GRS-IBS...................................................15

3.1 OVERVIEW OF GRS-IBS DESIGN METHOD..........................................................15

3.2 EXTERNAL STABILITY ..............................................................................................16

3.3 INTERNAL STABILITY................................................................................................16

3.3.1 Ultimate Capacity .....................................................................................................18

3.3.2 Deformations.............................................................................................................21

3.3.3 Required Reinforcement Strength.............................................................................24

CHAPTER 4. CASE HISTORIES .............................................................................................27

4.1 INTRODUCTION............................................................................................................27

4.2 DEFORMATIONS ..........................................................................................................27

4.2.1 Vertical Deformation ................................................................................................27

4.2.2 Lateral Deformation..................................................................................................41

4.3 THERMAL CYCLES......................................................................................................42

APPENDIX A. PULL-OUT TEST RESULTS..........................................................................45

A.1 SETUP..............................................................................................................................46

A.2 PROCEDURE .................................................................................................................47

A.3 TEST RESULT FOR SRW BLOCKS ..........................................................................48

APPENDIX B. PREDICTION DATA FOR ANALYTICAL EQUATIONS.........................51

B.1 INTRODUCTION...........................................................................................................51

B.2 SOIL-GEOSYNTHETIC CAPACITY EQUATION...................................................51

B.3 REQUIRED REINFORCEMENT STRENGTH EQUATION ..................................52

ACKNOWLEDGEMENTS ........................................................................................................55

REFERENCES.............................................................................................................................57
iv

LIST OF FIGURES

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

Figure 2. Photo. MSE inextensible reinforcement—steel strips......................................................3

Figure 3. Photo. MSE inextensible reinforcement—wire mats.......................................................3

Figure 4. Photo. MSE extensible reinforcement (geogrid)..............................................................4

Figure 5. Illustration. Typical wrapped-face GRS structure............................................................4

Figure 6. Photo. Cut-away of GRS mass.........................................................................................5

Figure 7. Graph. General comparison of surcharges on MSE and GRS structures.........................6

Figure 8. Photo. Internally supported GRS structure.......................................................................9

Figure 9. Illustration. Idealized lateral earth pressure at the face of a GRS structure ...................17

Figure 10. Illustration. Bin pressure diagram for GRS structures .................................................17

Figure 11. Graph. Performance test results for different materials................................................19

Figure 12. Graph. Predictive capability of the soil-geosynthetic composite capacity equation.......21

Figure 13. Graph. Design envelope for vertical strain at 8-inch reinforcement spacing...............22

Figure 14. Graph. Deformation estimation from in-service GRS-IBS structures..........................23

Figure 15. Illustration. Lateral deformation of a GRS structure....................................................24

Figure 16. Graph. Predictive capability of the required reinforcement strength equation ............25

Figure 17. Photo. Vine Street Bridge.............................................................................................28

Figure 18. Graph. Vine Street GRS-IBS settlement versus time...................................................29

Figure 19. Photo. Glenburg Road Bridge under flood conditions.................................................29

Figure 20. Graph. Glenburg Road GRS-IBS settlement versus time ............................................30

Figure 21. Photo. Huber Road Bridge ...........................................................................................30

Figure 22. Graph. Huber Road GRS-IBS settlement versus time..................................................31

Figure 23. Photo. Bowman Road Bridge after 4.5 years...............................................................31

Figure 24. Graph. Bowman Road GRS-IBS settlement versus time .............................................32

Figure 25. Photo. Tiffin River Bridge............................................................................................32

Figure 26. Graph. Tiffin River GRS-IBS settlement versus time..................................................33

Figure 27. Graph. Settlement versus log-time to predict creep settlement for the Bowman 

Road Bridge at 100 years...............................................................................................................35

Figure 28. Photo. GRS-IBS tunnel at TFHRC...............................................................................36

Figure 29. Photo. Close-up of GRS-IBS tunnel at TFHRC...........................................................36

Figure 30. Graph. Settlement versus time for TFHRC tunnel .......................................................37

Figure 31. Graph. Settlement versus log-time to predict creep settlement for TFHRC tunnel 

at 100 years....................................................................................................................................38

Figure 32. Photo. GRS abutment behind historic stone abutment.................................................39

Figure 33. Photo. Concrete box bridge on GRS abutment in Mammoth Lake, CA ......................39

Figure 34. Illustration. Cross section of GRS abutment behind historic stone abutment..............39

Figure 35. Photo. Concrete box bridge on GRS abutment in Ouachita wildlife refuge ................40

Figure 36. Illustration. Cross section of GRS abutments in Ouachita wildlife refuge ..................41

Figure 37. Graph. Measured and calculated lateral deformation on the Tiffin River Bridge 

GRS abutment................................................................................................................................42

Figure 38. Illustration. Instrumentation for Tiffin River Bridge ...................................................43

Figure 39. Photo. Pressure cells behind back wall on Tiffin River Bridge ...................................43

Figure 40. Graph. Average lateral pressure on back wall for Tiffin River Bridge ........................44

Figure 41. Photo. TFHRC pier test................................................................................................45
v

Figure 42. Photo. Block pull-out test on GRS wall .......................................................................46

Figure 43. Illustration. Block pull-out test setup ...........................................................................46

Figure 44. Graph. Pull-out test results for an SRW block seven rows from the top......................48

Figure 45. Graph. Pull-out test results for an SRW block 11 rows from the top...........................49

Figure 46. Graph. Pull-out test results in terms of normal force for SRW blocks.........................50
vi

LIST OF TABLES

Table 1. Large-scale GRS tests........................................................................................................7

Table 2. Bridge information summary...........................................................................................28

Table 3. Movement information for five bridges...........................................................................34

Table 4. Vertical settlement and strain information for five bridges.............................................34

Table 5. Summary of in-service GRS abutments...........................................................................40

Table 6. Predicted lateral deformations of five in-service bridges................................................41

Table 7. Prediction data for large-scale tests.................................................................................51

Table 8. Soil-geosynthetic capacity equation validation results....................................................52

Table 9. Required reinforcement strength equation validation results..........................................53
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 synthesis report 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) This synthesis report provides

the background and research behind the recommended design of GRS-IBS outlined in the interim

implementation manual.

(1) In-service performance of GRS-IBS and other GRS applications is 

also discussed through case histories.

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 the 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.

1.2 BACKGROUND

Mark Twain said, “The ancients have stolen all our best ideas.” Reinforced soil technology is not

modern. The ancients used native material such as straw, tree branches, and plant material to 

reinforce the earth. The reinforcement provides tensile resistance to soil that is weak in tension 

but relatively strong in compression and shear. Through soil reinforcement interface bonding, the

reinforcement restrains lateral deformation of the surrounding soil, increases its confinement, reduces

its tendency for dilation, and consequently, increases the stiffness and strength of the soil mass. 

In ancient Babylonia, the technology was used to construct the Aqar Quf ziggurat in Iraq around 

1440 B.C. The stepped pyramid was built using compacted layers of plant material and soil blocks.

The Great Wall of China also used reinforced earth to construct some sections. The fact that 

these structures are still visible today is a tribute to the durability of reinforced soil technology.

Modern reinforced soil technology has evolved into two primary methods for the stabilization of 

earth: mechanically stabilized earth (MSE) and GRS. Today, the predominant method of building

reinforced soil is MSE, established in the early 1960s when Henri Vidal patented Reinforced Earth®.

The method incorporates discrete steel strips embedded within a soil mass. Since then, other types

of reinforcement materials, classified as either inextensible or extensible, have been used to reinforce

earth. Berg et al. defined inextensible reinforcement as a material that deforms considerably less 

than the surrounding soil at failure and extensible reinforcement as a material that deforms as 

much as the surrounding soil.(3) In fact, MSE technology has branched off into two primary 
3

pathways: proprietary structures built with metallic (inextensible) reinforcements and proprietary 

structures built with geosynthetic (extensible) reinforcements.

MSE structures built with inextensible reinforcement such as discrete metallic strips or welded 

wire mats have a unique combination of precast panels, reinforcement, and connection details. 

The vertical spacing of the reinforcement (Sv) is typically about 30 inches, and the typical size of 

the precast panels is about 5 ft high by 5 to 10 ft wide (see figure 2 and figure 3).

Figure 2. Photo. MSE inextensible reinforcement—steel strips.

