Development of Protocols for 

Confined Extension/Creep Testing of 

Geosynthetics for Highway 

Applications 

PUBLICATION NO. FHWA-RD-97-143 MARCH 1998 

US Department of Transportam 

Fedeml HkJhwoy Administmtion 

Research and Development 

Turner-Fairbank Highway Research Center 

6300 Georgetown Pike 

McLean, VA 22101-2296 
FOREWORD 

The report, Development of Protocols for Confined ExtensionKreept Testing of Geosynthetics 

for Highway Applications, is part of a comprehensive study on the Durability of Geosynthetic 

Materials for Highway Construction. The report presents the development and verification of 

testing protocol and protocol equipment for confined extension testing and confined creep tests 

for geosynthetic reinforcement materials. The protocols are presented in ASTM format for 

possible ASTM consideration. 

qg$&ig$w 

Director, Oflice of ingineering 

Research and Development 

NOTICE 

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

interest of information exchange. The United States Government assumes no liability for its 

contents or use thereof This report does not constitute a standard, specification, or regulation. 

The United States Government does not endorse products or manufacturers. Trademarks or 

manufacturers’ names appear herein only because they are considered essential to the object of 

this document. 
Technical Report Dwumenttion Page 

I- I. Report No. 2. Government Accession No. 6. Recfplent’a Catalog No, 

FHWA-RD-97-143 

4. Tltle and Subtitle 5. Report Date 

. 

DEVELOPMENT. OF PROTOCOLS FOR CONFINED EXTENSION/ March lgg8 

CREEP TESTING OF GEOSYNTHETICS FOR HIGHWAY 6. Performlng Organization Code 

APPLICATIONS 

8. Performfng~Organiratlon Report No. 

7.Author(s) Victor Elias, iehong Yuan, Robert H Swan,Jr. 

and Robert C. Bachus 

9l-329 

g. PerformIng Organlzatlon Name and Address 10. Work Unit No. fTRAlSl 

3E3B0772H 

1 

Earth Engineering and Sci,ences, Inc. 

3401 Carlins Park Drive 11, Contract or Grant No. 

Baltimore, MD 21215 DTFH61-91-R-00054 

13. Type of. Report and Period Covered 

12. Sponsoring Agenoy Name and Address 

Office of Engineering R & D 

Final Report 

Federal Highway Administration 

April 1993 - June 1997 

6300 Georgetown Pike 14. Sponsoring Agency Code 

McLean, VA 22101-2296 

I 5. Supplementary Notes 

FHWA Contract Manager : A.F. DiMillio (HNR-30) 

Technical Consultant : Barry Christopher, Ph.D. 

Research Subcontractor : Geosyntec Consultants 

16. Abstract 

1 

1 

This ‘report’ presents the development and verification of a testing protocol and 

protocol equipment for confined extension testing and confined creep testing for 

geosynthetic reinforcement materials. 

The developed data ind:cate that confined response significantly improves stress 

strain response of geosynthetic materials, especially nonwoven geotextiles. 

Current use of uncotimed stress-strain response appears overly conservative. 

This report is part of a comprehensive study on the “Durability of Geosynthetic 

Materials for Highway Applications. ” The developed protocols in the Appendix 

have been developed in the ASTM format and may be forwarded for ASTM 

consideration. 

COVER PHOTO: Nonwoven polyester geosynthetic 

1’ fiey Words 

Geogrid; Geotextile; Confir 

c 

., ,. A r. I_ 

( 

. 

led Extension;, 

;eosy,ntnetics, LonTlnea Lreep, Confined No restrictions.This document is 

Stress-Strain. available to the public through the 

'National Technical Information Service, 

, Springfield, Virginia 22161. 

’ 1 18. Distribution Statement 

l! 2. Security Classff. (of this report) 

I 

‘20. Security Classlf. (of this page) 21. No. of Pages 22. Price 

Unclassified 

FOITII DOT F 1700.7 (a-nj 

(PF V2.1, 12/13/w) 

.'Unclassified 

Reproduction of oompleted page authorized 

211 
APPROXIMATE CONVERSIONS TO SI UNITS APPROXIMATE CONVERSIONS FROM SI UNITS 

Symbol When You Know Multlply By To Flnd Symbol 

in 

ft 

yd 

mi 

inches 

feet 

Y=JS 

miles 

LENGTH 

25.4 

0.305 

0.914 

1.61 

AREA 

millimeters 

meters 

meters 

kilometers 

in* 

fF 

Y@ 

ac 

mP 

square inches 

square feat 

square yards 

aaes 

square miles 

645.2 

0.093 

0.836 

0.405 

2.59 

VOLUME 

square millimeters 

square meters 

square meters 

hectares 

square kilometers 

fl 02 fluid ounces 29.57 milliliirs 

gal gallons 3.785 liters 

fP cubic feet 0.028 cubic meters 

Ye cubii yards 0.765 cubic meters 

NOTE: Volumes greater than 1000 I shall be shown in m3. 

MASS 

pbz 

ounces 28.35 

pounds ~0.454 

grams 

T short tons (2000 lb) 0.907 

kilograms 

megagrams 

(oi ‘metric ton”) 

TEMPERATURE (exact) 

“F Fahrenheit 5( F-32)/9 Celcius 

temperature or (F-32yl.8 temperature 

ILLUMINATION 

fc 

II 

foot-candlas 10.76 lux 

loot4amberts 3.426 candela/m* 

FORCE and PRESSURE or STRESS 

Ibf 

IbfW 

pou~force 

poundforce per 

square inch 

4.45 newtons 

6.89 kilopascals 

mm 

m 

En 

mm* 

m* 

m* 

ha 

k@ 

mL 

L 

m3 

m3 

N 

kPa 

Symbol When You Know Multiply By To Find Symbol 

mm 

m 

rrn 

mm* 

m* 

m2 

ha 

km* 

mL 

L 

m3 

m3 

millimeters 

meters 

meters 

kilometers 

LENGTH 

0.039 

3.28 

1.09 

0.621 

AREA 

inches 

feet 

Y&S 

miles 

square millimeters 

square meters 

square meters 

hectares 

square kilometers 

0.0016 

10.764 

1.195 

2.47 

0.366 

square inches 

square feet 

square yards 

acres 

square miles 

VOLUME 

milliliters 

liters 

cubic meters 

cubic meters 

0.034 

0.264 

35.71 

1.307 

fluid ounces 

gallons 

cubic feet 

cubic yards 

MASS 

grams 

kilograms 

0.035 

2.202 

ounces 

pounds 

in 

It 

yd 

mi 

in* 

fP 

Y@ 

ac 

mi2 

fl 02 

gal 

fP 

Y8 

Oi! 

lb 

megagrams 

(or ‘metric ton’) 

1.103 short tons (2000 lb) T 

TEMPERATURE (exact) 

Celcius 

temperature 

1.8C +32 

ILLUMINATION 

Fahrenheit 

temperature 

“F 

lux 0.0929 foot-candles fc 

candela/m* 0.2919 foot-Lamberts 6 

FORCE and PRESSURE or STRESS 

N 

kPa 

newtons 

kilopascals 

0.225 

0.145 

poundforce 

poundforce per 

square inch , 

Ibf 

IbfM 

SI is the symbol for the lntematkmal System of Units. Appropriate 

rounding should be made to comply with Section 4 of ASTM E380. 

(Revised September lQQ3) 
TABLE OF CONTENTS 

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

1.1 Objective and Scope ............................. 1 

1.2 Test Materials ................................. 2 

L: 

Geosynthetic Materials ....................... 2 

Soil Materials ............................. 3 

1.3 Organization of Report ........................... 3 

1.4 Reviews .................................... 7 

CHAPTER 2 INITIAL TESTING AND SELECTION OF TEST EQUIPMENT . 9 

2.1 Overview and Literature Review ...................... 9 

k: 

Test Equipment ............................ 9 

Range of Test Results ........................ 22 

2.2 Initial Baseline Testing - Pullout Tests .................. 25 

it: 

Test Equipment ............................ 25 

Test Procedures ........................... 25 

Ii: 

Test Results and Observation ................... 29 

Interpretation of Test Results ................... 33 

e. Conclusions .............................. 38 

2.3 Initial Baseline Testing - Confined Wide-Width Tes .......... 38 

:: 

Equipment ............................... 38 

Testing Procedures ......................... 41 

:: 

Test Results .............................. 42 

Conclusion .............................. 42 

CHAPTER 3 DEVELOPMENT OF CONFINED EXTENSION/ 

CREEP TEST EQUIPMENT . . . . . . . . . . . . . . . . . . . . . . . 45 

3.1 Overview .................................... 45 

3.2 Protocol Equipment ............................. 45 

;: 

Description of Protocol Equipment ................ 45 

Calibration of Protocol Equipment ................ 48 

3.3 Summary .................................... 52 

CHAPTER 4 VERIFICATION OF CONFINED EXTENSION/CREEP 

TEST EQUIPMENT . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57 

4.1 Overview . . . . . . . . , . . . . . . . . . . . . . . . . . , , . . . . . . . . 57 

.a. 

111 
TABLE OF CONTENTS (continued) 

4.2 

4.3 

4.4 

Verification Testing Program ........................ 57 

E: 

General Testing Procedures .................... 57 

Pretension Force ........................... 59 

i: 

Strain Rate .............................. 60 

Aspect Ratio ............................. 61 

f: 

Initial Loading Rate ......................... 61 

Confining Soil Thickness ...................... 64 

Friction Along the Geosynthetic Specimen ............... 65 

a. Strain Compatibility Between Confining Soil and 

Geosynthetic ............................. 65 

b. Evaluation of Different Methods for Reducing Friction ... 67 

C. Assessment of Test Results .................... 71 

Summary and Conclusions ......................... 73 

CHAPTER 5 CONFINED EXTENSION/CREEP TESTING PROGRAM . . . 75 

5.1 Introduction .................................. 75 

5.2 Confined and Unconfined Extension Testing Program ......... 75 

:: 

Test Matrix .............................. 75 

Test Results .............................. 75 

C. Interpretation ............................. 78 

5.3 Confined and Unconfined Creep Testing Program ........... 86 

:: 

Test Matrix .............................. 86 

Test Results .............................. 86 

C. Interpretation ............................. 89 

5.4 Summary and Conclusions ......................... 94 

CHAPTER 6 CONFINED EXTENSION TESTS USING TRIAXIAL-TYPE 

TESTDEVICE..........,...............,..... 97 

6.1 Introduction ................................... 97 

6.2 Laboratory Testing Equipment ....................... 97 

F. 

Testing Equipment .......................... 97 

Equipment Calibration ....................... 98 

6.3 Test Procedure ............................... 102 

6.4 Confined Extension Testing Program .................. 103 

k: 

Test Matrix ............................. 103 

Test Results and Interpretation ................. 103 

iv 
TABLE OF CONTENTS (continued) 

6.5 

6.6 

Comparison Of Confined Extension Test Equipment and Results . 108 

:: 

Comparison of Test Devices ................... 108 

Comparison of Test Results ................... 110 

Summary Comments and Recommendations .............. 116 

CHAPTER 7 DEVELOPMENT OF WRITTEN PROTOCOL FOR CONFINED 

EXTENSION/CREEP TESTING . . . . . . . . . . . . . . . . . . . 117 

7.1 Overview ................................... 117 

7.2 Discussion .................................. 117 

7.3 Future Use .................................. 119 

CHAPTER 8 CONCLUSIONS AND RECOMMENDATIONS . . . . . . . . . . 121 

8.1 Summary and Conclusions ........................ 121 

8.2 Design Implications ............................ 124 

8.3 Recommendations .............................. 125 

APPENDICES 

A - Confined Extension Test Results ..................... 127 

B - Confined Creep Test Results ....................... 138 

c - Protocol for Confined Extension/Creep Testing ......... 169 

REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 200 

V 
Fimre 

LIST OF FIGURES 

1 

2 

3 

4a 

4b 

5 

6 

7 

8 

9 

10 

11 

12 

13 

14 

15 

16 

17 

18 

19 

20 

21 

22 

23 

24 

25 

26 

27 

28 

29 

Particle-size distribution curves for beach sand and silty sand ......... 6 

In-soil test apparatus (after McGown et al. 1982) ................ 10 

Zero-span test apparatus (after Christopher et al. 1986) ............ 12 

Pullout test apparatus (cross-section view) .................... 13 

Pullout test apparatus (plan view) ......................... 13 

Modified triaxial apparatus (after Wu, 1991) ................... 14 

Plane strain unit cell device (UCD) (Boyle, 1995) ............... 16 

Automated plane strain reinforcement (APSR) cell (Whittle et al. 1993) , , 17 

Modified direct shear test machine (Leshchinsky and Field, 1987) ...... 19 

Soil confinement effect on secant moduli at 5 percent strain for 

nonwoven geotextiles ................................. 24 

Schematic diagram of pullout test device ..................... 26 

Schematic diagram showing tell-tail wires .................... 28 

Baseline pullout testing - test reproducibility ................... 30 

Baseline pullout testing - effect of specimen width at a normal stress 

of7psi ....................................... ..3 1 

Baseline pullout testing - effect of specimen width at a normal stress 

ofl4psi.........................................3 2 

Displacements at select nodes along a test specimen using 

tell-tail wires ...................................... 34 

Schematic diagram of deformed Geosynthetic’ PP-15 after pullout test ... 35 

Tension versus strain curves using the “constant load” approach ....... 36 

Tension versus strain curves using the “linearly -distributed load” 

approach ......................................... 37 

Strain distribution along 6-in-long front portion of pullout specimen 

at various pullout forces ............................... 39 

Schematic diagram of prototype confined wide-width test device ....... 40 

Tensile force versus strain curves for Geosynthetic PP-15 confined 

in beach sand using prototype confined wide-width test device ........ 43 

Tensile force versus strain curves for Geosynthetic PP- 16 confined 

in beach sand using prototype confined wide-width test device ........ 44 

Detailed drawing of confined extension/creep testing device ......... 47 

Friction between confinement box and rollers in confined extension test . . 49 

Schematic diagrams showing four tell-tail wires ................. 51 

Strain distribution over each element along the specimen length . , . . , . . 53 

Variation of tensile loads in confined creep test ................. 54 

Reproducibilty of confined extension test results ................ 55 

Results of unconfined wide-width tensile tests using specimens with 

different aspect ratios ................................. 62 

vi 
LIST OF FIGURES (continued) 

Figre 

30 

31 

32 

33 

34 

35 

36 

37 

38 

39 

40 

41 

42 

43 

44 

45 

46 

Patze 

Results of two confined creep tests having different initial loading rates . . 63 

Results of confined extension tests when confined by various 

thicknesses of soil layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . , . , 66 

Cross-section of confined extension testing with lubricated boundary 

conditions: (a) initial configuration, (b) deformed configuration . . . . . . . 68 

Confined response of Geosynthetic PP-10 under lubricated and 

unlubricated boundary conditions . . . . . . . . . . . . . . . . . . . . . . . . . . 70 

Confined response of Geosynthetic PP-11 under lubricated and 

unlubricated boundary conditions . . . . . . . . . . . . . . . . . . . . 

Soil confinement effect on secant moduli at 5 percent strain for 

selected geosynthetic materials confined in beach sand , . , , . . . . 

Soil confinement effect on secant moduli at 5 percent strain for 

selected geosynthetic materials confined in silty sand . . . . . . . , , 

Comparison of secant moduli at 5 percent strain for Geosynthetic 

PP-10 confined in beach sand and silty sand . I . . . . . . . . . . . , 

Comparison of secant moduli at 5 percent strain for Geosynthetic 

PP-10 confined in beach sand and silty sand with data of similar 

. . * . . . 72 

I . . . 81 

. . . . 82 

. . . * 83 

geosynthetic materials reported by other researchers . . . , . . . , . , , . . 

Schematic diagram of triaxial-type device developed by 

Polytechnic University . . . . . . . . . . . . . . . . . . , . . . . . . . . . , , , 

Comparison of loads measured by top and bottom load cells in 

unconfined wide-width tensile test . . . . . . . . . . . . . . . . . . . . . . . , 

Load versus displacement responses from confined extension test 

on Geosynthetic PP-15 at a normal stress of 10 psi . . . . . . . . . . . . . , 

Friction versus displacement response from confined extension test 

on Geosynthetic PP-15 at a normal stress of 10 psi . , . . . . , . . , , . , , 

Comparison of GeoSyntec and Polytechnic test results (secant 

moduli of Geosynthetics PP-10 and PP-15 confined in beach sand) , . . . 

Comparison of GeoSyntec and Polytechnic test results (secant 

moduli of Geosynthetics PP-10 and PP-15 confined in silty sand) , . . , , 

Comparison of secant moduli at 5 percent strain for Geosynthetics PP-10 

and PP-15 confined in beach sand obtained by GeoSyntec and Polytechnic 

with data of similar geosynthetic materials reported by other 

researchers , . , . . . . , . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

Comparison of GeoSyntec and Polytechnic test results 

(peak strengths of Geosynthetics PP-10 and PP-15 confined in 

beach sand and silty sand) . . . . . . . . . . . . I . . . . . . . . . I . . I . . . 

. 84 

. 99 

101 

105 

107 

112 

113 

114 

115 

vii 
LIST OF FIGURES (continued) 

Figure Page 

A-l 

A-2 

A-3 

A-4 

A-5 

A-6 

A-7 

A-8 

A-9 

A-10 

B-l 

B-2 

B-3 

B-4 

B-5 

B-6 

B-7 

B-8 

B-9 

B-10 

Tensile force versus strain response for Geosynthetic PP-10 

confined in beach sand . . . . . . . . . . . . . . . . . . . . . . , . . . . . . . . 128 

Tensile force versus strain response for Geosynthetic PP- 11 

confined in beach sand . . . . . . . . . . . . . . . . . . . . , . . . . . . . . . . 129 

Tensile force versus strain response for Geosynthetic PP-12 

confined in beach sand . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130 

Tensile force versus strain response for Geosynthetic PE-13 

confined in beach sand . . , . , . . . . . . . . , . . . . . . . . . . . , . . . . . 131 

Tensile force versus strain response for Geosynthetic PET-14 

confined in beach sand . . . . . . . . . . . . , . . . . . . . . . . , . , , . . . , 132 

Tensile force versus strain response for Geosynthetic PP-10 

confined in silty sand . . . . . . . . . . . . . , . . . . . . , . . , . . . . . . . . 133 

Tensile force versus strain response for Geosynthetic PP-11 

confined in silty sand . . . . . . . . . . . . . . . . , . . . . . . . . . . . . . . . 134 

Tensile force versus strain response for Geosynthetic PP-12 

confined in silty sand . . . . . . . . . . . , . . . . . . . . . . . . . . . . . . . . 135 

Tensile force versus strain response for Geosynthetic PE- 13 

confined in silty sand . , . , , . . . . . . . . . . . . . . . . . . . . . . . . . . . 136 

Tensile force versus strain response for Geosynthetic PET-14 

confined in silty sand , , . . . . . . . . . , . . . . . . . . . . . . . . . . . . . . 137 

Strain versus time response for Geosynthetic PP-10 

confined in beach sand . . . . . . . , . . . . . . , , . . . . . . . . . . . . . . . 139 

Incremental strain rate versus time response for Geosynmetic PP-10 

confined in beach sand . . . . . . . . . . . . . , , . , . . . . . . . , . . . . . . 140 

Incremental strain rate versus strain response for Geosynthetic PP-10 

confined in beach sand , . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141 

Strain versus time response for Geosynthetic PP-11 

confined in beach sand . , . . , , , . . . . . . , , . . . . . . . . . , . . . . . . 142 

Incremental strain rate versus time response for Geosynthetic PP- 11 

confined in beach sand . . . , . . . . . . . . . . . . . . . . . . . . . . . . . . . 143 

Incremental strain rate versus strain response for Geosynthetic PP-11 

confined in beach sand . , , . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144 

Strain versus time response for Geosynthetic PP-12 

confined in beach sand . . . . . . . . . . . . . . . . . . . . . . . . . . . . , . . 145 

Incremental strain rate versus time response for Geosynthetic PP-12 

confined in beach sand , . , . , . . . . . . . . , . . . . . . . . . . . . . . . . . 146 

Incremental strain rate versus strain response for Geosynthetic PP-12 

confined in beach sand . . . . . . . . . . . . . . . . . . . , , . . . . . . . , . . 147 

Strain versus time response for Geosynthetic PE-13 

confined in beach sand . . , . . . . . . . , , , . . . . . . . . . . . . . , . . . . 148 

a*. Vlll 
LIST OF FIGURES (continued) 

Fimre 

B-11 

B-12 

B-13 

B-14 

B-15 

B-16 

B-17 

B-18 

B-19 

B-20 

B-21 

B-22 

B-23 

B-24 

B-25 

B-26 

B-27 

B-28 

B-29 

B-30 

Incremental strain rate versus time response for Geosynthetic PE-13 

confined in beach sand . . . . . . . . . . . . . . , , , . , . . . . . . . . . . . . 149 

Incremental strain rate versus strain response for Geosynthetic PE-13 

confined in beach sand . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 150 

Strain versus time response for Geosynthetic PET-14 

confined in beach sand . . . . . . . . . . . . . . . . . . . . . . . . . . . , . . . 151 

Incremental strain rate versus time response for Geosynthetic PET-14 

confined in beach sand . . . . . . . . . . . , . #. . . . . . . . . . . , . . . . . . 152 

Incremental strain rate versus strain response for Geosynthetic PET-14 

confined in beach sand . . . . , . . . . . . . . . . . . . . . . . . . . . . . . , , 153 

Strain versus time response for Geosynthetic PP-10 

confined in silty sand . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154 

Incremental strain rate versus time response for Geosynthetic PP-10 

confined in silty sand . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155 

Incremental strain rate versus strain response for Geosynthetic PP-10 

confined in silty sand . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156 

Strain versus time response for Geosynthetic PP-11 

confined in silty sand . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157 

Incremental strain rate versus time response for Geosynthetic PP- 11 

confined in silty sand . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158 

Incremental strain rate versus strain response for Geosynthetic PP- 11 

confined in silty sand . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159 

Strain versus time response for Geosynthetic PP-12 

confined in silty sand . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160 

Incremental strain rate versus time response for Geosynthetic PP-12 

confined in silty sand . . . . . . . . , . . . . . . . . . . . . . . . . . . . . . . . 161 

Incremental strain rate versus strain response for Geosynthetic PP-12 

confined in silty sand . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 162 

Strain versus time response for Geosynthetic PE-13 

confined in silty sand . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163 

Incremental strain rate versus time response for Geosynthetic PE-13 

confined in silty sand . . . , . . . . . . . . . . . . . . . . . . . . . . . . . . . . 164 

Incremental strain rate versus strain response for Geosynthetic PE-13 

confined in silty sand . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165 

Strain versus time response for Geosynthetic PET-14 

confined in silty sand . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166 

Incremental strain rate versus time response for Geosynthetic PET-14 

confined in silty sand . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 167 

Incremental strain rate versus strain response for Geosynthetic PET-14 

confined in silty sand . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 168 

ix 
Table 

LIST OF TABLES 

Pape 

5 

6 

7 

8 

9 

10 

11 

12 

13 

14 

15 

Summary of geosynthetic material properties . . . . . . , , , . . . . . . . . . . . . 4 

Summary of soil properties and shear strength test results , , , , . . . . . , . . , 5 

Summary of selected references in confined extension and creep tests . . . 20-21 

Summary of test conditions for confined and unconfined extension tests in 

beach sand . , , . . . . . . , . . . . . . . . . . . . . . . . . . . . , . . , . . . . . . . 76 

Summary of test conditions for confined and unconfined extension tests in 

silty sand . . . . , , , . . . , . . . . . . . . . . . . . . . . . , , . . . . . . . . . . . . 77 

Summary of secant moduli at select strains, peak strength, and strain at 

peak for unconfined and confined extension tests in beach sand , , . . , . , . . 79 

Summary of secant moduli at select strains, peak strength, and strain at 

peak for unconfined and confined extension tests in silty sand . , . . . . . , . , 80 

Summary of test conditions for confined and unconfined creep tests in 

beach sand . , , , . . . . . , , , . . . . . . . . . . . . . . . , . , , . . . . . . . . . . 87 

Summary of test conditions for confined and unconfined creep tests in 

silty sand . . , . , , . . . . . , , + . . . . . . . . . . . . . , , , , , , , . . . . . . , . 88 

Summary of total strains at select times for unconfined and 

confined tension creep tests in beach sand . . . . , . . . . . , , , . . . . . . . . . 90 

Summary of total strains at select times for unconfined and 

confined tension creep tests in silty sand . . . . . . . . . . . . , , , . . , , , , . . 91 

Summary of incremental strain rates at select times for confined and 

unconfined tension creep tests in beach sand . . . . . , , . , . , , , . . . . . , . 92 

Summary of incremental strain rates at select times for unconfined 

and confined tension creep tests in silty sand . . . . . , , . , . . , , . , , . , . . 93 

Summary of unconfined and confined extension test conditions 

conducted at Polytechnic University . . . . . . . . . . . , , . , . . . . . . . . . . 104 

Summary of unconfined and confined extension test results 

conducted at Polytechnic University . . . . . . . . . . . . . , , . . . . . , . , . . 109 
INTRODUCTION 

This report presents development and verification of the testing protocol and protocol 

equipment for confined extension testing and confined creep testing (Tasks C and G) for the 

research program titled, “Durability of Geosythetics for Highway Applications” DTFH61- 

91-R-00054. The research was primarily conducted by GeoSyntec Consultants. 

1.1 OBJECTIVE AND SCOPE 

The laboratory testing program presented was developed to characterize the confined stressstrain response of geosynthetic materials in engineering design applications. The results are 

anticipated to improve the characterization that is currently used in engineering and analysis 

and to allow considerable material savings in load bearing applications. 

The specific objectives of the research program were to: (i) develop the protocol test 

equipment for evaluating confined stress-strain properties of geosynthetics; (ii) establish 

protocol testing procedures for conducting confined extension and confined creep tests on 

geosynthetics; (iii) verify the protocol test equipment and procedures through testing of 

representative geosynthetics; and (iv) develop a written protocol for confined extension and 

creep testing. In addition, a complementary confined extension testing program was 

conducted at Polytechnic University (Polytechnic) to evaluate the confined response of 

selected geotextile materials using a modified triaxial-type test apparatus. 

A scope of work was developed to achieve these objectives and to meet the stated purpose. 

This scope of work included the following four-phased technical approach: 

a Phase I: Initial Testing and Selection of Test Equipment - This initial phase included 

extensive review of the technical literature regarding the evaluation of soil 

confinement effects on the stress-strain properties of geosynthetics. This phase also 

included the selection of testing equipment and testing materials, and preliminary 

confined and unconfined testing using a large pullout test device and a prototype 

confined extension testing device. 
l Phase II: Development of Confined Extension/Creep Test Equipment - This phase 

included the design, fabrication, and calibration of the protocol test equipment for 

conducting confined extension/creep tests. Calibration tests were conducted to 

evaluate the overall performance of the protocol equipment. 

l Phase III: Verification of Confined Extension/Creep Test Equipment and Procedures - 

This phase included establishment of test procedures, selection of test conditions, 

verification of the protocol equipment through testing of representative geosynthetics 

under a specific set of test conditions, and assessment of the test results using the 

triaxial-type test device developed by Polytechnic. 

l Phase IV: Development of Protocol for Confined Extension/Creep Testing and 

Equipment - This phase included a discussion on the key factors considered in 

developing the written testing protocol. 

1.2 TEST MATERIALS 

a. Geosynthetic Materials 

Seven geosynthetic materials were used in the testing program. These materials are 

referenced by name in this report, and include: 

l a polypropylene staple fiber needle-punched nonwoven geotextile having a mass per 

unit area of 8 oz/yd2 (272 g/m2), referred to as Geosynthetic PP-10; 

l a polypropylene slit-film multi-filament woven geotextile, referred to as Geosynthetic 

PP-11; 

l a polypropylene woven mono-filament geotextile, referred to Geosynthetic PP-12; 

l a uniaxial extruded polyethylene geogrid, referred to Geosynthetic PE-13; 

l a polyester multi-filament woven geogrid with PVC coating, referred to as 

Geosynthetic PET-14; 

2 
0 a polypropylene staple fiber needle-punched nonwoven geotextile having a mass per 

unit area of 8 oz/yd2 (272 g/m2), referred to as Geosynthetic PP-15; and 

l a polypropylene continuous fiber spun-bond nonwoven geotextile having a mass per 

unit area of 9 oz/yd2 (305 g/m2), referred to as Geosynthetic PP-16. 

The dimensional and mechanical properties of each geosynthetic material are summarized in 

table 1. Bulk samples of the seven geosynthetic materials were obtained directly from the 

respective geosynthetic manufacturers. 

b. Soil Materials 

A fine sand and a silty sand were used in the verification testing program. Both sand 

materials were obtained from project sites located in the southeastern United States and are 

herein referred to as beach sand and silty sand, respectively. Sufficient bulk quantities of the 

two soil materials were obtained to complete the entire testing program. Particle-size 

distribution, compaction characteristics, and internal shear strength of the two soil materials 

were evaluated and are summarized in table 2. The particle-size distribution curves for the 

two soil materials are presented in figure 1. 

1.3 ORGANIZATION OF REPORT 

The report is organized as follows: 

l The results of Phase I: Initial Testing and Selection of Test Equipment are presented 

in chapter 2. 

0 The results of Phase II: Development of Confined &tension/Creep Test Equipment are 

presented in chapter 3. 

a The results of Phase III: Verification of Confined Extension Creep Test Equipment 

and Procedures are presented in chapters 4, 5, and 6, as follows: 

0 Results of the verification testing program using confined wide-width 

equipment are presented in chapter 4. 

3 
Table 1. Summary of geosynthetic material properties.“) 

P 

Geosynthetic 

Materials 

VW of Structure or Polymer Mass Per Unit Direction of Aperture Size Ultimate 

Filament’*’ Manufacturing Composition Area”’ Testingc4) (in) Tensile 

Process (oz/yd2) 

CMD 

Strength 

(lb/ft) 

Geosynthetic PP- 10 Staple 

filament 

Nonwoven Polypropylene 8.0 MD NA NA 1 9 352(” 

Geosynthetic PP-11 Slit-film 

multi-filament 

Woven Polypropylene NA CMD NA NA 4,800 

Geosynthetic PP-12 Monofilament Woven Polypropylene NA MD NA NA 2,760 

Geosynthetic PE- 13 NP 

Extruded 

Punched and 

Drawn Sheet 

Polyethylene NA MD 4.0(5) 0.7@) 5,892 

Geosynthetic PET-14 Multi-filament Woven Polyester NA MD 3.2c5) 0.5@ 5,856(5) 

Geosynthetic PP-15 Staple Nonwoven Polypropylene 8.0 MD NA N/A 1 9 126c5) 

filament 

Geosynthetic PP- 16 Continuous 

filament 

Nonwoven Polypropylene 9.0(5) MD NA N/A 890c5) 

lin = 25.4 mm 

1 lb/ft = 14.59 N/m 

1 ozlyd* = 33.91 grim* 

Notes: (1) 

(2) 

(3) 

(4) 

(5) 

Information presented in this table was obtained from the 1996 Specifier’s Guide, Geotechnical Fabrics Report, December 1995. 

NP refers to the fact that the property is not applicable to the product. 

NA refers to the fact that the property was not reported. 

MD and CMD refer to the machine direction and cross-machine direction, respectively. 

The material properties were measured by GeoSyntec. 
Table 2. Summary of soil properties and shear strength test results. 

Soil 

Material 

Beach San 

Compaction Characteristics Shear Strength Parameters”) 

ASTM D 698 ASTM D 3080 

Particle Size Coefficient Coefficient 

ASTM D 422 of Uniformity of Curvature sop&ss~fi2~~ll Maximum Dry Optimum 

K”) (C3 Unit Weight Water Content Peak Residual 

WI mo) 

0% gravel SP (Poorly Graded 

98% Saud 2.2 1.2 Sand) 102.6 11.5 30” and 10 psf 29” and 0 psf 

2% fines 

1% gravel 

77% sand 

22% fines 

16.0 2.6 SM (Silty Sand) 98.0 21.5 

34” and 160 psf 30” and 95 psf 

1 psf = 47.88 Pa 

Note: (1) Shear strength parameters were determined from direct shear testing conducted on remolded beach sand or silty sand specimens that were 

compacted to 95 percent of their standard Proctor maximum dry unit weight at their optimum moisture content. Each test series was 

conducted at normal stresses ranging from 2 to 9 psi (14 to 63 kPa). 
100 

90 

80 

c4 

!2 

50 

z 

k 40 

20 

10 

0 

U.S. STANDARD SIEVE SIZES AND NUMBERS 

80 

Figure 1. Particle-size distribution curves for beach sand and silty sand. 
0 Results of the production testing program using confined wide-width equipment 

are presented in chapter 5. 

0 Results of confined extension testing using triaxial equipment are presented in 

chapter 6. 

l The results of Phase IV: Development of Protocol for Confined Extension/Creep 

Testing and Equipment are presented in chapter 7. 

l The conclusion and recommendations are presented in chapter 8. 

0 The following appendices are included: 

0 Confined extension test results are presented in appendix A. 

0 Confined creep test results are presented in appendix B. 

0 The protocol for confined extension/creep testing is presented in appendix C. 

1.4 REVIEWS 

The research tasks reported were initially developed by a project Interdisciplinary Advisor 

Team and formalized on a Task A Final Report, which formed the basis of the research 

program. 

The Task A Final Report was then reviewed by a Peer Advisory Group whose valuable 

suggestions were incorporated whenever possible prior to the commencement of the research. 

The Peer Advisory Group consisted of: 

1. Dr. Robert Koerner - Geosynthetic Research Institute 

2. Dr. Robert Holtz - University of Washington 

3. Dr. Robert Duvall - Engineering Systems Inc. 

4. Dr. Barry Christopher - Geoconsultant 

Prior to finalizing the results of the research, a draft report was further reviewed by the Peer 

Advisory Group, which made a number of suggestions to clarify some issues and improve 

the final product. These suggestions have been incorporated whenever possible in this final 

report. 

