Composite Behavior of Geosynthetic Reinforced Soil Mass

CHAPTER 4. GSGC TESTS

The understanding of GSGC behavior in reinforced soil structures has been lacking. As a result, current design methods have considered the geosynthetic layers simply as added tensile elements and have failed to account for the interaction between soil and geosynthetics. A series of GSGC laboratory tests were designed and conducted to examine the behavior of a GSGC with varying spacing and strength of reinforcement, provide test data for verifying the analytical model for calculating strength properties of a GRS composite as described in chapter 5, and provide test data for calibration of an FE model for a GRS mass. The GSGC tests were conducted at TFHRC in McLean, VA.

4.1 DIMENSIONS OF THE PLANE-STRAIN GSGC TEST SPECIMEN

A soil mass reinforced by layers of geosynthetic reinforcement is not a uniform mass. To investigate the behavior of GSGCs by conducting laboratory tests, it is necessary to determine the proper dimensions of the test specimen so that the test provides an adequate representation of GSGC behavior.

A number of factors were considered prior to determining the test specimen dimensions of the GSGC test, including the following:

• Plane-strain condition: Because most GRS structures resemble a plane-strain condition, the test should be conducted in a plane-strain condition.
  
  
• Backfill particle size: To alleviate the effects of particle size on the test specimen, the dimensions of a generic GRS mass should be at least 6 times as large as the maximum particle size of the soil specimen, as suggested by the U.S. Army Corps of Engineers, and 15 times larger than the average particle size (D₅₀ ).⁽⁶¹⁾ The recommended maximum particle size for the backfill of GRS structures is 0.75 inches (19 mm).⁽⁶²⁾ The specimen dimension, therefore, should be at least 4.7 inches (120 mm).
  
  
• Reinforcement spacing: The reinforcement spacing plays an important role in the deformation behavior of GRS structures and the load-transfer mechanism of reinforced soil masses.⁽⁶³⁾ The height of a generic GRS mass should be able to accommodate the typical reinforcement spacing of 8 to 12 inches (200 to 300 mm) for GRS walls.
  
  
• Size of reinforcement sheet: The specimen dimensions in the plane-strain direction, referred to as the width, W, and in the longitudinal direction, referred to as length, L, should be sufficiently large to provide adequate representation of the geosynthetic reinforcement. For polymer grids, enough grid cells need to be included for a good representation of the polymer grid. For nonwoven geotextiles, the aspect ratio of the reinforcement specimen (i.e., the ratio of width to length) should be sufficiently large (e.g., greater than 4) to alleviate significant necking effect. There will be a little necking effect for woven geotextiles regardless of the aspect ratio.

Using Plaxis Version 8.2, a series of FE analyses were conducted to examine the effect of specimen dimensions on the resulting global stress-strain and volume change relationships of the composites. The objective of the FE analyses was to determine proper dimensions of a GSGC that will produce load-deformation behavior sufficiently close to that of a large mass of soil-geosynthetic composite, referred to as the reference composite.

Figure 99 shows the typical geometric and loading conditions of the GSGC tests. The reference GSGC is taken as a reinforced soil mass with dimensions of 23 ft (7.0 m) high and 16 ft (4.9 m) wide in a plane-strain condition. Four different dimensions of GSSCs were analyzed with varying specimen heights (23, 6.6, 3.3, and 1.6 ft (7.0, 2.0, 1.0 and 0.5 m), while the width of the test specimen was kept as 0.7 × H. In these analyses, the soil was a dense sand. The sand was reinforced by a medium-strength woven geotextile (Geotex^® 4×4) at 0.7 ft (0.2 m) vertical spacing. Table 2 lists the conditions and properties of the soil and reinforcement used in the analyses.

1 m =3.28 ft
Figure 99. Illustration. Typical geometric and loading conditions of a GSGC.

 

Table 2. Conditions and properties of the backfill and reinforcement used in FE analyses.

Element	Description
Soil	A dense sand where unit weight = 17 kN/m3; cohesion = 5 kPa; angle of internal friction ( Φ ) = 38 degrees; angle of dilation ( Ψ ) = 8 degrees; soil modulus (E50 ) = 40,000 kPa; and Poisson’s ratio = 0.3
Reinforcement	Geotex® 4×4 axial stiffness (EA) = 1,000 kN/m; ultimate strength (Tult) = 70 kN/m; and reinforcement spacing = 0.2 m
Confining pressure	Constant confining pressures of 0 and 30 kPa
1 kN/m3 = 0.006 kip/ft3 1 kPa = 0.145 psi 1 kN/m = 0.068 kip/ft 1 m = 3.28 ft

The global stress-strain curves obtained from the analyses are shown in figure 100 and figure 101 for confining pressures ( σ_c ) of 0 and 4.4 psi (0 and 30 kPa), respectively (a confining pressure of 4.4 psi (30 kPa) is representative of the lateral stress at the mid-height of a 23-ft (7.0-m)-high wall). The corresponding global volume change curves are shown in figure 102 and figure 103. The global vertical strain ( ε_v ) was calculated by figure 104.

1 kPa = 0.145 psi
1 m = 3.28 ft
Figure 100. Graph. Global stress-strain curves for GSGCs of different dimensions under a confining pressure of 0 psi (0 kPa).

 

1 kPa = 0.145 psi
1 m = 3.28 ft
Figure 101. Graph. Global stress-strain curves for GSGCs of different dimensions under a confining pressure of 4.4 psi (30 kPa).

 

1 m = 3.28 ft
Figure 102. Graph. Global volume change curves for GSGCs of different dimensions under a confining pressure of 0 psi (0 kPa).

 

1 m = 3.28 ft
Figure 103. Graph. Global volume change curves for GSGCs of different dimensions under a confining pressure of 4.4 psi (30 kPa).

 

 
Figure 104. Equation. Global vertical strain

Where:

ΔH = Total vertical displacement of the specimen.
H = Initial height of the specimen.

Figure 100 and figure 101 indicate that a composite with a height of 6.6 ft (2.0 m), width of 4.6 ft (1.4 m), and confining pressure of 4.4 psi (30 kPa) yielded stress-strain and volume change relationships that were sufficiently close to those of the reference composite. Specimen sizes with heights of 3.3 and 1.6 ft (1.0 and 0.5 m) appeared too small to provide an adequate representation of the reference composite.

For comparison, additional analyses were conducted on unreinforced soil. Figure 105 and figure 106 show the global stress-strain curves and global volume change curves of the soil masses without any reinforcement for the different specimen dimensions. The results indicate that a specimen height as small as 1.6 ft (0.5 m) will yield nearly the same stress-strain and volume change relationships as the reference soil mass of height 23 ft (7.0 m) when reinforcement is not present.

1 kPa = 0.145 psi
1 m = 3.28 ft
Figure 105. Graph. Global stress-strain curves of unreinforced soil under a confining pressure of 4.4 psi (30 kPa).

 

1 m = 3.28 ft
Figure 106. Graph. Global volume change curves of unreinforced soil under a confining pressure of 4.4 psi (30 kPa).

Based on the results of the FE analyses, a specimen height of 6.6 ft (2.0 m) and depth of 4.6 ft (1.4 m) with 0.7-ft (0.2-m) reinforcement spacing was selected for the GSGC tests. The actual specimen dimensions for the GSGC tests are shown in figure 107. Figure 108 and figure 109 show the front and plan views of the GSGC test setup.