(3)

Figure 3. Photo. MSE inextensible reinforcement—wire mats.(3)

MSE structures built with extensible reinforcement such as geosynthetics were introduced in the 

mid-1980s when geogrids were used to reinforce or stabilize the fill behind structures constructed 

with concrete modular blocks (see figure 4). Today, these proprietary modular block structures are 

typically built with a unique combination of the block, geogrids, and connection details. The 

vertical spacing of the reinforcement (Sv) is typically 24 inches, or one layer of reinforcement for 

every three courses of 8-inch modular block facing. The facing block has typical dimensions of 

8 inches by 16 inches by 12 inches and a weight of about 80 lb.
4

Figure 4. Photo. MSE extensible reinforcement (geogrid).

The first documented use of alternating layers of geosynthetic and soil, referred to as GRS

technology, was by the U.S. Forest Service in the 1970s.(4) The Forest Service used the technology

to build logging roads on steep mountain terrain. These GRS structures utilized a wrapped face—

the geosynthetic was wrapped up and around the face of the individual soil layers and anchored 

by the overburden of the subsequent layer of soil (see figure 5). Many of these wrapped-face GRS

structures (also called burrito walls) are still in service.

Figure 5. Illustration. Typical wrapped-face GRS structure.

Later, the Colorado Department of Transportation (CDOT) developed a low-cost generic wall system 

using lightweight concrete modular blocks. Rather than securing the blocks to the reinforcement 

with connections, as in MSE technology, the concrete facing blocks were frictionally connected 

to the GRS mass (see figure 6). The interface between the blocks and the geosynthetic provided

enough friction to resist block movement. This method of connection in combination with closely
5

spaced reinforcement layers created a facing system that adjusts to relieve stress without attracting

loads. The FHWA refined the CDOT method to account for vertical load-bearing applications,

resulting in the development of GRS abutments, followed by GRS-IBS (see figure 1).

(5,6)

Figure 6. Photo. Cut-away of GRS mass.

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.

As of 2010, 45 bridges utilizing GRS abutments had been built in the United States. Of these 

bridges, IBS had been employed on 28 bridges, all built over water crossings. The hydraulic 

environments at these locations is such that scour is shallow, making the system feasible for

these sites. Refer to the interim implementation manual for special hydraulic design requirements

before selecting a water crossing for GRS-IBS deployment.(1)

1.3 COMPOSITE BEHAVIOR

GRS abutments built with a reinforcement spacing less than or equal to 12 inches behave as a 

composite mass with predictable behavior. They can be built to economically support a bridge

superstructure bearing directly on the reinforced soil behind the facing block. GRS can be used 

to integrate the superstructure with the approach and substructure to create a jointless bridge

system (GRS-IBS).

Many studies have been conducted on the composite behavior of GRS structures. These

investigations have concluded that existing design methods do not adequately characterize 

the interactive behavior between the soil and the closely spaced reinforcement of a GRS
6

abutment.(3,7) As a result, the interim implementation manual was developed.(1) The manual 

is largely based on the observed composite behavior of GRS and is substantiated by empirical 

evidence of in-service GRS-IBS.

1.3.1 Reinforcement Spacing

A degree of composite behavior results from reinforcement frequency. For larger-spaced reinforced

soil systems, the composite behavior diminishes with increased reinforcement spacing. It is important

to note that the transition into GRS behavior is not dependent solely on reinforcement spacing; the

aggregate size and friction angle are also contributing factors, as explained in chapter 3.

Closer reinforcement spacing creates more soil-geosynthetic interaction. In GRS, the reinforcement 

not only serves to resist tensile forces but also functions to restrain lateral deformation of the soil,

increase lateral confinement of the soil, generate apparent cohesion in a granular fill (while

maintaining all desirable characteristics of granular soil), suppress dilation of the soil, enhance

compaction-induced stresses, increase ductility of the soil mass, and reduce migration of fines, 

depending on the reinforcement type selected. These added benefits develop because of the 

close reinforcement spacing.

The frequency of reinforcement spacing in GRS allows for compaction of the soil directly behind 

the facing, producing the capacity for load bearing at this location. The spacing of the reinforcement

also has a significant impact on the strength and behavior of GRS performance. This is illustrated

in figure 7, which shows the load-carrying capacity of GRS with a typical spacing of 8 inches.(8)

The figure shows that working loads for bridges are about 4 ksf.(9)The ultimate capacity of GRS, 

however, can be as high as 25 ksf. The ultimate capacity of GRS is a function of the reinforcement 

spacing, the reinforcement strength, and the soil conditions, including maximum particle size and 

friction angle.

Figure 7. Graph. General comparison of surcharges on MSE and GRS structures.
7

The ultimate capacity of GRS is influenced more by the reinforcement spacing than by the

reinforcement strength (Tf).(10) This is apparent in the results from large-scale tests, shown in table 1.

The results indicate that the response is not the same at the same Tf /Sv ratio. The strength of the 

GRS mass is 1.5 times higher and the stiffness 1.3 times higher at lower spacing and reinforcement

strength (test 2) than at higher spacing and reinforcement strength (test 3). Reduced spacing at equal

reinforcement strengths (tests 2 and 4) increases the strength of the GRS mass by 2.1 times and

the stiffness by 1.3 times. Increasing the reinforcement strength at equal spacing, however, only

increases the strength of the GRS mass by 1.4 times and the stiffness by 1.0 times (tests 3 and 4).

This shows that spacing has a larger effect than reinforcement strength.

Table 1. Large-scale GRS tests.

(10)

Test

Reinforcement

Confining 

Stress

Ultimate 

Applied 

Vertical Load

Modulus at 

1 Percent 

Vertical Strain

Strain at 

Spacing Strength Ratio Failure

Sv

(inches)

Tf

(lb/ft) Tf/Sv

c

(ksf)

qult

(ksf)

E @ v = 1% 

(ksf)

v

(Percent)

1 – 4,800 – 0.7 16 700 3

2 8 4,800 600 0.7 56 1,300 6.5

3 16 9,600 600 0.7 36.5 1,000 6.1

4 16 4,800 300 0.7 27 970 4

5 8 4,800 600 0.0 40 1,100 6

1.3.2 Reinforcement Strength

Since the reinforcement strength and spacing are not proportional, a new equation for required 

reinforcement strength was needed. The analytical equation shown in equation 1 was developed by

Wu et al. to incorporate the effect of confinement (i.e., facing rigidity, c), reinforcement spacing, 

and aggregate size (dmax).(10) The equation has been validated against numerous large-scale

experiments that were tested to failure (see appendix B). Equation 1 will be discussed in detail

in chapter 3.

(1)

The required reinforcement strength calculated using equation 1 accounts for compaction-induced 

stresses (CIS), which increase the lateral stress in a GRS mass.(10) CIS are locked in and stiffen 

the composite. This increased lateral stress must be overcome in order to initiate movement of 

the GRS mass.

In addition to calculating the required reinforcement strength, a factor of safety (or reduction factor) 

is necessary to reduce the ultimate strength of the reinforcement used in design. In current design,

cumulative reduction factors for geosynthetics made of polypropylene have practically precluded 

their use in permanent GRS structures.(3) Numerous case histories and field observations have 

proven, however, that geotextiles can successfully be implemented in several applications, 

including walls and abutments.(10–13)

σ ε ε

σ

TTrrrrrr = � σσh−σσcc

0.7

� SSvv 6ddmmmmmm �

� SSvv
8

Because a GRS structure is a composite mass, the use of cumulative reduction factors for the

long-term strength of the reinforcement by itself is unnecessary. A single factor of safety of 3.5 

(or resistance factor of 0.4) for ultimate reinforcement strength should be used, which accounts 

for long-term degradation (creep, durability, and installation damage). This recommended value 

is derived from the cumulative long-term reduction factors for a GRS mass in conjunction with 

an overall uncertainty factor of 2.0.(14,10) This factor of safety is based on the results of several tests

conducted on different reinforcement materials within soil, including accelerated creep tests.

(15)

Note that creep deformation of a GRS wall is the result of soil-geosynthetic interaction. If the backfill

has a tendency to creep faster than the geosynthetic reinforcement, the creep rate of the geosynthetic

reinforcement will accelerate. Conversely, if the backfill has a tendency to creep slower than the 

geosynthetic reinforcement, the creep rate of the geosynthetic reinforcement will become smaller.