7 

CHAPTER 2 

INITIAL TESTING AND SELECTION OF TEST EQUIPMENT 

2.1 OVERVIEW AND LITERATURE REVIEW 

The initial component of this research program included an extensive review of the literature, 

an initial testing program using conventional pullout testing equipment, and an initial testing 

program using a prototype confined extension testing device. 

The literature review was conducted to identify and evaluate the testing equipment used by 

various researchers to measure the confined stress-strain properties of geosynthetic materials. 

The literature review also identified the range and variability of the confined extension test 

results obtained by these researchers. 

a. Test Equipment 

Confined extension tests have been previously conducted using various types of testing 

equipment. Characteristics of the testing equipment used for the confined extension testing in 

the selected references are described below: 

0 In-Soil Test Device by &Gown et al. [1982]: The apparatus consists of a confinement 

box containing two pressure bellows that are used for the application of normal stress 

to the test specimen as shown in figure 2. Testing with this type of equipment 

involves placing a geosynthetic specimen between two thin soil layers confined 

between the two pressure bellows. Confining pressure is applied to the test specimen 

by applying air pressure to the bellows. The box is then mounted vertically, and 

tensile loads are applied to the lower end of the geosynthetic specimen. This type of 

in-soil test apparatus was modified recently by Wilson-Fahmy et al. [1993]. Their 

modified apparatus was mounted in the horizontal direction to simplify testing; 

however, all of their other testing equipment conditions appear to be similar to those 

of the original McGown et al. [1982] device. 

9 
Fixed Clamp 

-Ai=? 

Pressure Beilow 

Confining Soil 

Geotextile 

Resin-Treated C 

Movable Clamp 

Load 

Figure 2. In-soil test apparatus (after McGown et al., 1982). 

10 
l Zero-Span Test by Christopher et al. [1986]: The device consists of a pair of 

pneumatic clamps that were used to confine the geosynthetic specimen as shown in 

figure 3. The two clamps are placed so that there is a zero gage length (i.e., no 

unconfined length of the test specimen) at the beginning of the test. Surface treatment 

is used on the clamp faces to reportedly simulate soil conditions. During testing, the 

upper clamp is designed to move apart from the lower clamp at a constant rate. 

Because separation between the two clamps gradually causes the central portion of the 

test specimen to be exposed to air (i.e., unconfined), the zero span test cannot be 

considered as a true confined test. 

l Pullout Test by Holtz [I9771 and Juran et al. [1991]: Testing with this type of 

equipment involves placing a geosynthetic specimen between two soil layers within a 

pullout box as shown in figures 4a and 4b. Confining pressure is then applied to the 

top of the upper soil layer using an air bag. During testing, tensile loads are applied 

to one end of the test specimen while the other end of the test specimen remains 

tension-free. Tell-tail wires or other deformation measuring devices may be placed 

along the geosynthetic specimen to monitor the development of deformation in the 

specimen during the test. It is noted that the pullout-type test is generally intended to 

measure the soil-geosynthetic interaction properties rather than the confined stressstrain properties of geosynthetic materials. However, the measured pullout response 

is related to the confined stress-strain properties of the geosynthetic specimen. With 

appropriate instrumentation and interpretation, it is expected that the confined stressstrain properties of the geosynthetic specimen can be explicitly decoupled from the 

pullout test results. 

l Triaxial Test by Ling et al. [I9911 and Wu [1991]: The device consists of two metal 

clamps, a pressure chamber, and a tension loading system as shown in figure 5. The 

metal clamps are fitted in the pressure chamber. The upper clamp is attached to the 

loading system and the lower clamp is fixed to the bottom of the pressure chamber. 

A typical test setup involves placing a geotextile specimen in a vertical orientation 

inside a cylindrical soil specimen that is encased by a rubber membrane. During 

testing, isotropic confining pressure is applied all around the test specimen, and a 

tensile load is applied to the upper end of the geosynthetic specimen. As reported by 

Ling et al. [1991], the confined extension testing using the modified triaxial apparatus 

achieves strain compatibility between the confining soil and the geosynthetic 

specimen. This simulates the operational conditions for many types of reinforced soil 

systems. Therefore, the tensile load along the geosynthetic specimen length is 

11 
-- H . 

n . 

---M--l 

Clamp 

G-textile 

Air Ram 

Figure 3. Zero-span test apparatus (after Christopher et al,, 1986). 

12 
Contining &ssure 

Figure 4a. Pullout test apparatus (cross-section view). 

(linearally variable differential transducer) 

LMT or 

Dial Gage 

I 

Figure 4b. Pullout test apparatus (plan view). 

13 
Load 

t 

Top Clamp 

Lucite Cylinder 

Rubber Membrane 

Bottom Clamp 

Steel Rod 

f 

-Cell Water (or Air) 

- Cylindrical .Soil Column 

- Geotextile 

- Resin-Treated 

Geotextlle 

Figure 5. Modified triaxial apparatus (after Wu, 1991). 

14 
constant and a true “element” test is possible. However, since the tensile strain at 

failure for many geosynthetics is tens or hundreds times the failure strain of soils, the 

strain compatibility conditions for working stress levels are only applicable during the 

initial part of the testing. Additionally, placing soils in a manner that is 

representative of field conditions is not possible with this device. 

0 The Plane Strain Unit Cell Device (UCD) by Boyle [199.5]: The UCD is a loadcontrolled test apparatus in which vertical pressures at the bottom and on the top of 

the test specimen are continuously increased until the test specimen reaches some limit 

state as shown in figure 6. The UCD simulates stress conditions that an element 

within a geosynthetic reinforced wall would likely experience during construction, 

The geosynthetic specimen is typically loaded at varying strain rates ranging from 

0.035 to 0.25 percent/min during the actual test, although different strain rates or 

stress-controlled loading is possible. Boyle concluded that the effective confining 

pressure in the soil increased as the strain in the reinforcement increased, thus 

explaining the increase in reinforced soil strength relative to unreinforced soil tested 

at constant confining pressure. Therefore, the UCD may more closely model 

reinforced soil behavior, but likely will not be able to assess the geosynthetic response 

under constant confining pressure. Consequently, it may be difficult, if not 

impossible, to make a direct comparison between the unconfined stress-strain 

properties obtained from the wide-width tensile test and the confined stress-strain 

properties obtained from the UCD test. It should be noted that the UCD device was 

specially developed to study the interaction of soil and geosynthetics and is likely too 

complex for routine testing, although the device presents significant advantages to the 

industry for studying soil-geosynthetic interaction. 

0 The Automated Plane Strain Reinforcement (APSR) Cell by Whittle et al. [1993]: The 

APSR cell measures major tensile forces transferred to a planar reinforcement as the 

surrounding soil matrix is deformed in a plane strain compression mode as shown in 

figure 7. Similar to the UCD, the APSR cell simulates stress conditions that a 

reinforcement element in a geosynthetic-reinforced wall would experience during 

construction. However, the APSR ceil was designed to study the load-transfer 

mechanism between the soil and the reinforcement and not specifically on the 

confined response of the reinforcement. Like the previously referenced UCD, the 

APSR is highly specialized and is not likely to be used for routine testing. 

15 
TOP DLnDDER VERTICAL DISPLACEMENT LVDT 

END OLADDER 

DO~OU DfJ,DDER J u VERTICAL DISPLACEMENT LVDT 

b) f 02 LOAD CEUS 

LOAD CELL1 

\ SOIL AND REINFORCING 

Figure 6. Plane strain unit cell device (UCD) (Boyle, 1995). 

16 
Aluminum 

PlZlk 

CIamping Mcchnism 

, 

Platf&n Jacks 

Air Prcssurc\ 

Figure 7. Automated plane strain reinforcement (APSR) cell (Whittle et al., 1993), 

17 
e Tlze Modified Direct Shear Test Machine by Leshchinsky and Field [1987]: The device 

is similar to the pullout test device in which the rear of a geotextile specimen is fixed 

while the front of the geotextile is subjected to tensile loads as shown in figure 8. 

This test is perhaps best characterized as a “single-sided anchored pullout” test. Due 

to the mobilized friction between the soil and the geotextile, the applied tensile load at 

the moving end of the test specimen is only partially transferred to the rear of the test 

specimen. The confined stress-strain properties were obtained by taking account of 

the soil-geotextile friction during interpretation of test results. 

0 The In-Soil Creep Test Device by Fock and &Gown [1987]: This device is essentially 

the same as the in-soil extension test apparatus developed by McGown et al. [1982] as 

shown in figure 2. The only difference is that a lever arm system was mounted into 

the test frame to allow the geosynthetic specimen to be tested under constant load 

conditions. 

A summary of the various test apparatus details that were used for reference in this research 

project is presented in table 3. 

The confined test devices used by these researchers differ with respect to how the applied 

loads are transferred through the confined geosynthetic. As far as the loading method is 

concerned, all of the referenced testing devices can be categorized into the following two 

types of devices: 

0 “Direct-Loading ” Device: In the direct-loading device, tension is “directly” applied to 

a geosynthetic specimen at a constant rate of displacement or constant rate of strain. 

This type of test device includes: (i) the in-soil confined extension/creep test 

apparatus; (ii) the zero-span test apparatus; (iii) the pullout test device; (iv) the 

modified triaxial test device; and (v) the modified direct shear test device. 

0 “Indirect-Loading” Device: In the indirect-loading device, pressures are applied to the 

boundary of the confining soil, and tension within a geosynthetic specimen is 

developed through the soil-geosynthetic interaction. This type of device includes: (i) 

the plane strain UCD and (ii) the APSR cell. 

18 
Fixed 

Moving End 

of Sample 

^L 

Greased Late 

Membrane - 

Load Applied by 

Screw Jack Annnnnfxnn c )’ - Linear Bearings 

+- Rail 

(Not to Scale) 

Figure 8. Modified direct shear test machine (Leshchinsky and Field, 1987). 
Table 3. Summary of selected references in confined extension and creep tests. 

Investigator Geosynthetic 

Types 

Soil”’ Geosynthetic Testing Device Description Comments 

Specimen Size 

WxL(mm) 

McGown et al. 

[1982] 

Wilson-Fahmy et al. 

[1993] 

Christopher et al. 

[1986] 

Woven and nonwoven geotextile 

Woven and nonwoven geotextile, 

geomembrane, geonet, and geosynthetic 

clay liner 

Woven and nonwoven geotextile 

Sand 

Sand 

N/A 

200x100 

200x100 

75x75 

Two pressurized soil boxes 

that are clamped together 

and sandwich geosynthetics 

Two pressurized soil boxes 

that are clamped together 

and sandwich geosynthetics 

Tensile test using large 

pneumatic clamps with 

textured surfaces 

Soil-geotextile interaction minimized by 

lubrication. Measured confined stress-strain 

response. 

Soil-geosynthetic interaction minimized by 

lubrication. Measured confined stress-strain 

response. 

No soil used. Clamps simulate soil 

confinement. 

Holtz 

[1977] 

Woven geotextile Sand 150x1100 Pullout test device Long slender specimen. Magnetic 

positioners used to evaluate differential 

strain. 

Fabian and Fourie 

[1988] 

Woven and nonwoven geotextile Clay 500x700 Pullout test with the 

geosynthetic fixed at the rear 

Tensile loads measured at both ends. 

Significant friction mobilized between soil 

and geotextile. 

Juran et al. [1991] Geogrids Sand 300x900 Pullout test device Measured pullout response of geogrids and 

calculated confined stress-strain properties 

of geogrids. 

Ling et al. [1991] Nonwoven geotextile NIA 300x38 Modified triaxial device Rubber membrane was in direct contact 

with geotextile. Measured confined stressstrain response. 

wu [1991] Nonwoven geotextile Sand 150x50 Modified triaxial device Strain compatibility between soil and 

geotextile. Measured confined stress-strain 

response. 
Table 3. Summary of selected references in confined extension and creep tests (continued). 

Investigator Geosynthetic 

Types 

Soil”’ Geosynthetic Testing Device Description Comments 

Specimen Size 

WxL(mm) 

Boyle [ 19951 Woven and nonwoven geotextile 

Steel sheet 

Sand 200x100 Unit cell device A load control test apparatus that increases 

vertical stress on a geosynthetic reinforced 

soil specimen until soil fails or reaches 

critical state. Confining stress varied 

during testing. 

Whittle et al. [1993] Steel Sheet Sand 360x50 Automatic plain strain Measured loads transferred from confining 

reinforcement cell soil to reinforcement as test specimen 

subjected to increasing vertical stress. 

Leshchinsky et al. 

[1987] 

Nonwoven geotextile Sand 200x100 Modified direct shear device Measured sand-geotextile interaction 

parameters and calculated confined stressstrain properties. 

Fock et al. [1982] Nonwoven geotextile Sand 200x100 In-soil creep test device Soil-geotextile interaction minimized by 

lubrication. Constant load applied using a 

lever arm system. 

Note: (1) N/A refers to the fact that the geosynthetic specimen was not confined in soils. 
Indirect-loading confined test devices were designed to simulate stress conditions that a 

geosynthetic reinforcement within a reinforced soil structure would typically experience 

during and/or after construction. However, relative to the unconfined response determined in 

accordance with ASTM D 4595 (the industry standard for assessing unconfined stress-strain 

characteristics of geosynthetics), the confined response measured in this type of test device 

reflects effects of test variables that may be difficult to quantify or isolate. It may be 

difficult to quantitatively separate the effect of soil confinement from the effects of other test 

variables. Boyle [1995] reported, that for two polypropylene woven geotextiles, the confined 

modulus at 5 percent strain measured in the plane strain UCD was between 50 and 80 

percent of that determined from the unconfined wide-width tensile tests. The indirect loading 

devices likely have the highest potential for assessing the actual soil-geosynthetic interaction 

mechanisms, because they rely on the soil applying load to the geosynthetic. This 

mechanism is typically what occurs in the field. These devices are, however, highly 

specialized and are not applicable for routine testing. One of the goals of this research 

program is to assess testing equipment specifically with the potential for comparing and 

contrasting confined/unconfined response. This research program should logically be 

extended to include comparison testing using indirect-loading test equipment. 

As far as the ability to provide a constant rate of strain or constant rate of displacement rate 

to the confined specimen is concerned, the direct-loading device appears superior to the 

indirect-loading device because it allows assessment of the confined material response. It is 

believed that the in-soil test apparatus and modified triaxial test device can provide a nearly 

constant strain rate over the test specimen length; the other three direct-loading devices 

provide a constant displacement rate to only one end of the geosynthetic specimen. The 

referenced authors believe that the confined response of a geosynthetic measured using the 

in-soil test apparatus and modified triaxial test device can be directly compared with the 

unconfined response (ASTM D 4595) of a geosynthetic material to assess the overall effect of 

soil confinement. 

b. Range of Test Results 

Although each of the previously introduced types of test equipment potentially influences the 

measured confined response of the geosynthetic specimen, each respective author discusses 

the potential influence resulting from the in-soil confinement. Most noteworthy is that each 

of the selected references reports some effect of confinement. Typically, the soil 

confinement effect on the stress-strain properties is realized by an increase in initial tangent 

modulus, secant moduli at various strain levels, and ultimate strength of the geosynthetic 

22 
specimen as compared with the results from in-air tension testing. To date, there has not 

been an attempt by any of the authors to directly compare the results found by the various 

investigators. For this report, a comparison was made of the test results reported in the 

selected references using the secant modulus at 5 percent strain for the various types of 

nonwoven geotextiles reported. The results of the improvement in the secant moduli at 5 

percent strain due to soil confinement for the nonwoven geotextiles reported in the selected 

references are shown in figure 9. 

Figure 9 shows a distinct influence of soil confinement on nonwoven geotextiles, but also 

indicates that the results, particularly the quantitative results, are scattered. As an example, 

for the needle-punched nonwoven geotextiles used during these investigations, the increase of 

moduli at 5 percent strain under a confining pressure of approximately 15 psi (104 kPa) 

varies from approximately 40 to 350 percent. For other geosynthetic materials, data are 

limited in the literature. However, Fabian and Fourie [1988] reported that the confined 

modulus at 5 percent strain increased approximately 15 percent in comparison with the 

unconfined modulus at the same strain level for a polypropylene woven geotextile confined in 

a silty clay under a confining pressure of 7 psi (48 kPa). For a polyester geogrid tested 

under a confining pressure of 7 psi (48 kPa), the confined modulus at 5 percent strain 

increased approximately 210 percent as reported by Juran et al. [ 19911. 

On the basis of the results of confined tests reported in the selected references, it appears 

that: 

0 There is a significant confinement effect on the modulus of nonwoven geotextiles. 

This is supported by the results presented in figure 9, which summarizes the increase 

in secant moduli at 5 percent strain due to soil confinement effects for several 

nonwoven geotextiles. 

a There may be a confinement effect on the tensile strength of nonwoven geotextiles. 

l There may (or may not) be confinement effects on the stress-strain properties of 

polyester geogrids and woven geotextiles; observed effects may be induced by the 

testing equipment. 

Each of the referenced different types of test equipment offers various advantages regarding 

ease of testing and ability to simulate actual field conditions. ,There are, however, 

disadvantages to each test device, particularly related to the ability to uncouple the confined 

23 
ooooo Christopher et al., 1986 

q oaao Christopher et al., 1986 

AAAAA td&own et al., 1984 

l .m.. Wilson-Fahmy et al., 1993 

n ==== Wilson-Fahmy et al., 1993 

AAAAA wu, 1991 

***** Boyle, 1995 

***** Boyle, 1995 

0 

0 

* 

. 

0 

* 

0 

0 

. 

00 5 10 15 20 25 

CONFINING PRESSURE (psi) 

1 psi = 6.89 kPa 

30 5 

Figure 9. Soil confinement effect on secant moduli at 5 percent strain for 

nonwoven geotextiles. 

24 
extension behavior of the geosynthetic specimen and the potential influence of the test 

equipment on the measured test results. These observations are based on the limited data 

presented in the selected references summarized in table 3. Furthermore, there was no 

reported instance where a direct comparison had been made of results from tests conducted 

on a common geosynthetic product using different test equipment or of a test program in 

which a particular test apparatus was used to conduct testing on a wide range of products. 

2.2 INITIAL BASELINE TESTING - PULLOUT TESTS 

The initial project goals were to: (i) select a testing device(s) that appeared to have 

significant promise with respect to the ability to assess the confined response of 

geosynthetics, and (ii) systematically evaluate the effects and influences on testing results of 

the equipment and the geosynthetic. Of particular importance was the ability to evaluate 

coupled and uncoupled material response using the selected test device(s). Two directloading devices were selected for this systematic study. The results of initial tests using the 

pullout test device are presented in this section; the results of initial tests using the confined 

wide-width test device are provided in section 2.3. 

a. Test Equipment 

The pullout testing device used for the initial baseline evaluation of confined stress-strain 

properties of geosynthetics consists of the following three major components: (i) a pullout 

box that has internal plan dimensions of 24 in by 60 in (610 mm by 1525 mm) and an overall 

depth of 12 in (305 mm); (ii) an air bladder system to apply confining pressure of up to 20 

psi (138 kPa) to a test specimen; and (iii) two 20,000-lb- (89-kN-) capacity hydraulic 

cylinders to apply pullout loads to the geosynthetic specimen. This device can be used for 

both in-air and in-soil testing under a wide range of confining pressures. The schematic 

diagram of the pullout test device used for the initial baseline ‘testing program is shown in 

figure 10. 

b. Test Procedures 

Three series of pullout tests were conducted on a needle-punched nonwoven geotextile 

(Geosynthetic PP-15) confined in beach sand. 

25 
Figure 10. Schematic diagram of pullout test device. 

tlllour Load 
For all the tests, the length of geosynthetic specimen was 48 in (1219 mm) while the width 

varied from 4 to 18 in (102 to 457 mm). The 48-in- (1219~mm) long specimen was selected 

with the intention to achieve a rupture failure mode for the geotextile specimen at the 

selected confining pressures based on experience. This would allow the ft.111 -range of stressstrain properties to be evaluated for the geotextile specimen in the pullout test. 

For each pullout test series, tests were conducted in accordance with the following 

procedures: 

Beach sand was placed in the lower half of the pullout box and compacted by hand 

tamping to form a 6-in- (152~mm-) thick sand bedding layer. The sand was 

compacted to approximately 95 percent of its maximum dry unit weight as determined 

in a standard Proctor compaction test, but under dry conditions. 

The geosynthetic specimen was trimmed from the bulk sample and placed on top of 

the compacted sand bedding layer. Four tell-tail wires were connected to the selected 

locations on the test specimen as shown in figure 11. The front end of the geotextile 

specimen was cast into a low-temperature curing epoxy to form the specimen clamp. 

Additional beach sand was then placed in the upper half of the pullout box and 

compacted by hand tamping to form a 6-in- (152~mm-) thick layer above the 

geotextile specimen. The sand was compacted to approximately 95 percent of its 

maximum dry unit weight under dry conditions. 

A load cell was attached to the pullout loading harness that was connected to the 

clamping end of the test specimen. 

The normal stress was then applied to the top of the upper sand layer through the air 

bladder loading system. 

The geotextile specimen was then pulled out at a constant displacement rate of 0.04 

in/min (1 mm/min) as measured on the specimen clamp. The specimen was loaded 

until a constant or decreasing pullout load was recorded. The pullout load and 

displacement were monitored continuously during the test. 

27 
(Note: Nodes are longitudinally spaced 2 in apart) 

1 in = 25.4 m 

Figure 11. Schematic diagram showing tell-tail wires. 
It should be noted that the four tell-tail wires were connected to the nodes located within the 

6-in (152~mm) length of the geotextile specimen behind the clamp as shown in figure 11. 

This arrangement was intended to measure displacement variation along this short portion 

where the average tensile load in the geotextile was expected to be close to the applied 

pullout load. 

C. Test Results and Observation 

Three pullout test series were performed on Geosynthetic PP-15 using the procedures 

described in the previous section. For each test, the pullout load and displacement at the 

specimen clamp and in-soil displacements of the geotextile at the four selected points were 

measured via the tell-tail wires. The pullout force versus displacement curves for the three 

test series are presented in figures 12 through 14. Following is a discussion of the test 

results: 

l Test Series Pl was conducted to evaluate the precision of the test device and confirm 

the reproducibility of the test results. Each of the three tests in the test series was 

conducted under identical conditions as described in figure 12. Results of the tests 

plotted in terms of pullout force versus displacement are shown in figure 12. These 

data indicate excellent reproducibility. It was found that variation of the pullout 

forces at any displacement was generally within 5 percent of the mean value. 

0 Test Series P2 was conducted to evaluate the effect of specimen width on the pullout 

response. Tests were conducted on specimens that were all 48 -in (1,219 -mm) long 

but were 4, 8, 12, and 18 -in (102, 203, 305, and 457 -mm) wide, respectively; and 

specimens were tested under a normal stress of 7 psi (48 kPa). The test results 

presented in figure 13 indicate that the pullout stiffness, defined as the slope of the 

pullout force versus displacement curve, as well as the pullout resistance, generally 

increase as the specimen width increases. 

l Test Series P3 was conducted on the same size specimens as those used for Test 

Series P2, but at a different normal stress. The results are presented in figure 14. A 

consistent increase was observed of the pullout stiffness and the pullout resistance as 

the width of the specimen increased. 

29 
2000 

1800 

3 1400 

z 

2 1200 

G 

W 

+ 600 

0’ 

3 

2 

400 

TEST SERIES NUMBER Pl : GEOSYNTHETIC PP-15 

CONFINED IN BEACH SAND UNDER A NORMAL STRESS 

OF 7 psi. 

GEOTEXTILE SPECIMEN DIMENSIONS 

----------------------- 

c+wee 48 in long by 12 in wide 

OBBBEI 48 in long by 12 in wide 

- 48 in long by 12 in wide 

200 

0 

DISPLACEMENT MEASURED AT CLAMP (in) 

NOTE: Each test specimen failed due to rupture. 

1 in = 25.4 mm 

1 lb/ft = 14.59 N/m 

1 psi = 6.89 kPa 

Figure 12. Baseline pullout testing - test reproducibility. 

30 
CONFINED IN BEACH SAND UNDER A NORMAL STRESS 

\ 

a 1600 3 

GEOTEXTILE SPECIMEN DlMENSlO@ 

----------------------- 

o++eee 48 in long by 4 in wide 

w-e-a-a 48 in long by 8 in wide 

- 48 in long by 12 in wide 

+++++ 48 in long by 18 in wide 

I I I I I I I I I I I I I I 

0.0 0.5 

DI~~iACEhk?klT MEk’kRED *G Cd; (in) 

3.5 4.0 

NOTE: Each test specimen failed due to rupture. 

1 in = 25.4 mm 

1 lb/ft = 14.59 N/m 

1 psi = 6.89 kPa 

Figure 13. Baseline pullout testing - effect of specimen width at a normal stress of 7 psi. 

31 
TEST SERIES NUMBER P3: GEOSYNTHETIC PP- 15 

CONFINED IN BEACH SAND UNDER A NORMAL STRESS 

OF 14 psi. 

800 

600 

3 

GEOTEXTILE SPECIMEN DIMENSIONS 

--------_------_---- 

weee 48 in long by 4 in wide 

~~S+EI 48 in long by 8 in wide 

A-M-A+ 48 in long by 12 in wide 

+-++-+e 48 in long by 18 in wide 

0 q I I I I I I I I I I I I I 

0.0 0.5 1 .o 1.5 2.0 2.5 3.0 3.5 4 

DISPLACEMENT MEASURED AT CLAMP (in) 

3 

NOTE: Each test specimen failed due to rupture. 

1 in = 25.4 mm 

1 lb/ft = 14.59 N/m 

1 psi = 6.89 kPa 

Figure 14. Baseline pullout testing - effect of specimen width at a normal stress of 14 psi. 

32 
The in-soil displacement data at each selected node on a geosynthetic specimen obtained from 

a typical pullout test during Test Series P2 are shown in figure 15. For all three test series, 

each geosynthetic specimen was inspected at the conclusion of the test. It was observed that 

each geosynthetic specimen deformed nonuniformly immediately behind the clamp as shown 

in figure 16. The maximum reduction of the specimen width occurred approximately 2 to 4 

in (51 to 102 mm) away from the clamp and was the location where the rupture occurred. 

This nonuniform deformation is herein referred to as necking. 

d. Interpretation of Test Results 

An interpretation procedure was developed to obtain the uncoupled confined stress-strain 

properties of a geotextile specimen subjected to pullout. The procedure involves the analysis 

of a pullout test in which a very short element (i.e., a short portion of the test specimen 

immediately behind the clamp) is used, and it is assumed that the tensile force in the short 

element is essentially constant across its length, herein referred to as “constant load” 

approach. Accordingly, the tensile force is assumed to be equal to the applied pullout force 

across this element. The confined tensile force versus strain curve can be readily obtained 

by plotting the pullout force versus the average strain over the first element. It is noted that 

the confined stress-strain properties obtained using this procedure vary with the first element 

length as shown in figure 17. Figure 17 indicates that, as the length of the first element 

increased from an assumed base length, the confined stress-strain properties of geosynthetics 

were overestimated (i.e., the calculated response indicates a progressively stiffer response as 

the element length increases). To use progressively shorter specimens introduces excessive 

calibration influence effects due to the clamping system. 

An alternative interpretation procedure was used in which it was assumed that the force was 

distributed linearly across the element, herein referred to as “linearly -distributed load” 

approach. This procedure involves the estimation of the friction force mobilized along the 

element. On the basis of an internal friction angle of 30 degrees for the beach sand and an 

assumed sand-geotextile interaction coefficient of 0.8, it was estimated that the maximum 

friction force on the upper or lower surface of the 2-in- (51~mm-) long element at a normal 

stress of 7 psi (48 kPa) would be approximately 155 lb/ft (2.3 kN/m), The average force 

along the 2-in- (5 1 -mm-) long element would be 78 lb/ft (1.1 kN/m) less than the maximum 

force. Furthermore, the average tensile force versus strain curve over a 2-in- (5 l-mm-) long 

element is shown in figure 18. For comparison, an average tensile force versus strain curve 

over a 6-in- (152~mm-) long element is also shown in figure 18. The comparison indicates 

that change in the first element’s length can also influence the confined response of the 

geotextile. 

33 
4.0 

3.5 

A 

C ;I; 

2 3.0 

E 

z 

0 2.5 

P 

ti 

d * 2.0 

‘- 

I5 1.5 

I 

W 

3 

El ‘.O 

0 

0.5 

0.0 

TEST NUMBER P2C: GEOSYNTHETIC PP-15 CONFINED 

IN BEACH, SAND UNDER A NORMAL STRESS OF 7 psi 

c-)888f) DISPLACEMENT AT NODE 1 

QEHXM DlSPlACEtvlENT AT NODE 2 

w DISPLACEMENT AT NODE 3 f 

++-+++ DISPLACEMENT AT NODE 4 

i 

lin = 25.4 mm 

1 lb/ft = 14.59 N/m 

1 psi = 6.89 kPa 

100 200 300 ,400 500 600 700 800 900 1000 

TENSILE FORCE AT SPECIMEN CUiMP (Ib/ft) 

Figure 15. Displacements at select nodes along a test specimen using tell-tail wires. 

34 
(Not To Scale) 

Minimum 

Deformed 

Width 

- 

Low Normal \ 

Stress High Normal 

Stress 

(Note: Typical response of Geosynthetic PP-15 specinien observed at the completion of pullout test) 

Figure 16. Schematic diagram of deformed Geosynthetic W-15 after pullout test. 

35 
2000 

1800 

TEST NUMBER PZC: GEOSYNTHEXC PP-15 CONFINED 

IN BEACH SAND UNDER A NORMAL STRESS OF 7 psi 

1600 eeeea AVERAGE STRAIN BETWEEN NODES 1 AND 2 

ma-as-a AVERAGE STRAIN BETWEEN NODES 1 AND 4 

2 1400 

K 

D 

= 1200 

3 800 

600 

400 

200 

0 

cl 10 20 30 40 50 SO 

STRAIN (x) 

1 lb/ft = 14.59 N/m 

1 psi = 6.89 kPa 

Figure 17. Tension versus strain curves using the “constant load” approach. 

36 
1800 

1600 

TEST NUMBER P2C: GEOSYNTHETIC PP-15 CONFINED 

IN BEACH SAND UNDER A NORMAL STRESS OF 7 psi 

= AVERAGE STRAIN BETWEEN NODES 1 AND 2 

~f~fefl AVERAGE STRAIN BETWEEN NODES 1 AND 4 

A 1400 

Y 

2 1200 

$ 

(fl 

E 

1000 

$j 800 

2 

Y 

4 600 

I I I I I I I I I I 

cl 10 20 30 40 50 60 

STRAIN (x) 

1 lb/ft = 14.59 N/m 

1 psi = 6.89 kPa 

Figure 18. Tension versus strain curves using the “linearly -distributed load” approach. 

37 
As described in the previous section, each geotextile specimen deformed nonuniformly 

immediately behind the clamp, resulting in the narrowing of the specimen width (i.e., 

necking). The effect of necking on the strain distribution was investigated by measuring 

displacements at four locations (i.e., nodes) on a 6-in- (152~mm-) long portion of the test 

specimen behind the specimen clamp as schematically shown in figure 11. It was anticipated 

that the tensile force and strain would decrease along the 6-in- (152~mm-) long portion, 

However, the actual strain distribution along the 6-in (152~mm) length immediately behind 

the specimen clamp obtained from a typical pullout test indicated that the maximum strain 

occurred in the center element at high tensile loads as shown in figure 19. This unexpected 

strain distribution is likely a result of the necking along the front portion of the geotextile 

specimen and a restraining effect of the reinforced clamped end. 

e. Conclusions 

Confined testing in a pullout device is convenient because the equipment is readily available 

in many laboratories throughout the United States. Tests conducted using the pullout device 

during this and other testing programs show that there is an effect of soil confinement on the 

pullout response of geosynthetic specimens. However, the pullout test results and laboratory 

observations indicate that there are four major problems in evaluating the confining stressstrain properties of a nonwoven geotextile using the pullout test method. The problems are: 

(i) the necking of the geotextile, which affects the response of the geotextile; (ii) the actual 

“mobilized” aspect-ratio (i.e., ratio of the mobilized specimen width to length), which varies 

during the pullout test and thus makes stress distribution difficult to quantify; (iii) the actual 

strain rate, which varies along the geosynthetic specimen during the pullout test; and (iv) the 

arbitrary selection of the size of the first element length for interpretation. Because of these 

observations associated with the pullout test, it was warranted to consider other alternative 

tests that could be used to assess the confined behavior of geosynthetics. 

2.3 INITIAL BASELINE TESTING - CONFINED WIDE-WIDTH TEST 

a. Equipment 

Since unconfined stress-strain properties of geosynthetics are obtained from the wide-width 

tensile test conducted at a constant strain rate, it is logical to consider a similar testing 

approach for evaluating the confined response of the geosynthetic materials. A prototype 

confined extension test device was designed and fabricated as part of this initial testing 

program. The schematic diagram is shown in figure 20. The device was designed to operate 

38 
60 

OFigure 19. 

0 

GEOSYNTHETIC PP-15 CONflNED IN BEACH SAND 

UNDER A NORMAL STRESS OF 7 psi 

SPECIMEN DIMENSIONS: 4-6 in long by 12 in wide 

v PULLOUT FORCE: 200 Ib/ft 

M PULLOUT FORCE: 400 Ib/ft 

w PULLOUT FORCE: 600 Ib/ft 

M PULLOUT FORCE: 800 Ib/ft 

- PULLOUT FORCE: 1000 Ib/ft 

Q 

I I I I I T I I I ? I 

1 2 3 4 5 6 

GEOSYNTHETIC SPECIMEN LENGTH (in) 

1 in. = 25.4 mm 

Ib/ft = 14.59 N/m 

Strain distribution along Gin-long front portion of pullout spekinlen 

at various pullout forces. 