1 m = 3.28 ft
Figure 107. Illustration. Specimen dimensions for the GSGC tests.

 

1 mm = 0.039 inches
Figure 108. Illustration. Front view of the test setup.

 

1 mm = 0.039 inches
Figure 109. Illustration. Plan view of the test setup.

4.2 APPARATUS FOR PLANE-STRAIN TEST

To maintain a plane-strain condition for the GSGC specimens throughout the tests, two major factors were considered: (1) the test bin needed to be sufficiently rigid to have negligible lateral deformation in the longitudinal direction (i.e., the length direction) and (2) the friction between the backfill and the side panels of the test bin needed to be minimized to nearly zero.

4.2.1 Lateral Deformation

Five GSGC masses were tested inside a test bin. The test bin was designed to experience little deformation for a surcharge pressure up to 410 psi (2,800 kPa). The test bin is shown in figure 110.

 
Figure 110. Photo. Test bin.

4.2.2 Friction

Two transparent Plexiglas^® panels were attached inside the steel tubing frame to form the side surfaces of the test bin. To minimize the friction between the Plexiglas^® and the backfill in these surfaces, a lubrication layer was created on the inside surfaces of the Plexiglas^® panels. The lubrication layer consisted of a 0.02-inch (0.5-mm)-thick latex membrane and an approximately 0.04-inch (1 mm)-thick lubrication agent (Dow Corning^® 4 Electrical Insulating Compound NSF 6). This procedure has been used successfully in many plane-strain tests conducted by Tatsuoka and his associates at University of Tokyo and by the lead author of this paper in many large-scale experiments. (See references 64–68.) The friction angle between the lubricant layer and the Plexiglas^®, as determined by direct shear test, was less than 1 degree.

4.3 TEST MATERIAL

The backfill and geotextile reinforcement employed in the tests are described in the following sections.

4.3.1 Backfill

The backfill was a crushed diabase from a source near Washington, DC. Before conducting the GSGC tests, a series of laboratory tests was performed to determine the properties of the backfill, including the following:

• Gradation test.
  
  
• Specific gravity and absorption test of the coarse aggregates.
  
  
• Moisture-density tests (Proctor compaction) with rock correction.
  
  
• Large-sized triaxial tests with specimen diameter of 6 inches (152 mm).

A summary of some index properties is given in table 3. The grain size distribution of the soil is shown in figure 111. Two gradation tests were performed, and the results agreed well with each other.

Table 3. Summary of index properties of backfill.

Classification	Well-Graded Gravel: Soil Classification A-1a per AASHTO M-15 and Soil Classification GW-GM per ASTM D 2487(69,70)
Maximum dry unit weight	0.15 kip/ft3 (24.1 kN/m3)
Optimum moisture content	5.2 percent
Specific gravity of soil solids	3.03

 

1 mm = 0.039 inches
Figure 111. Graph. Grain size distribution of backfill.

Four triaxial tests were conducted at confining pressures of 5, 15, 30, and 70 psi (34, 103, 207, and 482 kPa), and the results were compared with those performed at the University of Colorado Denver on the same soil. The soil specimen was approximately 6 inches (152 mm) in diameter and 12 inches (305 mm) in height. The shapes of a typical specimen before and after failure are shown in figure 112 and figure 113, respectively. Figure 114 and figure 115 present the stress-strain and volume change curves of the tests. The stress-strain curves obtained by Ketchart and Wu are included for comparison and to furnish a more complete set of data.⁽⁶⁸⁾ The stress-strain relationships agree well in trend with those by Ketchart and Wu.⁽⁶⁸⁾ The Mohr-Coulomb failure envelopes of the backfill are shown in figure 116. For confining pressures between 0 and 30 psi (0 and 103 kPa), the strength parameters are c = 10.3 psi (71.0 kPa) and Φ = 50 degrees. For confining pressures between 30 and 110 psi (103 and 758 kPa), the strength parameters are c = 35.1 psi (242 kPa) and Φ = 38 degrees.

 
Figure 112. Photo. Typical triaxial test specimen before test.

 

 
Figure 113. Photo. Typical triaxial test specimen after test.

 

1 kPa = 0.145 psi
Figure 114. Graph. Triaxial test results: stress-strain curves of backfill at 0.15 kip/ft³ (24.1 kN/m³) dry density and 5.2 percent moisture.

 

1 kPa = 0.145 psi
Figure 115. Graph. Triaxial test results: volume change curves of backfill at confining pressures of 5 and 30 psi (34.45 and 103.35 kPa).

 

1 kPa = 0.145 psi
Figure 116. Graph. Mohr-Coulomb failure envelopes of backfill.

4.3.2 Geosynthetics

The geosynthetic used in the experiments was Geotex^® 4×4 manufactured by Propex^® (formally known as Amoco 2044). This geosynthetic is a woven polypropylene geotextile. Table 4 shows its strength properties as provided by the manufacturer.

Table 4. Summary of Geotex^® 4×4 properties.

Property	Test Method	Machine Direction (i.e., Wrap Direction)	Cross Direction (i.e., Fill Direction)
Tensile strength (grab)	ASTM D4632(71)	0.6 kip (2.67 kN)	0.5 kip (2.22 kN)
Wide-width tensile ultimate strength	ASTM D4595(72)	400 lb/inch (70 kN/m)	400 lb/inch (70 kN/m)
Wide-width strength at 5 percent strain	ASTM D4595(72)	121 lb/inch (21 kN/m)	217 lb/inch (38 kN/m)
Wide-width ultimate elongation	ASTM D4595(72)	10 percent	10 percent
Puncture	ASTM D4833(73)	170 lb (0.8 kN)
Trapezoid tearing strength	ASTM D4533(74)	250 lb (1.11 kN)

 

Two types of geosynthetics were used for the experiments: a single sheet of Geotex^® 4×4 and a double sheet Geotex^® 4×4 (by gluing two sheets together using 3M^® Super 77 spray adhesive). The double sheet was used to create a geosynthetic that was approximately twice as stiff (and as strong) while maintaining the same interface condition as that of the single-sheet geosynthetic. Geotex^® 4×4 geotextile has been used in the construction of hundreds of GRS walls and in many full-scale experiments, including the FHWA GRS pier, Havana Yard Test abutment and pier, Blackhawk preloaded GRS bridge abutment, and National Cooperative Highway Research Program test abutments. (See references 6 and 75–77.)

Uniaxial tension tests were performed on both types of geosynthetic to determine the load-deformation behavior using a specimen 12 inches (305 mm) wide and 6 inches (152 mm) long (see figure 117). The stiffness and strength of the two geosynthetics are shown in table 5, and load-deformation curves are shown in figure 118. The stiffness and strength of the double-sheet Geotex^® 4×4 were approximately twice as much as those of the single-sheet Geotex^® 4×4 with the breakage strain almost the same.

 
Figure 117. Photo. Uniaxial tension test of Geotex^® 4×4

 

Table 5. Properties of Geotex^® 4×4 in fill direction.

Geosynthetic	Wide-Width Tensile Strength per ASTM D4595(72)
	Stiffness (kN/m) at 1 Percent Strain	Ultimate Strength (kN/m) (Percent at Break)
Single-sheet Geotex® 4×4	1,000	70 (12 percent)
Double-sheet Geotex® 4×4	1,960	138 (12 percent)
1 kN/m = 0.069 kip/ft

 

1 kN/m = 68.52 lbf/ft
Figure 118. Graph. Load deformation curves of the geosynthetics.