For a GRS wall with a well-compacted granular backfill, the time-dependent deformation will be 

very small, and the rate of deformation will typically decrease rapidly with time (the geosynthetic

cannot creep by itself). This means that creep will cease soon after construction. Moreover, the 

tensile forces induced in geosynthetic reinforcement at working stresses are typically very small 

due to stress redistribution. The very small tensile forces also contribute to very small creep 

deformation. GRS tests have shown that the soil and reinforcement strain together because the 

lower spacing confines the soil.(14)

For the recommended granular fill, damage to the reinforcement is not a concern.(1) If large

aggregate particles (greater than 3 inches in diameter) are used, however, considerable damage

to the reinforcement may occur. This would require reevaluation of the combined effects and 

may necessitate the use of a heavier reinforcement with a greater tensile strength. Such a situation 

is not discussed in this report.

1.3.3 Reinforcement Length

The base-to-height (B/H) ratio can be reduced to as low as 0.3 as long as external stability is 

satisfied.(16,17) This is because a GRS mass is freestanding and internally stable. Internally 

supported systems stabilize a soil mass by the inclusion of the reinforcement alone.(10) Figure 8

shows a stable, internally supported GRS mass without a facing.
9

Figure 8. Photo. Internally supported GRS structure.

The facing elements of a GRS abutment are not required for structural support and do not carry 

any appreciable load. GRS facing blocks are primarily a construction aid to provide a form for 

each lift of compacted fill, a protective barrier, and a façade for aesthetic purposes. To ensure 

that the face is not loaded, the superstructure is placed with a certain setback and clear space as 

recommended in the interim implementation manual.

(1)

11

CHAPTER 2. NOTATION, ABBREVIATIONS, AND TERMINOLOGY

2.1 NOTATION

Unit weight of soil [F/L3

]

fb Bulk unit weight of facing block [F/L3

]

Friction angle between the geosynthetic and the facing block [rad]

3 Change in lateral pressure due to the reinforcement [F/L2

]

S Differential bridge settlement [L]

Sabut Differential abutment settlement [L]

Vface Volume gained at the face of the GRS mass [L3]

Vtop Volume lost at the top of the GRS mass [L3]

L Lateral strain

V Vertical strain

c Lateral confining pressure [F/L2

]

h Lateral pressure [F/L2

]

h,bin Lateral pressure at the face due to bin pressure [F/L2

]

v Vertical earth pressure [F/L2

]

Soil friction angle [deg]

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

b Bearing width for bridge; beam seat [L]

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

c Cohesion [F/L2]

dmax Maximum grain size [L]

Df Depth of facing block unit [L]

DL Maximum lateral displacement [L]

DV Vertical settlement in the GRS mass [L]

E Modulus of GRS composite [F/L2

]

Fbin Thrust force found from bin theory [F/L]

γ

γ

δ

Δσ

Δ

Δ

Δ

Δ

ε

ε

σ

σ

σ

σ
12

H Height of GRS abutment [L]

Ka Coefficient of active earth pressure

Kar Coefficient of active earth pressure for the reinforced backfill

Kpr Coefficient of passive earth pressure for the reinforced backfill

L Length of the wall [L]

qb Equivalent superstructure DL pressure [F/L2]

qcalc Calculated ultimate capacity [F/L2

]

qmeasured Measured ultimate capacity [F/L2]

qrupture Measured vertical capacity at reinforcement rupture [F/L2]

qult Ultimate applied vertical load [F/L2]

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

]

qult,an,c Ultimate load-carrying capacity of GRS using the analytical method with 

cohesion [F/L2

]

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

Sv Reinforcement spacing [L]

T Reinforcement strength [F/L]

Tactual Actual reinforcement strength at rupture [F/L]

Tcalc Calculated reinforcement strength [F/L]

Tf Ultimate reinforcement strength [F/L]

Treq Required reinforcement strength [F/L]

Treq,c Required reinforcement strength including effect of cohesion [F/L]

w Factor accounting for reinforcement spacing and aggregate size

2.2 ABBREVIATIONS

AASHTO American Association of State and Highway Transportation Officials

ASD Allowable Stress Design

CDOT Colorado Department of Transportation

CIP Cast-in-place

CIS Compaction-induced stresses
13

CMU Concrete masonry unit

COB Center of bearing

EDM Electronic distance measurement

FHWA Federal Highway Administration 

GRS Geosynthetic Reinforced Soil

GSGC Generic Soil-Geosynthetic Composite

IBS Integrated Bridge System

LRFD Load and Resistance Factor Design

MSE Mechanically stabilized earth

NCHRP National Cooperative Highway Research Program

RSF Reinforced soil foundation

SRW Segmental retaining wall

TFHRC Turner-Fairbank Highway Research Center

2.3 TERMINOLOGY

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). 
14

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 live loads. 

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 

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.
15

CHAPTER 3. DESIGN METHODOLOGY FOR GRS-IBS

3.1 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. While GRS technology can provide solutions in a variety of applications

and under certain extreme conditions, the design method described in the interim implementation 

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

spans, and design loads.(1)

The design methods 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 

longer 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. 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 these

limits will produce safe, durable systems. 

This section focuses on stability within the design method of GRS-IBS.

(1) The main distinction 

with the GRS-IBS design method is the evaluation of internal stability for the GRS abutment,

which is different from other reinforced soil systems. External stability in this design method is 

largely unchanged from other abutment wall systems. However, overturning, or limiting

eccentricity, is not a failure mode for fully constructed GRS-IBS.

Two design philosophies are presented in the interim implementation manual: Allowable Stress 

Design (ASD) and American Association of State and Highway Transportation Officials (AASHTO)

Load and Resistance Factor Design (LRFD).(1) It is FHWA policy that design for Federal-aid funded

projects be conducted using the LRFD methodology. 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 LRFD-based methods. After sufficient data are

produced and collected as a result of this technology deployment and other efforts, a thorough 

statistical analysis will be performed to appropriately produce LRFD specifications for the 

design of GRS-IBS.
16

3.2 EXTERNAL STABILITY

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).

3.3 INTERNAL STABILITY

Internal stability for a GRS abutment consists of evaluating ultimate capacity, deformations, and 

required reinforcement strength. There are two approaches to internal stability: empirical and 

analytical. Empirically, an engineer can predict the ultimate capacity and the deformations of a 

GRS abutment. Analytically, an engineer can estimate the ultimate capacity and the required 

reinforcement strength. 

Connection strength and pullout are not evaluated in the design of the GRS-IBS as they are with 

other reinforced soil systems. The design of GRS structures assumes a relatively constant earth 

pressure with depth at the wall face (see figure 9). This method takes into account the tensile forces

in the reinforcement, which counteract the classical lateral earth pressure distribution. This occurs

because the reinforcement, not the wall face, acts to restrain lateral deformation of the soil. 
17

Figure 9. Illustration. Idealized lateral earth pressure at the face of a GRS structure.

Wu et al. proposed that the lateral pressure distribution between reinforcements is based on the

concept of bin pressure.(11) In an idealized bin pressure diagram, the pressure is zero at the depth 

of any reinforcement layer within a bin. The lateral earth pressure increases linearly with depth 

before decreasing to zero at the next reinforcement layer. Because reinforcement may deform 

slightly and the interface between the soil and reinforcement may not be perfectly bonded, the 

bin pressure diagram shown in figure 10 was developed.(11) The bin pressure is not a function of 

wall height. Instead, the bin pressure is only a function of reinforcement spacing and the strength 

parameters of the soil (see equation 2).

Figure 10. Illustration. Bin pressure diagram for GRS structures.

(11)
18

(2)

The thrust force against the wall face calculated using equation 2 is lower than that using the theory

of active earth pressure. This eliminates the need for mechanical connections between the wall face 

and the reinforcement. A frictional connection, recommended in GRS design, is sufficient to 

prevent connection failure. The facing blocks sit directly on the reinforcing geosynthetic and are 

held in place purely by the friction between the reinforcement and the concrete block. This is an

adequate connection because the GRS is internally supported by the closely spaced reinforcement

and does not need the facing block to resist lateral pressures. As such, connection strength is not 

an internal stability problem in GRS design. Nevertheless, the capacity of this simple method of 

connection is reported in appendix A.