39 
Fixed to 

, ,, Reaction-Frame cirQtinder ,, , 

With Load Cell 

(Note: Not To Scale) 

Figure 20. Schematic diagram of prototype confined wide-width test device. 
within the pullout box. Confining pressures were applied to the test specimen using an air 

cylinder, and tensile loads were applied to the front clamp of the test specimen using the two 

hydraulic cylinders of the pullout box at a constant rate of strain. 

b. Testing Procedures 

Two confined extension test series were conducted on two nonwoven geotextiles 

(Geosynthetics PP-15 and PP-16) using the confined wide-width test device shown in figure 

20. 

l 

0 

0 

l 

l 

l 

Each test was conducted using the following test procedures: 

Beach sand was placed in the lower half of the containment box and compacted by 

hand tamping to form a 3-in- (76~mm-) thick bedding layer. The sand was compacted 

to approximately 95 percent of its maximum dry unit weight (standard Proctor test) 

under dry conditions. 

A geotextile specimen was trimmed from the bulk sample and placed on top of the 

compacted sand. Each end of the geotextile specimen was clamped between two steel 

plates, which form the test specimen clamps. 

Additional beach sand was then placed in the upper half of the containment box and 

compacted by hand tamping to form a 3-in- (76~mm-) thick layer of sand above the 

geotextile specimen. The sand was compacted to approximately 95 percent of its 

maximum dry unit weight under dry conditions. 

A load cell was then attached to the loading harness and connected to the other end of 

the front test specimen clamp. The rear end of the test specimen was attached to the 

specimen clamp and fixed to the rear of the pullout box. 

The normal stress was then applied to the test specimen through an air cylinder. 

The geotextile specimen was loaded under a constant rate of strain until tensile failure 

occurred. 

For purposes of these initial tests, attempts were not made to eliminate friction effects within 

the testing device. If the confined wide-width device was shown to have potential for this 

study, this variable would be assessed in subsequent investigations. The initial tests were 

conducted using a fixed soil thickness and a fixed geosynthetic length and width. 

41 
C. Test Results 

For each test in the two-test series, tensile stresses and strains were calculated on the basis of 

the initial specimen width and length. The calculated stresses and strains were used to 

develop the confined stress-strain curves for each geotextile specimen as shown in figures 21 

and 22. For comparison, the unconfined stress-strain curve of each geotextile that was 

obtained using the same device are also shown in the corresponding figures. The results of 

the two confined test series indicate that soil confinement improves the stiffness (i.e., 

modulus) of nonwoven geotextiles. 

d. Conclusion 

The prototype confined extension test apparatus appears to offer several advantages over the 

pullout testing equipment. These advantages include: (i) the relative ease of test set-up and 

operation under controlled conditions; (ii) a relatively constant aspect ratio of the 

geosynthetic specimen during testing (i.e., no necking observed); and (iii) a constant strain 

rate that can be applied to the test specimen. Because the geosynthetic specimen is subjected 

to a constant aspect ratio and constant rate of strain conditions during the confined extension 

test using the prototype confined extension test device, the confined response can be directly 

compared with the unconfined response to evaluate the effect of soil confinement. On the 

basis of these findings, it was concluded that the confined extension/creep testing protocol be 

developed using a device similar to the prototype confined extension test apparatus. The 

design of this testing equipment is presented in the chapter 3. 

42 
2000 

1600 

40( 

Note: 

TEST SERIES NUMBER Cl: GEOSYNTHETIC PPSPECIMEN LENGTH: 5 in 

SPECIMEN WIDTH: 10 in 

15 

- IN AIR 

- CONFINED IN BEACH SAND AT 2 psi 

- CONFINED IN BEACH SAND AT 4 psi 

~8f3~fl CONFINED IN BEACH SAND AT 10 psi 

cmeea CONFINED IN BEACH SAND AT 20 psi 

10 20 

ST;: (%) 

40 50 60 

Each test specimen failed due to rupture. 

1 in = 25.4 mm 

1 lb/ft = 14.59 N/m 

1 psi = 6.89 kPa 

Figure 21. Tensile force vesurs strain curves for Geosynthetic PP-15 confined in beach 

sand using prototype confined wide-width test device. 

43 
1400 

TEST SERIES NUMBER C2: GEOSYNTHETIC PP-16 

SPECIMEN LENGTH: 5 in 

SPECIMEN WIDTH: 10 in 

- IN AIR 

.--- CONFINED IN BEACH SAND AT 2 psi 

--- CONFINED IN BEACH SAND AT 10 psi 

1000 

STRAIN (sa) 

Note: Each test specimen failed due to rupture. 

1 in = 25.4 mm 

1 lb/ft = 14.59 N/m 

1 psi = 6.89 kPa 

Figure 22. Tensile force versus strain curves for Geosynthetic PP-16 confined in beach 

sand using prototype confined wide-width test device. 

44 
CHAPTER 3 

DEVELOPMENT OF CONFINED EXTENSION/ 

CREEP TEST EQUIPMENT 

3.1 OVERVIEW 

The prototype confined extension test device described in the previous section was designed 

to operate within a pullout box and was equipped with only one load cell to measure tensile 

loads applied to the front of the geosynthetic specimen. Furthermore, the prototype device 

was designed for conducting confined extension testing under a constant rate of strain 

condition, and special precautions were not initially introduced to eliminate friction. It was 

decided to design and fabricate a new test apparatus with two load cells and other new 

features so that it can be used for both confined extension and confined creep testing. The 

development of this new equipment is described in the following section. 

3.2 PROTOCOL EQUIPMENT 

a. Description of Protocol Equipment 

A new confined extension/creep test device (protocol device) was designed and fabricated. 

The protocol device is conceptually similar to the device presented in McGown et al. 

[1982], as will be discussed subsequently. The device consisted of the following major 

components: 

0 A rigid supporting table consisting of a 0.5-in- (13~mm-) thick steel plate welded to a 

rigid steel frame. The overall plan dimensions of the supporting table were 42 in 

(1067 mm) in length by 28 in (711 mm) in width. 

l A confinement box made of 0.5-in- (13~mm-) thick steel plates. The internal 

dimensions of the confinement box were 17 in by 12 in (432 mm by 305 mm) in plan 

and 6 in (152 mm) in depth. An air bladder system was incorporated into the top and 

bottom of the confinement box and was used for applying normal stresses to the test 

specimen. The air bladder was made of 0.125-in- (3-mm-) thick gum rubber. 

45 
0 A 6-in (152mm) diameter air cylinder for applying tensile loads to the geosynthetic 

specimen. The air cylinder was mounted on the front of the supporting table. For 

the confined extension test, a constant rate of strain was achieved by manually 

adjusting the rate of air flow into the air cylinder. For the confined creep test, a 

constant load was maintained by applying a constant air pressure into the air cylinder. 

A detailed, cross-section drawing of the test equipment is shown in figure 23. As shown, 

two steel rollers are placed between the bottom of the confinement box and the top of the 

supporting table, This setup allowed the confinement box to move together with the 

geosynthetic specimen in the horizontal direction during the confined extension/creep tests. 

The protocol device was designed for performing confined extension tests (i.e., constant rate 

of strain tests) and confined creep tests (i.e., constant load tests). For the confined extension 

testing, a constant strain rate was achieved by manually adjusting the rate of air flow into the 

air cylinder. For the confined creep testing, a constant load was achieved by maintaining a 

constant air pressure within the air cylinder. The protocol device was designed to have the 

following capacities: (i) a maximum tensile loading capacity of 10,000 lb (44.5 kN); (ii) a 

maximum confining stress capacity of 50 psi (345 kPa); (iii) a maximum geosynthetic 

specimen size of 8 in (203 mm) in width by 12 in (305 mm) in length; and (iv) a maximum 

confining soil thickness of 3 in (76 mm) above and below the geosynthetic specimen. 

The protocol device was equipped with two load cells and two linear variable differential 

transformers (LVDTs). As shown in figure 23, the front load cell was attached to the air 

cylinder and connected to the specimen clamp with the use of a clevis to allow direct 

measurement of tensile loads applied to the front of the geosynthetic specimen. The rear 

load cell was mounted on a steel plate that was welded to the rear reaction frame. The rear 

load cell was connected to the specimen clamp with a clevis and was used to measure the 

tensile loads transmitted to the rear of the test specimen. The two LVDTs were used to 

measure displacements at select locations along the specimen length. The load and 

displacement data were recorded with the use of a computer data acquisition system. 

The protocol test device is similar in principle to that used by McGown et al. [ 19821 and 

Wilson-Fahmy et al. [1993]. However, the device developed by McGown et al. and 

modified by Wilson-Fahmy et al. consisted of a stationary box rather than a moving box. 

For testing that utilized the stationary box, the soil-geosynthetic interaction was minimized by 

using lubricated membranes placed between the confining soil and pressure bellows or 

bladders. For the protocol test device, the soil-geosynthetic interaction was minimized by 

46 
Rear Lo 

LVDTs 

ell Upper Air Bladdy 

Confmement Box 

Front Load Cell 

\ 

Air Cylinder 

\ 

Supporting 

Table 

Stbl 

Roller Roller 

I+- 18" - 

l 42” 

1 in = 25.4 mm 

Figure 23. Detailed drawing of confined extension creep testing device. 
placing the entire box on low-friction steel rollers, as shown in figure 23. This modification 

greatly simplifies the test set-up procedures and allows for either constant rate of strain or 

constant load tests to be performed. 

b. Calibration of Protocol Equipment 

To ensure that the protocol confined extension/creep test device worked as designed, the test 

device was evaluated through calibration tests conducted in accordance with the following 

procedures: 

calibrate the pressure gages, load cells, and LVDTs that were to be mounted on the 

test device; 

adjust elevation of the air cylinder and the rear load cell so that a common center line 

was established through the loading harness and test specimen, as shown in figure 23; 

calibrate the mobilized friction that develops between the confinement box and steel 

rollers during a confined extension test; 

calibrate the confining pressure; 

calibrate strain distribution along the geosynthetic specimen to ensure uniformity of 

strain distribution along the test specimen length; 

conduct a confined creep test to evaluate variation of an applied constant load during 

the duration of testing; and 

conduct three confined extension tests on Geosynthetic PP-10 under the same 

confining pressure of 10 psi (69 kPa) to check reproducibilty of the test results. 

To evaluate the mobilized friction between the confinement box and steel rollers during 

testing, a test specimen was fabricated using Geosynthetic PP-10, and a tensile load was 

applied to the front of the test specimen. Tensile loads were simultaneously measured by the 

two load cells. The mobilized friction was obtained by subtracting the load measured by the 

rear load cell from the load measured by the front load cell. The results of this test are 

shown in figure 24. The maximum measured friction was approximately 9 lb (40 N). The 

curve in figure 24 indicates that the friction reached its peak shortly after application of 

48 
12 

10 

2 

0 

GEOSYNTHETIC PP-10 IN MACHINE DIRECTION 

CONFINED IN BEACH SAND AT A NORMAL 

STRESS OF 10 psi 

I I I I I I I I I I I I I 

40 80 120 160 200 240 2 

TIME (s) 

10 

1 si 

lpb 

= 6.895 kPa 

= 4.448 N 

NOTE: The friction curve was obtained by subtracting the load measured 

by the rear load cell from the load measured by the front load cell. 

Figure 24. Friction between confinement box and rollers in confined extekion 

test. 

49 
tensile load and reduced to a steady value of approximately 8 lb (36 N). The results of 

additional tests indicate that the mobilized frictions were approximately the same under 

different confining pressures. This observation is supported by the fact that the contact force 

between the steel rollers and confinement box does not vary under the applied air pressures. 

The mobilized friction simply equals the sum of the total weight (i.e., weight of soil and 

box) times the friction coefficient between steel rollers and confinement box. To further 

understand the friction developed between the confinement box and the steel rollers, the 

confined box was filled with soil and was pulled at the front end while the rear remained 

unattached to the supporting table. The peak load measured in this simple pull test was 8 lb 

(36 N), which is in good agreement with the results shown in figure 24. 

During the confined extension testing, a constant air pressure is applied into the air bladders 

and then transmitted to the geosynthetic specimen through the confining soil. Friction is 

anticipated to develop at the interface between the soil and the internal surfaces of the 

confinement box (i.e., side wall friction), and it is believed that this friction would reduce 

the overall pressure on the center plane of the soil specimen under which the geosynthetic 

specimen is located. A series of tests were performed to assess the overall pressure loss for 

the 6-in- (152~mm-) thick soil specimen. During each test, the upper bladder was filled with 

tap water and connected to a pressure gage but not directly connected to a pressure source. 

A constant air pressure was applied to the lower bladder, which pushed the soil against the 

upper bladder. Measured pressures in the upper bladder indicate a typical loss of 5 to 10 

percent of applied pressures ranging from 5 to 20 psi (35 to 138 kPa) in the lower bladder. 

However, based on experience in measuring confining pressures on geosynthetic specimens 

in a pullout box using earth pressure cells, the pressure in the central portion of the soil 

specimen is nearly the same as the applied pressure. A direct measurement of soil presslres 

immediately above or below the geosynthetic specimen was not performed during this 

calibration due to both the unavailability and reliability of small diameter earth pressure cells. 

Although it has been verified that the applied tensile load was transmitted to the rear of the 

geosynthetic specimen with a minor friction loss, a question remained as to how the applied 

tensile load is transmitted to the rear end of the test specimen. There was a concern about 

the uniformity of tension or strain distribution over the specimen length. This concern was 

addressed by evaluating the strain distribution along the geosynthetic specimen in a confined 

creep test (Test No. ClB) conducted as part of the confined creep testing program. Four 

tell-tail wires were attached to specific locations along the geosynthetic specimen as shown in 

figure 25. Displacements at these locations were measured by LVDTs connected to the telltail wires. The measured displacements were used to calculate average strains over each 

50 
LVDTs hn 

TeEWires 

Node 2 

Connected A\\\\\\\\\; 

Rear Clamp -iE$rngq1 1.33” 1 1.33” 

Constant 

Applied L 

I 

rY\\\\\\\\y 

Elem 1 

1.33” 

Front Clamp 

1 in = 25.4 mm 

Figure 25. Schematic diagrams showing four tell-tail wires. 
element. To show the strain distribution along the specimen length, the average strain over 

each element at an elapsed time of 100 hours is presented in figure 26. As shown in figure 

26, the strain distribution over the specimen length is approximately uniform. A larger strain 

within the center element (Element 2) is likely due to the necking ,effect of the test specimen 

that was observed after completion of the test. 

To evaluate the potential for the prototype equipment to maintain a constant load during a 

creep test, a test was conducted on Geosynthetic PP-10. Tensile loads were measured by the 

two load cells and recorded by the computer data acquisition system. The tensile load 

history of the front load cell is shown in figure 27. The maximum variation was 

approximately 410 lb (&45 N) at the tested load level of 1,000 lb (4.5 kN). With respect 

to the applied load level in this particular test, the variation of tensile load during the test 

was f 1 percent. 

The test reproducibility was assessed by conducting three tests on Geosynthetic PE-13, an 

unaxially drawn, high-density polyethylene geogrid, confined in the beach sand under a 

normal stress of 10 psi (69 kPa). Each specimen was tested under the same conditions. The 

results of the three tests are shown in figure 28 and indicate that the differences between the 

three confined stress-strain curves are relatively small. 

3.3 SUMMARY 

The test equipment was designed and fabricated for the confined extension/creep tests. 

Pressure gages, load cells, and LVDTs were calibrated by an external calibration agency 

prior to being installed onto the test equipment. Specific calibration tests were conducted, 

and the results of these specific calibration tests are summarized below: 

a The peak friction between the confinement box and steel rollers is on the order of 1 

percent of anticipated maximum tension in both the confined extension test and the 

confined creep test. 

l The loss of confining pressure because of side wall friction for a 6-in- (152mm-) 

thick soil specimen was 5 to 10 percent of the applied pressures in the range of 5 to 

20 psi (35 to 138 kPa). However, the confining pressure transmitted to the 

geosynthetic specimen in the confined extension and confined creep tests was 

considered the same as the pressure applied to the bladders on the basis of our 

experience. 

52 
m 28 Iy: 

p 24 

$20 

l- 

<I6 

7 

d 

pG 12 

u-l 

GEOSYNTHETIC PP- 10 IN MACHINE DIRECTION 

CONFINED IN SILTY SAND AT 20 psi 

TENSILE FORCE: 25% OF ULTIMATE STRENGTH 

AVERAGE STRAIN 

0 

------------------------------------------ 

0 0 

._-- 

b- ra m 

I w I 1 Wire 1 1 Wire 2 2 Wire 3 3 Wife 4 

SPECIMEN LENGTH (in) 

1 in = 25.4 mm 

1 psi = 6.89 kPa 

Figure 26. Strain distribution over each element along the specimen length. 

53 
1400 

TARGET CONSTANT LOAD: 1000 lb 

1200- AVERAGE LOAD OVER ENTlRE DURATION: 998 lb 

MINIMUM LOAD: 989 lb 

MAXIMUM LOAD: 1009 lb 

g IOOO-* 

2 

9 800 - 

!?I 

!z 

P 600 - 

cl 

W 

? 

k 400 - 

4 

200 - 

0 II II II II II II I I I I II l 

0 10 20 30 40 50 60 70 80 90 1 

TIME (h) 

i0 

1 lb = 4.448 N 

NOTE: It appears that the tensile load varied periodically. This eriodic 

variation is more like1 

x 

related to compression cycles of t R e air 

compressor used in t is testing program, 

Figure 27. Variation of tensile loads in confined creep test. 

54 
:’ ‘\ 

-+-J 

Y- 

> 

10000 

GEOSYNTHEK, PE- 13 IN MACHINE DIRECTION 

CONFINED IN BEACH SAND AT A NORMAL 

STRESS OF 10 psi 8000 

SPECIMEN WIDTH: 5 ribs (4.4 in ) 

SPECIMEN LENGTH: 2 aperture (13.1 in ) 

7000 GAGE LENGTH: 2 a er-ture 

STRAIN RATE: 10% P 

(13.1 in ) 

min 

6000 

5000 

4000 

3000 

2000 

1000 

0 

00000 TEST NO. 1 

000~0 TEST NO. 2 

*- TEST NO. 3 

0 2 4 :TRAl: (zl” 12 14 16 

1 in = 25.4mm 

1 lb/ft = 14.59 N/m 

1 psi = 6.89 kPa 

Figure 28. Reproducibility of confined extension test results. 

I I I I I I I I I I I I I I I 

55 
0 The distribution of tensile strain along the specimen length was nearly uniform. 

l The test equipment was able to provide a relatively constant tensile load to the test 

specimen for the creep tests, with a variation on the order of 1 percent of the 

anticipated creep loads. 

l The test equipment has the ability to produce consistent test results. 

It is noted that the allowable peak friction between the steel rollers and confinement box 

(i.e., the difference between the loads measured by the front and rear load cells) and the 

variation of a desired constant load during a creep test have not been established for the 

confined extension and creep tests so far. It appears that a relative measurement for each of 

these two variables is appropriate. Specifically, the friction is measured relative to an 

anticipated maximum tensile load of the geosynthetic test specimen, and the variation of 

desired constant load is measured relative to the applied creep load. As described 

previously, the friction is approximately 1 percent or less of anticipated maximum tensile 

load to be used in the confined extension and creep tests, and the variation of the applied 

creep load is approximately f 1 percent of anticipated creep load for the confined creep tests. 

These measured values are herein considered within an acceptable range and will be 

controlled within these limits during the actual confined extension/creep testing program. 

Based on the results of the Phase II program, the confined wide-width device, herein referred 

to as the protocol device, appears to offer the greatest potential to achieve the goals of the 

research program. As discussed in this section, this device is conceptually similar to devices 

presented by McGown et al. [1982] and Wilson-Fahmy et al. [1993]. The authors of this 

report note that these investigators reported that specific testing procedure efforts (i.e., 

lubricated bladders, thin soil specimens, etc.) influence the measured test results. These 

procedural effects were explicitly not assessed during Phase II but will be a specific focus of 

the Phase III program presented in the subsequent chapters. 

56 
VERIFICATION OF CONFINED EXTENSION/ 

CREEP TEST EQUIPMENT 

4.1 OVERVIEW 

It has been shown in the preceding chapter that the protocol confined extension/creep test 

device functioned appropriately and can be used for the verification testing program. The 

measured response of confined geosynthetics, however, may be influenced by other factors, 

which may not be related to the test device itself. These factors and their potential effects on 

the stress-strain properties of the confined geosynthetics comprise Phase III of the program 

and are discussed in the following sections. 

4.2 VERIFICATION TESTING PROGRAM 

a. General Testing Procedures 

A general test procedure was developed to evaluate the confined extension and creep 

behavior of geosynthetics using the protocol test equipment. This test procedure was used 

for each test conducted during the confined extension/creep testing program and is described 

below : 

l The soil material (i.e., beach sand or silty sand) was compacted into the lower half of 

the confinement box to form a lower confining layer. The upper surface of the lower 

confining layer was placed so that it would be level with the center line of the loading 

harness (i.e., air cylinder and the rear load cell). Each soil was compacted by hand 

tamping to a desired unit weight under specific moisture conditions; the beach sand 

was placed at 95 percent of its maximum dry unit weight under dry conditions and the 

silty sand was placed at 95 percent of its maximum dry unit weight at its optimum 

moisture content. The maximum dry unit weights for the two soil materials were 

determined in accordance with ASTM D 698. 

57 
l A geosynthetic specimen was trimmed from one of the bulk samples of the 

geosynthetic material. For the three geotextile materials, the test specimen was 

trimmed to have an aspect ratio (i.e., width to length ratio) of approximately 2: 1. 

For the two geogrid materials, the test specimen was trimmed in such way that it had 

a length containing two complete apertures in the direction of manufacture (i.e., 

machine direction) and a width containing five ribs in the cross-machine direction. 

The end of each geosynthetic specimen was cast in a low-temperature curing epoxy 

resin that was reinforced using layers of impregnated nonwoven geotextile to facilitate 

clamping of each geosynthetic. The epoxy resin was prepared by mixing equal 

volumes of epoxy compound and curing agent. During the 24-hour-long curing 

period, each end of the specimen was confined between smooth steel plates under a 

normal stress of 0.15 psi (1 k.Pa) to ensure a uniform casting, which facilitates 

clamping. 

0 After preparing the geosynthetic specimen, the test specimen was then placed on top 

of the lower confining soil layer. The two cast ends of the geosynthetic specimen 

were then connected to the front clevis and rear clevis components of the loading 

system, as shown previously in figure 23. 

l Two tell-tail wires were used to monitor displacement of the geosynthetic specimen 

within the confinement box. The wires were connected to the two ends of the 

geosynthetic specimen. Each wire was then connected to LVDTs mounted to the rear 

of the supporting table. 

l A pretension force was applied to the geosynthetic specimen to eliminate slack within 

the geosynthetic specimen. The amount of pretension force was preselected for each 

test. 

0 Additional soil was placed above the geosynthetic in the upper half of the confinement 

box and compacted by hand tamping to form the upper confining layer above the 

geosynthetic specimen. The soil was compacted by hand tamping to the same unit 

weight and was at the same specific moisture content as the lower confining soil 

layer. 

0 A normal stress was then applied to the test specimen by pressurizing the air bladder 

loading systems. A predetermined source pressure was applied simultaneously 

through the lower and upper air bladders. 

58 
l For the confined extension testing, each geosynthetic specimen was loaded under a 

constant rate of displacement, as measured on the front specimen clamp. For the 

confined creep testing, a constant load was applied to the test specimen in a relatively 

fast but controlled manner. 

The general test procedure involves the selection of specific values for five major test 

variables. These test variables include: 

the level of pretension force to be used in the confined extension and creep tests; 

the strain rate to be used in the confined extension test; 

the aspect ratio to be used in the confined extension and creep tests; 

the initial loading rate (rate of application of the constant tension load) to be used in 

the confined creep testing; 

the thickness of confining soil to be used in the confined extension and creep tests; 

and 

the magnitude of friction along the geosynthetic specimen during the confined 

extension tests. 

These factors (variables) may influence the confined response of geosynthetics. An 

appropriate value for each variable was selected on the basis of existing ASTM standards or 

was determined from conducting specific verification tests. 

b. Pretension Force 

Similar to what is already known about the in-air wide-width stress-strain properties of 

geosynthetics, the confined stress-strain response of a geosynthetic is also influenced by the 

pretension force applied to the geosynthetic specimen prior to initiation of the testing. The 

pretension force reduces or eliminates slack within the test specimen, provides the same 

initial stress-strain conditions for similar test specimens, and allows the results of the 

confined extension/creep tests to be compared with those of unconfined tests. It is noted that 

application of a pretension force on a test specimen is a laboratory testing procedure 

established for conditioning the test specimen so that it will have a repeatable starting point, 

59 
thus contributing to the comparison of the test results. Pretensioning may or may not 

simulate the actual behavior of the geosynthetic material deployed in a reinforced soil 

structure since the reinforcing geosynthetic may not be subjected to a specific amount of 

tensile force prior to the construction loading. 

Since the results of the confined extension and confined creep testing are to be compared 

with the results of standard wide-width tensile testing and unconfined creep testing, 

respectively, it was logical to use the procedures established in the current in-air wide-width 

tensile testing standard (Le., ASTM D 4595) for selection of the pretension force for the 

various geosynthetics. Following the procedures described in ASTM D 4595, a pretension 

force equal to 1.25 percent of the expected breaking force was applied to each geosynthetic 

specimen. However, at any given time, the total applied pretension force was not less than 

10 lb (45 N) or greater than 50 lb (222 N), as recommended in ASTM D 4595. It should be 

noted that the pretension force was applied to the geosynthetic specimen prior to placement 

of the soil above the geosynthetic specimen. 

C. Strain Rate 

The effect of strain rate on the unconfined stress-strain properties of geosynthetic materials 

has been studied by McGown et al. [1984]. The reported results indicate that a geosynthetic 

exhibits a higher modulus and greater ultimate tensile strength when tested at high rates of 

strain. The strain rate is likely to have a similar effect on the confined stress-strain 

properties of geosynthetics .For the confined creep testing, the strain rate also influences the 

total strain versus time response since it is recognized that a constant load can be applied to 

the test specimen under different strain/loading rates. 

A constant strain rate of 10 percent/min was selected to load each test specimen during this 

testing program. Selection of this specific rate was based on the procedures established in 

ASTM D 4595. It should be noted that geosynthetic materials used in reinforced soil 

structures may be subjected to faster or slower loading rates during construction and much 

slower strain rates during their service life time. Furthermore, for a safely designed 

reinforced soil structure, the strain rate in the reinforcing geosynthetic decreases with time 

after construction. Therefore, the constant strain rate test was designed to evaluate the 

stress-strain properties under a specific reproducible strain rate rather than to simulate actual 

loading conditions that the reinforcing geosynthetic within a reinforced soil structure would 

experience during its construction and/or service life time. 

60 
d. Aspect Ratio 

The effect of aspect ratio on measured stress-strain properties of a nonwoven geotextile was 

studied by conducting a four-test series of tensile tests on Geosynthetic PP-15 using aspect 

ratios varying from 1: 1 to 5: 1. The results of this test series are shown in figure 29; these 

results indicate that, as the aspect ratio increases, the nonwoven geotextile exhibits a higher 

modulus and a slightly higher strength. For woven geotextiles and geogrids, however, it has 

been reported that the increase of aspect ratio results in a reduction of strength (Myles and 

Carswell, 1986). 

For all of the geotextile materials used subsequently in this confined extension/creep testing 

program, an aspect ratio of approximately 2: 1 was selected as the test aspect ratio, based on 

the recommendations presented in ASTM D 4595. For the geogrid materials, a constant 

aspect ratio was not able to be used among the products tested in order to maintain complete 

apertures (i.e., a whole number of ribs) within the test specimens. It was decided that each 

geogrid specimen was to be trimmed in such way that it had two complete apertures in the 

machine direction and five ribs in the cross-machine direction. 

e. Initial Loading Rate 

Since the stress-strain properties of geosynthetic materials are strain-rate dependent, it is 

expected that variation of the initial loading rate will affect the total strain versus time curve 

in a long-term constant load creep test. To evaluate the effect of initial loading rates on the 

confined creep response of geosynthetics, two confined creep tests were conducted on 

Geosynthetic PE-13. For each test, the geosynthetic specimen was confined within the beach 

sand at a normal stress of 10 psi (69 kPa). In one test, a target load corresponding to 50 

percent of the ultimate tensile strength of the geogrid was applied to the test specimen at a 

strain rate of approximately 1 percent/min. In the other test, the same target load was 

applied to the test specimen at a strain rate of approximately 50 percent/min. The results of 

the two confined creep tests are shown in figure 30. As anticipated, figure 30 indicates that 

the initial portion of the total strain versus time curve is significantly affected by the loading 

rate. However, the difference between the two creep curves becomes smaller as time 

increases. More tests at different load rates need to be conducted in a future study to 

definitively confirm that the initial loading rate effect does not significantly affect the total 

creep response curve. 

61 
2800 

2400 - 

2 2000 - 

K 

D - 

; 1600 - 

is 

F? 1200 - 

W 

-I 

iz 

800 - 

400 - 

GEOSYNTHETIC PP-15 IN MACHINE DIRECTION 

-W/L = 1.0 - - I3-eaaa W/L = 2.0 

- W/L = 3.3 

m W/L = 5.c 

, 

1 lb/ft = 14.59 N/m 

Figure 29. Results of unconfined wide-width tensile tests using specimens with 

different aspect ratios. 

62 
16 

GEOSYNTHEllC PE-13 IN MACHINE 

14- DIRECTION UNDER 50% OF ULTlMATE LOAD 

CONFINED IN BEACH SAND AT 10 psi 

IZ- - LOADED AT A STRAIN RATE OF APPROX. l%/min 

-- - - LOADED AT A STRAIN RATE OF APPROX. 50w/min. 

10 -' 1 

TIME (h) 

1 psi = 6.895 kPa 

Figure 30. Results of two confined creep tests having different initial loading 

rates. 

63 
It has been shown that the initial strain rate at least influences the initial portion of total 

strain versus time curve obtained from the confined creep test. A specific loading rate is 

required for conducting the confined and unconfined creep tests so that the results of the 

creep tests conducted during this testing program are comparable. On the basis of the test 

procedure established in ASTM D 5262, a constant strain rate of 10 i-3 percent/per min was 

selected to load each creep test specimen. 

f. Confining Soil Thickness 

In the confined extension and creep tests, an applied normal stress is transferred to the 

geosynthetic specimen through the confining soil. The friction between the confining soil 

and the internal lateral surfaces of the confinement box (i.e., soil/side wall interface friction) 

may influence the normal stress transmitted to the geosynthetic specimen and thus affect the 

measured confined stress-strain properties of the geosynthetic. It was anticipated that there 

would be increasing soil/side wall friction for increasingly thicker confining soil layers, 

Therefore, the effect of confining soil thickness on the confined stress-strain properties of a 

nonwoven geotextile was investigated by conducting a series of tests, each conducted at a 

specific soil thickness. 

Geosynthetic PP-10 was selected for studying the thickness effect of the confining soil on the 

confined stress-strain properties of geosynthetics. The selection was based on the fact that 

the behavior of nonwoven geotextiles may be significantly affected by soil confinement. 

Therefore, the effect of soil thickness on the confined behavior should be readily defined by 

a series of tests in which this type of geotextile is confined within different thicknesses of 

soil. 

Four tests were performed using Geosynthetic PP-10. Each geosynthetic specimen was 

confined in the beach sand under a total normal stress of 10 psi (69 kPa) and tested under a 

constant strain rate of 10 percent/min. Four different thicknesses of the beach sand were 

used: 

0 soil thickness of 0.4 in (10 mm) above and below the geotextile specimen; 

0 soil thickness of 1.0 in (25 mm) above and below the geotextile specimen; 

0 soil thickness of 2.0 in (5 1 mm) above and below the geotextile specimen; and 

0 soil thickness of 3.0 in (76 mm) above and below the geotextile specimen. 

64 
It should be noted that the confinement box was designed to contain a typical 3.0-in- (76- 

mm-) thick soil specimen above and below the geosynthetic specimen. To confine the 

geosynthetic between thinner soil layers, a rigid substrate of appropriate thickness was first 

placed between the lower soil layer and lower air bladder. After compaction of the upper 

soil layer, another rigid substrate of appropriate thickness was placed between the upper soil 

layer and upper air bladder. 

The confined stress-strain curves obtained from the four tests are presented in figure 31, 

These data show that the confined stress-strain curves are almost identical for the tests 

conducted at soil thicknesses of 1, 2, and 3 in (25, 5 1, and 76 mm). It appears that varying 

the confining soil thickness from 1 to 3 in (25 to 76 mm) has little effect on the confined 

stress-strain behavior of the geosynthetic. Further, the geotextile specimens were each 

placed 2 in (51 mm) away from the side walls of the confinement box, which appears to be 

far away enough to eliminate or at least minimize the effect of the side wall friction. 

However, for the test conducted at a soil thickness of 0.4 in (10 mm), the results indicate the 

soil confinement effect was significant. Inspection of the test specimen after testing indicates 

that the geotextile specimen was only partially confined within the soil due to the fact that 

“cracks” developed through the thickness of the upper and lower thin soil layers. This likely 

explains why the confined stress-strain response was different, and specifically is less stiff 

and weaker than the response of geotextiles between the thicker soil layers. To avoid 

potential interpretation difficulties caused by cracking of the thin soil layer, it was 

preliminarily concluded that a soil thickness of 3 in (76 mm) above and below the geotextile 

specimen would be used for all of the confined extension and creep tests. 

4.3 FRICTION ALONG THE GEOSYNTHETIC SPECIMEN 

The uncharacteristic cracking observed when thin soil specimens were used, coupled with a 

corresponding difference in confined geosynthetic response, warranted additional 

investigation. It is believed that the observed confined response is due at least in part to the 

friction mobilized at the soil-geosynthetic interface. A study was conducted to investigate the 

effect of friction. Results are presented in this section. 

a. Strain Compatibility Between Confining Soil and Geosynthetic 

In the confined extension test, a geosynthetic specimen confined between two soil layers is 

65 
4000 

GEOSYNTHETIC PP- 10 IN MACHINE D 

CONFINED IN BEACH SAND AT A 

3500 NORMAL STRESS OF 10 psi 

2 

- THICKNESS OF SAND SPECIMEN: 0.4 in 

3000 e-eas+a THICKNESS OF SAND SPECIMEN: 1 .O in 

\ - THICKNESS OF SAND SPECIEMN: 2.0 in 

CXMXB THICKNESS OF SAND SPECIMEN: 3.0 in 

g 2500 SPECIMEN WIDTH: 8 in 

SPECIMEN LENGTH: 5 in 

8 GAGE LENGTH: 1 4 in 

fy 2000 STRAIN RATE: Ow/min 

2 

WJ 1500 

Gi 

6 1000 

I-- 

0 

d . lb . 2b . 3b * 40 ’ 50 

STRAIN (w) 

1 lb/ft = 14.59 N/m 

Figure 31. Results of confined extension tests when confined by various 

thickness of soil layer. 