4.3.3 Facing Block

Blocks used for the facing of the GSGC mass during specimen preparation were hollow concrete blocks with dimensions of 15.625 by 7.625 by 7.625 inches (397 by 194 by 194 mm) and an average weight of 40 lb/block (18.1 kg/block).

4.4 TEST PROGRAM

The test program comprised five GSGC tests. Their test conditions are shown in table 6. The plate compactor used for the tests (MBW GP1200) had a weight of 120 lb (54 kg), plate dimensions of 12 by 21 inches (305 by 533 mm), centrifugal force of 1,500 lbf (6,700 N), rotation speed of 5,000 vibrations/min, and moving speed of 70 ft/min (21 m/min).

Table 6. Test program for GSGC tests.

Test Designation	Geosynthetic Reinforcement	Confining Pressure (kPa)	Wide-Width Strength of Reinforcement (kN/m)	Reinforcement Spacing, Sv (m)
Test 1	None	34	None	No reinforcement
Test 2	Geotex® 4×4	34	70	0.2
Test 3	Double-sheet Geotex® 4×4	34	140	0.4
Test 4	Geotex® 4×4	34	70	0.4
Test 5	Geotex® 4×4	0 (unconfined)	70	0.2
1 kPa = 0.145 psi 1 kN/m = 0.069 kip/ft 1 m = 3.28 ft

 

4.5 TEST CONDITIONS AND INSTRUMENTATION

4.5.1 Vertical Loading System

The vertical loads were applied to the test specimens by using a 1,000,000-lb- (454,000-kg)-capacity loading frame with a 1,000,000-lb (454,000-kg) hydraulic jack. Loads were measured by load cells and by hydraulic jack pressure gauges. For test 1, two load cells of 100,000 and 300,000 lb (45,400 and 136,200 kg) were used to measure the loads. For tests 2 through 5, a 1,000,000-lb (454,000-kg) load cell was used to measure the loads. A 12-inch (305-mm)-thick concrete pad was placed on top of the specimen before loading. Vertical loads were applied in equal increments with 10 min of elapsed time between increments to allow for equilibrium. The elapsed time also allowed manual recording of displacements of the test specimen. The vertical loads were applied until a failure condition was reached to determine the strength of the composite specimen. The applied pressures on the composite specimens were determined from the applied vertical loads divided by the surface areas of the composite specimens.

4.5.2 Confining Pressure

The confining pressure on the test specimens was applied by vacuuming. The entire surface area of the test specimen was vacuum-sealed with a 0.02-inch (0.5-mm)-thick latex membrane. A prescribed confining pressure of 5 psi (34 kPa) was applied for tests 1 through 4 by connecting the latex membrane to a suction device through two 0.234-inch (6-mm)-diameter flexible plastic tubes. Only test 5 was conducted without confining pressure.

4.5.3 Instrumentation

The specimens were instrumented to monitor their performance during tests. The instruments used include the following:

• Vertical movement: Three linear variable displacement transducers (LVDT) and two digital dial indicators were installed on the top of the concrete pad to measure the vertical movement of the specimen during loading. The vertical movement was measured along the top surface of the concrete pad.
  
  
• Lateral movement: Ten LVDTs and two digital dial indicators were installed along the height of the specimen (two open sides of the specimen) to measure the lateral movement of the specimen. The location of the LVDTs and digital dial indicators are shown in figure 119.
  
  
• Internal movement: The internal movement of the soil at selected points in the soil mass was traced by marking the locations of preselected points on a 2-by-2-inch (51-by-51‑mm) grid system drawn on the membrane.
  
  
• Reinforcement strain: To measure the strains in the geotextile, a number of high elongation strain gauges manufactured by Measurements Group, Inc. (type EP-08-250BG-120) were used. Each strain gauge was glued to the geotextile at only two ends to avoid inconsistent local stiffening of geotextile due to the adhesive. The strain gauge attachment technique was developed at the University of Colorado Denver. The gauge was first mounted on a 0.97-by-3.0-inch (25-by-76-mm) patch of a lightweight nonwoven geotextile (see figure 120 and figure 121). A microcrystalline wax and rubber coating (M-Coat B Nitrile Rubber) were used to protect the gauges from moisture. To check the effectiveness of the moisture protection technique, the geotextile specimens with the strain gauges were tested after immersion in water for 24 h. Before placing the reinforcement sheet in the test specimen, an M-Coat FB-2 butyl rubber tape was used to protect the gauges during compaction (see figure 121). To measure the strain distribution of the reinforcement, six strain gauges were mounted on each Geotex^® 4×4 sheet (see figure 122).

 
Figure 119. Illustration. Locations of LVDTs and digital dial indicators.

 

 
Figure 120. Photo. Strain gauge on Geotex^® 4×4 geotextile before applying protection tape.

 

 
Figure 121. Photo. Strain gauge on Geotex^® 4×4 geotextile after applying protection tape.

 

 
Figure 122. Photo. Strain gauges mounted on Geotex^® 4×4 geotextile.

Due to the presence of the lightweight geotextile patch, calibration of the strain gauge was needed. The calibration tests were performed to relate the strain obtained from the strain gauge to the actual strain of the reinforcement. Figure 123 and figure 124 show the calibration curves along the fill direction of Geotex^® 4×4 geotextile for the single-sheet and double-sheet specimens, respectively.

 
Figure 123. Graph. Calibration curve for single-sheet Geotex^® 4×4.

 

 
Figure 124. Graph. Calibration curve for double-sheet Geotex^® 4×4.

4.5.4 Preparation of Test Specimen for GSGC Tests

The preparation procedure of a typical composite mass with the dimensions of 6.6 by 4.6 by 3.9 ft (2.0 by 1.4 by 1.2 m) is as follows:

1. Mark the anticipated location of the GSGC mass on the Plexiglas^®. 
   
   
2.  Apply approximately 0.04-inch (1-mm)-thick lubricating agent (Dow Corning^® 4 Electrical Insulating Compound NSF 61) evenly on the inside surfaces of the Plexiglas^® (see figure 125).
   
   

   
    
   Figure 125. Photo. Applying grease on Plexiglas^® surfaces.

3. Attach a sheet of membrane (with 2.0-by-2.0-inch (51-by-51-mm) grid system predrawn on membrane) over each Plexiglas^® and at the bottom of the specimen (see figure 126). 
   
   

   
    
   Figure 126. Photo. Attaching membrane.

    

4. Lay a course of the facing blocks on the open sides of the specimen (see figure 127). 
   
   

   
    
   Figure 127. Photo. Placement of the first course of facing block.

    

5. Place the backfill in the test bin and compact in 0.7-ft (0.2-m) lifts (see figure 128 and figure 129). If needed, check and adjust the backfill moisture before compaction to achieve the target moisture of 5.2 percent. 
   
   

   
    
   Figure 128. Photo. Compacting the first lift of backfill.
   
   

    

   
    
   Figure 129. Photo. Placing backfill for the second lift.

6.  Check the water content and dry unit weight of each lift by using a nuclear density gauge (Troxler 3440) by the direct transmission method.
   
   
7.  Place the next layer of geosynthetic reinforcement (with strain gauges already mounted), covering the entire top surface area of compacted fill and the facing blocks (see figure 130).
   