In a GRS structure, there is no thrust on the wall face due to surcharge or bridge loads. This is 

because the GRS mass is internally supported by the closely spaced reinforcement layers of the

bearing reinforcement bed. Because the facing elements (e.g., concrete masonry unit (CMU) blocks)

are frictionally connected and the bulk unit weight of the facing is smaller or about the same as that

of the GRS mass, the reinforcement, soil, and facing elements all strain together. The interaction 

between the reinforcement and the soil is such that the soil is strong in compression while the

reinforcement provides the tensile capacity, creating the composite mass. Any thrust that occurs 

due to the surcharge and bridge loads is absorbed to some extent by the reinforcement and does 

not transmit to the face.

3.3.1 Ultimate Capacity

The ultimate vertical capacity of a GRS mass 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 empirically if the reinforced fill is different

from the reported performance test in this report. 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.

3.3.1.1 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 test mass strains 5 percent 

vertically. Several performance tests have been conducted with different materials, as shown in 

figure 11. In all examples, the facing elements were frictionally connected to the GRS mass.

FFbbbbbb = 0.72γγKK SSvv

2
19

Figure 11. Graph. Performance test results for different materials.

Note that three of the materials in figure 11 have cohesion (c). Relatively clean granular fills 

(sands, gravels, and rock fills) share two common characteristics in terms of Mohr-Coulomb 

strength parameters: (1) no cohesion and (2) a curved failure envelope over a wide range of 

confining pressure. Depending on how the strength parameters are evaluated, one may come up 

with a set of c and values for a granular soil. The cohesion should be ignored in design if it 

will not be there during the service life of the structure. Cohesion may be apparent (due to

capillary tension in a moist soil that will vanish when wetted or dry) or the small amount of 

clay binder may disappear when the granular soil becomes submerged for a while. If this is the 

case, refer to the analytical method that ignores the effect of cohesion.

3.3.1.2 Analytical Method

The load-carrying capacity of a GRS wall and abutment (qult,an,c) can be evaluated using an analytical

formula (see equation 3).

(10) The analytical formula was originally developed for GRS walls, but 

it is also applicable to GRS abutments. Note that the analytical method assumes that the backfill 

satisfies the criteria outlined in the interim implementation manual.

(1)

(3)

Where c is the lateral confining pressure, 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 

uuuuuu, ,cc = �σσcc + 0.7

� SSvv

6ddmmmmmm � TTff

SSVV

�KKpp + 2cc�KKpp

σ
20

the coefficient of passive earth pressure for the reinforced fill. For a GRS wall with dry-stacked 

modular block facing, the value of c can be estimated according to equation 4. 

(4)

Where fb is the bulk unit weight of the facing block, Df is the depth of the facing block unit (in 

the direction perpendicular to the wall face), and is the friction angle between the geosynthetic 

reinforcement and the top or bottom surface of the facing block.

The ultimate capacity (qult,an,c) was determined using Bell’s relation for lateral earth pressure, 

shown in equation 5.(18) The equation was then rearranged to solve for the vertical pressure ( v). 

(5)

Where h is the lateral earth pressure (see equation 6), v is the vertical pressure, Ka is the active 

earth pressure coefficient, and c is the cohesion.

(6)

Where c is the lateral confining pressure (equation 4) and 3 is the change in lateral pressure 

due to the reinforcement (see equation 7).

(7)

Where w is a factor accounting for the effect of reinforcement spacing and aggregate size (see 

equation 8), T is the reinforcement strength, and Sv is the reinforcement spacing. 

(8)

Where Sv is the reinforcement spacing and dmax is the maximum aggregate size. The w factor was 

determined based on several large-scale GRS experiments.(10) Inputting the ultimate strength of 

the reinforcement (Tf) for the reinforcement strength (T) in equation 7 and solving equation 5 for 

the vertical pressure ( v) will yield the ultimate capacity of the GRS abutment (equation 3).

Figure 12 shows the predictive capability of equation 3, where the calculated ultimate load-carrying 

capacity is compared to measured values for a number of full-scale experiments and in-service 

GRS structures. The complete set of data, showing the range of fills, reinforcement strength, and 

spacing, can be found in appendix B. Note that equation 3 should not be used in design. 

σ

σσcc = γγff DDff tt

γ

δ

σ

σσh = σσvvKK − 2cc�KK

σ σ

σσh = σσcc + ∆σσ3

σ Δσ

∆σσ3 = ww

TT

SSvv

ww = 0.7

SSvv

6ddmmmmmm

σ
21

Figure 12. Graph. Predictive capability of the soil-geosynthetic composite capacity equation.

To estimate the ultimate capacity used in design, equation 3 is modified to neglect confining 

stress ( c) and cohesion (c) and is referred to as the soil-geosynthetic composite capacity equation

(see equation 9).

(1,10) The magnitude of c is typically very small and should conservatively be 

assumed to equal zero. The effect of cohesion is also removed in design as it cannot be counted 

on in the long-term strength of the GRS abutment.

(9)

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.

3.3.2 Deformations

The vertical deformation involves empirically finding the strain from an applicable performance 

test curve. The lateral strain is then analytically found assuming the theory of zero volume change.

σ

σ

uuuuuu, = �0.7

� SSvv

6ddmmmmmm � TTff

SSVV

�KKpp
22

3.3.2.1 Vertical

The vertical strain of the GRS mass is found from the intersection of the applied vertical stress due

to the dead load (qb) and the stress-strain curve found from a performance test (see figure 13). The

vertical strain should be less than about 0.5 percent. 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 with a granular fill, the majority of settlement within the GRS mass will 

occur immediately after the placement of dead loads (qb) and before the bridge is opened to traffic. 

Figure 13. Graph. Design envelope for vertical strain at 8-inch reinforcement spacing.

The settlement of the underlying foundation soils is determined separately using classic soil

mechanics theory for immediate (elastic) or consolidation settlement. Factors such as excavation 

and the RSF should be taken into account, because 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.

(19)

Figure 14 shows the results of the previously described method for five in-service GRS abutment 

walls in Defiance County, OH. The measured vertical strain at the applied dead loads fits well 

with the curve. More information on these five structures is given in chapter 4.
23

Figure 14. Graph. Deformation estimation from in-service GRS-IBS structures.

3.3.2.2 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 mass and represents a worst-case 

scenario. The maximum lateral displacement of the abutment face wall can be estimated using 

the following procedure.

(20)

Assuming a uniformly loaded vertical wall in plane strain conditions (such as with a strip footing),

the maximum lateral displacement (DL) occurs in one direction along one wall face (see figure 15).

It is found by assuming the volume lost at the top ( Vtop) due to settlement is equal to the volume

gained at the face ( Vface) due to lateral deformation (see equation 10).

(10)

Where bq,vol is the width of the load along the top of the wall including the setback, L is the 

unit length of the wall, Dv is the vertical settlement in the GRS mass, and H is the wall height. 

Equation 10 can then be solved for the maximum lateral displacement (see equation 11). The 

lateral strain ( L) is then found using equation 12. The lateral strain should be limited to around 

1 percent.

Δ

Δ

∆VVtt = bb ,vv LLDDvv = ∆VVff = 1

2

HH DDLL

ε
24

(11)

(12)

Where V is the vertical strain at the top of the wall. Equation 10 comes from the assumptions 

of a triangular lateral deformation and a uniform vertical deformation. 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 at the top. 

Figure 15. Illustration. Lateral deformation of a GRS structure.

3.3.3 Required Reinforcement Strength

The required reinforcement strength in the direction perpendicular to the wall face (Treq,c) can be 

determined analytically using equation 13. Note that equation 13 is equation 3 rearranged to

solve for the required reinforcement strength to prevent failure. In the design, the effect of

confinement ( c) and cohesion (c) are ignored to find the required reinforcement strength (Treq) 

to ensure long-term strength and durability (see equation 14).

(13)

(14)

Where c is the lateral confining pressure of GRS mass (equation 4), assumed to equal zero for a 

CMU block face wall; Sv is the reinforcement spacing; dmax is the maximum grain size of backfill; 

Kar is the active earth pressure coefficient for the reinforced fill; and h is the total lateral stress 

within the GRS mass at a given depth and location.

DDLL = 2bb ,vv DDvv

HH 

εεLL = DDLL

bb ,vv

= 2DDvv

HH = 2εεvv 

ε

σ

TTrrrrrr ,cc = �

σσh−σσcc−2cc�KK

0.7

� SSvv 6ddmmmmmm � � SSvv

TTrrrrrr = � σσh

0.7

� SSvv 6ddmmmmmm �

� SSvv

σ

σ
25

Equation 13 was validated by comparing it to the measured reinforcement strength at failure, as 

shown in figure 16. The required reinforcement strength at failure matches well with the actual 

values of strength at rupture. The input data for each case in figure 16 are shown in appendix B. 