66 
typically loaded in tension at a constant rate of strain (or an incrementally increasing load). 

As the geosynthetic specimen deforms upon application of tensile forces, the confining soil 

may deform with the geosynthetic specimen at the same rate (i.e., strain compatibility) or the 

slower rate (i.e., strain incompatibility), depending on: 

l strains of the geosynthetic and the confining soil at the failure (i.e., rupture of the 

geosynthetic and development of active state for the confining soil); and 

l boundary conditions imposed on the confining soil specimen. 

For a majority of geosynthetic materials used for reinforcement/stabilization, strains at failure 

are within the range of 10 to 20 percent while the confining soil requires only a small tensile 

strain (typically less than 0.5 percent) in granular media prior to the development of a peak 

failure condition or an “active” state-of-stress condition. The strain compatibility between 

the confining soil and the geosynthetic may exist during the initial stage of confined 

extension test. 

However, strain incompatibility between the confining soil and the geosynthetic is likely to 

develop during the later stage of the test. Therefore, the relative movement between the 

confining soil and geosynthetic specimen will occur in the later stage of the test, as shown in 

figure 32, and the friction will be mobilized along the soil-geosynthetic interface. 

It appears that the friction along the soil-geosynthetic interface cannot be totally eliminated 

due to the fact that there is a significant difference between the failure strains of the 

confining soil and the geosynthetic material. However, this friction can be reduced by two 

different methods: (i) imposing low-friction boundary conditions on the confining soil using 

procedures recommended by McGown et al. [1982] and Wilson-Fahmy et al. [1993], and/or 

(ii) allowing the confinement box to move during confined extension testing. Both of these 

methods for reducing the friction were evaluated and results are presented in the following 

section. 

b. Evaluation of Different Methods for Reducing Friction 

A testing program was conducted to evaluate the two different methods for reducing the 

friction between the confining soil and the geosynthetic specimen. Specifically, a series of 

confined extension tests was conducted to determine the confined response of a nonwoven 

geotextile (i.e., Geosynthetic PP-10) under the same normal stress condition of 10 psi (70 

67 
Specimen 

Clamp 

Lo 

IFigure 32. Cross-section of confined extension testing with lubricated boundary 

conditions: (a) initial configuration, (b) deformed configuration. 

68 
kPa) but with different lateral boundary conditions. Details of the lateral boundary 

conditions for each test are described below: 

l Test Number IA: The geotextile specimen was confined between two 0.4-in- (lo-mm-) 

thick layers of beach sand. The confinement box was placed on steel rollers and 

allowed to move during testing. No lubricated rubber sheets and latex membranes 

were placed on the top of the upper soil layer and below the bottom of the lower soil 

layer. 

l Test Number IB: The geotextile specimen was confined between two 0.4-in- (lo-mm-) 

thick layers of beach sand. The confinement box was placed on steel rollers and 

allowed to move during testing. Lubricated rubber sheets and latex membranes were 

placed on the top of the upper soil layer and below the bottom of the lower soil layer. 

l Test Number IC: The geotextile specimen was confined between two 0.4-in- (lo-mm-) 

thick layers of beach sand. Lubricated rubber sheets and latex membranes were 

placed on the top of the upper soil layer and below the bottom of the lower soil layer, 

The confinement box was fixed to the top of its supporting table. 

The results of the three confined extension tests are presented in a plot of tensile force versus 

average strain, as shown in figure 33. The comparison of the confined stress-strain curves in 

figure 33 indicates that the methods developed by McGown et al. [1982] and Wilson-Fahmy 

et al. [1993] for reducing the friction between the confining soil (i.e., lubricated rubber 

sheets) and the methods developed under this research (i.e., low-friction movable confining 

box) produce similar results. This indicates that the two methods can effectively reduce the 

friction between the confining soil and the geosynthetic specimen. In fact, it follows that the 

friction can be further reduced by imposing lower friction boundary conditions on the 

confining soil and allowing the confinement box to move during testing, as indicated by the 

results of Test 1B. 

An additional confined test (i.e., Test 1D) was conducted on Geotextile PP-10 with a 

confining soil thickness of 3 in (75 mm). No lubricated rubber sheets and latex membranes 

were placed on the top of the upper soil layer or below the bottom of the lower soil layer. 

The confined response from Test 1D is also shown in figure 33. Compared with the 

confined responses from the other three tests (i.e., Test lA, lB, and lC), the soil 

confinement effect was significantly improved. This is due to the fact that the geotextile 

specimen was fully confined within the soil during the entire test when the confining soil was 

69 
TEST NO. SOIL THICKNESS LUBRICATION BOX 

______________-_-___-------------- 

SPECIMEN WIDTH: 8 in 

SPECIMEN LENGTH: 5 in 

GAGE LENGTH: 5 in 

STRAIN l?ATE: lO%/min 

1 in = 25.4 mm 

1 lb/ft = 14.59 N/m 

Figure 33. Confined response of GeoSynthetic PP-10 under lubricated and 

unlubricated boundary conditions. 

70 
3-in (75mm) thick. As the confining soil thickness was reduced to 0.4 in (10 mm), cracks 

were observed to occur within the confining soil layer at the end of the test. It is believed 

that the geotextile specimen that is initially confined between two thin layers of soil may 

become only partially confined within the soil in the later stage of the test. This likely 

explains why the soil confinement effect was significantly reduced when the geotextile was 

confined between two 0.4-in- (lo-mm-) thick soil layers. A similar, although more subdued, 

response was observed when this test series was repeated using a woven geotextile (i.e., 

Geotextile PP-12). Results are presented in figure 34. 

C. Assessment of Test Results 

Assessment of results from confined tests, regardless of the type of laboratory test device and 

test configuration, ideally involves consideration of the degree to which the laboratory test 

simulates conditions within actual geosynthetic-reinforced structures under working 

conditions. It was demonstrated in chapter 2 that test devices exist, specifically the UCD and 

the APSR, which offer a potentially more realistic model of actual reinforced soil response 

than the relatively simple confined extension device used with the testing protocol developed 

herein, Specifically, the developers of the UCD and APSR note that the normal stress and 

shear stress along the confined geosynthetic specimen varies as the confining soil moves 

relative to the geosynthetic during the test. In contrast, the confined extension device 

provides a constant normal stress, which is selected prior to testing. 

While the UCD and APSR may offer a potential advantage with respect to the ability to 

model actual reinforced soil response, this potential advantage is tempered by the fact that 

actual stress/confinement conditions within geosynthetic-reinforced structures are usually 

uncertain. The UCD and APSR are also sophisticated devices that may not be accessible to 

most nonresearch organizations wanting to conduct confined creep testing. On the basis of 

these considerations, the confined extension device was selected for this program because of 

its demonstrated capability to measure the effect of controlled confinement on the response of 

geosynthetics to tensile loading. Designers of reinforced-soil structures will likely debate the 

most appropriate use of measured values from the confined extension device. Nevertheless, 

it is believed that, by using the confined extension device and selecting reasonable testing 

conditions, designers will obtain practical test results needed for design or for a comparative 

evaluation of candidate soil and geosynthetic materials. 

The confined extension device can model either full or partial confinement conditions and a 

protocol for either confinement condition was developed as part of this project. The 

71 
10000 

f-Y 

;ti 

1;- 8000 

- 

- 

Y v, 4000 

7 

P 

2000 

~Eosmn-imc w-1 1 (CROSS-MACHINE) 

IN BEACH SAND 

TEST NO. SOIL THICKNESS LUBRICATION BOX 

___---_-_--__-----__-------------- 

Q+=+=m 3.0-in NO MOVlNG 

- 2B: 0.4-in NO MOVING 

- 2c: 0.4-in 

- 20: 0.4-in E 

;;TA&IOtARY 

SPECIMEN WIDTH: 8 in 

SPECIMEN LENGTH: 5 in 

GAGE LENGTH: 5 in 

STRAIN RATE: lOw/min 

IllI III11 II 11 II II I 

0 2 4 6 8 10 12 14 16 18 ; 

STRAIN (w) 

1 in = 25.4 mm 

1 Ib/ft = 14.59 N/m 

Figure 34. Confined response of geosynthetic PP-11 under lub@cated and 

unlubricated boundary conditions. 

0 

72 
selection of testing conditions for the confined extension device therefore includes using 

partial or full confinement. A discussion regarding the difference between partial or full 

confinement is given below. 

0 Partial Confinement: It was demonstrated that the thin-soil test specimen coupled 

with lubricated rubber sheets provide full confinement in the early stages of the 

confined extension tests. However, as deformation continues, strain incompatibility 

of the soil and geotextile causes cracking of the soil and only partial confinement to 

the geotextile. It has been argued that this is an appropriate model for the reinforced 

soil within the active zone of a reinforced backfill. Since the partially confined 

response also shows a smaller effect of confinement compared with the fully confined 

response, it has been suggested that the geosynthetic response in the partially confined 

condition be utilized to provide a conservative design. 

* Full Conjinement: It was similarly demonstrated that, unless relatively extreme 

measures are taken to reduce soil-geosynthetic interface friction, only a relatively thin 

layer of soil (i.e., only 1 in (25 mm) is needed to provide full confinement and 

restrict the actual development of cracks in the soil. Therefore, this may better model 

the actual condition in the field. Furthermore, it can be argued that the fully confined 

condition models the anchorage zone of the reinforcement. 

It is noted that, for the production testing component of the testing program, a fully confined 

condition was utilized because of the consistency of the test results and the belief that partial 

confinement (and certainly no confinement) may provide an overly conservative stress-strain 

response. 

4.4 SUMMARY AND CONCLUSIONS 

A general test procedure was developed for conducting the confined extension/creep tests 

using the protocol confined extension/creep test equipment. The test procedure involved the 

selection of specific values for: (i) pretension force; (ii) strain rate for extension testing; (iii) 

aspect ratio; (iv) initial rate of loading for conducting creep testing; and (v) thickness of the 

confining soil. All of these variables except the confining soil thickness are associated with 

the test procedures for unconfined extension and unconfined creep tests. Standard values for 

these common variables have been established in ASTM D 4595 for the wide-width tensile 

testing and ASTM D 5262 for the unconfined tension creep testing. These standard values 

73 
were adopted in this test procedure for the confined extension/creep tests and are summarized 

below : 

l Pretension Force: A total pretension force equal to 1.25 percent of the expected 

breaking force, but not less than 10 lb (45 N) or greater than 50 lb (222 N), is to be 

applied to the geosynthetic specimen in the confined extension and creep tests. 

l Strain Rate: A constant strain rate of 10 percent/min is to be applied to the 

geosynthetic specimen during the confined or unconfined extension test. 

l Aspect Ratio: An aspect ratio of approximately 2: 1 for geotextiles is to be used in 

both the confined or unconfined extension and creep tests. For geogrids, each test 

specimen is to be trimmed in such way that it has two complete apertures in the 

machine direction and five ribs in the cross-machine direction. 

l Initial Loading Rate: A constant strain rate of 10 &3 percent/min. was selected to 

load each creep test specimen to a specific target load level. 

For the confining soil thickness, its potential effect on confined stress-strain properties was 

studied by performing four confined extension tests, each conducted at a different soil 

thickness ranging from 0.4 to 3 in (10 to 76 mm). The results of the tests conducted at soil 

thicknesses of 1 to 3 in (25 to 76 mm) were approximately the same, indicating that varying 

the confining soil thickness from 1 to 3 in (25 to 76 mm) has little effect on the confined 

stress-strain behavior of the tested geotextile. However, when the soil thickness was reduced 

to 0.4 in (10 mm), cracks developed through the thickness of the soil layers, and the soil 

confinement effect was significantly reduced. Because of the potential effect of test 

reproducibility and because a partially confined condition may provide an overly conservative 

estimate of the confined response, a 3-in (76~mm) thickness of confining soil was selected for 

the confined extension/creep testing program, which will be discussed in chapter 5. The 

selected testing device, however, can readily be used to conduct partially confined tests 

incorporating relatively thin soil specimens. 

74 
CHAPTER 5 

CONFINED EXTENSION/CREEP TESTING PROGRAM 

5.1 INTRODUCTION 

The test procedure for conducting the confined extension and creep tests was developed as 

described in chapter 4. Using this established test procedure, a confined extension/creep 

testing program was conducted. The testing program and the results of the confined 

extension/creep testing program are presented in the following sections. 

5.2 CONFINED AND UNCONFINED EXTENSION TESTING PROGRAM 

a. Test Matrix 

Ten test series were conducted in the confined and unconfined testing program. Each test 

series consisted of one unconfined and two confined extension tests on one of the five 

selected geosynthetic materials (i.e., Geosynthetics PP-10, PP-11, PP-12, PE-13, and PET14). For the confined extension tests, each geosynthetic material was confined within beach 

sand (Test Series TlA through T5A) or silty sand (Test Series TlB through TSB) under two 

different normal stresses (i.e., 10 and 20 psi (69 and 138 kPa)). A summary of test 

conditions used during the testing program is presented in table 4 (Test Series TlA through 

T5A) and table 5 (Test Series TlB through T5B). It should be noted that only one 

unconfined extension test wa., conducted for each geosynthetic material; the test result is 

presented in each of the two tables. 

b. Test Results 

For each confined extension test, the tension load and displacement on the test specimen 

were measured continuously during the test. The test data were then plotted as tension force 

versus strain. The tension force was defined as the total measured load divided by the initial 

width of the geosynthetic specimen (i.e., units of F/L (lb&t (1 lb/f? = 14.59N/m)) and the strain was 

defined as the total measured deformation of the test specimen withing the gage length deivided 

by the initial gage length of the geosynthetic specimen. The results of each confined extension 

test are presented as tension force versus strain curves in Appendix A. For each test series, the 

75 
Table 4. Summary of test conditions for confiied and unconfined extension tests in beach sand. 

Test Series Geosynthetic Confiing collfillillg Test Test Specimen Strain Rate 

Number Material Material Pressure Specimen Gage Length (%/min) 

(psi) Width 

Geosynthetic PP-10 in In-air 0 8.0 in 4.0 in 10 

TlA Machine Direction Beach Sand 10 8.0 in 4.0 in 10 

Beach Sand 20 8.0 in 4.0 in 10 

Geosynthetic PP-11 in In-air 0 8.0 in 4.0 in 10 

T2A Cross-Machine Beach Sand 10 8.0 in 4.0 ill 10 

Direction Beach Sand 20 8.0 in 4.0 in 10 

Geosynthetic PP-12 in In-air 0 8.0 in 4.0 in 10 

T3A Machine Direction Beach Sand 10 8.0 in 4.0 in 10 

Beach Sand 20 8.0 in 4.0 in 10 

Gemynthetic PE-13 in In-air 0 5 ribs 2 apertures 10 

T4A Machine Direction Beach Sand 10 5 ribs 2 apertures 10 

Beach Sand 20 5 ribs 2 apertures 10 

Geosynthetic PET-14 In-air 0 5 ribs 2 apertures 10 

T5A in Machine Direction Beach Sand 10 5 ribs 2 apertures 10 

Beach Sand 20 5 ribs 2 apertures 10 

1 in = 25.4 mm 

psi = 6.895 kJ?a 
Table 5. Summary of tests conditions for confined and unconfined extension tests in silty sand. 

Test Series 

Number 

TlB 

T2B 

T3B 

T4B 

T5B 

lin = 25.4 mm 

psi = 5.895 kPa 

Geosynthetic 

Material 

Geosynthetic PP-10 in 

Machine Direction 

Geosynthetic PP- 11 in In-air 

Cross-Machine Silty Sand 

Direction Silty Sand 

Geosynthetic PP- 12 in 

Machine Direction 

Geosynthetic PE- 13 in 

Machine Direction 

Geosynthetic PET-14 

in Machine Direction 

collfii 

Material 

In-air 

Silty Sand 

Silty Sand 

In-air 

Silty Sand 

Silty Sand 

In-air 

Silty Sand 

Silty Saud 

In-air 

Silty Sand 

Silty Sand 

0 8.0 in 

10 8.0 in 

20 8.0 in 

0 8.0 in 

10 8.0 in 

20 8.0 in 

0 8.0 in 

10 8.0 in 

20 8.0 in 

0 5 ribs 

10 5 ribs 

20 5 ribs 

0 5 ribs 

10 5 ribs 

20 5 ribs 

1 

4.0 in 10 

4.0 in 10 

4.0 in 10 

4.0 in 10 

4.0 in 10 

4.0 in 10 

4.0 in 10 

4.0 in 10 

4.0 in 10 

2 apertures 10 

2 apertures 10 

2 apertures 10 

2 apertures 10 

2 apertures 10 

2 apertures 10 
tension force versus strain curves at the three normal stresses (i.e., 0, 10, and 20 psi (0, 69, 

and 138 kPa)) are presented in a single plot. This presentation allows direct comparison 

between the measured confined and unconfined response of the geosynthetic material. 

For Geosynthetic PP-10, figures A-l and A-6 indicate that the soil confinement improves 

both the modulus and peak strength and significantly reduces the total strain at both the peak 

strength and failure. For Geosynthetics PP-11, PP- 12, PE-13, and PET-14, the results 

shown in figures A-2 through A-5 and A-7 through A-10 also indicate a noticeable 

improvement in modulus and strength resulting from soil confinement. However, the relative 

increase of modulus and strength resulting from soil confinement for these four geosynthetic 

materials appears to be much less than the increase noted for Geosynthetic PP-10 with the 

least effect noted for the uniaxial geogrid (Le., Geosynthetic PE-13). To further quantify the 

improvement resulting from soil confinement, secant moduli at 1, 2, 5, and 10 percent strain 

levels, peak strength, and strain at peak strength were calculated for each test. The results of 

these calculations are summarized in table 6 (Test Series TlA through T5A) and table 7 

(Test Series TlB through T5B). 

The calculated secant moduli at 5 percent strain presented in tables 6 and 7 were selected to 

further demonstrate the soil confinement effect on the confined response. The improvement 

of secant moduli at 5 percent strain was plotted versus confining pressure for each 

geosynthetic material confined in the beach sand and in the silty sand as shown in figures 35 

and 36, respectively. In these plots, “improvement” is defined as the ratio of the confined 

secant modulus to the unconfined secant modulus at the selected strain level. These figures 

clearly demonstrate that the nonwoven geotextile is significantly affected by soil confinement 

while the other four geosynthetic materials are much less affected by soil confinement. 

Figure 37 shows a comparison of the secant moduli at 5 percent strain for the nonwoven 

geotextile confined in the beach sand and the silty sand. The comparison indicates that the 

confinement effect of the beach sand is greater than that of the silty sand. The secant moduli 

at 5 percent strain of the nonwoven geotextile confined in the beach sand and the silty sand 

are also compared with those of similar nonwoven geotextiles reported by other researchers; 

this comparison is graphically presented in figure 38. The results from this testing program 

are similar. 

C. Interpretation 

The test results presented in the previous section clearly demonstrate that soil confinement 

improves the confined response of the needle-punched nonwoven geotextile; the observed 

78 
Table 6. Summary of secant moduli at select strains, peak strength, and strain at peak 

for unconfined and confined extension tests in beach sand. 

Modulus at Modulus at Modulus at 

2% Strain 5% Strain 10% Strain 

(lb/ft) (lblft) (lb/ft) 

Strain 

at Peak 

6) 

Strength 

(lb/ft) 

5141 4035 4292 1352 38.5 

12259 15915 11112 1813 33.8 

23201 22979 16891 2542 28.0 

Geosynthetic PP-11 in In-air 0 46174 45875 42298 38354 6020 17.9 

Cross-Machine Beach Sand 10 56703 53805 57533 4837 1 6547 17.1 

Direction Beach Sand 20 54068 54746 57265 48945 7432 19.0 

In-air 0 38295 38219 28548 22344 3400 22.6 

Beach Sand 10 53611 44944 32902 26085 3845 21.0 

Beach Sand 20 65556 50076 36063 29067 4228 20.6 

T2A 

Geosynthetic PP-12 in 

T3A Machine Direction 

In-air 0 77738 70917 56987 46568 5856 14.0 

Beach Sand 10 83608 68855 59444 47454 5879 14.0 

Beach Sand 20 88314 85284 64909 46220 5936 14.6 

In-air 0 67734 62210 63669 57281 6320 14.0 

Beach Sand 10 62109 63597 75103 62548 6469 13.2 

Beach Sand 20 65846 90986 93152 67298 7097 11.9 

T5A 

Geosynthetic PET-14 

in Machine Direction 

Ib/ft = 14.59 N/m 

psi = 6.895 kPa 
Table 7. Summary of secant moduli at select strains, peak streq$h, and strain at peak 

for mxotimed and confined extension tests in silty sand. 

Geosynthetic PP-11 in 

Cross-Machine 

Direction 

T3B 

Geosynthetic PP- 12 in 

Machine Direction 

T4B 

Geosynthetic PE-13 

in Machine Direction 

T5B 

Geosynthetic PET-14 

in Machine Direction 

Confming 

Material 

In-air 

Silty Soil 

Silty Soil 

In-air 

Silty Soil 

Silty Soil 

In-air 

Silty Soil 

Silty Soil 

In-air 

Silty Soil 

Silty Soil 

In-air 

Silty Soil 

silty soil 

Confmiug Modulus at Modulus at Modulus at Modulus at Peak Strain 

Pressure 1% Strain 2% Strain 5% Strain 10% Strain Strength at Peak 

@SO (lbs/ft) ObW (lblft) (lblft) (lb/ft) m 

0 5602 5141 4035 4292 1352 38.5 

10 8000 7981 10490 7401 1664 37.8 

20 19011 18446 19574 15999 2154 25.3 

0 46174 45875 42298 38354 6020 17.9 

10 63943 62391 50052 42473 6751 18.2 

20 74566 72536 60220 52654 7145 16.1 

0 38295 38219 28548 22344 3400 22.6 

10 44096 44598 31990 24944 3612 20.8 

20 51610 49856 35620 26899 4109 20.5 

0 77738 70917 56987 46568 5856 14.0 

10 88717 76885 58447 49686 5780 15.7 

20 90534 92230 67860 52586 5980 12.9 

0 67734 62210 63669 5728 1 6320 14.0 

10 80403 85332 71604 59930 6688 12.4 

20 95665 94743 85572 67404 7196 12.4 

Ib/ft = 14.59 N/m 

psi = 6.895 kPa 
- 800 

2?5 

$ TOOki 

83 600- 

G 

2 500- 

ii 

g 400- 

!z Q 

y 300- 

v) 

!5 200- Id 

m 

4 

6 IOOz 

SELECT GEOSYNTHETICS CONFlNED IN BEACH SAND 

- GEOSYNTHETIC PP-10 

- GEOSYNTHETIC PP-11 

- GEOSYNTHEI-IC PP-12 

r*r~ww~ GEOSYNTHETIC PE- 13 

- GEOSYNTHETIC PET- 14 

I 4 8 12 16 20 1 c

CONFINING PRESSURE (psi) 

4 

1 psi = 6.895 kPa 

Figure 35. Soil confinement effect on secant moduli at 5 percent strain for 

selected geosynthetic materials confined in beach sand. 

81 
- 800 

-!s 

$ 700 

! 

SELECT GEOSYNTHETlCS CONFlNED IN SILTY SAND 

l.z 

8 600 

y 500 

ti 200 

2;: 

s 

g 100 

z 

- GEOSYNTHETIC PP- 10 

- GEOSYNTHETIC PP-I 1 

- GEOSYNTHETIC PP-12 

WI-W-M+ GEOSYNTHEUC PE-13 

m GEOSYNTHFTIC PET-l 4 

0 

CONFINING PRESSURE (psi) 

1 psi = 6.895 kPa 

Figure 36. Soil confinement effect on secant moduli at 5 percent strain for 

selected geosynthetic materials confined in silty sand. 

82 
- 800 

23 

z 

$ 700 

El 

IF! 600 

5 500 

8 ZE 400 

5 

25 ii 300 

if5 200 

t;: 

i.5 

5 100 

Z - 

0 

GEOSYNTHETlC PP-10 CONFINED IN 

TWO SOIL MATERlALS 

- IN BEACH SAND 

- IN SILTY SAND 

I I I I I I I I I I I 

4 8 12 ,P 1, 20 ; 

CONFINING PRESSURE (usi) 

4 

1 psi = 6.895 kPa 

Figure 37. Comparison of secant moduli at 5 percent strain for Geosynt.hetfc 

PP-10 confined in beach sand. 

83 
- 800 

25 

,’ 

Z 

$ 700- 

Li 

8 600- 

% 

2 500- 

-I 

i$ 

x 400- 

z= 

$ 300- 

v) 

b 200 - 

W 

cn 

4 

5 IOOz 

- 

ooooo Christopher et al., 1986 

aoooa Christopher et al., 1986 

AAAAA McGown et al., 1984 

00000 Wilson-Fahmy et al., 1993 

-t+i+t- Wilson-Fahmy et al., 1993 

xxxxx wu, 1991 

***** Boyle, 1995 

***** Boyle, 1995 

- GeoSyntec: Geosynthetic PP-10 in Beach Sand 

- GeoSyntec: Geosynthetic PP- 10 in Silty Sand 

0 

0" 5 10 15 20 25 

CONFINING PRESSURE (psi) 

1 psi = 6.89 kl?a 

30 35 

Figure 38. Comparison of secant moduli at 5 percent strain for Geosynthetic PP-10 

confined in beach sand and silty sand with data of similar 

geosynthetic materials reported by other researchers. 

84 
improvement is smaller on the confined response of the other four geosynthetic materials 

(i.e., two woven geotextiles and two geogrids). It is not well understood why soil 

confinement can improve the stress-strain properties of geosynthetics or why different types 

of geosynthetics have different responses to soil confinement. Three mechanisms of 

interaction between the soil and geosynthetic during a confined extension test are proposed to 

provide a framework for qualitatively understanding the confined extension test results: 

0 Internal Friction: referred to as the friction between the basic components (i.e., fibers 

or yarns) within a geosynthetic product during confined extension testing. Internal 

friction constrains reorientation and alignment of fibers or yarns and slippage between 

fibers or yarns. 

l Tortuosity: referred to as the curvature of the basic components (i.e., fibers or yarns) 

within a geosynthetic product. Soil confinement constrains alignment of curved fiber 

or yarns. 

l Interlocking: referred to as strike-through or penetration of soil into openings or 

apertures of a geosynthetic product during confined extension testing. Interlocking 

constrains reorientation of fibers or yarns. 

For a nonwoven geotextile, discrete fibers or continuous yarns are randomly placed and 

interconnected by needle-punching or another process during manufacturing. Deformation is 

primarily caused by reorientation of fibers and interfiber slippage. It is anticipated that all 

three mechanisms are well developed during confined extension testing and therefore the soil 

confinement can significantly improve the confined stress-strain properties of a nonwoven 

geotextile. 

For a woven geotextile and PET geogrid, fibers or yarns are continuous in the direction of 

loading. It is anticipated that each fiber or yarn carries approximately the same level of load 

as that applied to the entire test specimen. There likely exists little interfiber friction. 

However, the actual fibers or yarns within these types of geosynthetic materials are 

somewhat curved (i.e., tortuosity), there are fill fibers and cross-overs at intersections 

between the warp and fill fibers, and there exist certain degrees of interlocking that depend 

on the ratio of soil-particle size to geotextile openings or geogrid apertures. It is anticipated 

that soil confinement improves the confined stress-strain properties of these types of 

geosynthetics but to a lesser degree than for a nonwoven geotextile. 

85 
For the PE geogrid, each rib is simply a solid piece and little internal friction is anticipated 

to be mobilized within the ribs during a confined extension test. However, soil can strike 

through apertures of the geogrid. Consequently, while soil confinement may improve the 

confined stress-strain properties of the PE geogrid, it does so to a lesser degree in 

comparison with the other types of geosynthetics tested in this program. 

5.3 CONFINED AND UNCONFINED CREEP TESTING PROGRAM 

a. Test Matrix 

Ten test series were conducted in the confined and unconfined creep testing program. Each 

test series consisted of an unconfined tension creep test and two or three confined tension 

creep tests conducted on one of the five geosynthetic materials and one of the two soil 

materials. For the confined tension creep tests, herein referred to as creep tests, each 

geosynthetic material was confined within the beach sand (Test Series ClA through C5A) or 

silty sand (Test Series ClB through C5B) under two different normal stresses (i.e., 10 and 

20 psi (69 and 138 kPa)). A summary of test conditions used during the testing program is 

presented in table 8 (Test Series ClA through C5A) and table 9 (Test Series ClB through 

C5B). It should be noted that the unconfined creep test on the specific geosynthetic material 

presented in each of the two tables is actually the same test. 

b. Test Results 

For each confined or unconfined creep test, elongations of the geosynthetic specimen within 

the gage length were monitored over time by the use of two LVDTs. Measured elongations 

were used to calculate the total strain and incremental strain, rate for each test specimen, The 

total strain was defined as the total deformation of the test specimen within the gage length 

divided by the initial gage length of the geosynthetic specimen. The incremental strain rate 

was defined as the change in strain with respect to time between two successive monitoring 

periods. For each test, the calculated strains were plotted on a graph of total strain versus 

logarithm of time and the calculated incremental strain rates were plotted on a graph of 

logarithm of incremental strain rate versus total strain. The incremental strain rates were 

also plotted on a graph of logarithm of incremental strain rate versus logarithm of time. The 

results of the 10 creep test series are presented graphically in appendix B. 

86 
Table 8. Summary of test conditions for confined and unconfined creep tests in beach sand. 

Test Series 

Number 

ClA 

C2A 

C3A 

C4A 

C5A 

Geosyntbetic Confmiug 

Material Material 

Geosynthetic PP- 10 in 

Machine Direction 

In-air 0 8.0 in 4.0 in 25 100 

Beach Sand 10 8.0 in 4.0 in 25 100 

Beach Sand 20 8.0 in 4.0 in 25 100 

Beach Sand 10 8.0 in 4.0 in 25 1000 

Geosynthetic PP-11 in 

Cross-Machine 

Direction 

In-air 0 8.0 in 4.0 in 

Beach Sand 10 8.0 in 4.0 in 

Beach Sand 20 8.0 in 4.0 in 

Beach Sand 10 8.0 in 4.0 in 

Geosynthetic PP-12 in 

Machine Direction 

In-air 0 8.0 in 

Beach Sand 10 8.0 in 

Beach Sand 20 8.0 in 

Geosyntbetic PE-13 in 

Machine Direction 

In-air 0 

Beach Sand 10 

Beach Sand 20 

Geosyntbetic PET-14 in 

Machine Direction 

In-air 0 

Beach Sand 10 

Beach Sand 20 

5 ribs 2 apertures 

5 ribs 2 apertures 

5 ribs 2 apertures 

5 ribs 2 apertures 

5 ribs 2 apertures 

5 ribs 2 apertures 

Applied 

Tensile 

Load 

(% of uk) 

Total 

Duration 

(hrs) 

50 100 

50 100 

50 100 

50 1000 

+ 

50 100 

50 100 

50 100 

50 100 

50 100 

50 100 

+ 

60 100 

60 100 

60 100 

1 in = 25.4 mm 
Table 9. Summary of test conditions for confined and unconfined creep tests in silty sand. 

Test Series Geosynthetic collf%lillg 

Number Material Material 

Confining 

Pressure 

(psi) 

Geosynthetic PP-10 in In-air 

ClB Machine Direction Silty Sand 

Silty Sand 

Geosynthetic PP-11 in In-air 

C2B Cross-Machine Silty Sand 

Direction Silty Sand 

Geosynthetic PP-12 in In-air 

C3B Machine Direction Silty Sand 

Silty Sand 

Geosynthetic PE-13 in In-air 

C4B Machine Direction Silty Sand 

Silty Sand 

Geosynthetic PET-14 In-air 

C5B in Machine Direction Silty Sand 

Silty Saud 

1 in = 25.4 mm 

psi = 6.895 kPa 

0 

10 

20 

0 

10 

20 

0 

10 

20 

0 

10 

20 

0 

10 

20 

8.0 ill 4.0 in 25 100 

8.0 in 4.0 in 25 100 

8.0 in 4.0 in 25 100 

8.0 in 4.0 in 50 100 

8.0 in 4.0 in 50 100 

8.0 in 4.0 in 50 100 

8.0 in 4.0 in 50 100 

8.0 in 4.0 in 50 100 

8.0 in 4.0 in 50 100 

5 ribs 2 apertures 50 100 

5 ribs 2 apertures 50 100 

5 ribs 2 apertures 50 100 

5 ribs 2 apertures 60 100 

5 ribs 2 apertures 60 100 

5 ribs 2 aperhues 60 100 
A summary of the test results is presented in tables 10 through 13. These tables summarize 

the total strain and incremental strain rate at 1, 10, 100, 500, and 1,000 hours of elapsed 

time for each test. It is noted that the values of total strain and incremental strain rate at the 

selected times presented in tables 10 through 13 were derived by linear interpolation between 

the actually measured data points at each specific time period of interest. 

The results presented in appendix B and in tables 10 and 11 indicate that the soil confinement 

significantly reduces the total strain for the nonwoven geotextile (i.e., Geosynthetic PP-10). 

For the other four geosynthetic materials (Geosynthetics PP-11, PP- 12, PE-13, and PET-14), 

soil confinement also reduces the total strain but the decrease is less than that measured for 

the nonwoven geotextile. 

C. Interpretation 

Since a confined creep test specimen is initially subjected to loading conditions similar to 

those in the confined extension testing, it is anticipated that all three mechanisms previously 

.discussed can be used to explain the soil confinement effect on the initial portion (i.e., 

loading phase) of confined creep response for each of the five geosynthetic materials tested. 