   

   
    
   Figure 130. Photo. Placing a reinforcement sheet.

    

8.  Repeat steps 4 to 8 until the full height of the composite mass is reached.
   
   
9.  Sprinkle a 0.2-inch (5-mm)-thick fine sand layer over the top surface of the completed composite mass to level the surface and protect the membrane from being punctured by gravel in the backfill (see figure 131).
   
   

   
    
   Figure 131. Photo. Completion of compaction of the composite mass and leveling the top surface with 0.2-inch (5-mm)-thick sand layer.

10. Place a geosynthetic sheet on top of the composite mass (see figure 132). 
    
    

    
     
    Figure 132. Photo. Completed composite mass with a geotextile sheet on the top surface.

11. Glue a sheet of membrane to the top edge of the side membrane sheets (see figure 133). 
    
    

    
     
    Figure 133. Photo. Top surface of the composite mass covered with a sheet of membrane.

12. Remove all facing blocks and trim off the excess geotextile (see figure 134). 
    
    

    
     
    Figure 134. Photo. Removing facing blocks and trimming off excess geosynthetic reinforcement.

13. Insert strain gauge cables through the plastic openings that were already attached on the membrane sheets at prescribed locations (see figure 135). 
    
    

    
     
    Figure 135. Photo. Inserting strain gauge cables through the membrane sheet.

14.  Glue membrane sheets to enclose entire composite mass.
    
    
15. Apply vacuum to the composite mass at a low pressure of 2.0 psi (14 kPa) (see figure 136). 
    
    

    
     
    Figure 136. Photo. Vacuuming the composite mass with a low pressure.

16. Seal the connection between cables and membrane with epoxy to prevent air leaks (see figure 137). The low vacuum pressure allows the epoxy to seal the connection well. 
    
    

    
     
    Figure 137. Photo. Sealing the connection between cable and membrane with epoxy to prevent air leaks.

17. Raise the vacuum pressure to 4.9 psi (34 kPa) and check air leaks under vacuuming (see figure 138). Measure the specimen dimensions (see table 7 for specimen dimensions of five tests).
    
    

    
     
    Figure 138. Photo. Checking air leaks under vacuuming.

 

Table 7. Dimensions of the GSGC specimens before loading.

Test	Height, inches (m)	Width, inches (m)	Length, inches (m)
Test 1	76.25 (1.937)	57.00 (1.448)	47.00 (1.194)
Test 2	76.35 (1.939)	54.00 (1.372)	46.75 (1.187)
Test 3	76.35 (1.939)	53.00 (1.346)	46.75 (1.187)
Test 4	76.30 (1.938)	58.75 (1.492)	46.75 (1.187)
Test 5	76.35 (1.939)	49.00 (1.245)	46.75 (1.187)

Figure 139 shows the LVDTs to monitor the lateral movement of a test specimen. The locations of the selected points for the tests are depicted in figure 140. The measured dry unit weights of five tests are shown in figure 141.

 
Figure 139. Photo. The LVDTs on an open side of test specimen.

 

1 mm = 0.039 inches
Figure 140. Illustration. Locations of selected points to trace internal movement of tests 1–5.

 

1 m = 3.28 ft
1 kN/m³ = 0.006 kip/ft³
Figure 141. Graph. Soil dry unit weight results during specimen preparation.

 

4.6 TEST RESULTS

4.6.1 Test 1—Unreinforced Soil

Test 1 is perhaps the largest plane-strain test for soil with a confining pressure. It was conducted as the baseline for the other four GSGC tests.

The loading sequence of the soil mass was as follows:

• Loading up to a vertical pressure of 36 psi (250 kPa) (nearly 1 percent vertical strain).
  
  
• Unloading to zero.
  
  
• Reloading until a failure pressure of 110 psi (770 kPa) was reached.

The soil mass at failure is shown in figure 142. Figure 143 shows the global vertical stress-strain relationship, and figure 144 shows the volume change relationships of the soil mass. The average lateral displacements on the open faces of the soil mass under different vertical stresses, measured by LVDTs, are presented in figure 145. The internal displacements of the soil at selected points under vertical applied pressures of 28, 45, 90, and 110 psi (190, 310, 620, and 770 kPa) are shown in figure 146. The test 1 results for unreinforced soil are summarized in table 8.

 
Figure 142. Photo. Soil mass at failure of test 1.

 

1 kPa = 0.145 psi
Figure 143. Graph. Test 1 unreinforced soil mass global vertical stress/vertical strain relationship.

 

 
Figure 144. Graph. Test 1 unreinforced soil mass global volume change strain relationship.

 

1 m = 3.28 ft
1 kPa = 0.145 psi
Figure 145. Graph. Lateral displacements on the open face of test 1.

 

1 N = 0.00022 kip
1 mm = 0.039 inches
Figure 146. Illustration. Internal displacements of test 1.

 

Table 8. GSGC test 1 result summary.

Parameter		Measurement
Test condition	Geosynthetic reinforcement	None
	Wide-width strength of reinforcement	None
	Reinforcement spacing	No reinforcement
	Confining pressure	34 kPa
Test results	Applied stress at vertical strain of 1 percent	335 kPa
	Ultimate applied pressure	770 kPa
	Vertical strain at failure	3 percent
	Maximum lateral displacement of the open face at failure	47 mm
	Stiffness at 1 percent vertical strain (Eat 1%)	33,500 kPa
	Stiffness for unloading-reloading (Eur)	87,100 kPa
1 mm = 0.039 inches 1 kPa = 0.145 psi

 

4.6.2 Test 2—GSGC Test (T, S_v )

In the second test, the GSGC mass was reinforced by nine sheets of single-sheet Geotex^® 4×4 with spacing of 0.7 ft (0.2 m). The soil layer was compacted at 0.7-ft (0.2-m)-thick lifts. Each reinforcement sheet was mounted with 54 strain gauges.

The failure load in this test was 1,000,000 lb (454,000 kg). All nine reinforcement sheets were ruptured after testing. The composite mass after testing is shown in figure 147. The shear bands of the composite mass after testing are visible through the diagonal lines of the mass (see figure 147 and figure 148). Along the shear bands, the square grids of 2 by 2 inches (51 by 51 mm) were severely distorted after testing (see figure 148). These shear bands correspond exactly with the failure surfaces shown in figure 149.

 
Figure 147. Photo. Composite mass at failure of test 2.

 

 
Figure 148. Photo. Close-up of shear bands at failure of area A in figure 147.

 

 
Figure 149. Photo. Failure planes of the composite mass after testing in test 2.

The measured data from test 2 were as follows:

• Global stress-strain relationship: Figure 150 and figure 151 show the global stress-strain relationship of the composite up to and after failure. The maximum applied vertical pressure was about 390 psi (2,700 kPa), and the corresponding vertical displacement was 4.88 inches (125 mm) (6.5 percent vertical strain).
  
  
• Lateral displacement: The average lateral displacement profiles are on the open faces of the composite under different vertical pressures, as shown in figure 152. The lateral displacements were nearly uniform along the height of the composite up to a pressure of about 87 psi (600 kPa). At vertical pressures between 110 and 220 psi (770 and 1,500 kPa), the maximum lateral displacement occurred at about ³/₈ H from the base, where H is the height of the composite mass. The locations of the maximum displacements were about the same as those of test 1 (unreinforced). The maximum lateral displacement at the mid-height of the composite under the applied pressure of 390 psi (2,700 kPa) was 2.3 inches (60 mm).
  