Figure 16. Graph. Predictive capability of the required reinforcement strength equation.

27

CHAPTER 4. CASE HISTORIES

4.1 INTRODUCTION

This chapter presents the performance of in-service GRS abutments, specifically, bridges built with

GRS-IBS between 2005 and 2010. The performance of the first series of GRS-IBS indicates that 

the method has considerable potential to advance the state of the practice. Included in this chapter is

a performance overview of several other bridges built with GRS abutments in the private and public

sectors within the United States and Canada. The chapter also lists various case histories of many 

GRS walls built during the past 20 years to validate the long-term performance of these structures.

The elimination of the joint at the bridge ends helps improve the durability of both the beam ends 

and the bearings in conventional bridge systems. 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, hopefully as a long-term solution to the bump at the end of the bridge. 

Reducing the bump at the end of bridge will improve the bridge’s overall performance and 

serviceability. The bump not only creates a chronic maintenance issue but also induces an

amplification of live load on the superstructure, creating fatigue on bridge elements.

4.2 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 conventional bridges structurally and functionally in addition to eliminating the bump 

at the end of the bridge that often results from conventional construction. At the time of this

report, the suppression of the bump had been maintained for all in-service GRS-IBS bridges. The 

first bridge constructed with the IBS method, Bowman Road Bridge, has been in service since

2005 without the development of a crack in the asphalt layer from the road to the bridge.

Of the nearly 30 bridges built with GRS-IBS, 5 bridges have current data from monitoring of

settlement at the abutments. The total settlement and deformation (and, thus, vertical strain) of the

GRS abutment due the bridge load is recorded using either a standard survey level and rod system

or an electronic distance measurement (EDM) survey referenced off a permanent survey pole 

and benchmarks. The methods to monitor performance are discussed in detail in the interim 

implementation manual.

(1)

In both methods, settlement is recorded for both the abutment face wall and the superstructure. The

difference between the settlement measured on the abutment face wall and on the superstructure 

is the vertical deformation within the GRS mass due to the bridge load. This value can be divided 

by the height of the abutment face wall to compute the vertical strain within the GRS abutment.

The lateral strain and deformation can then be estimated.

4.2.1 Vertical Deformation

Table 2 gives general information about five bridges built using GRS-IBS. The settlement of three 

of the bridges, Vine Street, Glenburg Road, and Huber Road, was monitored with the survey level

system over the course of 3.2, 3.6, and 2.4 years, respectively (see figure 17 through figure 22).
28

Lateral deformation was not measured on these bridges, but it can be estimated knowing the 

vertical deformation (see chapter 3).

Table 2. Bridge information summary.

Bridge Date Built Abutment

Abutment 

Height

(ft)

Dead Load

(kips/ft2)

Span Length 

(COB to COB)

(ft)

Width of 

Bridge

(ft)

Vine 

Street

October 

2006

North 12.36

South 10.36 2.37 50.0 32.67

Glenburg 

Road

May 

2006

North 13.22

South 12.80 4.53 30.6 28

Huber 

Road

August 

2007

North 17.30

South 16.16 1.53 28.0 28

Bowman 

Road

October 

2005

East 16.91

West 16.47 3.46 79.0 34

Tiffin 

River

July 

2009

North 20.52

South 18.00 3.69 134.0 36

COB = Center of bearing.

Figure 17. Photo. Vine Street Bridge.
29

Figure 18. Graph. Vine Street GRS-IBS settlement versus time.

Figure 19. Photo. Glenburg Road Bridge under flood conditions.
30

Figure 20. Graph. Glenburg Road GRS-IBS settlement versus time.

Figure 21. Photo. Huber Road Bridge.
31

Figure 22. Graph. Huber Road GRS-IBS settlement versus time.

The settlement of two of the bridges, Bowman Road and Tiffin River, was monitored with the 

EDM and total station system over the course of 1.5 years (see figure 23 through figure 26). 

Lateral deformation was also recorded using this method for both bridges, but the noise level on 

Bowman Road Bridge was considered too high to be of any significant use. 

Figure 23. Photo. Bowman Road Bridge after 4.5 years.
32

Figure 24. Graph. Bowman Road GRS-IBS settlement versus time.

Figure 25. Photo. Tiffin River Bridge.
33

Figure 26. Graph. Tiffin River GRS-IBS settlement versus time.

For the majority of these cases, the bridges were built with concrete box beams, so the settlement 

was essentially instantaneous with one large load increment. The settlement history for Tiffin River 

Bridge, shown in figure 26, is slightly different than the settlement of the other bridges because 

the bridge was assembled in stages. The record began following the erection of the steel girders 

on day 0. The plot clearly shows the settlement due to the weight of the concrete deck (installed 

on day 30). The bridge was opened to traffic on day 60, but this is not apparent in the settlement 

record. The postconstruction settlement, from when the Tiffin River Bridge was opened to traffic 

until the last settlement reading, was 0.023 ft on the north abutment and 0.027 ft on the south

abutment. Compared to the settlement due to casting the concrete deck (0.079 and 0.06 ft for the 

north and south abutments, respectively), the settlement after traffic is minimal and reaches the 

secondary settlement stage. In fact, in all of the bridges, the primary settlement essentially ceased

before the bridges opened to traffic.

The differential settlement for an individual abutment, the uniformity of abutment settlement, the 

differential settlement between opposite abutments (i.e., bridge differential settlement), and the 

angular distortion were calculated for each of the five bridges (see table 3). The total vertical 

settlement and strain (including the GRS abutment and the foundation settlement) along with

just the GRS abutment settlement and strain are shown in table 4.
34

Table 3. Movement information for five bridges.

Bridge Abutment

Abutment 

Height

(ft)

Abutment

Differential 

Settlement

( Sabut)

(ft)

Uniformity of 

Abutment 

Settlement

( Sabut/

width of bridge)

Bridge 

Differential 

Settlement

( S)

(ft)

Angular 

Distortion

( S/

span length)

Vine 

Street

North 12.36 0.024 0.0007

South 10.36 0.015 0.0005 0.009 0.00018

Glenburg 

Road

North 13.22 0.020 0.0006

South 12.80 0.008 0.0002 0.012 0.00039

Huber 

Road

North 17.30 0.011 0.0004

South 16.16 0.021 0.0008 0.01 0.00036

Bowman 

Road

East 16.91 0.022 0.0007

West 16.47 0.003 0.0001 0.019 0.00024

Tiffin 

River

North 20.52 0.003 0.0001

South 18.00 0.005 0.0003 0.033 0.00025

Table 4. Vertical settlement and strain information for five bridges.

Bridge

Average 

Abutment 

Height

(ft)

Average 

Total 

Settlement

(ft)

Average 

Total Vertical 

Strain

(percent)

Average 

GRS 

Settlement

(ft)

Average 

GRS Vertical 

Strain

(percent)

Vine Street 11.36 -0.035 0.31 0.023 0.20

Glenburg Road 13.01 -0.107 0.82 0.083 0.64

Huber Road 16.73 -0.004 0.024 0.015 0.09

Bowman Road 16.69 -0.07 0.42 0.047 0.28

Tiffin River 19.26 -0.175 0.91 0.106 0.55

The long-term settlement can be forecasted by plotting the secondary settlement in log-time 

scale. For example, the settlement of Bowman Road Bridge is plotted in log-time scale with the 

appropriate trend lines included in figure 27. After 100 years (the service life of the bridge), the 

average predicted creep settlement is 0.09 ft for the bridge and 0.035 ft for the abutment walls. 

The difference between these two values, 0.055 ft, represents the creep settlement of the GRS 

abutment alone after 100 years.

Δ Δ Δ Δ
35

Figure 27. Graph. Settlement versus log-time to predict creep settlement for the 

Bowman Road Bridge at 100 years.

Another example comes from a long-term settlement study of a GRS-IBS structure built at FHWA’s

Turner-Fairbank Highway Research Center (TFHRC). This 12-year settlement study represents the

longest settlement record of a GRS abutment. Within the embankment, a tunnel was constructed 

to serve several purposes, including measuring deformation, observing the versatility of GRS, 

understanding reinforcement strength on deformation, and providing a walkway to move people

and equipment through the embankment (see figure 28 and figure 29). The overburden on the

tunnel is equal to about 3,800 lb/ft2

, about the same as a typical bridge load.