As discussed previously, deformation of a nonwoven geotextile is primarily caused by 

reorientation of fibers and interfiber slippage. Elongation of the fibers may contribute little 

to the total deformation due to the fact that the actual tension within a fiber may be a small 

percentage of its tensile strength even when rupture of the nonwoven geosynthetic specimen 

occurs. The tensile creep of the nonwoven geotextile, as defined in current engineering 

practice, may actually be time-dependent slippage (i.e., shear creep) between fibers. This 

time-dependent interfiber slippage almost disappears shortly after application of a constant 

load to the nonwoven geotextile, as indicated in figures B-l and B-4. This may be due to the 

fact that the confining pressure of 10 or 20 psi (69 or 138 kPa) is high enough to prevent any 

time-dependent interfiber slippage within the nonwoven geotextile. 

For the woven geotextiles and PET geogrid, fibers or yarns within the two woven geotextiles 

and PET geogrid are continuous in the direction of loading. It is anticipated that each fiber 

or yarn carries approximately the same level of load as that applied to the entire test 

specimen. Therefore, the measured time-dependent behavior truly reflects the tension creep 

behavior of these types of geosynthetic materials. When these geosynthetic materials are 

confined in a soil, they are likely to creep as they do under unconfined conditions. 

89 
Table 10. Summary of total strains at select times for unconfined and corrfined tension creep tests in beach sand. 

Test Series Geosyntbetic 

Number Material 

Gemynthetic PP-10 in 

ClA Machine Direction 

Geosynthetic PP-11 in 

C2A Cross-Machine Direction 

Geosynthetic PP-12 in 

C3A Machine Direction 

Geosynthetic PE-13 in 

C4A Machine Direction 

Geosynthetic PET-14 in 

C5A Machine Direction 

psi = 6.895 kPa 

Confming 

Material 

In-air 

Beach Sand 

Beach Sand 

Beach Sand 

In-air 

Beach Sand 

Beach Sand 

Beach Sand 

In-air 

Beach Sand 

Beach Sand 

In-air 

Beach Sand 

Beach Sand 

In-air 

Beach Sand 

Beach Sand 

confwng 

Pressure 

(Psi) 

0 

10 

20 

10 

0 

10 

20 

10 

1: 

20 

0 

10 

20 

0 

Percentage of 

Ultimate 

Strength(‘) 

25 

25 

25 

25 

50 

50 

50 

50 

50 

50 

50 

50 

50 

50 

60 

60 

60 

1 

15.8 

11.2 

10.6 

11.5 

6.2 

5.5 

5.1 

5.7 

10.7 

9.3 

8.3 

8.3 

8.1 

7.7 

4.8 

4.1 

3.7 

Total Strain (%) at Elapsed Ties @rs) 

10 100 500 

16.9 20.7 (2) 

11.2 11.4 (2) 

10.6 10.6 (2) 

11.6 11.7 11.7 

8.0 12.1 (2) 

7.0 9.3 (2) 

6.4 8.5 c-3 

7.1 8.8 10.1 

12.6 19.2 (2) 

11.7 14.9 (2) 

10.4 12.6 (2) 

9.4 11.3 (2) 

9.3 10.8 (2) 

8.9 10.3 (2) 

5.1 -5.3 (2) 

4.4 4.6 (2) 

4.0 4.2 (2) 

1000 

(2) 

(2) 

(2) 

11.8 

(2) 

(2) 

(2) 

10.5 

(2) 

(2) 

(2) 

(2) 

(2) 

(2) 

(2) 

(2) 

(2) 

Notes: (1) Ultimate strength was determined in accordance with ASTM D 4595. 

(2) The test was terminated at an elapsed time of 100 hours. 
Table 11. Summary of total strains at selected times for unconfined and confined creep tests in silty sand. 

Test Series Geosynthetic Confming Confmiug Percentage of 

Number Material Material Pressure Ultimate 

@SO Strength(‘) 

Geosynthetic PP-10 in In-air 0 25 

ClB Machine Direction Silty Soil 10 25 

Silty Soil 20 25 

Geosynthetic PP-11 in In-air 0 50 

C2B Cross-Machine Direction Silty Soil 10 50 

Silty Soil 20 50 

Geosynthetic PP-12 in In-air 0 50 

C3B Machine Direction Silty Soil 10 50 

Silty Soil 20 50 

Geosynthetic PE-13 in In-air 0 50 

4B Machine Direction Silty Soil 10 50 

Silty Soil 20 50 

Geosynthetic PET-14 in In-air 0 60 

C5B Machine Direction Silty Soil 10 60 

Silty Soil 20 60 

psi = 6.895 kPa 

Notes: (1) Ultimate strength was determined in accordance with ASTM D 4595. 

(2) The test was terminated at an elapsed time of 100 hours. 

1 

15.8 

11.5 

10.8 

6.2 

5.4 

4.8 

10.7 

9.9 

8.7 

8.3 

f :i 

4.8 

4.3 

3.8 

Total Strain (%) at Elapsed Ties (hrs) 

10 100 500 

16.9 20.7 (2) 

11.5 11.7 

10.8 10.8 

g; 

8.0 12.1 (2) 

6.9 9.7 (2) 

6.0 7.8 (2) 

12.6 19.2 (2) 

12.4 15.9 (2) 

11.0 13.3 (2) 

9.4 11.3 (2) 

9.5 11.1 (2) 

9.3 10.8 (2) 

5.1 5.3 (2) 

4.5 4.7 (2) 

4.0 4.3 (2) 
Table 12. Summary of incremental strain rates at select times for cotimed and uncotimed tension creep tests in beach sand. 

Test Series Geosyntbetic 

Number Material 

Geosynthetic PP-10 in 

ClA Machine Direction 

Geosynthetic PP-11 in 

C2A Cross-Machine Direction 

Geosynthetic PP-12 in 

C3A Machine Direction 

Geosynthetic PE-13 in 

C4A Machine Direction 

Geosynthetic PET-14 in 

C5A Machine Direction 

psi = 6.895 kPa 

Confming 

Material 

In-air 

Beach Sand 

Beach Sand 

Beach Saud 

In-air 

Beach Sand 

Beach Sand 

Beach Sand 

In-air 

Beach Sand 

Beach Sand 

In-air 

Beach Sand 

Beach Sand 

In-air 

Beach Sand 

Beach Sand 

Confming Percentage of 

Pressure Ultimate 

WI Strength 

0 25 

10 2.5 

20 25 

10 25 

0 50 

10 50 

20 50 

10 50 

0 50 

10 50 

20 50 

0 50 

10 50 

20 50 

0 60 

10 60 

20 60 

Incremental Strain Rates (%/min) at Elapsed Ties (hrs) 

1 10 100 500 1000 

6.67~10~~ 3.54x10-3 4.51x104 (1) (1) 

1.36~10” 652x10” 3.33x10-5 (1) (1) 

2.00x10” 1.33x104 1.75x10” (1) (1) 

2.09~10~ 4.66~10” 5.91x10-5 1.21x10-5 8.14xlO~ 

1.33x10-2 1.14x10-3 3.67~10~ (1) (1) 

7.31x10-3 1.65~10” 1.51x104 (1) (1) 

5.72~10” 1.86~10” 1 .20x104 (1) (1) 

9.30x10-3 1.19x10-3 1 .56x10A 2.85~10‘~ 1.16~10-~ 

2.50~10-~ 1.29x10-* 8.30~10~ (1) (1) 

1.47x10-2 2.22x10-3 3.43x10” (1) (1) 

1.31x10-2 1.98~10.~ 2.22x10‘4 (1) (1) 

1.51x10-2 4.75x10-3 2.42~10~ (1) (1) 

1.40x10-2 2.80~10‘~ 1.21x104 (1) (1) 

1.67~10-~ 2.70~10” 1.1Sx10~ (1) (1) 

1.63~10” 7.13x10-5 2.20x10-5 (1) (1) 

2.34~10-~ 3.30x104 1 .22x1o-5 (1) (1) 

2.03~10-~ 1 .88x104 1.11x10” (1) (1) 

Note: (1) The test was terminated at an elapsed time of 100 hours. 
Table 13. Summary of incremental strain rates at select times for unconfined and confined tension creep tests in silty sand. 

Test Series Geosynthetic 

Number Material 

Geosynthetic PP-10 in 

ClB Machine Direction 

Geosynthetic PP-11 in 

C2B Cross-Machine Direction 

Geosynthetic PP-12 in 

C3B Machine Direction 

Geosynthetic PE-13 in 

C4B Machine Direction 

Geosynthetic PET-14 in 

C5B Machine Direction 

psi = 6.895 kPa 

Confining 

Material 

In-air 

Silty Sand 

Silty Sand 

In-air 

Silty Sand 

Silty Sand 

In-air 

Silty Sand 

Silty Sand 

In-air 

Silty Sand 

Silty Sand 

In-air 

Silty Sand 

Silty Sand 

Confining Percentage of Total Strain (%) at Elapsed Tiies (hrs) 

Pressure Ultimate 

@si) Strength 1 10 100 500 1000 

0 25 6.67~10-~ 3.54x10-3 4.51x10A (1) 

10 25 4.OOxlO~ 3.51x10” 0.00x10~ (1) ::; 

20 25 2.00x10’ 1.3OxlO~ 1.75x10” (1) (1) 

0 50 1.33x10-2 1.14x10-3 3.67~10” (1) 

10 50 6.11~10-~ 1.04x10” 1.50x104 (1) I:; 

20 50 5.36~10-~ 1.74x10-3 4.11x10-4 (1) (1) 

0 50 2.50x10-* 1.29x10-* 8.30x104 (1) (1) 

10 50 1 .56x105 2.36~10.~ 3.68~10~ (1) (1) 

20 50 1.38x10-2 2.09~10” 1 .94x104 (1) (1) 

0 50 1.51x10‘* 4.75x10-3 2.42~10~ (1) (1) 

10 50 1.75x10‘* 2.58~10” 1.31x10-5 (1) (1) 

20 50 1.68~10-~ 2.70~10-~ 1.19x10A (1) (1) 

0 60 1.63~10‘~ 7.13x10-5 2.2OxlW (1) (1) 

10 60 2.28~10~~ 9.22~10” 1.50x10-5 (1) (1) 

20 60 1.16~10-~ 1.45x10’ 1.47x10-5 (1) (1) 

Note: (1) The test was terminated at an elapsed time of 100 hours. 
However, during the creep phase, there are still fill fibers and cross-overs at intersections 

between the warp and fill fibers and certain degrees of interlocking even though the 

tortuosity may disappear during the initial loading phase. It is anticipated that the creep 

strain may also be reduced because of soil confinement, as supported by the test results. 

For the PE geogrid, little internal friction is anticipated to be mobilized between or within 

ribs. Therefore, the creep behavior of the PE geogrid is likely to be least affected by the 

soil confinement. This observation is also supported by the test results. 

5.4 SUMMARY AND CONCLUSIONS 

Confined and unconfined extension and creep tests were conducted on the five geosynthetic 

materials with the use of the confined extension/creep testing device discussed in chapter 4. 

The geosynthetic materials included a needle-punched nonwoven PP geotextile, two woven 

PP geotextiles, a PET geogrid, and a PE geogrid. These geosynthetic materials were tested 

in an unconfined mode and subsequently confined in beach sand and silty sand. The 

following conclusions can be drawn on the basis of the obtained test results: ’ 

a Soil confinement significantly improves the moduli and strength of Geosynthetic PP10 (i.e., a needle-punched nonwoven geotextile). The relative improvement of 

confined modulus at a specific level of strain varied from approximately 200 to 400 

percent. The relative increase of strength varied from approximately 25 to 85 

percent. As the confining pressure increased, the improvement in the moduli and 

strength also increased for the nonwoven geotextile. 

0 For tests conducted at a specific confining pressure for Geosynthetic PP-10, confined 

extension tests conducted using beach sand showed a more significant effect of 

confinement than tests conducted using silty sand. 

l There was a noticeable confinement effect for the other four geosynthetic materials 

when confined in the beach sand or silty sand. The improvement of confined moduli 

at 5 percent strain ranged from approximately 5 to 30 percent for the two woven 

geotextiles and two geogrids at a normal stress of 20 psi (138 kPa). The increase can 

be considered significant with respect to unconfined moduli of these materials, 

although the improvement is not as significant when compared with the response for 

the nonwoven geotextile. 

94 
l Improvement of the confined stress-strain properties for a given geosynthetic material 

is believed to be primarily related to the reorientation of fibers and interfiber friction 

within the geosynthetic material. There are significant interfiber interactions for the 

nonwoven geotextile, slight interactions between fibers and yarns for the woven 

geotextile and PET geogrid, and little interaction between or within the ribs for the 

PE geogrid. This leads to an important conclusion that soil confinement has much 

less of an effect on the woven geotextiles and PET geogrid and an even lower effect 

on the PE geogrid when compared with the response of the nonwoven geotextile. 

0 Soil confinement significantly reduces the measured time-dependent total strain of 

Geosynthetic PP-10 relative to the unconfined response. There is a large reduction in 

the measured total strain for this material relative to the response of the other 

geosynthetics at high confining pressures. 

l Soil confinement does not significantly reduce the measured time-dependent total 

strain for Geosynthetic PP-11, PP-12, PE-13, and PET-14 relative to the unconfined 

response. 

95 

CHAPTER 6 

CONFINED EXTENSION TESTS USING 

TRIAXIAL-TYPE TEST DEVICE 

6.1 INTRODUCTION 

One of the primary purposes of this research program was to assess the influence of 

confinement on the tensile load versus strain (i.e., stress-strain) response of geosynthetic 

materials. One of the key steps in making this assessment was to evaluate and develop the 

most appropriate equipment for conducting confined extension tests in the laboratory. To 

this end, Polytechnic University (Polytechnic) participated in a collaborative effort to develop 

alternate and appropriate laboratory testing equipment to conduct confined extension and 

potentially confined creep testing of geosynthetics. The results of the Polytechnic triaxial 

testing program, which focused exclusively on confined extension testing using a modified 

triaxial apparatus (i.e., triaxial-type device), are summarized in this section. In addition, a 

comparison of the two confined extension test results from the testing programs is presented. 

6.2 LABORATORY TESTING EQUIPMENT 

a. Testing Equipment 

To meet the objectives of this research program, it is necessary to compare the response of 

geosynthetic materials under confined and unconfined conditions while test “variables” (Le., 

pretension force, aspect ratio, and strain rate) remain unchanged. Specifically, the stressstrain properties of geosynthetics have been shown to vary with respect to soil confinement, 

pretension force, aspect ratio, and strain rate, and possibly other unknown test variables. 

Therefore, with two or more test variables changing simultaneously in a test, it would be 

difficult to isolate the influence of a specific variable on the test results. As was 

demonstrated previously, any test device developed for conducting the confined extension 

testing should at least meet the following two requirements: (i) provide a constant strain rate 

to the test specimen; and (ii) maintain a constant aspect ratio of the test specimen throughout 

the test. 

97 
On the basis of the results of a literature review and the initial pullout testing program, the 

following two confined extension test devices were identified to have the potential to meet 

these two requirements: 

0 In-Soil Test Apparatus developed by McGown et al. [1982] and modified by WilsonFahmy et al. [1993]; and 

0 Modified Triaxial Apparatus developed by Ling et al. [ 19911 and Wu [ 19911. 

The triaxial-type device used for this program consists of a triaxial (extension) cell and a 

tension loading system. A schematic diagram of this triaxial-type device is shown in figure 

39. A typical test setup using this equipment involves placing a geotextile specimen in a 

vertical orientation inside of a cylindrical soil specimen that is encased in a latex membrane, 

Each end of the geotextile specimen is fixed within a metal clamp. The lower clamp is then 

fit through a slot in the bottom of the triaxial cell and connected to the bottom load cell. The 

bottom load cell is fixed to the platform of the triaxial frame. Similarly, the upper clamp is 

fit through a slot in the cover of the triaxial cell and connected to the upper load cell. The 

upper load cell is connected to the tension loading system. An LVDT is attached to the 

upper clamp to measure test specimen displacement. With reference to figure 39, it is 

explicitly noted that the cylindrical soil specimen is prepared in such a way that its top and 

bottom are in direct contact with the top and bottom of the triaxial cell, respectively. 

Therefore, during the extension testing, the confining soil is constrained in the vertical 

direction while the geotextile moves through the soil. 

The triaxial-type device can accommodate 8-in- (203~mm-) wide and 4-m (102mm-) long 

specimens and can provide up to 22,000 lb (98 kN) of tensile loading capacity on specimens 

confined under a maximum confining stress of 100 psi (690 kPa). 

b. Equipment Calibration 

The triaxial-type test device was calibrated electronically and mechanically. In electronic 

calibration, the electronic data measurement components (i.e., an LVDT and two load cells) 

were calibrated to verify that the loads or displacements applied to the instruments were 

properly conditioned and correctly recorded in the data acquisition computer. In mechanical 

calibration, tests were conducted on the equipment to assess friction losses in the equipment 

and/or effects due to the influence of the equipment and to check reproducibilty of the test 

results. The calibration of the triaxial-type device included the following steps: 

98 
TriaxialCell 1 

TOP 

Upper End Cap 

--EtLVD 

bber O-Ring 

Latex Membrane 

r 

h- Geotextile 

Perforated Plastic 

Glass Mold 

(Note: Not to Scale) 

Figure 39. Schematic diagram of triaxial-type device developed by Polytechnic University, 

99 
l Calibration of an LVDT and two load cells. 

l Calibration of the “triaxial cell effect” on the in-air tensile test results by conducting 

two in-air tensile tests on Geosynthetic PP-15: one with the triaxial cell and the other 

without the triaxial cell. 

l Evaluation of the reproducibility of the test results by conducting duplicate in-air 

tensile tests on Geosynthetics PP-15 and PP-16 under nearly identical test conditions. 

l Evaluation of the friction between the upper clamp and soil. 

The results of the calibration tests are summarized as follows: 

l Both the top and bottom load cells had a precision of f2.2 lb (+ 10 N); the LVDT 

had a precision of f0.0005 in (kO.01 mm). 

l No influence of the triaxial cell on the unconfined extension test results was detected. 

The movement of the upper specimen clamp through the cover of the cell did not 

influence the tension measurements. Loads measured by the two load cells were very 

consistent, as shown in figure 40. 

l Duplicate in-air tensile tests were conducted on Geosynthetics PP-15 and PP- 16. The 

results of two sets of tests indicate that tensile forces varied about 5 percent for 

Geosynthetic PP-15 and 20 percent for Geosynthetic PP-16 for strains up to 10 

percent. 

l Friction developed between the upper clamp and confining soil when the clamp was 

pulled out of the soil. A friction of approximately 25 lb (110 N) was measured when 

the upper clamp alone was confined in the beach sand at confining pressures in the 

range of 2 to 20 psi (14 to 138 kPa). The friction was subtracted from tensile loads 

during reduction of confined extension test data. Little friction was anticipated to 

develop between the lower clamp and soil because the lower clamp is attached to the 

stationary bottom load cell during testing. 

100 
1000 - 

400 - 

200 - 

UNCONFINED TENSILE TESTING ON GEOSYNTHETIC PP- ‘I 5 

SPECIMEN LENGTH: 4 in 

SPECIMEN WIDTH: 8 in 

eeeea FIXED BOTTOM LOAD CELL 

mxaea MOVING UPPER LOAD CELL 

DISPLACEMENT AT UPPER SPECIMEN CLAMP (in) 

1 in = 25.4m 

1 lb = 0.454 Kg 

Figure 40. Comparison of loads measured by top and bottom load cells 

in unconfined wide-width tensile test. 

101 
6.3 TEST PROCEDURE 

A test procedure was developed for conducting the confined extension tests using the triaxialtype device. With reference to figure 39, the step-by-step test procedure is described as 

follows: 

0 Stretch of Membrane: An g-in- (203mm-) diameter latex membrane is stretched to fit 

inside a perforated plastic glass mold. Each end of the membrane is folded back over 

the outside of the mold and fastened with a rubber O-ring. The mold is then placed 

on the lower end cap. 

a Preparation of Geosynthetic Specimen: A geotextile specimen is trimmed from the 

bulk sample and each end of the geotextile specimen is fixed within metal clamps. 

The lower clamp is then fit through the slot in the bottom of the triaxial cell and 

connected to the bottom load cell. The upper clamp is fit through the slot in the 

cover of the triaxial cell and attached to the top load cell. The geotextile is then 

slightly pretensioned to ensure that the geotextile stands vertically inside the mold. 

0 Preparation of Soil Specimen: The beach sand or silty sand is placed within the mold 

around the geotextile specimen in layers. Each layer of soil is compacted to 

approximately 70 to 75 percent relative density. 

l AssembZy of Triuxial Cell: The triaxial cell is placed around the mold containing the 

test specimen. Subsequently, each end of the triaxial membrane is fitted onto the 

lower or upper end cap and fastened with the rubber O-ring. The top of the triaxial 

cell is placed in direct contact with the upper end cap. The top and bottom of the 

triaxial cell is rigidly attached to the loading frame. 

0 Attachment of LVDT: An LVDT is attached to the upper clamp to measure 

displacements of the geotextile specimen. 

l Application of Confining Pressure: An isotropic confining pressure is applied to the 

cylindrical soil specimen by pressurizing the triaxial cell and consolidating the soil 

specimen. 

0 Extension of Geosynthetic Specimen: After application of the confining pressure, the 

upper clamp is pulled at a constant displacement rate, corresponding to an average 

strain rate of 10 percent/min relative to the initial specimen length. 

102 
l Monitoring of Test: Loads and displacements measured by the load cells and LVDT 

are recorded using a computer data acquisition system. Monitoring of the test is 

continued until rupture of the geotextile specimen occurs. 

It should be noted that the extension tests reported in this section were conducted on the 

geotextile specimens having initial dimensions of g-in (203~mm) wide by 4-in (102~mm) 

long. 

6.4 CONFINED EXTENSION TESTING PROGRAM 

a. Test Matrix 

The confined extension testing program consisted of four test series conducted on 

Geosynthetics PP-15 and PP-16 confined in the beach sand and silty sand. Each test series 

consisted of an unconfined tension test and several confined extension t.ests under confining 

pressures ranging from 2 to 20 psi (14 to 138 kPa). At each test confining pressure, two or 

five “identical” tests were conducted, and the average results of the tests were used to assess 

the confined stress-strain properties of the geotextile. A summary of test conditions used for 

conducting these four test series is presented in table 14. 

b. Test Results and Interpretation 

For each confined or unconfined extension test, the directly measured results include: 

e load measured by the top load cell at the upper specimen clamp versus displacement 

measured by the LVDT attached to the upper specimen clamp, referred to as the top 

load versus displacement data; and 

l load measured by the bottom load cell at the lower specimen clamp versus 

displacement measured by the LVDT attached to the upper specimen clamp, referred 

to as the bottom load versus displacement data. 

The results of a confined extension test on Geosynthetic PP-15 confined in the beach sand at 

a confining pressure of 10 psi (69 kPa) is presented in figure 41. This result is typical of the 

test results obtained in the triaxial apparatus. The top load versus displacement data and 

bottom load versus displacement data are graphically represented. It is observed from figure 

41 that there is a significant difference between the two curves. The difference or 

103 
Table 14. Summary of unconfined and confined extension test conditions conducted at Polytechnic University. 

;5 P 

Test Series 

Number 

TX1 

TX2 

TX3 

TX4 

Geosynthetic confining confining Number Test Average Strain 

Material Material Pressure of Tests(‘) SGEen Specimen Rate”) 

(psi) Width Lengjh (%/min) 

(in) (in) 

Geosynthetic PP- 15 In-air 0 2 8 4 10 

in Machine Direction Beach Sand 2 2 8 4 10 

Beach Sand 4 2 8 4 10 

Beach Sand 10 5 8 4 10 

Beach Sand 20 2 8 4 10 

Geosynthetic PP- 15 In-air 0 2 8 4 10 

in Machine Direction Silty Sand 4 2 8 4 10 

Silty Sand 10 2 8 4 10 

Geosynthetic PP- 16 In-air 0 2 8 4 10 

in Machine Direction Beach Sand 2 2 8 4 10 

Beach Sand 4 2 8 4 10 

Beach Sand 10 5 8 4 10 

Beach Sand 20 2 8 4 10 

Geosynthetic PP- 16 In-air 0 2 8 4 10 

in Machine Direction Silty Soil 4 2 8 4 10 

Silty Soil 10 2 8 4 10 

1 in = 25.4 mm 

psi = 6.895 kPa 

Note: (1) Number of tests refers to number of tests conducted at the same confining pressure under nearly identical conditions. 

(2) Average strain rate was defined as total displacement measured at the upper clamp divided by the initial specimen length. 
1200 

1000 

800 

- 

n 

ci 

0 

3 600 

w -I 

ii5 

ifI 

t400 

200 

0 

GEOSYNTHETIC PP-15 CONFINED IN BEACH SAND 

SPECIMEN LENGTH: 4 in 

SPECIMEN WIDTH: 8 in 

CONFINING PRESSURE: 10 psi 

= FIXED BOllOM LOAD CELL 

m MOVING UPPER LOAD CELL 

, 

0.0 0.2 0.4 0.6 0.8 1.0 1.2 

DISPIACEMENT AT UPPER SPECIMEN CLAMP (in) 

1 in = 25.4 mm 

1 lb = 0.454 Kg 

1 psi = 6.89 kPa 

Figure 41. Load versus displacement responses from confined extension test on 

Geosynthetic PP-15 at a normal stress of 10 psi. 

105 
differential load is caused by the friction mobilized between the constrained soil and the 

moving geotextile specimen. The friction response can be readily obtained by plotting 

differential load versus displacement, as shown in figure 42. The friction versus 

displacement curve shown in figure 42 is similar to that of a typical pullout response for a 

geosynthetic material. 

Examining the results of the confined extension tests conducted in the triaxial cell indicates 

the friction developed between the constrained soil and moving geotextile increases with 

increasing confining pressures for a given geosynthetic material. At low confining pressures 

of 2 to 4 psi (14 and 28 kPa), the friction is in the range of 10 and 20 lb (45 to 90 N). 

However, at high confining pressures of 10 and 20 psi (69 and 138 k.Pa), the friction is in 

the range of 200 to 800 lb (890 to 3560 N). 

It is believed that the top load versus displacement response obtained from this type of 

confined extension testing is a coupled response tif geotextile extension and soil-geotextile 

interface friction, At low confining pressures, the friction developed between the constrained 

soil and moving geotextile is relative small, and distribution of tension along the geotextile 

specimen is relatively uniform. For these considerations, tensile forces and strains can be 

calculated directly from the upper load versus displacement data as follows: 

0 divide the displacement at the top by the initial specimen length to obtain the strain; and 

a divide the top load by the initial specimen width to obtain the tensile force (Le., load 

per unit width). 

At high confining pressures, the interface friction becomes a significant component. An 

appropriate interpretation procedure is required to calculate tensile forces and strains of a 

confined geotextile specimen. However, due to the lack of displacement data along the 

length of the geotextile specimen (between the two ends of the geotextile specimen), the 

actual strain distribution cannot be adequately evaluated. At best, an average strain along the 

specimen can be estimated. The authors of this report believe that use of an interpretation procedure to 

define the confined stress-strain properties involves significant assumptions that limit the 

utility of these data, as described previously in the discussion on the pullout test results, For 

the triaxial tests, the tensile forces and strains of the geotextile confined in soil at high 

confining pressures were estimated using the same method that was used for interpreting the 

results of confined extension tests at low confining pressures. 

106 
ow .$ 1000 

P 

z - 

GEOSYNTHETIC PP-15 CONFINED IN BEACH SAND 

SPECIMEN LENGTH: 4 in 

SPECIMEN WIDTH: 8 in 

CONFINING PRESSURE: 10 psi 

cl.2 0.4 0.6 0.8 1.0 1.2 

DlSPlACEMENT AT UPPER SPECIMEN CLAMP (in) 

1 in = 25.4 mm 

1 lb = 0.454 Kg 

1 psi = 6.89 kPa 

Figure 42. Friction versus displacement response from confined extension test on 

Geosynthetic PP-15 at a normal stress of 10 psi. 

107 
Using the tensile force and strain data from each confined or unconfined extension test, the 

following properties were calculated to characterize the confined or unconfined response: (i) 

secant modulus at 5 percent strain; (ii) secant modulus at 10 percent strain; (iii) peak 

strength; and (iv) strain at the peak strength (i.e., failure strain). It is noted that two or five 

confined extension tests were conducted on the same geotextile under nearly identical test 

conditions. An average value of each of these four characterization properties was calculated 

from the results of the duplicate tests. A summary of the average values of these 

characterization properties is presented in table 15. 

6.5 COMPARISON OF CONFINED EXTENSION TEST EQUIPMENT AND 

RESULTS 

a. Comparison of Test Devices 

The triaxial-type device and the protocol confined. extension/creep test device are similar in 

the following aspects: 

0 capability to provide flexible boundary conditions over a major portion of the soil 

specimen surface; in the extension/creep device, the top and bottom of the soil 

specimen are in contact with air bladders; in the triaxial device, the soil specimen is 

encased in a latex membrane; 

l capability to apply a constant confining pressure to the test specimen; 

l capability to apply a constant strain rate to the test specimen; and 

l potential to maintain an approximately constant aspect ratio for the geosynthetic 

specimen. 

However, there are the following two major differences between the two test devices: 

l The triaxial device applies an isotropic pressure to the entire test specimen while the 

confined/creep device applies a vertical (normal) pressure to the test specimen. It is 

anticipated that the geosynthetic specimen encased in the cylindrical specimen in the 

triaxial device is under compression in all directions. Compression in the direction of 

width (i.e., in-plane compression) may influence the confined response of the geotextile. 

108 
Table 15. Summary of unconfined and confined extension test results conducted at Polytechnic University. 

Test Series Geosynthetic 

Number Material 

Confining 

Material 

confining Number Modulus at Modulus at Peak Strain 

Pressure 

T:(l) 

5% Strain(2) 10% Strength0 at Peakt4) 

(psi) (lblft) Strain(2) (Iblft) (%I 

(lb/ft) 

TX1 

TX2 

Geosynthetic 

PP-15 in 

Machine 

Direction 

Geosynthetic 

PP-15 in 

Machine 

Direction 

In-air 0 2 2,352 1,848 948 65 

Beach Sand 2 2 1,800 1,692 768 44 

Beach Sand 4 2 3,300 2,292 888 51 

Beach Sand 10 5 9,840 6,240 1,008 43 

Beach Sand 20 2 15,840 8,760 996 36 

In-air 0 2 2,352 1,848 948 65 

Silty Sand 4 2 2,700 4,656 888 88 

Silty Sand 10 2 1,572 2,772 756 61 

TX3 

TX4 

Geosynthetic 

PP-16 in 

Machine 

Direction 

Geosynthetic 

PP-16 in Machine 

Direction 

In-air 0 2 2,328 2,076 804 112 

Beach Sand 2 2 4,080 2,640 888 135 

Beach Sand 4 2 1,800 1,344 840 140 

Beach Sand 10 5 2,784 1,800 900 123 

Beach Sand 20 2 7,344 4,572 948 97 

In-air 0 2 2,328 2,076 804 112 

Silty Soil 4 2 1,044 528 444 150 

Silty Soil 10 2 6,144 3,027 1,104 120 

psi = 6.895 kPa 

lblft = 14.59 N/m 

Notes: (1) 

(2) 

(3) 

(4) 

Number of tests refers to number of tests conducted at the same confining pressure under nearly identical conditions. 

Modulus is an average modulus calculated from two or five duplicate tests conducted under the same conditions. 

Peak strength was measured at the upper clamp. 

Strain rate was defined as total displacement measured at the upper clamp divided by the initial specimen length. 
l In the triaxial device, the geotextile moves within the cylindrical soil specimen, which 

is constrained in the vertical direction. A friction is developed at the soil-geosynthetic 

interface, which causes a nonuniform distribution of tension along the specimen 

length. As a result, strain and strain rate are anticipated to decrease from the upper 

to lower end of the test specimen. In the confined extension/creep device, the soil 

tends to move together with the geotextile specimen. The mobilized soil-geosynthetic 

interface friction is relatively small. Therefore, the distribution of tension along the 

geosynthetic specimen is nearly uniform in this device. 

b. Comparison of Test Results 

In the context of the entire research program, there is only one component of the program 

that leads to a direct comparison between the two devices. To this end, a comparison is 

presented of the short-term confined and unconfined extension test results for Geosynthetics 

PP-15 and PP-10 confined in the beach sand and silty sand. Geosynthetics PP-15 and PP-10 

were two nonwoven geotextiles produced by the same manufacturer. On the basis of 

information provided by the manufacturer, the two geotextiles were considered as the same 

material as far as their average properties are concerned. The following summary comments 

reflect this comparison: 

l Unconfined Tensile Strength: In the confined extension/creep device, a peak strength 

of 1,352 lb/ft (lg.,8 kN/m) at a strain of 39 percent for Geosynthetic PP-10 was 

measured; the triaxial device measured a peak strength of 948 lb/ft (13.9 kN/m) at a 

strain of 65 percent for Geosynthetic PP-15. A direct comparison indicates that there 

is a significant difference between the two peak strengths and failure strains of the 

two geotextiles. To understand what caused the difference between the results 

obtained by the two organizations for these unconfined test results, two explanations 

were explored: (i) the difference between the two materials as measured by mass per 

unit area; and (ii) inherent variability of Geosynthetic PP-15. Geosynthetic PP-15 

tested at Polytechnic had a mass per unit area of 8.3 oz/yd* (282 g/m2) while 

Geosynthetic PP-10 tested at GeoSyntec had a mass per unit area of 10.1 oz/yd2 (343 

g/m*). It is expected that Geosynthetic PP-10 would be stronger than Geosynthetic 

PP-15; this is confirmed by the test results. 