  
• Internal displacement: The internal displacements of the composite mass at selected points under vertical applied pressures of 39 to 390 psi (270 to 2,700 kPa) at increments of 39 psi (270 kPa) are shown in figure 153. At points near the open faces—1 and 3, 4 and 6, and 7 and 9—the displacements moved downward and outward with angles that were measured after testing of about 67, 47, and 31 degrees, respectively, to the horizontal. The vertical displacements of the points at the upper part of the soil mass were greater than those at the lower part. Along the center line, there were only vertical displacements for points 2, 5, and 8. There was almost no displacement at point 8 near the bottom and at the center line. The maximum lateral displacement in the soil body was 3.3 inches (60 mm) at mid-height on the open sides, and the maximum vertical displacement was 4.87 inches (125 mm) at the top of the specimen.
  
  
• Reinforcement strain: Figure 154 shows the locations of the strain gauges on the geosynthetic sheets. The strain in the reinforcement of the GSGC mass is shown in figure 155 through figure 163. Most of the strain gauges performed well at strains less than 4 percent. All reinforcement layers were found ruptured after the test was completed. The locations of the rupture lines can be seen from the aerial view of the reinforcement sheets exhumed from the composite after testing (see figure 164). Based on the locations of the rupture lines, the rupture planes can be constructed as shown in figure 165. This figure agrees perfectly with the shear bands shown in figure 149. The maximum strain in reinforcement at rupture was about 12 percent, while the measured data from the strain gauges were less than 4 percent. The strain distributions in figure 155 through figure 163 show that the locations of the maximum strain in reinforcement were different between layers. In reinforcement layers near at the mid-height of the GSGC mass (2.6 and 3.3 ft (0.8 and 1.0 m) from the base), the maximum reinforcement strains were close to the centerline. In the reinforcement layers near the top and the base of the GSGC mass (0.7 and 5.9 ft (0.2 and 1.8 m) from the base), the maximum reinforcement strains were at about 1 ft (0.3 m) from the edge of the composite mass. The maximum strain locations in all reinforcement layers were at the ruptured lines of reinforcement, as shown in figure 164 and figure 165.

1 kPa = 0.145 psi
Figure 150. Graph. Test 2 reinforced soil mass global vertical stress-vertical strain relationship.

 

 
Figure 151. Graph. Test 2 reinforced soil mass global volume change strain relationship.

 

1 m = 3.28 ft
1 mm = 0.039 inches
1 kPa = 0.145 psi
Figure 152. Graph. Lateral displacements on the open face of test 2.

 

1 mm = 0.039 inches
1 kPa = 0.145 psi
Figure 153. Illustration. Internal displacements of test 2.

 

1 mm = 0.039 inches
Figure 154. Illustration. Locations of strain gauges on geosynthetic sheets in test 2.

 

1 m = 3.28 ft
1 kPa = 0.145 psi
Figure 155. Graph. Reinforcement strain distribution of the composite mass in layer 1 of test 2 0.7 ft (0.2 m) from the base.

 

1 m = 3.28 ft
1 kPa = 0.145 psi
Figure 156. Graph. Reinforcement strain distribution of the composite mass in layer 2 of test 2 1.3 ft (0.4 m) from the base.

 

1 m = 3.28 ft
1 kPa = 0.145 psi
Figure 157. Graph. Reinforcement strain distribution of the composite mass in layer 3 of test 2 2 ft (0.6 m) from the base.

 

1 m = 3.28 ft
1 kPa = 0.145 psi
Figure 158. Graph. Reinforcement strain distribution of the composite mass in layer 4 of test 2 2.6 ft (0.8 m) from the base.

 

1 m = 3.28 ft
1 kPa = 0.145 psi
Figure 159. Graph. Reinforcement strain distribution of the composite mass in layer 5 of test 2 3.3 ft (1.0 m) from the base.

 

1 m = 3.28 ft
1 kPa = 0.145 psi
Figure 160. Graph. Reinforcement strain distribution of the composite mass in layer 6 of test 2 3.9 ft (1.2 m) from the base.

 

1 m = 3.28 ft
1 kPa = 0.145 psi
Figure 161. Graph. Reinforcement strain distribution of the composite mass in layer 7 of test 2 4.6 ft (1.4 m) from the base.

 

1 m = 3.28 ft
1 kPa = 0.145 psi
Figure 162. Graph. Reinforcement strain distribution of the composite mass in layer 8 of test 2 5.2 ft (1.6 m) from the base.

 

1 m = 3.28 ft
1 kPa = 0.145 psi
Figure 163. Graph. Reinforcement strain distribution of the composite mass in layer 9 of test 2 5.9 ft (1.8 m) from the base.

 

 
Figure 164. Photo. Aerial view of the reinforcement sheets exhumed from the composite mass after test 2 (numbers indicate sheet number).

 

1 mm = 0.039 inches
Figure 165. Illustration. Locations of rupture lines of reinforcement in test 2 based on figure 164.

The results of test 2 are summarized in table 9.

Table 9 . GSGC test 2 results summary.

Parameter		Measurement
Test condition	Geosynthetic reinforcement	Geotex® 4×4
	Wide-width strength of reinforcement	70 kN/m
	Reinforcement spacing	0.2 m
	Confining pressure	34 kPa
Test results	Ultimate applied pressure	2,700 kPa
	Vertical strain at failure	6.5 percent
	Maximum lateral displacement of the open face at failure	60 mm
	Stiffness at 1 percent vertical strain (Eat 1%)	61,600 kPa
	Maximum strain in reinforcement at ruptured	12 percent
	Maximum measured strain in reinforcement	4.0 percent
1 kN/m = 0.069 kip/ft 1 m = 3.28 ft 1 kPa = 0.145 psi 1 mm = 0.039 inches

 

4.6.3 Test 3—GSGC Test (2T, 2S_v )

In this test, the GSGC mass was reinforced by four double sheets of Geotex^® 4×4 at a 1.31-ft (0.4-m) spacing. The strength and stiffness of the double-sheet reinforcement were nearly doubled compared to those of the single-sheet reinforcement used in test 2. The GSGC mass after testing is shown in figure 166.

 
Figure 166. Photo. Composite mass after testing of test 3.

The measured data of test 3 were as follows:

• Global stress-strain relationship: The maximum applied vertical pressure was about 254 psi (1,750 kPa), as shown in figure 167. The vertical displacement at the failure pressure was 4.6 inches (118 mm) (6.1 percent global vertical strain).
  
  
• Lateral displacement: The average lateral displacement profiles on the open faces of the composite under different vertical pressures are shown in figure 168. The maximum lateral displacement under the failure pressure of 254 psi (1,750 kPa) was 2.1 inches (54 mm).
  
  
• Internal displacement: Internal displacements at selected points are shown in figure 169. The trend of the internal movements in test 3 was nearly the same as in test 2. For points 1, 3, 4, 5, 6, 7, and 9 near the open faces, the displacements moved downward and outward with angles from 49 to 71 degrees to the horizontal. For points 2, 5, and 8 along the center line, the displacements were almost vertical. The maximum vertical displacement at the top of the specimen was 4.6 inches (118 mm).
  