-0.150

-0.130

-0.110

-0.090

-0.070

-0.050

-0.030

-0.010

0.010

1 10 100 1000 10000 100000

Settlement (Ft)

Time (Days)

East Beam Avg

West Beam Avg

East Face Avg

West Face Avg

100 Years
36

Figure 28. Photo. GRS-IBS tunnel at TFHRC.

Figure 29. Photo. Close-up of GRS-IBS tunnel at TFHRC.

Settlement was measured on both sides of the tunnel wall: the abutment side and the embankment

side. Each side of the wall had different reinforcement strengths. The abutment side was reinforced

with 4,800 lb/ft geotextile, and the embankment side was reinforced with 2,100 lb/ft geotextile. The

measurements over 12 years show that the side with 4,800 lb/ft reinforcement has settled about the 

same as the side with 2,100 lb/ft reinforcement (see figure 30). The settlement difference between 

the two sides is only about 0.0024 ft. The fabric strength did not play a role in the GRS deformation.
37

Figure 30. Graph. Settlement versus time for TFHRC tunnel.

Figure 31 is a log-time scale plot of figure 30 used to provide an estimate of vertical deformation

within the GRS abutment at 100 years. The plot predicts the settlement at 100 years on the 

abutment side of the tunnel walls at 0.033 ft and on the embankment side at 0.030 ft.
38

Figure 31. Graph. Settlement versus log-time to predict creep settlement 

for TFHRC tunnel at 100 years.

Several concrete box beam bridges were built by the Office of Federal Lands Highways and the 

U.S. Forest Service. The Lake Mamie and Twin Lake Bridges were built in Mammoth Lakes, CA,

and have spans of 67 and 71 ft, respectively. Figure 32 and figure 33 are photos of these bridges. 

Figure 34 shows a cross sectional sketch of the GRS abutment behind the old stone abutment wall.(21)

Only short-term settlement information is available for the bridges built in Mammoth Lakes, CA

(see table 5).
39

Figure 32. Photo. GRS abutment behind historic stone abutment.

Figure 33. Photo. Concrete box bridge on GRS abutment in Mammoth Lake, CA.

Figure 34. Illustration. Cross section of GRS abutment behind historic stone abutment.
40

Table 5. Summary of in-service GRS abutments.

Bridge

Date 

Built Abutment

Average 

Total 

Settlement

(ft)

Differential 

Settlement

(ft)

Uniformity of 

Abutment 

Settlement

( Sabut/

width of bridge)

Angular 

Distortion

( S/

span length)

Lake 

Mamie

Fall

2000 Both

-0.013

at 3 days No data No data No data

Twin 

Lake

Fall 

2000 Both

-0.021

at 6 days No data No data No data

Cecil 

Creek

June 

2005

A -0.098 0.013 0.0007

B -0.02 0.002 0.0001 0.001

Big 

Lake

June 

2005

A 0.007 0.040 0.002

B -0.029 0.075 0.004 0.0005

Cut Off 

Creek

June 

2005

A 0.078 0.050 0.003

B 0.072 0.001 0.00006 0.0001

Three bridges were also built in the Ouachita wildlife refuge in Louisiana (see figure 35 and

figure 36). Each bridge has a span of 76 ft. The bridges are typically submerged under water for 

6 months each year. Table 5 provides the settlement data for these bridges after 147 days of beam

placement. The settlement performance of these bridges was well within acceptable criteria.

Figure 35. Photo. Concrete box bridge on GRS abutment in Ouachita wildlife refuge.

Δ Δ
41

Figure 36. Illustration. Cross section of GRS abutments in Ouachita wildlife refuge.

4.2.2 Lateral Deformation

Lateral deformation has not been widely measured on in-service bridges because of difficulties 

in obtaining accurate long-term information. Previous GRS walls and abutments have performed 

well and have shown no reason for concern with lateral deformation. Theoretical calculations

given in chapter 3 provide a method to determine the lateral deformation and strain based on an 

assumption of a zero loss of volume. The predicted average lateral deformation and lateral strain 

for the five previously discussed in-service bridges are shown in table 6.

Table 6. Predicted lateral deformations of five in-service bridges.

Bridge

Average 

Abutment 

Height

H

(ft)

Width of 

Bridge Bearing 

Area + Setback

b + ab

(ft)

Average 

GRS Mass 

Settlement

Dv

(ft)

Calculated Lateral 

Deformation

DL = (2(b+ ab)*Dv)/H

(ft)

Calculated

Lateral 

Strain

Vine Street 11.36 2.64 0.023 0.011 0.004

Glenburg Road 13.01 2.14 0.083 0.027 0.013

Huber Road 16.73 2.64 0.015 0.005 0.002

Bowman Road 16.69 3.64 0.047 0.02 0.005

Tiffin River 19.26 5.14 0.089 0.047 0.009

Of the five in-service bridges shown in table 3, lateral deformation was only measured for Tiffin 

River Bridge. Measuring the horizontal movement was difficult due to the seasonal movement of 

the survey pole. The pole foundation extended approximately 4 ft below the ground surface. In dry

periods, the data suggest that the pole settled and tipped. The pole then rebounded after soil 

moisture returned to normal levels. Attempts were made to correct the pole location by using 

back-site readings. However, due to the fact that many of the measured movements were smaller 

than the apparent movement of the survey pole over the long term, it is difficult to get a precise 

absolute movement. This is particularly true for the lateral deformation movement of abutment 

wall faces. By comparing the distance between both abutment faces, however, the effect of pole 

movement is minimized. While this does not allow the movement of each abutment to be evaluated

independently, it does provide a measure of the average wall maximum bulge between the two 

abutment wall faces. Figure 37 shows a comparison between the calculated and measured lateral 

deformation as previously explained for Tiffin River Bridge (equation 11). The theoretical 

calculation matches very closely, within the ±0.005-ft error of the EDM system.

εL
42

Figure 37. Graph. Measured and calculated lateral deformation on the 

Tiffin River Bridge GRS abutment.

4.3 THERMAL CYCLES

Thermal cycles occur on every bridge structure due to daily and seasonal temperature variations. 

The severity of the expansion and contraction depends on the coefficient of thermal expansion of 

the bridge. However, based on the experience of in-service GRS-IBS bridges with spans of up to 

140 ft, the performance of GRS-IBS is not affected by thermal cycles. GRS-IBS accommodates 

movement related to thermal cycles, so the effect on the abutment is minimal.

GRS-IBS accommodates movement through the integrated transition behind the beam ends. The 

road base is wrapped with geosynthetic and is then well-compacted directly against the beam end. 

The wrapped face confines the soil and allows for the beam to contract without the fill behind the 

beam ends sloughing off to fill the void. As a result, 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. 

The performance of Tiffin River Bridge, the longest GRS-IBS steel bridge span (140 ft), has been

monitored since construction.(22) Vibrating wire earth pressure cells were installed on the back 

wall of each abutment to measure lateral pressures between the superstructure and the integrated 

approach due to expansion and contraction of the steel girders (see figure 38 and figure 39). The 

average lateral earth pressure changes in magnitude with both daily and seasonal temperature 

effects (see figure 40). During the fall and winter (contraction), the lateral pressure decreased to 
43

about 62 lb/ft2. During the spring and summer (expansion), the lateral pressure increased to as

high as about 700 lb/ft2

.

(22) After one thermal cycle, it appears that the lateral pressure is nonlinear 

with temperature and displays hysteretic behavior. Monitoring continues, however, to determine 

the effect after multiple thermal cycles.

Figure 38. Illustration. Instrumentation for Tiffin River Bridge.

Figure 39. Photo. Pressure cells behind back wall on Tiffin River Bridge.
44

Figure 40. Graph. Average lateral pressure on back wall for Tiffin River Bridge.
45

APPENDIX A. PULL-OUT TEST RESULTS

As described in chapter 1, block pull-out and connection force is not an issue in GRS walls and 

abutments built with modular block facings because of the frequency of reinforcement between 

each layer of block. Many case studies have shown that a frictional connection between the 

modular blocks and the geotextile is compatible with deformations of the GRS composite mass. 

The facing blocks strain with the GRS mass, adjusting to lateral loads in order to maintain

equilibrium with lateral thrust. In other words, if the GRS mass strains laterally 0.5 percent, 

then the facing will also displace that amount without attracting loads against the face.