Inherent variability of Geosynthetic PP-15 was studied by analyzing a set of in-air test 

data (10 tests) on Geosynthetic PP-15 provided by the manufacturer. The results of 

statistical analysis are as follows: (i) strengths in the machine direction in the range 

110 
from 940 to 1,421 lb/ft (13.7 to 20.7 kN/m) with a mean of 1,043 lb/ft (15.2 kN/m); 

(ii) a calculated standard deviation of 175 lb/ft (2.6 kN/m); and (iii) a calculated 

coefficient of variation of 17 percent. The statistical analysis indicates that 

Geosynthetic PP-15 is highly variable in terms of tensile strengths. The unconfined 

tensile strengths of the two geotextiles measured by the two organizations fall in the 

range of the strength data provided by the manufacturer, yet the differences make a 

direct comparison difficult. 

l Confined Modulus at 5 and IO Percent Strains: Figures 43 and 44 present a summary 

plot of the laboratory-measured moduli at 5 and 10 percent strain for the geotextiles 

confined in the two soil materials. The results are generally consistent and indicate 

that: (i) confinement of each soil significantly improves the modulus; (ii) relative to 

the improved modulus, the higher the confining pressure, the greater the 

improvement; and (iii) under the same confining pressure, the confinement effect of 

the beach sand is greater than that of the &y sand. The triaxial results, however, 

indicate a potential slight reduction in stiffness compared with the in-air stiffness at 

low confining pressures. This may be due to inherent variability in the geotextile 

specimen, as previously described. The improvement in moduli at 5 percent strain 

for the nonwoven geotextiles confined in the beach sand obtained by both techniques 

are also compared with those for similar materials obtained by other researchers,as 

shown in figure 45. 

l Confinement EfSect on Strength: Figure 46 presents a summary plot of the laboratorymeasured peak strength for the confined test specimen. The confined extension creep 

device results indicate that there is consistent improvement in the peak strength as a 

result of soil confinement. The triaxial results indicates essentially no increase in 

peak strength. The internal consistency in both test results indicate that these results 

are likely not the result of inherent sample variability, but rather an effect of the 

specific test conditions (i.e., effect of clamping system, specimen geometry, testing 

equipment, etc.). At this stage, however, it is premature to provide a detailed 

evaluation of the observed response. 

Regardless of the detailed observation described above, the results indicate that: (i) 

soil confinement imposes beneficial effects on the stress-strain response of the 

nonwoven geotextile; (ii) a higher confining pressure results in a greater improvement 

of the confined secant modulus; and (iii) the confinement effect of the beach sand is 

greater than that of the silty sand. 

111 
32000 

GEOSYNTHETICS PP-10 AND PP-15 CONFINED IN 

BEACH SAND. 

28000 - 

$ 16000 

mee+~ GeoSyntec: Modulus at 5% strain 

~~~ff3~l Polytechnic: Modulus at 5% strain 

l -*** GeoSyntec: Modulus at 10% strain 

=W-W Polytechnic: Modulus at 10% strain 

0 I I I I I I I I I 

0 

I 

4 8 12 16 20 : 

CONFINING PRESSURE (psi) 

1 lb/ft = 14.59 N/m 

1 psi = 6.89 kPa 

1 

Figure 43. Comparison of GeoSyntec and Polytechnic test results (secant moduli of 

Geosynthetics PP-10 and PP-15 confined in beach sand. 

112 
32000 

28000 

GEOSYNTHETICS PP-10 AND PP-15 CONFINED IN 

SILTY SAND 

0 GeoSyntec: Modulus at 5% strain 

A 24000 

w+em Polytechnic: Modulus at 5% strain 

l MM GeoSyntec: Modulus at 10% strain 

5 

‘*a** Polytechnic: Modulus at 10% strain 

= 20000 

WI 

3 

$ 16000 

I 

l5 12000 

z11 

cn 

8000 

0 

0 4 8 12 16 20 24 

CONFINING PRESSURE (psi) 

1 lb/ft = 14.59 N/m 

1 psi = 6.89 kPa 

Figure 44. Comparison of GeoSyntec and Polytechnic test results (secant moduli of 

Geosynthetics PP-10 and PP-15 confined in silty sand. 

113 
ooooo Christopher et al., 1986 

00000 Christopher et al., 1986 

AAAAA McGown et al., 1984 

00000 Wilson-Fahmy et al., 1993 

-t+t+i- Wilson-Fahmy et al., 1993 

xxxxx wu, 1991 

**I** Boyle, 1995 

***** Boyle, 1995 

- GeoSyntec: Geosynthetic PP-10 in Beach Sand 

=*a** Polytechnic: Geosynthetic PP-15 in Beach Sand 

P / / / / / 

CONFINING PRESSURE (psi) 

1 psi = 6.89 kPa 

Figure 45. Comparison of secant moduli at 5 percent strain for Geosynthetics PP-10 

and PP-15 confined in beach sand obtained by GeoSyntec and 

Polytechnic with data of similar geosynthetic materials reported 

by other researchers. 

114 
3600 

3200 

2800 

n 

-v Cc 

c 2000 

Z 

W 

E 1600 

cn 

0 I I I I I I I I I I I 

0 4 

CONFYhNG 

\ 

Pk2ESSUR1E6 (psi) 

20 : 4 

1 lb/ft = 14.59 N/m 

1 psi = 6.89 kPa 

Figure 46. Comparison of GeoSyntec and Polytechnic test results (peak strengths of 

Geosynthetics PP-10 and PP-15 confined in beach sand and silty sand. 

115 
6.6 SUMMARY COMMENTS AND RECOMMENDATIONS 

The results of the collaborative project provide somewhat comparable results; specifically, 

there is a beneficial effect of confinement on the confined stress-strain behavior of a 

nonwoven geotextile. A detailed comparison of test results indicates that there is a noticeable 

difference between the stress-strain properties of the nonwoven geotextiles obtained by the 

two types of equipment. The difference is likely due to the fact that: (i) the two nonwoven 

geotextiles are highly variable, as suggested by the manufacturer data; and (ii) there are 

significant differences the two test devices. 

It was observed from the test results that the triaxial device works in such a way that a 

potentially significant friction is developed at the soil-geotextile interface at high confining 

pressures. The interface friction causes a nonuniform distribution of tension along the 

geosynthetic specimen similar to the authors’ reported assessment of the pullout test. It is 

difficult to account for the effect of friction on the confined extension properties of 

geosynthetics through an interpretation procedure. The authors believe that it is premature to 

adopt the triaxial-type device for evaluating confined stress-strain properties of geosynthetics 

unless an appropriate procedure is developed to account for the soil-geosynthetic friction 

effect during interpretation and subsequently verified. However, the triaxial device provides 

an alternative for measuring the pullout response of geosynthetics under saturated conditions, 

which cannot be easily simulated in the traditional pullout test device. Soil-geosynthetic 

interaction parameters determined from the triaxial device may have direct applications in 

design and analysis of reinforced soil structures, but additional comparison testing is needed 

prior to adopting this test. 

As indicated previously, the results of the confined extension tests conducted in the modified 

in-soil device were internally consistent. The utilized equipment has been found to be 

appropriate for confined extension tests. As a component of this project, a testing protocol 

for conducting short-term confined extension tests and long-term confined creep tests has 

been developed. At this stage of development, the authors believe that the confined 

extension, also referred to as a confined wide-width device, offers more potential than the 

triaxial-type device. It is recommended that the confined extension testing equipment and the 

testing protocol be utilized to produce, at the very least, a baseline reference for evaluating 

the confined response of geosynthetics. Additional testing is warranted and recommended. 

It is recommended that this additional testing focus on: (i) confined extension for a wide 

range of soil materials; (ii) confined creep at different tensile stress levels; (iii) confined 

creep beyond 1000 hours; (iv) detailed testing of geosynthetics that are used primarily for 

reinforcement; and (v) identifying mechanisms that lead to the understanding of the soil 

confinement effects on the confined response of geosynthetic materials. 

116 
CHAPTER 7 

DEVELOPMENT OF WRITTEN PROTOCOL FOR CONFINED 

EXTENSION/CREEP TESTING 

7.1 OVERVIEW 

The final component of this program is the development of a protocol for conducting 

confined extension and creep tests. The protocol was developed on the basis of chapters 3, 

4, and 5 of this report. The test protocol was written in accordance with the format 

established by the American Society for Testing and Materials (ASTM) for the preparation of 

a standard test method, and is presented in appendix C. 

7.2 DISCUSSION 

Using the confined extension equipment developed and the results of the verification testing 

program, a protocol has been established for conducting confined extension and creep tests. 

The protocol presented in appendix C of this report is essentially a “stand-alone” document. 

Several major factors were considered in the development of this protocol. These factors 

were discussed in the previous sections of this report and are summarized below: 

0 Friction Loss: It was found that the tensile load applied to the front of the 

geosynthetic specimen was not fully transferred to the rear of the geosynthetic 

specimen. The difference between the front and rear loads was mainly caused by the 

mobilized friction between the soil and geosynthetic specimen and specimen clamp. 

The friction loss was minimized by placing steel rollers between the confinement box 

and the supporting table. The overall friction loss should be measured through 

calibration. The friction should be controlled within 2 percent of the anticipated 

maximum load in the confined extension test or the applied creep load in the confined 

creep test in order to achieve a nearly uniform distribution of tensile loads along the 

geosynthetic specimen length. Uniformity should be checked by monitoring the strain 

distribution over the length of a test specimen. 

117 
l Strain Measurement: The tensile strain is to be measured by the use of two tell-tail 

wires connected to the geosynthetic specimen and monitored by electronic 

measurement devices such as LVDTs; however, dial gages could also be used. The 

location of the two tell-tail wires defines the gage length. A gage length of 4 in (102 

mm) should be used for geotextiles. For the geogrids, the gage length should include 

at least two full geogrid apertures but should not be less than 4 in (102 mm) in 

length. In cases where it is necessary to verify the uniformity of the strain 

distribution across the geosynthetic, a minimum of four tell-tails should be mounted 

over the gage length of the test specimen. 

l Pretension Force: In accordance with the procedure established in ASTM D 4595 for 

in-air tensile testing of geosynthetics, a total pretension force equal to 1.25 percent of 

the expected breaking force should be applied to each geosynthetic specimen. 

However, the total pretension force should not be less than 10 lb (45 N) or greater 

than 50 lb (222 N) in any case. The pretension force should be applied to the 

geosynthetic specimen prior to placement of the upper confining soil layer. This 

allows examination of the geosynthetic specimen to assess whether the geosynthetic is 

in uniform contact with the top of the lower confining soil layer. 

l Strain Rate: A constant rate of strain of 10 percent/min should be applied to the 

geosynthetic specimen in the confined extension test. Selection of this constant rate of 

strain is based on the standard loading rate specified in ASTM D 4595. Use of this 

strain rate allows for a direct comparison between the confined and unconfined stressstrain properties of geosynthetics. 

l Aspect Ratio: An aspect ratio of 2: 1 should be used for geotextile specimens. For 

geogrid specimens, a constant aspect ratio cannot be specified for various currently 

available geogrid products. A geogrid specimen should be trimmed in such a way 

that it has a length containing at least two complete apertures in the direction of 

testing (i.e., typically the machine direction) and a width containing at least five ribs 

in the cross-test direction (i.e., typically the cross-machine direction). For any 

geogrid, a minimum width of 6 in (152 mm) is recommended. 

l Initial Loading Rate for Creep Tests: A strain rate of 10 f3 percent/min should be 

used to load each creep test specimen. Selection of this initial loading rate is based 

on the standard loading rate in ASTM D 5262 for conducting unconfined tension 

creep tests of geosynthetics. 

118 
l Confining Soil Thickness: It has been shown that the confined stress-strain curves 

were almost identical for the geotextile specimens when confined in thicknesses of soil 

ranging from 1 to 3 in (25 to 76 mm) and when the geotextile specimens were placed 

at a distance of 2 in (51 mm) away from side walls of the confinement box. For ease 

of the test set-up, a soil thickness of 3 in (76 mm) should be used to assess the fully 

confined response of an g-in- (203-mm-) wide geosynthetic specimen placed within a 

12-in- (305~mm-) wide confinement box. To assess the partially confined response, a 

0.4-in- (41~mm-) thick soil layer should be placed above and below the geosynthetic. 

l Friction Along The Geosynthetic Specimen: It has been shown that the friction along 

the soil-geosynthetic interface cannot be totally eliminated due to the fact that there is 

a significant difference between the failure strains of the confining soil and the 

geosynthetic material. However, the friction along the soil-geosynthetic interface can 

be reduced by imposing low friction boundary conditions to the confining soil similar 

to that which was used by McGown et al. [1982] and Wilson-Fahmy et al. [1993]. 

The proposed test protocol identifies two different test procedures for conducting a 

test. Procedure A consists of a nonlubricated interface between the soil and 

geosynthetic where the test specimen is set up to model a fully confined condition. 

Procedure B consists of a lubricated interface between the soil and geosynthetic to 

model a partially confined condition. The selection of the specific test procedure 

should be based on the designers’ selected design approach. 

7.3 F'UTUREUSE 

As stated, the testing protocol presented in appendix C has been prepared in the format of an 

ASTM standard test method. The protocol in its current state should be offered to the 

ASTM D-35 committee on geosynthetics for consideration as a potential new test method. If 

accepted by the committee, a task group will be formed by ASTM to further review and test 

the method with the hope of eventually approving it as an ASTM standard. 

119 

CHAPTER 8 

CONCLUSIONS AND RECOMMENDATIONS 

8.1 SUMMARY AND CONCLUSIONS 

The objectives of the four-phase research program were to: (i) develop and assess the 

performance of test equipment for evaluating confined stress-strain properties of 

geosynthetics; (ii) establish testing procedures for conducting confined extension and confined 

creep tests on geosynthetics; (iii) verify the performance of the test equipment and confirm 

the validity of the procedures by testing representative geosynthetics; and (iv) develop a 

written protocol for conducting confined extension and confined creep testing. These 

objectives were achieved through the completion of several specific tasks, A brief discussion 

is presented of the most significant conclusions of each task: 

l Literature Review: The literature review identified 12 distinct references in which 

confined extension and/or creep testing equipment and/or test results were described. 

This review indicated that several types of laboratory testing equipment, including the 

pullout device, in-soil test device, and triaxial-type device, have been used to assess 

the confined stress-strain response of geosynthetics. A summary of the test equipment 

and test results reported in the references was presented. The pullout device 

developed by Juran et al. [ 19911, in-soil test device developed by McGown et al. 

[ 19821, and modified triaxial device developed by Ling et al. [1991] were identified 

as having a high potential for measuring a confined response of a geosynthetic that 

can be directly compared to an unconfined response of the geosynthetic in order to 

evaluate the effect of soil confinement. The reported results in these references 

indicate that soil confinement significantly improves the stress-strain properties of 

nonwoven geotextiles but does not significantly affect the stress-strain properties of 

woven geotextiles. 

0 Initial Base-Line Testing: Tests conducted utilizing the pullout device showed that 

there was an effect of soil confinement on the pullout response of various geosynthetic 

products. However, the results from the pullout testing indicate that the confined 

extension pullout test has the following four problems: (i) necking of the nonwoven 

geotextile specimen; (ii) variation of the “mobilized” aspect-ratio during a test; (iii) 

variation of the strain rate along the test specimen; and (iv) arbitrary selection of the 

121 
size of the first element length for interpretation. Because of these problems, it was 

concluded that it would be difficult to “uncouple” the confined stress-strain response 

and provide a measure of the actual confined geosynthetic response. Therefore, the 

pullout test was abandoned in favor of the in-soil apparatus developed by McGown et 

al. [1982], which was anticipated to overcome the problems experienced using the 

pullout testing device. A prototype confined extension testing device, similar to the 

in-soil test apparatus, was subsequently developed. The prototype confined extension 

testing device had an ability to provide: (i) a constant strain rate to a confined 

specimen; and (ii) maintain a constant aspect ratio during testing. 

l Development of Test Equipment: On the basis of the success of the prototype confined 

extension testing device, a test apparatus that could be used for production tests was 

designed, fabricated, and calibrated. The advantages of the test apparatus otherwise 

known as the protocol device fabricated for this project include: (i) ease of the test 

set-up; (ii) well-defined boundary conditions; and (iii) ease of test interpretation. The 

friction along the soil-geosynthetic interface was minimized by placing the entire 

confinement box on steel rollers so that a nearly uniform distribution of tension along 

the specimen length was achieved. Because of this uniform tension distribution, the 

confined stress-strain response of the geosynthetic was readily obtained from the 

measured data and used for a direct comparison with the unconfined response of the 

geosynthetic to evaluate the effect of soil confinement. Tests were conducted under 

constant strain rate and constant tensile load conditions. 

l Verification of Test Equipment: There were five test variables identified in the 

confined extension and confined creep testing program that required verification, 

These variables were: (i) pretension force; (ii) strain rate; (iii) aspect ratio; (iv) initial 

rate of loading for the creep tests; and (v) thickness of the confining soil. The first 

four variables were selected in accordance with ASTM D 4595 (wide-width tensile 

testing of geotextiles) and ASTM D 5262 (unconfined creep testing of geosynthetics). 

The effect of soil thickness on the confined response of geosynthetics was investigated 

through a series of confined extension tests on Geosynthetic PP-10 that was confined 

by various layers of beach sand with thicknesses ranging from 0.4 to 3 in (10 to 76 

mm). The test results indicate that there was little difference when the geosynthetic 

was confined within l-to 3-in- (25- to 76-mm-) thick soil layers. However, the effect 

of soil confinement was reduced when the thickness of the soil layer was reduced to 

0.4 in (10 mm) because cracks developed throughout the thickness of the soil layers, 

A thickness of 3 in (76 mm) was then selected for all production confined extension 

and confined creep tests to provide full confinement. 

122 
l Production Confined Extension and Creep Tests: On the basis of the results of 

verification confined extension and creep tests, production testing was conducted. 

Results based on the following conclusions can be drawn: 

1. Soil confinement significantly improves the moduli and strength of 

Geosynthetic PP-10 (Le., a needle-punched nonwoven geotextile). As the 

confining pressure increases, the moduli and strength of this material similarly 

increases. 

2. Under a constant confining pressure for Geosynthetic PP-10, the confined 

response (Le., modulus and strength) of the geosynthetic specimen in beach 

sand is greater than the response of the geosynthetic specimen in silty sand. 

3. There is a noticeable effect of confinement with respect to the confined 

response for the two woven geotextiles and two geogrids (Geosynthetics PP11, PP-12, PE-13, and PET-14) when confined in the beach sand and in the 

silty sand; the increase in confined moduli at 5 percent strain ranged from 

approximately 5 to 30 percent among Geosynthetics PP- 11, PP-12, PE-13, and 

PET-14 at a normal stress of 20 psi (138 kPa). These increases can be 

considered significant with respect to unconfined moduli of the same materials 

although less significant when compared with the increase of the 5 percent 

moduli for the nonwoven geotextile (Geosynthetic PP-10). 

4. Soil confinement significantly reduced the measured time-dependent total 

strain of Geosynthetic PP-10; the measured total strain reduced in direct 

proportion to the increase at higher confining pressures. 

5. Reduction of measured time-dependent total strains resulting from soil 

confinement for Geosynthetics PP-11, PP-12, PE- 13, and PET- 14 was less 

than that for Geosynthetic PP-10. 

6. Improvement of the confined stress-strain properties for a given geosynthetic 

material is believed to be primarily related to: (i) internal friction between 

fibers or yarns; (ii) alignment of curved fibers or yarns (i.e., tortuosity); and 

(iii) interlocking of soil within openings or apertures, of geosynthetics, 

123 
0 

l 

8.2 

Confined Extension Tests Using a Triaxial-Type Apparatus: The triaxial-type confined 

extension testing device developed at Polytechnic was used for evaluating the confined 

extension responses of Geosynthetic PP-15 and Geosynthetic PP- 16. The results of 

the confined extension tests conducted on Geosynthetic PP-15 are generally consistent 

with those found for Geosynthetic PP-10 at GeoSyntec in that the effect of soil 

confinement is significant and quantifiable. A detailed comparison of test results, 

however, indicates that there is a noticeable difference between the stress-strain 

properties of the nonwoven geotextiles obtained by the two organizations. The 

difference is likely due to the following two facts: (i) the strengths of nonwoven 

geotextiles tested by Polytechnic and GeoSyntec are highly variable, as suggested by 

the manufacturer’s data; and (ii) there are fundamental differences in the test devices. 

The triaxial-type device works in such a way that friction is developed at the soilgeotextile interface, causing a nonuniform distribution of tension along the 

geosynthetic specimen length. The response of the geosynthetic in this device is 

similar to the response in a pullout test. Accordingly, it is difficult to account for the 

effect of friction on the confined stress-strain properties of geosynthetics through an 

interpretation procedure. The authors of this report believe that it is premature to adopt 

the triaxial device for evaluating confined stress-strain properties of geosynthetics. 

Testing Protocol: A written testing protocol has been developed in the format of an 

ASTM standard test method. It is recommended that the testing protocol in its 

current state be offered to the ASTM D-35 committee on geosynthetics for 

consideration as a potential new test method. 

DESIGN IMPLICATIONS 

In the design of geosynthetic-reinforced soil walls and slopes, it is the state-of-practice to use 

the unconfined stress-strain properties of geosynthetic materials to evaluate the short-term 

and long-term stability of the geosynthetic-reinforced structures. The results of this research 

program indicate, that soil confinement imposes beneficial effects on the stress-strain response 

of the nonwoven geotextiles and the other geosynthetic materials tested in the program; 

furthermore, results indicate that the use of unconfined stress-strain properties of these 

geosynthetic materials in the design may be conservative. With specific reference to the 

nonwoven geotextile, the test results indicate that the stiffness is significantly enhanced as a 

result of soil confinement. Use of the unconfined stress-strain properties for the nonwoven 

geotextile in design may be overly conservative. Results of this program indicate that the 

124 
confined response of geosynthetics can be significant in some cases. Therefore, when using 

geosynthetic materials in the design, it is recommended that: 

0 confined extension tests be performed to determine the modulus and peak strength of 

nonwoven geotextiles and other geosynthetic materials; 

0 confined creep tests be performed to determine the long-term strength of the 

nonwoven geotextiles and other geosynthetic materials; 

0 confined extension and/or confined creep tests be performed to obtain serviceability 

values for woven geotextiles and geogrids; and 

0 confined extension and/or confined creep tests be conducted for calibrating 

instrumentation to be used in field monitoring and for assessing input parameters for 

numerical analysis. 

As discussed and supported in this document, the authors believe that the protocol confined 

extension test device is appropriate for assessing the confined properties of geosynthetic 

materials. The testing protocol can be utilized to assess geosynthetic stress-strain 

characteristics under either fully confined or partially confined conditions. The results of this 

study indicate that full confinement provides a larger improvement relative to the unconfined 

response when compared with partial confinement. Depending on the specific design 

condition assessed by the designer, either a fully or partially confined test can be conducted 

using the protocol testing device to obtain parameters to be used in’the design. 

8.3 RECOMMENDATIONS 

The test results from this study regarding the short-term and long-term confined response of 

various geosynthetic materials are encouraging. The test results indicate that soil 

confinement imposes beneficial effects on the stress-strain response of the nonwoven 

geotextile and other geosynthetic materials tested in the program. It is recommended that the 

Federal Highway Administration pursue a more comprehensive study of the confined 

behavior of geosynthetic materials with the use of the protocol confined extension/creep test 

device developed, with the ultimate goal of incorporating the confined stress-strain properties 

into future design procedures. The following recommendations are made to achieve this 

ultimate goal: 

125 
Confined extension tests are recommended to be performed for a wide range of soil 

materials. 

Confined extension tests are recommended to be performed on geosynthetics with the 

use of detailed instrumentation to measure the stiffness (modulus) of the confined 

geosynthetic materials at low strain levels. 

Confined creep tests are recommended to be performed at different tensile stress 

levels, especially at high stress levels to evaluate the confined creep rupture strengths. 

Confined creep tests are recommended to be performed for a duration of 10,000 

hours. 

Detailed tests are recommended to be performed on geosynthetics that are used 

primarily for reinforcement. 

Confined extension and confined creep tests should be conducted under both fully or 

partially confined conditions and the results compared. 

Select geosynthetic materials should be tested using the protocol testing equipment and 

the more complex UCD test device developed by Boyle [1995] and the APSR test 

device developed by Whittle et al. [1993] to assess how the confined extension test 

results relate to the actual response of geosynthetic reinforcement in simulated field 

conditions. 

It should be noted that the protocols developed will likely still result in fairly 

conservative confined creep results, although improved economy will be realized. 

126 
APPENDIX A 

CONFINED EXTENSION TEST RESULTS 

127 
4500 

4000 - 

3500 - 

q 3000 - 

n 

c; 

8 2500 - 

OL 

Lo 2000 - 

5 

G 

z 1500- 

k 

IOOOTEST SERIES NUMBER TIA: GEOSYNTHETIC PP-10 

(MACHINE DIRECTIQN) IN BEACH SAND 

~1888~ IN-AIR 

H-W-H IN BEACH SAND AT IO psi 

- IN BEACH SAND AT 20 psi 

SPECIMEN WIDTH: 8 in 

SPECIMEN LENGTH: 5 in 

GAGE LENGTH: 4 in 

STRAIN RATE: lO%/min 

0 5 10 15 20 25 30 35 40 45 E 3 

STRAIN (w) 

1 in. = 25.4 mm 

= 14.59 N/m 

6.895 kPa 

Figure A-l. Tensile force versus strain response for Geosynthetic PP-10 confined 

in beach sand. 

128 
12000 I 1 TEST SERIES NUMBER T2A: GEOSYNTHEK PP-11 

(CROSS-MACHINE DiRECTIoN) lN BEACH SAND 

10000 - 

B 8000- 

n G 

8 

!i%j 6000- 

IL 

!3 

E 2 4000- 

r 

2000 - 

- IN-AIR 

- IN BEACH SAND AT 10 psi 

- IN BEACH SAND AT 20 psi 

SPECIMEN WIDTH: 8 in 

SPECIMEN LENGTH: 5 in 

GAGE LENGTH: 4 in 

STRAlN RATE: lO%/min 

00 

0 2 4 6 8 10 12 14 16 18 : 0 

STRAIN (w> 

1 in. = 25.4 mm 

= 14.59 N/m 

= 6.895 kPo 

Figure A-2. Tensile force versus strain response for Geosynthetic PP-11 confined 

in beach sand. 

129 
TEST SERIES NUMBER T3A: GEOSYNTHETIC PP-12 

(MACHINE DIRECTION) IN BEACH SAND 

5000 - 

% 4000 - 

G 

8 

g 3000 - 

G 

3 

iz 

z 2000 - 

F 

IOOOM IN-AIR 

- IN BEACH SAND AT 10 psi 

- IN BEACH SAND AT 20 psi 

SPECIMEN WIDTH: 8 in 

SPECIMEN LENGTH: 5 in 

GAGE LENGTH: 4 in 

STRAIN RATE: 1 O%/min 

STRAIN (w) 

1 in. = 25.4 mm 

= 14.59 N/m 

= 6.895 kPa 

Figure A-3. Tensile force versus strain response for Geosynthetic PP-12 confined 

in beach sand. 

130 
8000 

c 7000 

s 

= 6000 

TEST SERIES NUMBER T4A: GEOSYNTHETIC PE-13 

(MACHINE DIRECTION) IN BEACH SAND 

- IN-AIR 

- IN BEACH SAND AT 10 psi 

- IN BEACH SAND AT 20 psi 

SPECIMEN WIDTH: 5 ribs (4.4 in) 

SPECIMEN LENGTH: 2 apertures (13.1 

GAGE LENGTH: 2 a ettures (13.1 in) 

in) 

STRAIN RATE: lO%/!nin 

0 2 4 6 8 10 12 14 1 

STRAIN (w) 

1 in. = 25.4 mm 

= 14.59 N/m 

= 6.895 kPa 

Figure A-4. Tensile force versus strain response for Geosynthetic PE13 confined 

in beach sand. 

131 
11000 1 

10000 

9000 Ii -I 

- 8000- 

c 

> 7000- ;3 

8 6000- 

f5 k 5000- 

Y i7j 4000- 

z 

)- 3000- 

2000 

1000 1 00 

0 2 4 6 8 10 12 14 I 

STRAIN (w) 

TEST SERIES NUMBER T5A: GEOSYNTHETIC Pm-14 

(MACHINE DiREcno~) IN BEACH SAND 

- IN-AIR 

- IN BEACH SAND AT 10 psi 

- IN BEACH SAND AT 20 psi 

6 

1 in. = 25.4 mm 

= 14.59 N/m 

= 6.895 kPa 

Figure A-5. Tensile force versus strain response for Geosynthetic PET-14 

confined in beach sand. 

132 
4500 

4000 

3 

8 2500 

5 

IL 2000 

41 

G 

z 1500 

F 

1000 

500 

0 

TEST SERIES NUMBER TlB: GEOSYNTHETIC PP-10 

(MACHINE ~l~EcnoN) IN sm SAND 

w IN-AIR 

- IN SILTY SAND AT 10 psi 

- IN SILTY SAND AT 20 psi 

SPECIMEN WIDTH: 8 in 

SPECIMEN LENGTH: 5 in 

GAGE LENGTH: 4 in 

STRAIN RATE: 1 Ow/min 

STRAIN (w) 

1 in.-= 25.4 mm _ 

= 14.59 N/m 

6.895 kPa 

Figure A-6. Tensile force versus strain response for Geosynthetic PP-10 confined 

in silty sand. 

133 
10000 - 

5 8000- 

e 

8 

E 6000 - 

kl 

!z 4000- 

F 

2000 - 

TEST SERIES NUMBER T2B: GEOSYNTHETlC PP-11 

(CROSS-MACHINE DIRECTION) IN SILTY SAND 

~~~880 IN-AIR 

- IN SILTY SAND AT 10 psi 

- IN SILTY SAND AT 20 psi 

SPECIMEN WIDTH: 8 in 

SPECIMEN LENGTH: 5 in 

GAGE LENGTH: 4 in 

STRAIN RATE: lO%/min 

STRAIN (w) 

1 in. = 25.4 mm 

= 14.59 N/m 

6.895 kPa 

Figure A-7. Tensile force versus strain response for Geosynthetic PP-11 coufined 

in silty sand. 

134 
6000 

5000 

- 

q4000 

4 G 

ow 

5 3000 

L 

w 

1 

g 2000 

2 

1000 

C 

TEST SERIES NUMBER T3l3: GEOSYNTHETIC PP-12 

(MACHINE D~REC~~N) IN BEACH SAND 

w IN-AIR 

- IN SILTY SAND AT 10 psi, 

- IN SILTY SAND AT 20 psi 

SPECIMEN WIDTH: 8 in 

SPECIMEN LENGTH: 5 in 

f?Al!F I FNPTH* A in - 

I I I I I I I I I I I 

4 8 12 16 20 : 

STRAIN (w) 

1 in. = 25.4 mm 

= 14.59 N/ 

6.895 kPa 

4 

n 

Figure A-8. Tensile force versus strain response for Geosynthetic PP-12 confined 

in silty sand. 

135 
10000 

TEST SERIES NUMBER T4B: GEOSYNTHETIC PE-13 

9000 (MACHINE r11~1xnoN) IN slLTy SAND 

8000 

- 

- IN-AIR SILTY SAND AT 10 

IN SILM SAND AT 20 

psi 

- psi 

c 7000 

s 

= 6000 

6 

i? K5000 

3 4000 

z 

P 3000 

2000 

1000 

0 

0 2 4 6 8 IO 12 14 

STRAIN (w) 

1 in. = 25.4 mm _ 

1 Ib/ft =s 14j59kp~/m 

1 psi = . 

Figure A-9. Tensile force versus strain response for Geosynthetic PE13 confined 

in silty sand. 

136 
11000 1 

10000 

9000 

1 

- 8000 

c 

> 7000 5 3 

TEST SERIES NUMBER T5B: GEOSYNTHETIC PET-14 

(MACHINE DIRECTION) IN SILTY SAND 

eeeea IN-AIR 

- IN SlLlY SAND AT 10 psi 

- IN SILTY SAND AT 20 psi 

SPECIMEN WIDTH: 5 ribs (4.8 in) 

SPECIMEN LENGTH: 2 apet-hres (7,7 in) 

‘H: 2 aoertures (7.7 I 

K 8 6000 2 5000 3 i7j 4000 El I- 3000 

2000 

I I I I I I I I I I I I I I 

0 2 4 6 8 10 12 14 ' 

STRAIN (w) 

6 

1 in. = 25.4 mm 

= 14.59 N/m 

6.895 kPa 

Figure A-10. Tensile force versus straiu response for Geosynthetic PET-14 

confined in silty sand. 

137 
APPENDIX B 

CONFINED CREEP TEST RESULTS 

138 
32 

TEST SERIES NUMBER CIA: GEOSYNTHETIC PP-10 

IN MACHINE DIRECTION UNDER 25% OF 

28- ULTlMATE LOAD 

24 

eeeee IN-AIR 

- IN BEACH SAND AT 10 

- IN BEACH SAND AT 20 

psi 

- IN BEACH SAND AT 10 

psi 

psi 

20 

sic 

Z 16 

is I- 

(f) 12 

8 

SPECIMEN WIDTH: 8 in 

SPECIMEN LENGTH: 5 in 

4 

GAGE LENGTH: 4 in 

0 t I I I I llll~ I I I I llll~ I I llllll( I I Illlll~ I lllllr 

10 --2 10 -' 1 

TIME 

IO2 

1 in = 25.4 mm 

1 psi = 6.895 kPa 

Figure B-l. Strain versus thne response for Geosynthetic PP-10 confined in 

beach sand. 

139 
lo2 

TEST SERIES NUMBER CIA: GEOSYNTHETIC PP-IO 

10 IN MACHINE DIRECTION UNDER 25% OF 

z 

ULTIMATE LOAD 

-5 1 w IN-AIR 

25 - IN BEACH SAND AT 10 psi 

- IN BEACH SAND AT 20 psi 

* IN BEACH SAND AT 10 psi 

SPECIMEN WIDTH: 8 in 

SPECIMEN LENGTH: 5 in 

GAGE LENGTH: 4 in 

lo-’ 1 I I I I llll( I I I I llll( I l I I Ill11 I ‘I I lllll( I I I Ill1 T 

10 -2 IO -' 1 10 IO2 IO3 

TIME (h) 

1 in = 25.4 mm 

1 psi = 6.895 kPa 

Figure B-2. Incremental strain rate versus time response for Geosynthetic PP-10 

confined in beach sand. 