  
• Reinforcement strain: Figure 170 shows the locations of the strain gauges on the geosynthetic sheets. The strain in the reinforcement of the GSGC mass is shown in figure 171 through figure 174. Three reinforcement layers near the top of the composite were ruptured after testing. Figure 175 shows an aerial view of the locations of the rupture line on the reinforcement sheets exhumed from the composite after testing. Based on the locations of the rupture line, the rupture planes can be constructed, as shown in figure 176. This rupture line agrees perfectly with the failure line in figure 166. The maximum strain in reinforcement was 4 percent and was located near the ruptured line, as shown in figure 176.

1kPa = 0.145 psi
Figure 167. Graph. Global stress-strain relationship of test 3.

 

1 m = 3.28 ft
1 mm = 0.039 inches
1 kPa = 0.145 psi
Figure 168. Graph. Lateral displacements on the open face of test 3.

 

1 mm = 0.039 inches
1 kPa = 0.145 psi
Figure 169. Illustration. Internal displacements of test 3.

 

1 mm= 0.039 inches
Figure 170. Illustration. Location of strain gauges on geosynthetic sheets in test 3.

 

1 m = 3.28 ft
1 kPa = 0.145 psi
Figure 171. Graph. Reinforcement strain distribution of the composite mass in layer 1 of test 3 1.3 ft (0.4 m) from the base.

 

1 m = 3.28 ft
1 kPa = 0.145 psi
Figure 172. Graph. Reinforcement strain distribution of the composite mass in layer 2 of test 3 2.6 ft (0.8 m) from the base.

 

1 m = 3.28 ft
1 kPa = 0.145 psi
Figure 173. Graph. Reinforcement strain distribution of the composite mass in layer 3 of test 3 3.9 ft (1.2 m) from the base.

 

1 m = 3.28 ft
1 kPa = 0.145 psi
Figure 174. Graph. Reinforcement strain distribution of the composite mass in layer 4 of test 3 5.2 ft (1.6 m) from the base.

 

 
Figure 175. Photo. Aerial view of the reinforcement sheets exhumed from the composite mass after test 3 (numbers indicate sheet number).

 

1 mm = 0.039 inches
Figure 176. Illustration. Locations of rupture lines of reinforcement in test 3 based on figure 175.

The results of test 2 are summarized in table 10.

Table 10. Test 3 result summary.

Parameter		Measurement
Test conditions	Geosynthetic reinforcement	Geotex® 4×4
	Wide-width strength of reinforcement	140 kN/m
	Reinforcement spacing	0.4 m
	Confining pressure	34 kPa
Test results	Ultimate applied pressure	1,750 kPa
	Vertical strain at failure	6.1 percent
	Maximum lateral displacement of the open face at failure	54 mm
	Stiffness at 1 percent vertical strain (Eat 1%)	48,900 kPa
	Maximum strain in reinforcement at rupture	12 percent
	Maximum measured strain in reinforcement	4.0 percent
1 kN/m = 0.069 kip/ft 1 m = 3.28 ft 1 mm = 0.039 inches 1 kPa = 0.145 psi

 

4.6.4 Test 4—GSGC Test (T, 2S_v )

The reinforcement used in this test was a single sheet of Geotex^® 4×4 at spacing of 1.3 ft (0.4 m). The composite mass after testing is shown in figure 177 and figure 178. The failure surfaces can be seen clearly in the figures.

 
Figure 177. Photo. Front view of failure planes of the composite mass after test 4.

 

 
Figure 178. Photo. Back view of failure planes of the composite mass after test 4.

The measured data for test 4 are as follows:

• Global stress-strain relationship: The maximum applied vertical pressure was about 190 psi (1,300 kPa), as shown in figure 179. The vertical displacement at the failure pressure was 3.0 inches (77 mm) (4.0 percent global vertical strain).
  
  
• Lateral displacement: The average lateral displacements on the open faces of the composite mass under different vertical pressures are shown in figure 180. The maximum lateral displacement under the failure pressure was 2.0 inches (52 mm).
  
  
• Internal displacements: Internal displacements at selected points are shown in figure 181. The trends of the internal movements in tests 2, 3, and 4 were identical. For points 1, 3, 4, 5, 6, 7, and 9 near the open faces, the displacements moved downward and outward with angles from 30 to 63 degrees to the horizontal. For points 2, 5, and 8 along the center line, the displacements were almost vertical. The maximum vertical displacement at the top of the specimen was 3.0 inches (77 mm).
  
  
• Reinforcement strain: Figure 182 shows the locations of the strain gauges on the geosynthetic sheets. The strain in the reinforcement of the GSGC mass is shown in figure 183 through figure 186. All reinforcement layers near the top of the composite were ruptured after the test was completed. The locations of the rupture lines are shown in figure 187. Based on the locations of the rupture line, the rupture planes can be constructed, as shown in figure 188. This rupture line agrees perfectly with the failure line in figure 177 and figure 178. The maximum strain in reinforcement at rupture was about 12 percent and located near the rupture line, as shown in figure 188.

1 kPa = 0.145 psi
Figure 179. Graph. Global stress-strain relationship of test 4.

 

1 m = 3.28 ft
1 mm = 0.029 inches
1 kPa = 0.145 psi
Figure 180. Graph. Lateral displacements on the open face of test 4.

 

1 mm = 0.039 inches
1 kPa = 0.145 psi
Figure 181. Illustration. Internal displacements of test 4.

 

1 mm = 0.039 inches
Figure 182. Illustration. Locations of strain gauges on geosynthetic sheets in test 4.

 

1 m = 3.28 ft
1 kPa = 0.145 psi
Figure 183. Graph. Reinforcement strain distribution of the composite mass in layer 1 of test 4 1.3 ft (0.4 m) from the base.

 

1 m = 3.28 ft
1 kPa = 0.145 psi
Figure 184. Graph. Reinforcement strain distribution of the composite mass in layer 2 of test 4 2.6 ft (0.8 m) from the base.

 

1 m = 3.28 ft
1 kPa = 0.145 psi
Figure 185. Graph. Reinforcement strain distribution of the composite mass in layer 3 of test 4 3.9 ft (1.2 m) from the base.

 

1 m = 3.28 ft
1 kPa = 0.145 psi
Figure 186. Graph. Reinforcement strain distribution of the composite mass in layer 4 of test 4 5.2 ft (1.6 m) from the base.

 

 
Figure 187. Photo. Aerial view of the reinforcement sheets exhumed from the composite mass after test 4 (numbers indicate sheet number).

 

1 mm = 0.039 inches
Figure 188. Illustration. Locations of rupture lines of reinforcement in test 4 based on figure 187.

 

The test 4 results are summarized in table 11.

Table 11. GSGC test 4 result summary.

Parameter		Measurement
Test conditions	Geosynthetic reinforcement	Geotex® 4×4
	Wide-width strength of reinforcement	70 kN/m
	Reinforcement spacing	0.4 m
	Confining pressure	34 kPa
Test results	Ultimate applied pressure	1,300 kPa
	Vertical strain at failure	4.0 percent
	Maximum lateral displacement of the open face at failure	53 mm
	Stiffness at 1 percent vertical strain (Eat 1%)	46,600 kPa
	Maximum strain in reinforcement at rupture	12 percent
	Maximum measured strain in reinforcement	2.0 percent
1 kN/m = 0.069 kip/ft 1 m = 3.28 ft 1 mm = 0.039 inches 1 kPa = 0.145 psi

4.6.5 Test 5—GSGC Test (Unconfined with T, S_v )

The configuration of this test was the same as test 2. The reinforcement was single-sheet Geotex^® 4×4 at a spacing of 0.6 ft (0.2 m). Confining pressure was not applied for this test. Without applying confining pressure, the soil on the open faces fell off continuously with increasing applied pressure. The composite mass and failure surfaces after testing are shown in figure 189 through figure 191.