(9)

The compatibility of the frictional connection method has been illustrated through performance 

tests. After a vertical stress of 26 kips/ft2, the performance test for Defiance County, OH, showed 

that the modular facing blocks remained frictionally connected to the GRS mass.(1) Similar results

were seen in other tests. For example, a GRS pier built at TFHRC was loaded to 19 kips/ft2 to 

show that the pier maintained its rectangular shape without the loss of any of the facing blocks 

(see figure 41). The frictional capacity between the blocks and the GRS mass was not exceeded 

even after extreme vertical loading conditions. 

Figure 41. Photo. TFHRC pier test.

Nevertheless, it is important to understand the general capacity of the frictional connection method 

and the overall mechanics of the GRS system. For this reason, the following block pull-out test

was devised to determine the amount of force required to mobilize a modular block out of the face

of the wall (the pull-out force). The test was performed on blocks at different heights along the 

GRS wall to determine the relationship between the normal force on top of the block and the 

required pull-out force.
46

A.1 SETUP

To evaluate the frictional capacity, block pull-out tests were conducted on an actual GRS wall to

quantify the amount of force necessary to move the block relative to the geotextile (see figure 42). 

The test was performed on segmental retaining wall (SRW) blocks frictionally connected to a 

2,400 lb/ft geotextile. The setup for the test is shown in figure 43. The block pull-out tests were 

performed for nearly every course of blocks on a 12-ft-high wall, as outlined in the following 

procedure. Note that the same setup procedure can also be used on CMU blocks.

Figure 42. Photo. Block pull-out test on GRS wall.

Figure 43. Illustration. Block pull-out test setup.
47

The test apparatus depicted in figure 42 and figure 43 includes the following:

10,000-lb load cell.

Strain indicator box to measure the load (connects to load cell).

Manual hydraulic pump with hoses for connecting the pump to the jacks.

Two small 20-kip jacks.

Two dial gauges for measuring displacement.

Two magnetic bases for holding and positioning the dial gauges.

0.375-inch-diameter drop-in anchors.

0.375-inch-diameter threaded rod.

Metal bar on which to connect the jacks.

C-clamp.

Metal bars.

Hammer drill with 0.5-inch drill bit.

The apparatus is connected to the block according to the following steps:

1. Using a 0.5-inch drill bit, drill a hole in the center of the block to a suitable depth 

(approximately 2.67 inches).

2. Set the anchor using the setting tool.

3. Screw the threaded rod into the anchor in the block and make sure it is tight.

4. Make two flat spots on the top and bottom of the face of the block (above and below the rod).

5. Attach metal plates to the flat spots using epoxy.

A.2 PROCEDURE

Once the test apparatus is set up and properly connected to the block, the following procedure 

can be used to determine the block pull-out force:

1. Zero the two dial gauges.

2. Zero the load gauge.

3. Take initial readings for the top and bottom dial gauges at zero load.
48

4. Record the values of the initial readings.

5. Turn on the pump.

6. Add 50 lb to the jacks by manually pumping the arm of the pump.

7. Record displacement values at 0, 1, and 3 min for load values beginning at 50 lb and 

increasing in increments between 50 and 200 lb depending on the rate of the block 

displacement.

8. Stop the test when the block is displaced outward approximately 0.5 inches.

9. Take a final displacement reading at a load of zero.

A.3 TEST RESULT FOR SRW BLOCKS

Block pull-out capacity was measured at various heights along the GRS wall (2, 3, 4, 7, 8, 9, 10, 

11, 12, 13, 15, 17, and 18 rows down). The load-displacement behavior for each test was recorded.

In all cases, the block did not displace until the frictional capacity was exceeded. For example, the 

load displacement behavior is shown in figure 44 for a block that was seven rows down from the 

top with a normal force equal to 425 lb on top of it. The same trend is shown in figure 45 for a 

block that was 11 rows down from the top with a normal force equal to 765 lb on top of it, except 

the force required to move the block was higher. The same trend was true for the remaining tests.

Figure 44. Graph. Pull-out test results for an SRW block seven rows from the top.
49

Figure 45. Graph. Pull-out test results for an SRW block 11 rows from the top.

The results show that a fairly linear relationship exists between the normal force on top of the block,

which corresponds to the number of blocks down from the top of the wall, and the amount of force

necessary to initiate block movement (see figure 46). The pull-out capacity for the experiment was 

chosen by reviewing the load versus displacement for the value directly before the greatest change

in displacement.
50

Figure 46. Graph. Pull-out test results in terms of normal force for SRW blocks.

The graph shows that the greater the normal force available due to the added weight of the blocks

from above, the greater the block pull-out capacity. In a typical abutment, the thrust at the face is 

considerably lower (see chapter 1). Therefore, block pull-out and connection force is not a concern

with GRS walls and abutments under normal conditions. It may, however, be a factor in designing

for extreme events, seismic or impact. To add conservatism to the design and prevent any issues 

at the top of the wall (where the normal forces are the lowest), the top three courses of blocks are 

filled with a concrete fill mix and rebar to tie these blocks together.(1) It should be noted that 

although the pull-out tests were performed on SRW blocks, similar results would occur with 

CMU facing elements. 
51

APPENDIX B. PREDICTION DATA FOR ANALYTICAL EQUATIONS

B.1 INTRODUCTION

Seventeen tests were used to validate the soil-geosynthetic capacity and required reinforcement 

strength equations. The complete set of data from these tests is presented in table 7.

Table 7. Prediction data for large-scale tests.

Test

Reference

No.

c

(lb/ft2)

Sv

(inch)

Tf

(lb/ft)

dmax

(inch)

c 

(lb/ft2) (degrees)

H 

(ft)

qrupture

(lb/ft2) (lb/ft3)

GSGC 1 10 710 None – 1.3 1,462 50 6.5 – 153

GSGC 2 10 710 8 4,800 1.3 1,462 50 6.5 56,403 153

GSGC 3 10 710 16 9,600 1.3 1,462 50 6.5 36,558 153

GSGC 4 10 710 16 4,800 1.3 1,462 50 6.5 27,157 153

GSGC 5 10 0 8 4,800 1.3 1,462 50 6.5 39,691 153

Elton and 

Patawaran 1 23 0 6 620 0.5 576 40 5 4,805 121

Elton and 

Patawaran 2 23 0 12 620 0.5 576 40 5 2,695 121

Elton and 

Patawaran 3 23 0 6 960 0.5 576 40 5 6,392 121

Elton and 

Patawaran 4 23 0 6 1,025 0.5 576 40 5 6,100 121

Elton and 

Patawaran 5 23 0 6 1,300 0.5 576 40 5 8,398 121

Elton and 

Patawaran 6 23 0 6 1,400 0.5 576 40 5 8,293 121

Elton and 

Patawaran 7 23 0 6 1,700 0.5 576 40 5 9,589 121

NCHRP 1 7 0 8 1,400 1 418 37.3 15 7,312 117

NCHRP 2 7 0 8 4,800 1 418 37.3 15 17,757 117

Defiance 1 24 0 8a 2,400 0.5 0 48.7 6.4 11,320 110

Defiance 2 24 0 8a 4,800 0.5 0 48.7 6.4 21,412 110

Vegas 

Mini Pier 20 0 6

a 2,400 1 576 40 8 20,890 121 a Two layers of a bearing bed reinforcement at 4 inches were placed at the top of the mass.

GSCG = Generic Soil-Geosynthetic Composite.

NCHRP = National Cooperative Highway Research Program.

B.2 SOIL-GEOSYNTHETIC CAPACITY EQUATION

The load-carrying capacity of a GRS wall and abutment (qult,an,c) can be evaluated using an analytical

formula (equation 3). For this equation, the calculated capacity matches well with the measured 

capacity from large-scale tests (see figure 12). The numerical results of this comparison are presented

in table 8. Note that for long-term design, cohesion (c) and confining stress ( c) are assumed to 

equal zero and should not be accounted for.

σ γ

σ
52

Table 8. Soil-geosynthetic capacity equation validation results.