140 
103g 

TEST SERIES NUMBER CIA: GEOSYNTHETIC PP-IO 

IN MACHINE DIRECTION UNDER 25% 01 

n ULTIMATE LOAD 

c 1n 7 ‘E IU’ w IN-AIR 

-2 

- IN BEi.-. KH SAND AT 10 psi 

- 

4n - IN BEACH si tND AT 20 psi 

IV - IN BEACH SAND AT 10 psi 

P 2 1 

z 10-l 

i55 

6 1p 

a 

2 1o-3 

iii I 

Id lo-$ (1 L 

is 

z 1cr5 4 1” 

SPECIMEN WIDTH: 8 in 

10 -6 ” tSPECIMEN LENGTH: 4 in 5 in 

GAGE LENGTH: 

low7 i 1 

I I I I I -1 I I I I I I I I I 

0 4 8 12 16 20 24 28 : 

TOTAL STRAIN (w) 

1 in = 25.4 mm 

1 psi = 6.895 kPa 

Figure B-3. Incremental strain rate versus strain response for Geosynthetic 

PP-10 confined in beach sand. 

141 
TEST SERIES NUMBER C2A: GEOSYNTHETIC PP-11 

IN CROSS-MACHINE DIRECTION UNDER 50% OF 

ULTIMATE LOAD 

- IN-AIR 

- IN BEACH SAND AT 10 psi 

- IN BEACH SAND AT 20 psi 

- IN BEACH SAND AT 10 psi 

SPECIMEN WIDTH: 8 in 

SPECIMEN LENGTH: 5 in 

GAGE LENGTH: 4 in 

1 IO 

TIME (h) 

1 in = 25.4 mm 

1 psi = 6.895 kPa 

Figure B-4. Strain versus time response for Geosynthetic PP-11 confined in 

beach sand. 

142 
1 

TEST SERIES NUMBER C2Az GEOSYNTHErIC PP-11 

IN CROSS-MACHINE DlRECTlON UNDER 50% OF 

ULTlMATE LOAD 

0 

0 

0 

0 

0 

3EACH SAND AT 10 psi 

3EACH SAND AT 20 psi 

- ii BEACH SAND AT 10 psi 

-l- 

-2, 

-3, 

4 

SPECIMEN WIDTH: 8 in 

SPECIMEN LENGTH: 5 in 

GAGE LENGTH: 4 in 

-5 

I I I lllll~ I I I I llll~ I I I I Ill11 I I I I Ill11 I IlllTl 

10 -2 10 -' 1 

TIME 

IO2 

1 in = 25.4 mm 

1 psi = 6.895 kPa 

Figure B-5. Incremental strain rate versus time response for Geosynthetic PP-11 

confined in beach sand. 

143 
10'3 

TEST SERIES NUMBER C2Az GEOSYNTHEI-IC PP-11 

IN CROSS-MACHINE DIRECTION UNDER 5oW OF 

T 10% ULTIMATE LOAD 

.- 

E 

\ t3eee.a IN-AIR 

-!!s - IN BEACH SAND AT 

1 

10 psi 

E IN BEACH SAND AT 20 psi 

W - IN BEACH SAND AT 10 psi 

Is 

z 10-l-g 

- 

$ lm lo-?? 

SPECIMEN WIDTH; 8 in 

SPECIMEN LENGTH: 5 in 

2 GAGE LENGTH: 4 in 

2 

g 1o-J% 

!z - 

10 -4r 

10 -+ II II II II II II II II 11 1 

0 2 4 6 8 10 12 14 16 18 ; 

TOTAL STRAIN (w ) 

1 in = 25.4 mm 

1 psi = 6.895 kPa 

Figure B-6. Incremental strain rate versus strain response for Geosynthetic 

PP-11 confined in beach sand. 

144 
32 

TEST SERIES NUMBER C3Az GEOSYNTHETIC PP-12 

IN MACHINE DIRECTION UNDER 50% OF 

U LTI MATE LOAD 

24 w IN-AIR 

- IN BEACH SAND AT 10 psi 

- IN BEACH SAND AT 20 psi 

g20 

z SPECIMEN WIDTH: 8 in 

z SPECIMEN LENGTH: 5 in 

g 16 GAGE LENGTH: 4 in 

10 -2 10 -' 1 

TIME 

IO2 

1 in = 25.4 mm 

1 psi = 6.895 kPa 

Figure B-7. Strain versus time response for Gosynthetic PP-12 confined in 

beach sand. 

145 
F 10 

.- 

E \ 

25 1 z 

F 

is 

z 10-l-g 

- 

$ 

ix 

a 

10 -2z 

!z 

; lO"z 

P - 

10 -4-g 

TEST SERIES NUMBER C3Az GEOSYNTHEl-IC PP-1 

IN MACHINE DIRECTION UNDER 50% OF 

ULTlMATE LOAD 

k M IN-AIR 

- IN BEACH SAND AT 10 psi 

- IN BEACH SAND AT 20 psi 

SPECIMEN WIDTH: 8 in 

SPECIMEN LENGTH: 5 in 

GAGE LENGTH: 4 in 

2 

IO" 1 I I I Illll~ I I I lllll~ I I I lllll( I I Illlll~ I llllll10 a 10 -’ 1 10 IO2 IO" 

TIME (h) 

1. in = 25.4 mm 

li psi = 6.895 kPa 

Figure B-8. Incremental strain rate versus time response for Geosynthetic PP-12 

confined in beach sand. 

146 
103S 

TEST SERIES NUMBER C3Az GEOSYNTHEl-IC PP-12 

- lop-g 

IN MACHINE DIRECTION UNDER 50% OF 

ULTlMATE LOAD 

.- 

E aeeea IN-AIR 

3 IQ 

- IN BEACH SAND AT 10 psi 

- IN BEACH SAND AT 20 psi 

g 12 SPECIMEN WIDTH: 8 in 

SPECIMEN LENGTH: 5 in 

Z - GAGE LENGTH: 4 in 

$ 

lz 

10 -‘z 

10 -6 I I I I I I I I I I I 

0 4 8 12 16 20 

TOTAL STRAIN (w ) 

1 in = 25.4 mm 

1 psi = 6.895 kPa 

Figure B-9. Incremental strain rate versus strain response for Geosynthetic 

PP-12 confined in beach sand. 

147 
16 

TEST SERIES NUMBER C4A: GEOSYNTHEI-IC PE-13 

IN MACHINE DIRECTION UNDER 5oW OF 

14 ULTlMATE LOAD 

12 - IN-AIR 

- IN BEACH SAND AT 10 psi 

- IN BEACH SAND AT 20 psi 

2 SPECIMEN WIDTH: 5 ribs (4.4 in) 

SPECIMEN LENGTH: 2 apertures (13.1 

GAGE LENGTH: 1 aperture (6.6 in) 

in) 

1 10 

TIME (h) 

1 in = 25.4 mm 

1 psi = 6.895 kPa 

Figure B-10. Strain versus time response for Geosynthetic PP-13 confined in 

beach sand. 

148 
lo2 

TEST SERIES NUMBER C4A: GEOSYNTHETIC PE-13 

IN MACHINE DIRECTION UNDER 50% OF 

U LTI MATE LOAD 

eeew~ IN-AIR 

- IN BEACH SAND AT 10 psi 

- IN BEACH SAND AT 20 psi 

z 10-l 

- 

iis 

ii 

d 

10 -2z 

t 

; 1o-3z 

5 

Z - 

lo-+% 

10m5 i I I I I Ill11 I I I I llll~ I I I I Illl~ I I Illlll~ I IllIf l-r 

10 -2 10 -' 1 10 IO2 IO3 

TIME (h) 

SPECIMEN WIDTH: 5 ribs (4.4 in) 

SPECIMEN LENGTH: 2 apertures (13.1 

GAGE LENGTH: 1 aperture (6.6 in) 

in) 

1 in = 25.4 mm 

1 psi = 6.895 kPa 

Figure B-11. Incremental strain rate versus time response for Geosynthetic PE13 

confiued in beach sand. 

149 
n 

.- 

E 

\ 

25 

P 

s 

1 

1 TEST SERIES NUMBER C4A: GEOSYNTHErlC PE-13 

-I 

10 2x 

IN MACHINE DIRECTION UNDER 50% OF 

ULTlMATE LOAD 

m IN-AIR 

10 E 

- IN BEACH SAND AT IO 

- IN BEACH SAND AT 20 

12 

0 -l0 -3- 

0 -4- 

i SPECIMEN WIDTH: 5 ribs (4.4 in) 

- SPECIMEN LENGTH: 2 apertures (13.1 

- GAGE LENGTH: 1 aperture (6.6 in) 

in) 

0 -5 

I I I I I I I I I I I I r 

0 2 4 6 8 10 12 

TOTAL STRAIN (w) 

psi 

psi 

Ii 

1 in = 25.4 mm 

1 psi = 6.895 kPa 

Figure B-12. Incremental strain rate versus strain response for Geosynthetic 

PE13 confined in beach sand. 

150 
9 

TEST SERIES NUMBER C5A: GEOSYNTHl3C PET-14 

IN MACHINE DIRECTION UNDER 60% OF 

ULTlMATE LOAD 

eeeee IN-AIR 

- IN BEACH SAND AT 10 psi 

- IN BEACH SAND AT 20 psi 

6 

1 IO 

TIME (h) 

1 in = 25.4 mm 

1 psi = 6.895 kPa 

Figure B-13. Strain versus time response for Geosynthetic PET-14 confined in 

beach sand. 

151 
1 

TEST SERIES NUMBER C5A: GEOSYNTHETlC Pm-14 

IN MACHINE DIRECTKIN UNDER 6th OF 

IO ULTIMATE LOAD 

eeeee IN-AIR 

1 - IN BEACH SAND AT 10 psi 

- IN BEACH SAND AT 20 psi 

0 -1 SPECIMEN WIDTH: 5 ribs (4.8 in) 

SPECIMEN LENGTH: 2 apertures (7.7 in) 

GAGE LENGTH: 1 aperture (4.0 in) 

0 -2 

1 IO 

TIME (h) 

1 in = 25.4 mm 

1 psi = 6.895 kPa 

Figure B-14. Incremental strain rate versus time response for Geosynthetic 

PET-14 confined in beach sand. 

152 
TEST SERIES NUMBER C5A: GEOSYNTHEllC PET-14 

IN MACHINE DIRECTION UNDER 60% OF 

ULTIMATE LOAD 

0 IN-AIR 

- IN BEACH SAND AT 10 psi 

- IN BEACH SAND AT 20 psi 

SPECIMEN WIDTH: 5 ribs (4.8 in) 

SPECIMEN LENGTH: 2 apertures (7.7 in) 

GAGE LENGTH: 1 aperture (4.0 In) 

10-y I 1 I 1 I 11 i-1 I 1 I ’ I I I I I ’ 

0123456789' 

TOTAL STRAIN (%) 

0 

1 in = 25.4 mm 

1 psi = 6.895 kPa 

Figure B-15. Incremental strain rate versus strain response for Geosynthetic 

PET-14 confined in beach sand. 

153 
32 

28 

TEST SERIES NUMBER Cl B: GEOSYNTHErlC PP-10 

IN MACHINE DIRECTION UNDER 25% OF 

ULTlMATE LOAD 

24 

m IN-AIR 

- IN SlLlY SAND AT 10 

- IN SILlY SAND AT 20 

psi 

psi 

20 

2 

Z 16 

i?s 

i.z 

12 

8 

SPECIMEN WIDTH: 8 in 

4 

SPECIMEN LENGTH: 5 in 

GAGE LENGTH: 4 in 

1 10 

TIME (h) 

1 in = 25.4 mm 

1 psi = 6.895 kPa 

Figure B-16. Strain versus time response for Geosynthetic PP-10 confined in silty 

sand. 

154 
TEST SERIES NUMBER Cl B: GEOSYNTHETIC PP-10 

IN MACHINE DIRECTION UNDER 25% OF 

ULTlMATE LOAD 

E IN-AIR IN SILTY SAND AT IO psi 

- IN SlLrY SAND AT 20 psi 

p 10-l 

$ 

'+ 1o-2 

ii! lo IO4 

s 

- i 

5 lo-$ 

zi 

g lo+ 

-I 

10 -I 

-4 

SPECIMEN WIDTH: 8 in 

SPECIMEN LENGTH: 5 in 

GAGE LENGTH: 4 in 

10m7 i I I 111111~ l I Ill1111 I I I I Illl~ I I I I Illll I IllIn 

I 

IO -2 IO -' 1 10 10 L IO" 

TIME (h) 

1 in = 25,4 mm 

1 psi = 6.895 kPa 

Figure B-17. Incremental strain rate versus time response for Geosynthetic PP-10 

confined in silty sand. 

155 
- il 10” 

TEST SERIES NUMBER Cl B: GEOSYNTHEl-IC PP-10 

IN MACHINE DIRECTION UNDER 25% OF 

ULTIMATE LOAD 

- 

E 

itis 1 

$10-l 

F fl IO" 

d !Ig 1o-s 

ii lo-$ 

f!5 

z 1o-5 

10 -I' 

IO -' 

0 

I 

- IN-AIR 

- IN SILTY SAND AT 

- IN SILTY SAND AT 

SPECIMEN WIDTH: 8 in 

SPECIMEN LENGTH: 5 in 

GAGE LENGTH: 4 in 

IA I I I I I - -1 I I I I I I I 

4 8 12 16 20 24 

TOTAL STRAIN (%) 

10 psi 

20 psi 

1 in = 25,4 mm 

1 psi = 6.895 kPa 

Figure B-18. Incremental strain rate versus strain response for Geosynthetic 

PP-10 confined in silty sand. 

156 
16 

14 

z I2 

Ti E 10 

cweee IN-AIR 

- IN SILTY SAND AT 10 psi 

- IN SILTY SAND AT 20 psi 

cweee IN-AIR 

- IN SILTY SAND AT 10 

- IN SILTY SAND AT 20 

psi 

psi 

SPECIMEN WIDTH: 8 in 

SPECIMEN LENGTH: 5 in 

GAGE LENGTH: 4 in 

SPECIMEN WIDTH: 8 in 

SPECIMEN LENGTH: 5 in 

GAGE LENGTH: 4 in 

6 

4 

2 

T 

10 -2 10 -’ 1 IO IO2 IO3 

TIME (h) 

TEST SERIES NUMBER C2B: GEOSYNTHETIC GEOSYNTHETIC PP-11 

IN CROSS-MACHINE CROSS-MACHINE DIRECTION DIRECTION UNDER 50% OF 

U LTl MATE LOAD 

1 in = 25.4 mm 

1 psi = 6.895 kPa 

Figure B-19. Strain versus time response for Geosynthetic PP-11 confined in silty 

sand. 

157 
n 

c _- 

10 -'z 

IO -2E 

10 -sz 

10 -4-g 

10 +j-- 

TEST SERIES NUMBER C2B: GEOSYNTHErIC PP-11 

IN CROSS-MACHINE DIRECTION UNDER 5oW OF 

ULlItvlATE LOAD 

v IN-AIR 

- IN SILTY SAND AT 10 psi 

- IN SILTY SAND AT 20 psi 

LENGTH: 5 in 

4GE LENGTH: 4 in 

I I llllll~ I I I I Ill11 I I llllll~ I I Illlll~ I lllllr 

IO -2 10 -' 1 IO IO2 IO= 

TIME (h) 

1 in = 25.4 mm 

1 psi = 6.895 kPa 

Figure B-20. Incremental strain rate versus time response for Geosynthetic Pk-11 

confined in silty sand. 

158 
102=J 

TEST SERIES NUMBER C2B: GEOSYNTHETIC PP-11 

IN CROSS-MACHINE DIRECTION UNDER 5oW OF 

ULTIMATE LOAD 

eeeea IN-AIR 

W 

I -g 

10 -‘z 

10 -2E 

10 -3z 

10 -4B 

c 

- IN SILTY SAND AT 10 psi 

- IN SILTY SAND AT 20 psi 

SPECIMEN WIDTH: 8 in 

SPECIMEN LENGTH: 5 in 

1 11 &AGE LENGTH: 4. h 

lO”i I 1 I 1 l 1 I 1 l 1 l 1 l 1 l 1 l 1 I 

0 2 4 6 8 10 12 14 16 18 : 

TOTAL STRAIN (w) 

1 in = 25.4 mm 

1 psi = 6.895 kPa 

Figure B-21. Incremental strain rate versus strain response for Geosynthetic 

PP-11 confined in silty sand. 

159 
TEST SERIES NUMBER C3B: GEOSYNTHFTIC PP-12 

IN MACHINE DIRECTION UNDER 50% OF 

28 ULTIMATE LOAD 

24 

A -i 

CSBBBEI IN-AIR 

- IN SILTY SAND AT 10 psi 

- IN SILTY SAND AT 20 psi P 

24 CSBBBEI IN-AIR 

- IN SILTY SAND AT 10 psi 

x20 

- IN SILTY SAND AT 20 psi 

Z SPECIMEN WIDTH: 8 in 

zz SPECIMEN LENGTH: 5 in 

El6 GAGE LENGTH: 4 in 

a 

6 12 

I8 

IT 

10 -2 IO -' 1 IO 10 2 IO3 

TIME (h) 

1 in = 25.4 mm 

1 psi = 6.895 kPa 

Figure B-22, Strain versus time response for Geosynthetic PP-12 confined in silty 

sand. 

160 
TEST SERIES NUMBER C3B: GEOSYNTHETIC PP-12 

IN MACHINE DIRECTION UNDER 50% OF 

ULTlMATE LOAD 

w IN-AIR 

- IN SILTY SAND AT 10 psi 

- IN SILTY SAND AT 20 psi 

SPECIMEN WIDTH: 8 in 

SPECIMEN LENGTH: 5 in 

GAGE LENGTH: 4 in 

IO -fl I I I Illll~ I I I I Ill11 I I I I Ill11 I I I IllIll I I Illlr 

10 -2 10 -’ 1 10 IO2 IO" 

TIME (h) 

1 in = 25.4 mm 

1 psi = 6.895 kPa 

Figure B-23. Incremental strain rate versus time response for Geosynthetic PP-12 

confined in silty sand. 

161 
TEST SERIES NUMBER C3B: GEOSYNTHEIIC PP-12 

- lo"? 

IN MACHINE DIRECTION UNDER 5Qw OF 

ULTlMATE LOAD 

eeeee IN-AIR 

- IN SILTY SAND AT 10 psi 

1 - IN SILTY SAND AT 20 psi 

Z - 

$ 

bi 

10 -' 

d 10-2 

!z 

i!i 

& IO5 

2 - ~ 

SPECIMEN WIDTH: 8 inSPECIMEN LLNGiHi, 5 In 

. 78 GAGE LENGT . 

IO -4-g 

IO -6 I I I I I I I I I I I I I 

0 4 

I 

8 

I 

12 16 20 24 28 Z 

TOTAL STRAIN (w ) 

1 in = 25.4 mm 

1 psi = 6.895 kPa 

Figure B-24. Incremental strain rate versus strain response for Geosynthetic 

PP-12 confined in silty sand. 

162 
16 

TEST SERIES NUMBER C4B: GEOSYNTHEl-IC PE-13 

14 

IN MACHINE DIRECTION UNDER 50% OF 

ULTlMATE LOAD 

12 OBBBF) IN-AIR 

- IN SILTY SAND AT 10 ps 

- IN SILTY SAND AT 20 ps 

IO A 

8 

z 8 

ii!! 

Li 

6 

2 SPECIMEN WIDTH: 5 ribs (4.4 in) 

SPECIMEN LENGTH: 2 apertures (13.1 

GAGE LENGTH: 1 aperture (6.6 in) 

in) 

0 T 

1 10 IO2 IO3 

TIME (h) 

1 in = 25.4 mm 

1 psi = 6.895 kPa 

Figure B-25. Strain versus time response for Geosynthetic PE13 coufined in silty 

sand. 

163 
TEST SERIES NUMBER C4B: GEOSYNTHETIC PEIN MACHINE DIRECTION UNDER 50% OF 

‘0 ULTlMATE LOAD 

= IN-AIR 

- IN SILTY SAND AT 10 osi 

0 -l0 -2, 

0 -3, 

a 

0 --+ SPECIMEN WIDTH: 5 ribs (4.4 in) 

SPECIMEN LENGTH: 2 apertures (13.1 

GAGE LENGTH: 1 aperture (6.6 in) 

in) 

813 

I I I I llll~ I I I I Ill11 I I I I llll~ I I Illlll~ I llllll 

10 -2 10 -' 1 IO 

TIME (h) 

1 in = 25.4 mm 

1 psi = 6.895 kPa 

Figure B-26. Incrernent.al strain rate versus time response for Geosynthetic PE13 

confined in silty sand. 

164 
IO33 

TEST SERIES NUMBER C4B: GEOSYNTHETIC PE-13 

A 102-g IN MACHINE DIRECTION UNDER 50% 0.F 

ULTIMATE LOAD 

.- 

L 0888f) IN-AIR 

- IN SlLlY SAND AT IO psi 

- IN SILTY SAND AT 20 psi 

IP -44 

rl 

J 3 SPECIMEN rk;WyH 5 

SPECIMEN 

2ri~p~~~~en~l 3 ’ , . * ,n) 

- 

10 -5 

GAGE LENGTH: 1 aperture (6.6 in) 

I I I I I I I I I I I I I I I 

0 2 4 TO:AL 8 10 12 14 1 

STRAIN (w) 

1 in = 25.4 mm 

1 psi = 6.895 kPa 

Figure B-27. Incremental strain rate versus strain response for Geosynthetic 

PE13 confined in silty sand. 

165 
-IN SILTYSANDAT IO ’ 

- IN SILTY S4ND AT 20 ,“:; 

0 ; , , , I,,,,, , , , ,,,,,, , , ,-J- ,,,,, 

IO -2 IO -* 1 

I I I I ir,q I I 

IO 

TIME (h) 

IO 2 

SPECIMEN WIDTH: 5 ribs (4.8 in) 

SPECIMEN LENGTH: 2 apertures (7.7 in) 

GAGE LENGTH: 1 aperture (4.0 in) 

1 in = 25.4 mm 

1 psi = 6.895 kPa 

166 
TEST SERIES NUMBER C5B: GEOSYNTHfllC PET-14 

IN MACHINE DIRECTION UNDER 60% OF 

10 ULTIMATE LOAD 

- IN-AIR 

- IN SILlY SAND AT 10 psi 

- IN SILTY SAND AT 20 psi 

SPECIMEN WIDTH: 5 ribs (4.8 in) 

SPECIMEN LENGTH: 2 apertures (7.7 in) 

GAGE LENGTH: 1 aperture (4.0 in) 

-2, 

-3, 

-4- 

-c -0 I 

I I I I I Ilil~ I I I I Ilil~ I I I I Illl~ I I llllll~ I I I1111 

1o-2 10-l 1 10 lo2 IO" 

TIME (h) 

1 in = 25.4 mm 

1 psi = 6.895 kPa 

Figure B-29. Incremental strain rate versus time response for Geosynthetic 

PET-14 confined in silty sand. 

167 
TEST SERIES NUMBER C5B: GEOSYNTHFTIC PET-14 

IN MACHINE DIRECTION UNDER 60% OF ULTIMATE LOAD 

c)8880 IN-AIR 

- IN SILTY SAND AT 10 psi 

- IN SILTY SAND AT 20 psi 

SPECIMEN WIDTH: 5 ribs (4.8 in) 

SPECIMEN LENGTH: 2 apertures (7.7 in) 

GAGE LENGTH: 1 aperture (4.0 in) 

10-y I I I 1 I 1 I 1-1 1 I I I I I I I I I 

0123456789' 

TOTAL STRAIN (w) 

1 in = 25.4 mm 

1 psi = 6.895 kPa 

D 

Figure B-30. Incremental strain rate versus strain response for Geosynthetic 

PET-14 confined in silty sand. 

168 
APPENDIX C 

PROTOCOL FOR CONFINED 

EXTENSION/CREEP TESTING 

169 
TEST PROTOCOL 

EVALUATING THE CONFINED EXTENSION/CREEP 

BEHAVIOR OF GEOSYNTHETICS FOR 

HIGHWAY APPLICATIONS 

1. Scope 

1.1 This test method is intended for use in determining the confined extension 

and/or confined creep behavior of geosynthetics used in highway applications when subjected 

to a sustained normal stress and tensile stress loading. This test method is applicable for 

geotextiles and geogrids. 

1.2 The test method can be used to measure the tensile force and strain properties 

of a geosynthetic specimen confined and tested at a constant deformation (i.e., strain) rate; 

the test method can also be used to measure the total strain of the geosynthetic test specimen 

tested under a constant sustained tensile load. 

1.3 The test method includes procedures for measuring the extension and creep 

behavior of a geosynthetic under two different confinement conditions. The selection of the 

test procedure should be based on the designer’s selected design approach. Procedure A 

provides full confinement to the soil and the geosynthetic by using a thick soil specimen and 

a nonlubricated interface between the soil and the confining air bladder; Procedure B 

provides partial confinement to the soil and the geosynthetic by using a thin soil specimen 

and a lubricated interface between the soil and the confining air bladder. 

1.4 The test method also provides guidance on the calculation and presentation of 

test data. In the extension test, data are used to calculate tensile strength, strain, tensile 

modulus (i.e., initial tensile modulus, offset tensile modulus, and secant tensile modulus), 

and force versus strain curves. In the creep test, data are used to calculate total strain, 

incremental strain rate, total strain versus time curves, incremental strain rate versus time 

curves, and incremental strain rate versus total strain curves. 

1.5 The values stated in SI units are to be regarded as the standard. The values 

given in parentheses are for information only. Within this protocol, the use of the term 

strain is considered as standard. 

170 
1.6 This test protocol may involve hazardous materials and equipment. This test 

protocol does not address all of the health and safety problems associated with its use when 

hazardous materials are involved. It is the responsibility of the user of this test protocol to 

establish appropriate health and safety practices and determine the applicability of regulatory 

limitations prior to use. 

2. Referenced Documents 

2.1 ASTM Standards: 

D 123 

D 653 

D 1776 

D 1909 

D 2435 

D 2990 

D 4354 

D 4439 

D 4595 

D 5262 

E6 

Terminology Relating to Textile Materials 

Standard Terminology Relating to Soil, Rock, and Contained 

Fluids 

Practice for Conditioning Textiles for Testing 

Table of Commercial Moisture Regains for Textile Fibers 

Standard Test Method for One-Dimensional Consolidation 

Properties of Soils 

Test Methods for Tensile, Compressive, and Flexural Creep and 

Creep Rupture of Plastics 

Practice for Sampling of Geotextiles for Testing 

Terminology for Geosynthetics 

Test Method for Tensile Properties of Geotextiles by the Wide 

Width Strip Method 

Standard Test Method for Evaluating the Unconfined Tension 

Creep Behavior of Geosynthetics 

Terminology Relating to Methods of Mechanical Testing 

171 
3. Terminology 

3.1 Definitions - For definitions of other terms used in this standard, refer to 

Terminology D 123, D 653, D 4439, and E 6. 

3.2 Descriptions of Terms Specific to This Standard: 

3.2.1 atmosphere for testing geosynthetics, n. - air maintained at a relative humidity 

between 50 to 70 percent and temperature of 21 O +2”C (70” f4 OF). 

3.2.2 confined extension tensile test, n. - an extension test in which a geosynthetic 

material is stretched in one direction while under confinement to determine the force versus 

strain characteristics, the tensile strength, tensile modulus (i.e., initial, offset, and/or secant), 

and the total strain. 

3.2.3 creep, n. - the time-dependent increase in accumulated strain in a material 

resulting from an applied constant force while under confinement (i.e., a confined creep 

test). 

3.2.4 creep strain, n. - the total strain (i.e., extension divided by gage length) at any 

given time produced by the applied tensile load during a confined creep test. 

3.2.5 corresponding force, n. - the force associated with a specific strain on the 

force per unit width strain curve. 

3.2.6 design load, n. - the load at which the geosynthetic is required to operate in 

order to perform its intended function. 

3.2.7 extension, n. - the change in the initial gage length of the geosynthetic 

specimen during the confined extension or confined creep test. 

3.2.8 failure, n. - the point during a confined extension or confined creep test at 

which a material ceases to be functionally capable of its intended use. 

3.2.9 gage length, n. - the initial length of the portion of the specimen over which 

strain (or creep strain) is to be measured. 

172 
3.2.10 geogrid, n. - a geosynthetic formed by a regular network of integrally 

connected elements with apertures greater than 6.35 mm (l/4 in) to allow interlocking with 

surrounding soil, rock, earth, and other materials to function primarily as reinforcement. 

3.2.11 geosynthetic, n. - a planar product manufactured from polymeric material 

used with soil, rock, earth, or other geotechnical engineering-related material as an integral 

part of a man-made project, structure, or system. 

3.2.12 geotextile, n. - a permeable geosynthetic comprised solely of textiles. Current 

manufacturing techniques produce nonwoven geotextiles, knitted (nontubular) geotextiles, and 

woven geotextiles. 

3.2.13 initial tensile modulus, n. - the ratio of the change in tensile force per unit 

width to a change in strain for the initial portion of a force per unit width strain curve. 

3.2.14 ofSset tensile modulus, n. - the ratio of the change in force per unit width to a 

change in strain below the proportional limit point and above the tangent point on the force 

per unit strain curve. 

3 -2.15 proportional limit, n. - the greatest stress that a material is capable of 

sustaining without any deviation from linear proportionality of stress to strain (Hooke’s 

Law). 

3.2.16 rate of creep, n. - the slope of the creep versus time curve at a given time. 

3.2.17 secant tensile modulus, n. - the ratio of the change in force per unit width to 

a change in strain between two points on a force per unit width strain curve. One of the two 

points will be the origin (or the offset of the origin). The second point will be the point on 

the curve corresponding to a specified strain. 

3.2.18 strain, n. - the total extension divided by the gage length produced by the 

applied tensile load during a confined extension or confined creep test. (Syn. total strain) 

3.2.19 tangent point, n. - the first point of the force versus strain curve at which a 

major increase in slope occurs. The tangent point is determined by extending the line on the 

force versus strain curve where Hooke’s Law is valid (see proportional limit) to the zero 

force axis. The point where the force versus strain curve first touches this line is the tangent 

point. 

173 
3.2.20 tensile creep rupture strength, n. - for geosynthetics, the force per unit width 

that will produce failure by rupture in a confined creep test in a given time in a specified 

constant environment. 

3.2.21 tensile creep strain, n. - the total strain at any given time during a confined 

creep test. 

3.2.22 tensile modulus, n. - the ratio of the change in tensile force per unit width to 

a corresponding change in strain. 

3.2.23 tensile strength, n. - the maximum resistance to deformation developed for a 

specific material when subjected to tension by an external force. Tensile strength of 

geosynthetics is the characteristic of a sample as distinct from a specimen and is expressed in 

force per unit width. 

3.2.24 wide-width strip tensile test, n. - for geosynthetics, a tensile test in which the 

entire width of a 200~mm (8.0-in) wide geosynthetic specimen incorporating a gage length of 

100 mm (4.0 in) is gripped in the clamps. 

4. Summary of Test Method 

4.1 Both ends of a relatively wide geosynthetic specimen are gripped across the 

entire specimen width in clamps. The specimen is placed in a confinement box between 

layers of a specified soil. Confining pressures are applied at the boundaries of soil to 

provide normal stress to the geosynthetic specimen. 

4.2 For both the confined extension and confined creep test, the thickness of soil 

adjacent to the geosynthetic can be controlled to provide either partial or full confinement to 

the geosynthetic. 

4.3 For a confined extension test, a constant rate of extension (CRE) type tensile 

testing machine is operated at a prescribed rate of extension to apply a tensile force to the 

confined geosynthetic specimen until the specimen ruptures, Tensile strength, strain, and 

initial, offset, and secant tensile modulus of the test specimen can be calculated from 

machine scales, dials, recording charts, or an interfaced computer. 

174 
4.4 For a confined creep test, the tension creep behavior of a confined 

geosynthetic specimen is measured by applying an initial load in one step and sustaining the 

load for the duration of the test. The total strain and creep strain of the test specimen as a 

function of time are measured while maintaining a specified confining stress and sustained 

tensile load. 

5. Significance and Use 

5.1 This test method is developed for use in the determination of tensile force and 

strain properties or creep properties that may occur in geosynthetics while under confined 

conditions. 

5.1.1 The test data can be used in conjunction with interpretive methods to evaluate 

tensile force versus strain and the creep strain at design loads. 

5.2 This test method is not intended for routine acceptance testing of 

geosynthetics. This test method should be used to characterize geosynthetics intended for use 

in reinforcement applications in which the confined stress-strain and creep behavior of the 

geosynthetic are of concern. Test Method D 4595 for the determination of the wide-width 

strip tensile properties of geotextiles should be used for the acceptance testing of commercial 

shipments of geosynthetics. 

5.3 The determination of the confined extension tensile force versus strain 

properties of geosynthetics under a confining stress provides design parameters for soil 

reinforcement applications. Example applications include reinforced embankments over soft 

subgrades, reinforced soil retaining walls, and reinforced slopes. 

5.4 Most geotextiles and geogrids can be tested by this test method. Some 

modification of clamping techniques may be necessary for certain geosynthetics depending on 

the structure of the geosynthetic. Special clamping adaption may be necessary for highstrength geosynthetics (i . e . , tensile strength greater than approximately 100 kN/m (600 

lbf/in)) or geosynthetics made from glass fibers to prevent them from slipping in the clamps 

or being damaged as a result of being gripped in the clamps. Specimen clamping may be 

modified at the discretion of the individual laboratory provided that a representative tensile 

strength is obtained. In any event, the procedure described in section 11 of this test method 

for obtaining confined extension/confined creep behavior must be maintained. 

175 
6. Apparatus 

6.1 Test Setup - The confined extension and/or confined creep test setup is similar 

to that described in ASTM D 4595 and ASTM D 5262 for unconfined testing, respectively. 