 
Figure 189. Photo. Composite mass at failure of test 5.

 

 
Figure 190. Photo. Front view of failure planes of the composite mass after test 5.

 

 
Figure 191. Photo. Back view of failure planes of the composite mass after test 5.

 

The measured data of test 5 are as follows:

• Global stress-strain relationship: The maximum applied vertical pressure was about 270 psi (1,900 kPa), as shown in figure 192. The vertical displacement at the failure pressure was 4.33 inches (111 mm) (6.0 percent global vertical strain).
  
  
• Lateral displacement: The average lateral displacements on the open faces of the composite mass under different vertical pressures are shown in figure 193. The maximum lateral displacement at the open faces under the failure pressure could not be measured because the soil at these faces dropped during testing under the high applied pressures.
  
  
• Internal displacements: Internal displacements at selected points are shown in figure 194. The trend of the internal movements in test 5 was nearly the same as that in tests 2, 3, and 4. For points 1, 3, 4, 5, 6, 7, and 9 near the open faces, the displacements move downward and outward with angles of 35 to 63 degrees to the horizontal. For points 2, 5, and 8 along the center line, the displacements were almost vertical. The maximum vertical displacement at the top of the specimen was 4.33 inches (111 mm).
  
  
• Reinforcement strain: Figure 195 shows the locations of the strain gauges on the geosynthetic sheets. The strain in the reinforcement of the GSGC mass is shown in Figure 196 through figure 198. Eight reinforcement layers near the top of the composite were ruptured after testing. Figure 199 shows the locations of the rupture lines from an aerial view of the reinforcement sheets exhumed from the composite after testing. Based on the locations of the rupture line, the rupture planes can be constructed (see figure 200). This rupture line agrees perfectly with the failure line in figure 189. The maximum strain in reinforcement measured was 3.2 percent and located at near the rupture line, as shown in figure 200.

1 kPa = 0.145 psi
Figure 192. Graph. Global stress-strain relationship of test 5.

 

1 m = 3.28 ft
1 mm = 0.039 inches
1 kPa = 0.145 psi
Figure 193. Graph. Lateral displacements on the open face of test 5.

 

1 mm = 0.039 inches
1 kPa = 0.145 psi
Figure 194. Illustration. Internal displacements of test 5.

 

1 mm = 0.039 inches
Figure 195. Illustration. Locations of strain gauges on geosynthetic sheets in test 5.

 

1 m = 3.28 ft
1 kPa = 0.145 psi
Figure 196. Graph. Reinforcement strain distribution of the composite mass in layer 1 of test 5 1.3 ft (0.4 m) from the base.

 

1 m = 3.28 ft
1 kPa = 0.145 psi
Figure 197. Graph. Reinforcement strain distribution of the composite mass in layer 2 of test 5 2.6 ft (0.8 m) from the base.

 

1 m = 3.28 ft
1 kPa = 0.145 psi
Figure 198. Graph. Reinforcement strain distribution of the composite mass in layer 3 of test 5 3.9 ft (1.2 m) from the base.

 

 
Figure 199. Photo. Aerial view of the reinforcement sheets exhumed from the composite mass after test 5 (numbers indicate sheet number).

 

1 mm = 0.039 inches
Figure 200. Illustration. Locations of rupture lines of reinforcement in test 5 based on figure 199.

The test results of test 5 are summarized in table 12.

Table 12. GSGC test 5 result summary.

Parameter		Measurement
Test conditions	Geosynthetic reinforcement	Geotex® 4×4
	Wide-width strength of reinforcement	70 kN/m
	Reinforcement spacing	0.2 m
	Confining pressure	0
Test results	Ultimate applied pressure	1,900 kPa
	Vertical strain at failure	6.0 percent
	Maximum lateral displacement of the open face at failure	Not measured
	Stiffness at 1 percent vertical strain (Eat 1%)	52,900 kPa
	Maximum strain in reinforcement at rupture	12 percent
	Maximum measured strain in reinforcement	3.2 percent
1 kN/m = 0.069 kip/ft 1 m = 3.28 ft 1 kPa = 0.145 psi

 

4.7 DISCUSSION OF THE RESULTS

The results of the GSGC tests are discussed in terms of the following:

• Effects of geosynthetic inclusion (comparison between tests 1 and 2).
  
  
• Relationship between reinforcement spacing and reinforcement strength (comparison between tests 2 and 3).
  
  
• Effects of reinforcement spacing (comparison between tests 2 and 4).
  
  
• Effects of reinforcement stiffness (comparison between tests 3 and 4).
  
  
• Effects of confining pressure (comparison between tests 2 and 5).
  
  
• Composite strength properties.
  
  

4.7.1 Effects of Geosynthetic Inclusion (Comparison Between Tests 1 and 2)

Table 13 shows the result comparisons between an unreinforced soil mass (test 1) and a soil mass reinforced by Geotex^® 4×4 at a 0.7-ft (0.2-m) spacing (test 2). The reinforced soil was much stronger than the unreinforced soil. The ultimate applied pressure for the GSGC mass was about 3.5 times as large as the strength of the soil mass without reinforcement. The stiffness of the unreinforced soil mass was 50 percent of that for the reinforced soil mass. In addition, the reinforced soil mass was much more ductile than the unreinforced soil mass. The global vertical strain was 6.5 percent at failure for test 2, whereas it was only 3.0 percent for test 1.

 

Table 13. Comparison between test 1 and test 2.

Parameter	Test 1	Test 2 (T, Sv )
Geosynthetic reinforcement	None	Geotex® 4×4
Wide-width strength of reinforcement	None	Tf = 70 kN/m
Reinforcement spacing	No reinforcement	Sv = 0.2 m
Confining pressure	34 kPa	34 kPa
Ultimate applied pressure	770 kPa	2,700 kPa
Vertical strain at failure	3 percent	6.5 percent
Maximum lateral displacement of the open face at failure	47 mm	60 mm
Stiffness at 1 percent vertical strain (Eat 1%)	33,500 kPa	61,600 kPa
Stiffness for unloading-reloading (Eur)	87,100 kPa	Not applied
1 kN/m = 0.069 kip/ft 1 m = 3.28 ft. 1 kPa = 0.145 psi 1 mm = 0.039 inches

 

4.7.2 Relationship Between Reinforcement Spacing and Reinforcement Strength (Comparison Between Tests 2 and 3)

Comparing the results of tests 2 and 3 demonstrates the relationship between reinforcement spacing and reinforcement strength (see table 14). As noted in section 2.1, the current design methods are based on the concept that reinforcement spacing and reinforcement strength play an equal role in the performance of a GRS mass. In other words, a GRS wall with reinforcement strength T_f at spacing S_v will behave the same as the one with reinforcement strength of 2 × T_f at spacing 2 × S_v. The results of tests 2 and 3 demonstrate that this assumption is incorrect. Even with the same T_f / S_v ratio (7.3 kip/ft² (350 kN/m²)) in tests 2 and 3, the stiffness and strength of test 2 (with T_f and S_v = 0.7 ft (0.2 m)) was much higher than that of test 3 (with 2T_f and S_v = 1.3 ft (0.4 m)). The strength of the composite mass in test 3 was only 65 percent of the strength in test 2. These results suggest that reinforcement spacing plays a more important role than strength of reinforcement in a reinforced soil mass.