Test

Reference

No.

qmeasured

(lb/ft2)

qcalc 

(lb/ft2)

GSGC 1 10 16,085 12,680

GSGC 2 10 56,403 51,181

GSGC 3 10 36,558 39,545

GSGC 4 10 27,157 26,113

GSGC 5 10 39,691 46,522

Elton and Patawaran 1 23 4,805 5,327

Elton and Patawaran 2 23 2,695 3,175

Elton and Patawaran 3 23 6,392 6,915

Elton and Patawaran 4 23 6,100 7,228

Elton and Patawaran 5 23 8,398 8,502

Elton and Patawaran 6 23 8,293 8,816

Elton and Patawaran 7 23 9,589 10,403

NCHRP 1 7 8,356 7,291

NCHRP 2 7 17,757 20,347

Defiance 1 24 13,370 11,322

Defiance 2 24 25,068 20,180

Vegas Mini Pier 20 20,890 18,258

GSCG = Generic Soil-Geosynthetic Composite.

NCHRP = National Cooperative Highway Research Program.

B.3 REQUIRED REINFORCEMENT STRENGTH EQUATION

For the required reinforcement strength equation (equation 13), the calculated strength matches 

well with the actual strength at rupture from large-scale tests (see figure 16). The biggest differences

occur with some of the GSCS tests, which had a uniform confining pressure applied.

(10) This 

confining stress will likely not be present in an actual GRS-IBS application. The numerical

results of this comparison are presented in table 9. Note that for long-term design, the cohesion (c)

and confining stress (σc) is assumed to equal zero and should not be accounted for.
53

Table 9. Required reinforcement strength equation validation results

Test

Reference

No.

Tactual

(lb/ft)

Tcalc

(lb/ft)

GSGC 2 10 4,800 5,483

GSGC 3 10 9,600 8,633

GSGC 4 10 4,800 5,274

GSGC 5 10 4,800 4,069

Elton and Patawaran 1 23 620 632

Elton and Patawaran 2 23 620 712

Elton and Patawaran 3 23 960 975

Elton and Patawaran 4 23 1,025 912

Elton and Patawaran 5 23 1,300 1,408

Elton and Patawaran 6 23 1,400 1,385

Elton and Patawaran 7 23 1,700 1,665

NCHRP 1 7 1,400 1,902

NCHRP 2 7 4,800 4,587

Defiance 1 24 2,400 2,857

Defiance 2 24 4,800 5,254

Vegas Mini Pier 20 2,400 2,944

GSCG = Generic Soil-Geosynthetic Composite.

NCHRP = National Cooperative Highway Research Program.

55

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 Pagán.

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.

57

REFERENCES

1. Adams, M., Nicks, J., Stabile, T., Wu, J., Schlatter, W., and Hartmann, J. (2010).

Geosynthetic Reinforced Soil Integrated Bridge System—Interim Implementation Guide,

Report No. FHWA-HRT-11-026, Federal Highway Administration, McLean, VA.

2. Geo-Institute, Committee on Shallow Foundations. (1999). Shallow Foundations on 

Reinforced Soil, American Society of Civil Engineers, Reston, VA.

3. Berg, R., Christopher, B., and Samtani, N. (2009). Design of Mechanically Stabilized Earth 

Walls and Reinforced Soil Slopes—Volume 1, Report No. FHWA-NHI-10-024, National 

Highway Institute, Federal Highway Administration, Arlington VA.

4. Wu, J.T.H. (1994). Design and Construction of Low Cost Retaining Walls: The Next Generation

in Technology, Report No. CTI-UCD-1-94, Colorado Transportation Institute, Denver, CO.

5. Adams, M.T., Ketchart, K., Ruckman, A., DiMillio, A., Wu, J.T.H., and Satyanarayana, R. 

(1999). “Reinforced Soil for Bridge Support Application on Low Volume Roads.” Presented 

at the Seventh International Conference on Low-Volume Roads, Transportation Research

Record 1652, pp. 150–160.

6. Koklanaris, M. (2000). “Geosynthetic Reinforced Soil Structures Can Carry the Load,” 

Public Roads, Federal Highway Administration, Washington, DC.

7. Wu, J.T.H., Lee, K.Z.Z., Helwany, S.B., and Ketchart, K. (2006). Design and Construction 

Guidelines for GRS Bridge Abutment with a Flexible Facing, Report No. 556, National 

Cooperative Highway Research Program, Washington, DC.

8. Adams, M.T. (2010). “GRS Technology and the GRS Integrated Bridge System.” Presented 

at Eugene Workshop, Local Technical Assistance Program, Eugene, OR.

9. Adams, M.T. (1997). “Performance of a Prestrained Geosynthetic Reinforced Soil Bridge 

Pier,” Mechanically Stabilized Backfill, Wu, J.T.H (Ed.), pp. 35–53, Balkema, Rotterdam, 

Netherlands.

10. Wu, J.T.H., Pham, T.Q., and Adams, M.T. (2011). Composite Behavior of GeosyntheticReinforced Soil (GRS) Mass, Draft, Federal Highway Administration, McLean, VA.

11. Wu, J.T.H. (2001). Revising the AASHTO Guidelines for Design and Construction of GRS 

Walls, Report No. CDOT-DTD-R-2001-6, Colorado Department of Transportation, Denver, CO.

12. Wu, J.T.H., Lee, K.Z.Z., and Pham, T. (2006). “Allowable Bearing Pressure of Bridge Sills 

on GRS Abutments with Flexible Facing,” Journal of Geotechnical and Geoenvironmental 

Engineering 132(7), pp. 836–841, American Society of Civil Engineers.

13. Adams, M.T. (2008). “The Bridge of Defiance County,” Geosynthetics, 26(2), pp 14–16, 18, 

20–21, Industrial Fabrics Association International, Roseville, MN.
58

14. Wu, J.T.H. and Adams, M.T. (2007). “Myth and Fact on Long-Term Creep of GRS Structures,”

Geotechnical Special Publication No. 165, Geosynthetics in Reinforcement and Hydraulic 

Applications, Proceedings, Geo-Denver 2007, American Society of Civil Engineers,

Denver, CO.

15. Ketchart, K. and Wu, J.T.H. (1996). Long-Term Performance Tests of Soil-Geosynthetic 

Composites, Report No. CDOT-CTI-96-1, Colorado Department of Transportation, Denver, CO.

16. Wu, J.T.J., Ketchart, K., and Adams, M. (2001). GRS Bridge Piers and Abutments, Report

No. FHWA-RD-00-038, Federal Highway Administration, McLean, VA.

17. Morrison, K.F., Harrison, F.E., Collin, J.G., Dodds, A., and Amdt, B. (2006). Shored 

Mechanically Stabilized Earth (SMSE) Wall Systems Design Guidelines, Report No. FHWACFL/TD-06-001, Federal Highway Administration, Central Federal Lands Highway 

Division, Lakewood, CO.

18. Bell, A.L. (1915). “The Lateral Pressure and Resistance of Clay and Supporting Power of 

Clay Foundations,” Min. Proceeding of Institute of Civil Engineers 199, pp. 233–272.

19. Samtani, N.C. and Nowatzki, E.A. (2006). Soils and Foundations Reference Manual, Report 

No. FHWA-NHI-06-088, National Highway Institute, Federal Highway Administration, 

Arlington, VA.

20. Adams, M.T., Lillis, C.P., Wu, J.T.H., and Ketchart, K. (2002). “Vegas Mini Pier Experiment

and Postulate of Zero Volume Change.” Proceedings, Seventh International Conference on 

Geosynthetics, pp. 389–394, Nice, France.

21. Keller, G.R. and Devin, S. (2003). “Geosynthetic Reinforced Soil Bridge Abutments,” 

Transportation Research Board 1819(2), pp. 362–368, Eighth International Conference on 

Low Volume Roads, Reno, NV. 

22. Warren, K.A., Schlatter, W., Adams, M., Stabile, T., and LeGrand, D. (2011). “Thermal 

Effects and Deformations Associated with a Large, Single Span Bridge Supported by a GRS 

Integrated Bridge System.” Proceedings, GeoFrontiers 2011, Dallas, TX.

23. Elton, D.J. and Patawaran, M.A.B. (2005). Mechanically Stabilized Earth (MSE) Reinforcement 

Tensile Strength from Tests of Geotextile Reinforced Soil, Alabama Highway Research Center,

Auburn University, Auburn, AL.

24. Adams, M.T., Schlatter, W., and Stabile, T. (2007). “Geosynthetic-Reinforced Soil Integrated 

Abutments at the Bowman Road Bridge in Defiance County, Ohio.” Proceedings, Geo-Denver

2007, American Society of Civil Engineers, Denver, CO.

HRDI-40/01-11(300)E Recycled

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