For confined testing, the specimen is placed within a horizontally aligned containment box 

that incorporates the use of two air pressure bladders such that normal stress can be applied 

to the test specimen from above and below the geosynthetic. Figure 1 shows the horizontal 

orientation of the test setup. 

6.1.1 For Procedure A, the setup consists of the geosynthetic test specimen mounted 

in suitable grips within the containment box, with a layer of soil having an approximate 

thickness of 75 -mm (3 -in) on both sides. The air bladders that are fitted to receive 

compressed air at a specified sustained pressure are contained within the confinement box, 

above and below the soil layers. The two air bladders interface directly onto the soil 

surfaces as shown in figure 2A. 

6.1.2 For Procedure B, the setup consists of the geosynthetic test specimen mounted 

in suitable grips within the containment box, with a thin layer of soil having an approximate 

thickness of 10 -mm (0.4 -in) on both sides. Covering each soil layer are two layers of thin 

lubricated latex membranes and a relatively thick rubber rub-sheet. Each of the two air 

bladders interface directly onto each of the rub-sheets, as shown in Figure 2B. Depending 

on the design of the air bladder systems, rigid spacers may be required, as discussed in note 

. 

1. 

Note 1 - It should be noted that rigid spacers may be required to be placed between 

the air bladder systems and soil confining layers, depending on the design of the air 

bladder systems and the selected test procedure (i.e., Procedure A or Procedure B), in 

order to fill the containment box to the proper level. 

6.2 Containment BOX - An open rigid box is used as the containment box. It is 

constructed of 13-mm- (O-50-in-) thick steel plates, consisting of two smooth parallel sides, 

front and back walls containing horizontal split adjustable doors at their center line (Le., 

mid-section of the wall), and a removable bottom and top plate. The internal dimensions of 

the containment box should have minimum dimensions of 430 mm (17 in) long by 305 mm 

(12 in) wide by 150 mm (6 in) deep. The air bladder systems used to provide the confining 

stress should be designed to be incorporated into the top and bottom of the containment box. 

176 
LVDTs 

Confinement Box 

Rear Load Cell 

\- ^_ 

Upper Air 

7 -7 

Bladder Front Load Cell Air Cylinder 

\ 

1 in = 25.4 mm 

Supporting 

Table 

Steel 

Roller Roller 

+ 42” 

Figure 1. Test setup configuration. 
1 in - 25.4 mm 

Figure 2A. Cross-section for Procedure A with full confinement to the soil and the 

geosynthetic by using a thick soil specimen and a nonlubricated 

interface between the soil and the confining air bladder. 

Figure 2B. Cross-section for Procedure B with partial confinement to the soil and the 

geosynthetic by using a thin soil specimen and a lubricated 

interface between the soil and the confining air bladder. 

178 
6.3 Rigid Supporting Table - A rigid supporting table consisting of a reaction 

frame with a smooth and polished tabletop should be constructed of structural steel tubing 

and a minimum 13-mm- (0.5-in-) thick steel plate rigidly attached to the frame to form the 

table. The overall minimum plan dimensions of the supporting table should be 1065 mm (42 

in) in length and 710 mm (28 in) in width. The structural frame should incorporate the 

tensile loading system and a very low friction bearing surface. As presented in this 

document, the table allows two 25-mm- (l-in-) diameter steel rollers to be placed between 

the bottom of the containment box and the polished surface of the tabletop. This setup is 

required to allow the containment box to move together with the geosynthetic specimen in the 

horizontal direction during the confined extension and confined creep tests. Other means of 

providing very low friction are acceptable, but are not required by this standard. 

6.4 Test Grips - The test grips must be smooth-surfaced thin steel plates measuring 

200~mm (&O-in) wide and of a length to extend out of each side of the containment box. 

The test specimen is held within these grips by a resin/epoxy to allow full and uniform 

tension to be applied during the test. The resin/epoxy should exit the grips within the 

containment box to leave, for geotextiles a minimum loo-mm- (4.0-in-) long gage length of 

test specimen or, for geogrids, a minimum of two complete apertures of the test specimen 

exposed for stress application. 

6.5 Normal Pressure Loading System - To maintain a uniform normal stress, a 

flexible pneumatic diaphragm loading device (air bladder) that is continuous over the entire 

containment box should be used. Air pressure within the upper and lower air bladders is 

used to simulate the normal stresses anticipated in the field. The magnitude of this pressure 

must be agreed upon by the parties using the test results. Air pressure in equal amounts 

above and below the specimen is applied before the testing tension force is applied. The 

normal pressure loading system should have an operating capacity of up to 345 Wa (50 psi). 

Note 2 - It is generally recognized that there can be a reduction of confining pressure 

applied to the specimen because of side wall friction for thick soil specimens, This 

reduction from side wall friction can be on the order of 5 to 10 percent of the applied 

normal pressure. It is believed that the effect of side wall friction on the soilgeosynthetic interface is minimized by maintaining a minimum distance of 50 -mm 

(2 -in) between the geosynthetic specimen and the side walls of the containment box. 

By maintaining this minimum distance, the confining pressure transmitted to the 

geosynthetic specimen in a confined extension or confined creep test is expected to be 

the same as the air pressure applied to the air bladders. 

179 
6.6 Tensile Loading System - The tension force is to be applied to the grips exiting 

the containment box. As shown in figure 1, a horizontally deployed test setup uses a loading 

system consisting of a “moving grip” and a “fixed grip.” A large-capacity hydraulic cylinder 

incorporating an electronic load cell is mounted on the front of the supporting table and 

placed in line with the center line of the horizontal containment box. This hydraulic cylinder 

is used to load the moving grip. The fixed grip is attached to the rear of the supporting table 

through the use of a second electronic load cell placed in line with the center line of the 

horizontal containment box. For a confined extension test, a constant rate of strain can be 

achieved by manually adjusting the rate of fluid flow into the hydraulic cylinder. For a 

confined creep test, a constant load can be maintained by applying a constant pressure into 

the hydraulic cylinder. The tensile loading system should have an operating capacity of at 

least 45 kN (10,000 lbf). Testing can be conducted using air or hydraulic fluid, although 

higher system capacity is generally available for tests conducted using hydraulic fluid. 

Note 3 - Even if specimens of the proper aspect ratio are used for testing, the tensile 

load applied to the front of the geosynthetic specimen may not be fully transferred to 

the rear of the geosynthetic specimen. The difference between the front and rear 

loads is mainly caused by the mobilized friction between the soil and geosynthetic 

specimen and the specimen clamp, as well as friction developing between the 

supporting table, steel rollers, and the containment box. The use of Procedure B can 

help to reduce the mobilized friction between the soil and geosynthetic specimen. 

The overall friction loss should be measured through calibration, and the total friction 

should be controlled to within 2 percent of the anticipated maximum load in the 

confined extension test or the applied creep load in the confined creep test. 

6.7 Extension Measurement - Extension of the geosynthetic specimen is measured 

at the moving grip and at several locations along the gage length of the confined portion of 

the geosynthetic specimen. Extensometers are preferred for the measurement of strain in 

geosynthetics when confined in soil. One such device utilizes tell-tail wires attached to the 

specimen and to externally mounted extensometers. The wires are protected from the 

influence of normal stress by a small-diameter tube surrounding the wire, which runs from a 

location on the test specimen to the extensometer mounted outside of the containment box. 

Whenever possible, other means of measuring strain should be calibrated against 

extensometers (see appendix Xl). In any case, the device chosen shall be capable of 

measuring deformations to an accuracy of at least +O. 03 mm ( fO.001 in). The means of 

measuring strain should be indicated clearly in the report. 

180 
6.8 Vibration Control - Confined creep tests are sensitive to shock and vibration. 

The location of the apparatus, test equipment, and mounting shall be designed so that the 

specimen is isolated from vibration. 

6.9 Time Measurement - The accuracy of the time measuring device shall be f I 

percent of the elapsed time of each creep measurement load increment. 

6.10 Temperature Control - The temperature in the containment box, especially that 

close to the gage length of the specimen, shall be maintained within +2”C (+4”F) of the 

targeted value by a suitable automatic device and shall be stated in the report. 

6.11 Temperature Measurements - Temperature measurements shall be recorded at 

frequent intervals, or recorded continuously, in order to ensure an accurate determination of 

the average test temperature and compliance with 6.10. 

6.12 Environmental Control - The test environment shall be maintained constant 

throughout the test. Safety precautions should be taken to avoid personal contact during the 

test. The area should be isolated adequately and fenced such ,mat only the test operator has 

access to the test station. 

6.13 Soil Preparation Equipment - Use equipment as necessary for the placement of 

soils at desired conditions. This may include compaction devices such as vibratory or 

“jumping-jack” type compaction or hand compaction hammers. Soil containers or hoppers, 

leveling tools, and soil placement/removal tools may be required. 

6.14 Miscellaneous Equipment - Measurement and trimming equipment as necessary 

for geosynthetic preparation, a timing device, and soil property testing equipment are 

necessary for testing, but are considered to be routine components of a geosynthetic testing 

laboratory. 

7. Sampling 

7.1 Lot Sample - For the lot sample, rolls of geosynthetics should be obtained as 

directed in an applicable material specification, or as agreed upon between the purchaser and 

the supplier. 

181 
7.2 Laboratory Sample - For the laboratory sample, a full-width swatch 

approximately l-m- (40-in-) long in the machine direction should be obtained from each roll 

in the lot sample. The sample may be taken from the end portion of a roll, provided there is 

no evidence that it is distorted or different from other portions of the roll. In general, and 

particularly in cases of dispute, it is recommended that a sample be obtained that will exclude 

material from the outer wrap of the roll or the inner wrap around the core. 

7.3 Test Specimens: 

7.3.1 Geotextiles - For tests in the machine and cross-machine directions, 

respectively, the number of specimens as directed in section 9.1 should be taken from each 

sample. The specimens should be taken along a diagonal across the width of the sample, 

with no specimens closer than l/10 the width of the roll or 150- mm (6- in) from the edge of 

the sample, whichever is smaller. 

7.3.2 Geogrids - For tests in the machine and cross-machine directions, 

respectively, the number of specimens as directed in section 9.1 should be taken from each 

sample. The specimens should be taken at random locations from the laboratory sample. 

Machine direction samples should be obtained from locations independent from the crossmachine samples. The test specimens should exclude the outermost longitudinal and 

transverse ribs. No specimens should be taken closer to the edge than l/10 the width of the 

roll or 150 mm (6 in) from the edge, whichever is smaller. 

8. Test Specimen Preparation 

8.1 Test Specimen Preparation: 

8.1.1 Geotextiles - An aspect ratio of 2: 1 should be used for all geotextile 

specimens. Prepare each finished specimen to 200~mm (8.0-in) wide by at least 200-mm 

(8.0-in-) long, with the length dimension being designated and accurately cut parallel to the 

direction for which the confined extension or confined creep behavior is being measured. 

Draw two parallel lines near the center of each specimen length that are separated by 100 

mm (4.0 in) and that extend the full width of the specimen. These lines should be exactly 

perpendicular to the length of the specimen. 

8.1.1-l The length of the specimen depends on the type of clamps being used. The 

specimen must be long enough to extend through the full length of both clamps, as 

182 
determined for the direction of the test. When specimen integrity is not affected, the 

specimen may be cut initially to the finished width. 

8.1.2 Geogrids - Prepare each finished specimen to contain a width of at least five 

ribs in the cross-test direction (i.e., typically the cross-machine direction) by a length of at 

least two complete apertures in the direction of testing (i.e., typically the machine direction), 

with the length dimension being designated and accurately cut parallel to the direction for 

which the confined extension or confined creep behavior is being measured, 

8.1.3 This test method may not be suitable for some woven geotextiles or geogrids 

that exhibit breaking strengths in excess of 100 kN/m (600 lbf/in), due to clamping and 

equipment limitations. In such cases, 100~mm- (4.0-in-) wide specimens may be substituted 

for 200~mm- (8.0-in-) wide geotextile specimens. For geogrids, a minimum of three ribs in 

the cross-test direction may be substituted for the five ribs in the cross-test direction to 

ensure proper load distribution. 

9. Number of Tests 

9.1 Confined Extension Tests - Unless otherwise agreed upon, confined extension 

tests shall be conducted at normal stress levels as specified by the designer. Three different 

normal stress levels are recommended for characterization of the interaction of the 

geosynthetic material and the confining soil. 

9.2 Confined Creep Tests - To evaluate design creep strains, it is recommended 

that a minimum of two different normal stress conditions be evaluated to cover the range of 

normal stresses anticipated in the design. For each normal stress condition, applied tensile 

loads shall be selected at intervals of approximately 10 to 20 percent of the maximum load 

per unit width, that is, 20, 30, 40 and 60 percent, as determined by applicable ASTM test 

methods. 

Note 4 - To properly characterize the confined behavior of a geosynthetic involves 

identification of the load levels at which there is no creep (no increase in strain with 

the log of time), low to moderate creep (linear increase in strain with the log of time), 

and high creep (exponential increase in strain with log of time). Therefore, a 

minimum of three tests are recommended for each normal stress. 

183 
10. Conditioning and Testing Atmosphere 

10.1 The specimens should be brought to moisture equilibrium in the atmosphere 

for testing geosynthetics. Equilibrium is considered to have been reached when the increase 

in mass of the specimen, in successive weighing, made at intervals of not less than 2 hours, 

does not exceed 0.1 percent of the mass measured at the previous time period. 

Note 5 - Geosynthetic materials are frequently not weighed to determine when 

moisture equilibrium has been reached. While such a procedure cannot be accepted 

in cases of dispute, in routine testing, it may be sufficient to expose the material to 

the standard atmosphere for testing for a reasonable time period before the specimens 

are tested. A period of 24 hours has been found acceptable in most cases. However, 

certain fibers may exhibit slow moisture equilibrium rates from the as-received wet 

side. When this is known, a preconditioning cycle, as prescribed in ASTM D 1776, 

may be agreed upon between contractual parties. 

11. Procedure 

11.1 Adequately conditioned specimens should be tested. Test are conducted at a 

temperature of 21’ +2”C (70” +4”F) and relative humidity of 50 to 75 percent. 

11.2 Procedure A or Procedure B should be selected on the basis of the designer’s 

design approach. The test setup is then constructed as discussed in section 6.1. 

11.3 The amount of soil should be determined to achieve the desired dry unit 

weight of the soil when placed in the lower half of the containment box above the lower air 

bladder system. The selected soil material is then compacted into the lower half of the 

confinement box to form the lower confining layer. The soil should be compacted by hand 

tamping or by some other means to the desired unit weight under the specified moisture 

condition. The upper surface of the lower confining layer should be placed so that it is level 

with the center line of the loading harness, (i.e., the hydraulic cylinder and the rear load 

cell). 

11.4 A test specimen should be obtained as described in section 8. The end of each 

geosynthetic specimen should be cast in a low-temperature curing epoxy, resin which should 

be reinforced using layers of impregnated nonwoven geotextile to facilitate clamping of each 

184 
geosynthetic. The epoxy resin should be prepared by mixing the components of the resin 

compound and curing agent as directed by the manufacturer. During the curing period, 

typically 24 -hours, each end of the specimen should be confined between smooth steel plates 

under a minimum normal stress of 1 kPa (0.15 psi) to ensure a uniform casting that 

facilitates clamping. 

11.5 After preparation of the geosynthetic specimen, the test specimen is then 

placed on top of the lower confining soil layer. The two cast ends of the geosynthetic 

specimen are then connected to the front moving grip through a clevis and the rear fixed grip 

through a clevis on the tensile loading system as shown in figure 1, 

11.6 At least two in-soil extension monitoring devices (typically tell-tail wires 

attached to extensometers) should be used to monitor displacement of the geosynthetic 

specimen within the containment box. When only two tell-tail wires are used, they should be 

connected to the two ends of the geosynthetic specimen gage length. The tell-tail wire gages 

can be attached by hooking the wire to a glued-on tab or by tying them directly onto the 

specimen. Care should be taken to ensure that any slack in the wire is eliminated and that 

the geosynthetic specimen is not damaged. The other end of each wire can then be 

connected to the extensometer mounted to the rear of the supporting table outside the 

containment box. See appendix X. 1 for information about extensometers. 

Note 6 - It is recommended that additional in-soil extension monitoring devices be 

used during testing to confirm that there is uniform strain distribution along the gage 

length of the confined portion of the geosynthetic specimen. 

11.7 A pretension force should be applied to the geosynthetic specimen to eliminate 

slack within the geosynthetic specimen. The amount of pretension force is preselected for 

each test in accordance with the procedure established in ASTM D 4595 for in-air tensile 

testing of geosynthetics. A total pretension force equal to 1.25 percent of the expected 

breaking force should be applied to each geosynthetic specimen. However, the total 

pretension force should not be less than 45 N (10 lb) or greater than 222 N (50 lb) in any 

case. The pretension force should be applied to the geosynthetic specimen prior to placement 

of the upper confining soil layer. This allows examination of the geosynthetic specimen to 

assess whether the geosynthetic is in uniform contact with the top of the lower confining soil 

layer. 

185 
11.8 Additional soil should now be placed above the geosynthetic in the upper half 

of the containment box and compacted by hand tamping or by some other means to form the 

upper confining layer above the geosynthetic specimen. The soil should be compacted to the 

same unit weight and at the same specified moisture content as the lower confining soil layer. 

11.9 The test setup is completed with the installation of the upper lubricated 

interface if Procedure B is used, upper air bladder system, and top plate (see note 1). 

11.10 A normal stress is then applied to the test specimen by pressurizing the upper 

and lower air bladder loading systems. The desired predetermined source pressure should be 

applied simultaneously through the lower and upper air bladders. 

11.11 For a confined extension test, the geosynthetic specimen should be loaded 

under a constant rate of strain of 10 percent/min as measured on the front specimen clamp. 

The selection of this constant rate of strain is based on the standard loading rate specified in 

ASTM D 4595 for conducting unconfined wide-width strip tension tests on geosynthetics. 

For a confined creep test, a load should be applied to the test specimen at an initial loading 

strain rate of 10 percent/min until the selected test load is achieved. Selection of this initial 

loading rate is based on the standard loading rate in ASTM D 5262 for conducting 

unconfined tension creep tests on geosynthetics. The variation of the applied constant load 

should be within & 1 percent of the desired load to be applied to the test specimen throughout 

the duration of the test. Readings should be taken regularly and, at a minimum at the same 

time the extension of the test specimen is measured, as discussed in section 11.11.1. 

11.11.1 During the confined creep test, the extension of the specimen should be 

measured in accordance with the following approximate time schedule: 1, 2, 6, 10, and 30 

min; and 1, 2, 5, 10, 30, 100, 200, 500, and 1000 hours, For creep tests longer than 1000 

hours, extension readings should be recorded every 500 hours until testing is complete. 

Note 7 - In design, it is generally accepted that creep data should not be extrapolated 

for time periods beyond one order of magnitude of the test time. In many cases, a 

test period of 1000 hours may not reflect the long-term creep behavior of the material 

accurately, as this reflects an extrapolation limit of approximately 10,000 hours or 

approximately 14 months. For such cases, creep tests should be conducted for more 

than 10,000 hours. 

11.11.2 Test results should be plotted on a graph as discussed in section 12.2. 

Results should be plotted regularly during the test. Readings should be recorded more 

186 
frequently if discontinuities in the creep strain versus log of time plot are suspected or 

encountered. To avoid such discontinuities, the use of automatic monitoring and measuring 

equipment is recommended. 

11.12 The test should be terminated when the specimen ruptures or at the end of the 

agreed upon time period. If the specimen ruptures, the type of failure, the location, and time 

to failure should be recorded. 

12. Calculations 

12.1 Confined Extension Tests: 

12.1.1 Tensile Strength - Calculate the tensile strength of individual specimens by 

reading the maximum force per unit width to cause a specimen to rupture from the testing 

instrument. Tensile strength is expressed in N/m (lbf/ft) of width, using Eq 1, as follows: 

af = F,/W, (1) 

where: 

af = tensile strength, N/m (lbf/ft) of width, 

Ff = observed breaking force, N (lbf’), and 

WS = initial specimen width, m (ft). 

2: 

12.1.1.1 For geogrids, the equivalent tensile strength is determined by the use of Eq 

a = (F/N,) x N, (2) 

where: 

a = equivalent tensile strength, N/m (lbf/ft) of width, 

F = observed breaking force, N (lbf), 

Nr = number of ribs tested, and 

w = number of ribs per unit width. 

12.1.2 Strain - Calculate the strain of individual specimens, expressed as the percentage 

increase in length, relative to the initial nominal gage length of the specimen, using Eq 3, as 

follows: 

E = (AL x 100)/L, (3) 

187 
where: 

E = strain, %, 

Lg = initial nominal gage length, mm (in), and 

AL = the unit change in length (extension) of the gaged portion of the specimen 

from zero force to the corresponding measured force, mm (in). 

12.1.3 Tensile Modulus: 

12.1.3.1 Initial Tensile Modulus - Calculate the initial tensile modulus using the initial 

straight portion of the force versus strain curve by drawing a line tangent to this curve. At 

any point on this tangent line, measure the slope by measuring the change in force and the 

corresponding change in strain. Calculate initial tensile modulus in N/m (lbf/ft) of width 

using Eq 4 (See appendix X2 and figure X2. l), as follows: 

Ji = (F X lOO)/(e X W,) (4) 

where: 

Ji = initial tensile modulus, N/m (lbf/ft) of width, 

F = determined force on the drawn tangent line, N (lbf), 

E = corresponding strain with respect to the drawn tangent 

line and determined force, %, and 

w, = initial specimen width, m (ft). 

12.1.3.2 Ofsset Tensile Modulus - Calculate the offset tensile modulus by determining the 

location where modulus is to be assessed and drawing a line tangent to the force versus strain 

curve between the tangent point and the proportional limit and through the zero force axis. 

Measure the force and the corresponding strain with respect to the force axis. Calculate 

offset tensile modulus in N/m (lbf/ft) of width using Eq 5 (See appendix X2 and X3 and 

figures X2.1 and X3. l), as follows: 

J, = (F x loo)& x W,) (5) 

where: 

J, = 

F = 

E = 

w,= 

offset tensile modulus, N/m (lbf/ft) of width, 

determined force on the drawn tangent line, N (Ibf), 

corresponding strain with respect to the drawn tangent 

line and determined force, %, and 

initial specimen width, m (ft). 

188 
12.1.3.3 Secant Tensile Modulus - Calculate the secant tensile modulus by determining 

the force for a specified strain, e2, and label that point on the force versus strain curve as P2. 

It is typical to report secant modulus for 2, 5, and/or 10 percent strain. Accordingly, P2 

would be located on the force versus strain curve at either 2, 5, and/or 10 percent strain, 

Likewise, label a second point, PI at a specified strain, el, usually 0 percent strain. Draw a 

straight line (secant) through points PI and P2 intersecting the zero force axis. The typical 

values are 0 and 10 percent strain for points PI and Pz, respectively, although other values 

may be used when required in an applicable material specification. The secant tensile 

modulus is calculated in N/m (lbf/ft) of width using Eq 6 (See appendix X4 and figure 

X4.1), as follows: 

Js = (F x lOO)/(e x W,) 

where: 

J, = secant tensile modulus, N/m (lbf/ft) between specified strains 

per m (ft) of width, 

F = determined force on the constructed line, N (lbf), 

E = corresponding strain with respect to the constructed 

line and determined force, % , and 

w, = initial specimen width, m (ft). 

(6) 

12.1.4 Force Versus Strain Curve - The standard confined extension curve is a graph of 

the force versus strain as measured on the geosynthetic while under a specified confining 

stress. A typical plot is presented in figure 3 and should be provided in the report if 

requested. 

12.2 Confined Creep Tests: 

12.2.1 Creep Curves - There are three standard confined creep curves, a graph of total 

strain versus log of time, a graph of log of incremental strain rate versus log of time, and a 

graph of log of incremental strain rate versus total strain. Typical plots of these three curves 

are presented in figures 4, 5, and 6. The data are prepared by use of the following 

calculations: 

12.2.1-l Time - Elapsed time intervals are converted to hours and converted to the log of 

time (in hours). 

12.2.1.2 Total Strain - The accumulated total strain at each interval is calculated (to the 

nearest 0.03 mm (0.001 in) of extension) using Eq 3. 

189 
12.2.1.3 Incremental Strain Rate - The incremental strain rate at each time interval is 

calculated as the change in strain, E, with respect to time between two successive monitoring 

periods. 

12.2.2 The data are then plotted as: (i) percent total strain as ordinate versus log of time 

as abscissa, (ii) log of incremental strain rate as ordinate versus log of time as abscissa and 

(iii) log of incremental strain rate as ordinate versus total strain as abscissa, see figures 4, 5, 

and 6). If several loads are used for testing, each plot shall be labeled clearly with the 

appropriate loading or force per unit width, expressed in N/m (lb/ft). 

4500 

4000 - 

3500 - 

A 

< 3000 - 

la 

ci 

w 2500 - 

2 

0 

h 2000 - 

W 

.-I 

G E 1500- 

1000 - 

500 - 

- IN-AIR 

- IN BEACH SAND AT 10 psi 

- IN BEACH SAND AT 20 psi 

SPECIMEN WIDTH: 8 in 

SPECIMEN LENGTH: 5 in 

GAGE LENGTH: 4 in. 

STRAIN RATE: 1 O%/min 

0 5 10 15 20 25 30 35 40 45 50 

STRAIN (w) 

1 in = 25.4 mm 

1 Ib/ft = 14.59 N/m 

1 psi = 6.89 kPa 

Figure 3. Typical force versus strain response for a geosynthetic material during 

confined extension testing. 

190 
18 

16 

6 

r 

10 -z 10 -' 1 10 IO = IO3 

TIME (h) 

- IN-AIR 

- IN BEACH SAND AT IO psi 

- IN BEACH SAND AT 20 psi 

- IN BEACH SAND AT 10 psi 

SPECIMEN WIDTH: 8 in 

SPECIMEN LENGTH: 5 in 

GAGE LENGTH: 4 in 

1 in = 25.4 mm 

1 psi = 6.895 kPa 

Figure 4. Typical total strain versus log of time response for a geosynthetic 

material during confined creep testing. 

191 
1 

10 

k 

0888fl IN-AIR 

- IN BEACH SAND AT 10 psi 

1 - IN BEACH SAND AT 20 psi 

- IN BEACH SAND AT 10 wi 

SPECIMEN WIDTH: 8 in 

O4 

SPECIMEN LENGTH: 5 in 

GAGE LENGTH: 4 in 

0 -3 

I rIrllll( I I I I lllll I I I I Ill11 

,,, ,h 

I IIII 1 

10 -z IO -' 1 10 IO2 IO" 

TIME (h) 

1 in = 25.4 mm 

1 psi = 6.895 kPa 

Figure 5. Typical log of incremental strain rate versus log of time response for 

a geosynthetic material during confined creep testing. 

192 
lop-\ 

cteetn, IN-AIR 

- IN E 

- IN BEACF 

- IN BEACH SA 

3EACH SAND AT 10 psi 

I SAND AT 20 psi 

ND;AT 10 psi 

SPECIMEN WIDTH: 8 in 

SPECIMEN LENGTH: 5 in 

GAGE LENGTH: 4 in 

lo-51 I 1 I 1 I 1 I 1 I (1 1 1 1 1 I 1 I 1 

0 2 4 6 8 IO 12 14 16 18 : 

TOTAL STRAIN (5%) 

1 in = 25.4 mm 

1 psi = 6.895 kPa 

Figure 6. Typical log of incremental strain rate versus total strain response for 

a geosynthetic material during confined creep testing. 

193 
13. Report 

13.1 Confined Extension Tests: The report should include the following information: 

13.1.1 Description of test apparatus. 

13.1.2 Test conditions, including which test procedure was used. 

13.1.3 Any departures from standard procedure. 

13.1.4 Notation that the specimens were tested as directed in this test method. 

Description of the material tested, including all pertinent information required for complete 

identification of the specimen. 

13.1.5 Dimensions of the test specimen. 

13.1.6 Identification of and description of soil, including soil classification, water 

content, unit weight, grain size, and other identifying information if available. 

13.1.7 All basic data including normal stresses, tensile strength, initial tensile modulus, 

offset tensile modulus, and/or secant tensile modulus, strain at rupture, and, if requested, 

include a force versus strain curve for each test. 

13.1.8 Description of the geosynthetic specimen conditions before and after testing. 

13.2 Confined Creep Tests: The report should include the following information: 

13.2.1 Description of test apparatus. 

13.2.2 Test conditions, including which test procedure was used. 

13.2.3 Any departures from standard procedure. 

13.2.4 Notation that the specimens were tested as directed in this test method. 

Description of the material tested, including all pertinent information required for complete 

identification of the specimen. 

194 
13.2.5 Dimensions of the test specimen. 

13.2.6 Identification of and description of soil, including soil classification, water 

content, unit weight, grain size, and other identifying information if available. 

13.2.7 Dates of the confined creep tests. 

13.2.8 For each test, plot of total strain in percent versus log of time in hours, log of 

incremental strain rate in percent/min versus log of time in hours, and log of incremental 

strain rate in percent/min versus total strain in percent under a given normal stress and creep 

load per unit width and as a percent of ultimate load as determined in appropriate ASTM test 

methods. 

13.2.9 Description of the geosynthetic specimen conditions before and after testing. 

14. Precision and Bias 

14.1 Precision - The precision of the procedure in this test protocol is being 

established. 

14.2 Bias - This test has no bias because the confined extension and creep behavior of 

geosynthetics are defined in terms of this test method. 

15. Key Words 

15.1 Geogrid; geosynthetics; geotextile; confined extension; confined creep; 

performance test. 

195 
APPENDIXES 

(Nonmandatory Information) 

Xl. EXTENSOMETERS 

Xl, 1 Three types of extensometers have been used successfully in testing 

geosynthetics .

Xl. 1.1 Direct reading extensometers are mounted directly on the geosynthetic. 

These extensometers typically consist of linear variable-differential transformers (LVDTs) 

units that read strain directly as the material extends. These units place an additional force 

(weight) on the material undergoing testing and may result in alteration of the force versus 

strain results. The user should determine that this additional force is or is not significant for 

the material being tested. This type of extensometer cannot typically be used during confined 

testing. 

Xl. 1.2 Semi-remote reading extensometers use clamps that are mounted directly 

on the geosynthetic. Wires, pulley systems, or other physical devices connect the clamps to 

LVDT units. This type of extensometer are appropriate for confined testing, but provisions 

must be provided to protect the wires, etc., from influences due to confinement. 

Xl. 1.3 Remote extensometers use clamps or markers that are mounted directly on 

the geosynthetic and sensing units that are mounted independently both of the geosynthetic 

and the clamps or markers. These sensing units use electromagnetic radiation, such as light, 

to sense the distance between the markers. These type extensometers may be inappropriate 

due to confinement. 

Xl.2 Users must bear in mind that clamps, markers, or other physical 

attachments can damage materials undergoing testing. This damage can cause premature 

failure in geosynthetics. It is of paramount importance to design and use clamps, markers, 

or other attachments in a manner that will not alter the test results by damaging the material 

undergoing testing. 

X2. INITIAL GEOSYNTHETIC TENSILE MODULUS 

X2.1 In a typical force versus strain curve (figure X2. l), there is usually a toe 

region AC that represents take up of slack, alignment, or seating of the specimen. For some 

196 
geosynthetics, this portion can represent a significant part of the strain characteristic of the 

specimen. This region is considered when determining the initial geosynthetic modulus. 

X2.1.1 The initial geosynthetic tensile modulus can be determined by dividing the 

force at any point along the straight-line projection of the force versus strain curve (i.e., line 

AG or its extension) by the strain at the same point (measured from point A, defined as zero, 

strain). 

Figure X2.1. Material with Hookean region. 

X3. OFFSET GEOSYNTHETIC TENSILE MODULUS 

X3.1 In the case of a geosynthetic exhibiting a region of Hookean (linear) 

behavior (figure X2.1) after the nonlinear initial region, a continuation of the linear region of 

the curve is constructed through the zero-force axis. This intersection point B is the zero 

strain point from which strain is measured. 

X3.1.1 The offset geosynthetic tensile modulus can be determined by dividing the 

force at any point along the line BD (or its extension) by the strain at the same point 

(measured from point B, defined as zero strain). The point where line BD first touches the 

force versus strain curve is the tangent point (i.e., point C). 

197 
X3.2 In the case of a geosynthetic that does not exhibit any linear region (figure 

X3. l), a line is constructed tangent to the point on the force versus strain curve exhibiting 

the maximum slope (i.e., point H). This is extended to intersect the zero force axis at point 

B’. This intersection point B’ is the zero strain point from which strain is measured. 

X3.2.1 The offset geosynthetic tensile modulus can be determined by dividing the 

force at any point along line Bk’ (or its extension) by the strain at the same point (measured 

from point B’, defined as zero strain). 

Figure X3.1. Material with no Hookean region. 

X4. SECANT GEOSYNTHETIC TENSILE MODULUS 

X4.1 In a typical force versus strain curve (figure X4. l), a straight line is 

constructed through the zero force axis, usually at zero strain point A’ and a second 

point usually at 10 percent strain, point M’. Point A” is the zero strain point from which 

strain is measured. 

X4.1.1 The secant geosynthetic tensile modulus at the selected strain level can be 

determined by dividing the force at any point along the line A”M’ (or its extension) by the 

strain at the same point (measured from point A’, defined as zero strain). 

198 
X4.1.2 Figure X4.1 also presents a straight line constructed through any two 

specified points where a secant modulus is to be calculated, point Q” and point R”, other than 

zero and 10 percent strain. In this case, the line is extended through the zero force axis at 

point B’. This intersection is the zero strain point from which strain is measured. The 

secant geosynthetic tensile modulus can be determined by dividing the force at any point 

along line Q”R” (or its extension) by the strain at the same point (measured from point B”, 

defined as zero strain). If this latter method is used, for example to account for zero-force 

offset due to removal of slack, etc., in the geosynthetic, the specified means for defining 

point Q” and R’ should be identified in the testing report. 

A- II- cFigure X4.1. Construction line for secant modulus. 

199 
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201