Table 14. Comparison between test 2 and test 3 with the same T_f /S_v

Parameter	Test 2 (T, Sv )	Test 3 (2T, 2Sv )
Wide-width strength of reinforcement (Tf)	70 kN/m	140 kN/m
Reinforcement spacing (Sv )	0.2 m	0.4 m
Tf /Sv	350 kPa	350 kPa
Confining pressure	34 kPa	34 kPa
Ultimate applied pressure	2,700 kPa	1,750 kPa
Vertical strain at failure	6.5 percent	6.1 percent
Maximum lateral displacement of the open face at failure	60 mm	54 mm
1 kN/m = 0.069 kip/ft 1 m = 3.28 ft. 1 kPa = 0.145 psi 1 mm = 0.039 inches

 

4.7.3 Effects of Reinforcement Spacing (Comparison Between Tests 2 and 4)

The effects of reinforcement spacing can be seen by comparing the results of tests 2 and 4 (see table 15). All test conditions in the two tests were the same except the reinforcement spacing. The spacing was 1.3 ft (0.4 m) in test 4 and 0.65 ft (0.2 m) in test 2. The results demonstrate the importance of reinforcement spacing on the behavior of a GRS mass. With reinforcement spacing of 0.7 ft (0.2 m), the strength of the GRS mass was about twice as high as the one with the 1.3-ft (0.4-m) spacing. The corresponding increase in stiffness at 1 percent strain was about 30 percent. The GRS mass at the 0.7-ft (0.2‑m) spacing also exhibited significantly higher ductility than at the 1.3-ft (0.4‑m) spacing.

Table 15. Comparison between test 2 and test 4.

Parameter	Test 2 (T, Sv )	Test 4 (T, 2Sv )
Wide-width strength of reinforcement (Tf )	70 kN/m	70 kN/m
Reinforcement spacing (Sv )	0.2 m	0.4 m
Tf /Sv	350 kPa	175 kPa
Confining pressure	34 kPa	34 kPa
Ultimate applied pressure	2,700 kPa	1,300 kPa
Vertical strain at failure	6.5 percent	4.0 percent
Maximum lateral displacement of the open face at failure	60 mm	53 mm
Stiffness at 1 percent vertical strain (Eat 1% )	61,600 kPa	46,600 kPa
1 kN/m = 0.069 kip/ft 1 m = 3.28 ft. 1 kPa = 0.145 psi 1 mm = 0.039 inches

4.7.4 Effects of Reinforcement Strength (Comparison Between Tests 3 and 4)

The effects of reinforcement strength can be seen by comparing the results of tests 3 and 4 (see table 16). All test conditions in the two tests were identical except that reinforcement strength in test 3 was almost twice as high as that in test 4. The results indicate that the increase in strength of the GRS mass due to doubling the reinforcement strength was about 35 percent. This increase was much smaller than doubling the reinforcement spacing, where the increase in strength of the GRS mass was over 100 percent. The increase in stiffness at 1 percent strain due to doubling the reinforcement strength was only about 5 percent, compared to about a 30 percent increase due to doubling the reinforcement spacing.

Table 16. Comparison between test 3 and test 4.

Parameter	Test 3 (T, Sv )	Test 4 (T, 2Sv )
Wide-width strength of reinforcement (Tf )	140 kN/m	70 kN/m
Reinforcement spacing (Sv )	0.4 m	0.4 m
Tf /Sv	350 kPa	175 kPa
Confining pressure	34 kPa	34 kPa
Ultimate applied pressure	1,750 kPa	1,300 kPa
Vertical strain at failure	6.1 percent	4.0 percent
Maximum lateral displacement of the open face at failure	54 mm	53 mm
Stiffness at 1 percent vertical strain (Eat 1%)	48,900 kPa	46,600 kPa
1 kN/m = 0.069 kip/ft 1 m = 3.28 ft. 1 kPa = 0.145 psi 1 mm = 0.039 inches

4.7.5 Effects of Confining Pressure (Comparison Between Tests 2 and 5)

The effects of confining pressure can be seen by comparing the test results of tests 2 and 5 (see table 17). All test conditions in the two tests were identical except that the confining pressure in test 2 was 4.9 psi (34 kPa) and test 5 was conducted without confinement. The results indicate that the increase in strength due to the confining pressure was about 40 percent. The increase in stiffness at 1 percent strain due to the confining pressure was about 15 percent.

Table 17. Comparison between test 2 and test 5.

Parameter	Test 2 (T, Sv )	Test 5 (T, Sv )
Wide-width strength of reinforcement (Tf )	70 kN/m	70 kN/m
Reinforcement spacing (Sv )	0.2 m	0.2 m
Tf /Sv	350 kPa	350 kPa
Confining pressure	34 kPa	0
Ultimate applied pressure	2.700 kPa	1,900 kPa
Vertical strain at failure	6.5 percent	6.0 percent
Maximum lateral displacement of the open face at failure	60 mm	Not measured
Stiffness at 1 percent vertical strain (Eat 1%)	61.600 kPa	52,900 kPa
1 kN/m = 0.069 kip/ft 1 m = 3.28 ft. 1 kPa = 0.145 psi 1 mm = 0.039 inches

 

4.7.6 Composite Strength Properties

Table 18 shows a comparison of the composite strength properties of the five GSGC tests as obtained from the measured data and calculated from Schlosser and Long’s method by assuming the friction angle remains the same as unreinforced soil (i.e., the same Φ value as in test 1).⁽²⁵⁾ The apparent cohesion, c_R, from Schlosser and Long’s method is calculated as shown in figure 201.⁽²⁵⁾

Table 18. Comparison of strength properties of five GSGC tests.

Parameter	Test 1 (unreinforced)	Test 2 (T, Sv )	Test 3 (2T, 2Sv )	Test 4 (T, 2Sv )	Test 5 (T, Sv )
Wide-width strength of reinforcement, Tf (kN/m)		70	140	70	70
Reinforcement spacing, Sv (m)		0.2	0.4	0.4	0.2
Tf /Sv (kPa)		350	350	175	350
Confining pressure (kPa)	34	34	34	34	0
Apparent cohesion, cR, (kPa) by Schlosser and Long’s method		550	550	310	550
Ultimate applied pressure (kPa) from measured data	770	2,700	1,750	1,300	1,900
Ultimate applied pressure (kPa) by Schlosser and Long’s method		3,250	3,250	1,930	3,030
Difference in ultimate pressure between measured data and Schlosser and Long’s method (percent)		20	86	48	59
1 kN/m = 0.069 kip/ft 1 m = 3.28 ft. 1 kPa = 0.145 psi Note: Blank cells indicate parameters for which there were no measurements.

 

 
Figure 201. Equation. Apparent cohesion from Schlosser and Long.⁽²⁵⁾

Where:

c = Cohesion of the backfill.
T_f = Strength of reinforcement.
S_v = Reinforcement spacing.
K_p = Coefficient of passive Earth pressure.

The values calculated from Schlosser and Long’s method were higher than the measured values by 20 to 86 percent.