Chapter 3 Material Testing and Properties
3.2 Soil Physical Properties
3.2.1 Specific Gravity Test
The specific gravity of the test soil was obtained by the specific gravity test according to ASTM D854. The value of the specific gravity was obtained from the average of three tests. Figure 3.3 shows the equipment of the specific gravity test, and the test results are presented in Table 3.1. The specific gravity of the pure sand and the silty sand is 2.65 and 2.62, respectively.
Table 3.1 Results of the specific gravity test
Soil type Test 1 Test 2 Test 3 Average Gs
Pure Sand 2.65 2.65 2.66 2.65
Silty Sand 2.63 2.60 2.62 2.62
Figure 3.3 Equipment of the specific gravity test 3.2.2 Sieve Analysis Test
Sieve analysis was conducted to obtain the soil grain size distribution curve in accordance with ASTM D293. Figure 3.4 shows the equipment used for the sieve analysis. Sieves No. 4, No. 10, No. 20, No. 40, No. 60, No. 100, and No. 200 were used in the test. Table 3.2 summarizes the test results, and Figure 3.5 shows the grain size distribution curve of the sand. Furthermore, the effective size (D10) of the testing soil is 0.16 mm while the coefficient of gradation (Cc) and the uniformity coefficient (Cu) is 0.89 and 0.66, respectively. Hence, one of the test soil, Vietnam quartz sand, is classified as poorly graded sand (SP) according to the Unified Soil Classification System (USCS).
The grain size distribution curve of the silty sand was obtained from the hydrometer tests.
Figure 3.4 Equipment for the sieve analysis
Figure 3.5 Grain size distribution curve of Vietnam quartz sand
Table 3.2 Results of the sieve analysis
Sieve No. Opening Size Percentage Retained Percent Finer
(mm) (%) (%)
According to ASTM D7928, hydrometer test was carried out to determine the grain size distribution curve for the silty sand of which the particle size is smaller than No. 200 sieve. A hydrometer and a sedimentation cylinder, as shown in Figure 3.6, was used to perform the hydrometer test. Figure 3.7 shows the grain size distribution curve of the testing soil (i.e., Vietnam quartz sand with 20% Kaolinite). The segment that has a particle diameter smaller than 0.075 mm was obtained from the hydrometer test, whereas the remainder was obtained from sieve analysis. The test soil is classified as silty sand (SM) according to the Unified Soil Classification System (USCS).
Figure 3.6 Apparatus of the hydrometer test
Figure 3.7 Grain size distribution curve of silty sand
3.2.4 Relative Density Test
Relative density can be calculated from the maximum and minimum dry unit weight or void ratio, as shown in Equation 3.1:
d d,min d,max max unit weight, γd dry unit weight at the target relative density, emax maximum void ratio, emin minimum void ratio, and e void ratio at the target relative density. The maximum
and minimum dry unit weight and void ratio were obtained using a vibrating deck according to ASTM D4523. Figure 3.8 shows the setup of the apparatus used for the relative density test. A mold with a volume of 2831 cm3 was filled with oven-dried testing soil by pluviation method to achieve the loosest state; the minimum dry unit weight and the maximum void ratio can be attained. The densest state was executed by mounting the mold on the vibratory table with a vertical surcharge (20.2 lb/in2). The volume at the densest state can thus be calculated by measuring the decrease of height after vibration; the maximum dry unit weight and the minimum void ratio can be known subsequently.
The study obtained the friction angle of sand by applying normal stress on samples at different relative density. The results indicate that the friction angle of sand slightly increases with the increase of relative density. Furthermore, the wetting process poses infinitesimal effect on the friction angle of sand with relatively high permeability.
Therefore, the target density of Dr 70% was selected for the GRS wall model using sand as backfill. Table 3.3 presents the results of the relative density test.
Figure 3.8 Apparatus for relative density test
Table 3.3 Results of the relative density test of sand
ASTM Loosest State Densest State Target State ( Dr = 70% )
Parameters γd,min emax γd,max emin γd e
(kN/m3) (kN/m3) (kN/m3)
Value 13.54 0.92 15.21 0.71 14.7 0.77
3.2.5 Compaction Test
Compaction test was carried out to determine the optimum water content and the maximum dry density of the testing soil (i.e., Vietnam quartz sand with 20% Kaolinite).
The standard Proctor compaction test was performed according to AASHTO T99, using a 5.5 lb hammer with a free-falling distance of 12 in, as shown in Figure 3.9 . The specimen was prepared by conducting three levels of compaction with 25 blows each layer. Figure 3.10 shows the results of the compaction curve of the testing soil. The maximum dry density is 18.1 kN/m3 while the optimum water content is 10.7%. In this study, the
specimen was compacted at the optimum water content (10.7%) to 90% of the maximum dry unit weight, on the basis of the construction regulation which specifies the compaction has to be at least 90% maximum dry density at 2% optimum water content.
Figure 3.9 Equipment for the compaction test
Figure 3.10 Compaction curve of the test soil
3.3 Engineering Properties of Testing Soil
In this study, permeability tests and triaxial tests were performed to determine the engineering properties, the permeability and shear strength, of the test sand and silty sand.
Specifically, constant head test and consolidated drained triaxial test were conducted for the testing soil with relatively large permeability (i.e., Vietnam quartz sand) while the falling head test and the consolidated undrained triaxial test were conducted for the testing soil with relatively low permeability (i.e., Vietnam quartz sand with 20% Kaolinite).
3.3.1 Constant Head Test
The permeability of the test sand was obtained from the constant head permeability test, conducted in accordance with ASTM D5084. The permeability can be determined by Darcy’s Law, as shown in Equation 3.2:
L length of the specimen which water seeps through, and h head difference.
Figure 3.11 shows the setup for the constant head permeability test. The specimen was mounted on a steel platform with an O-ring and Vaseline between the specimen and the platform to prevent the leakage of water. Two porous disks were capped at the top and bottom of the specimen, and non-woven geotextiles were also placed between the porous disk and the specimen to prevent soil particles from leaking out.
Figure 3.11 Apparatus for permeability tests
3.3.2 Falling Head Test
Falling head permeability test was conducted for silty sand. The permeability of the testing soil can be determined from Equation 3.3:
Figure 3.11 shows the apparatus for the falling head permeability test, and the test was carried out according to ASTM D5084. The specimen was vacuumed to be saturated before the test started. The permeability of the silty sand can be determined by measuring the head loss of water between a certain time interval. The results of the permeability tests are
Table 3.4 Results of the permeability tests
k (m/s) Test 1 Test 2 Test 3 Average Pure Sand 5.310-4 5.510-4 5.210-4 5.310-4 Silty Sand 3.910-6 3.910-6 3.810-6 3.910-6
3.3.3 Consolidated Drained Triaxial Test
Consolidated drained triaxial test was carried out to investigate the shear strength of the testing soil. The test apparatus, including the triaxial compression chamber, the axial loading device, and the pressure controlling devices, are shown in Figure 3.12. The preparation of the specimen, as shown in Figure 3.13, and the testing procedure were referred to ASTM D7181. Specifically, the specimen is allowed to drain during the shearing phase. Mohr circles along with the friction angle of the testing soil can be attained from the tests, and the results are presented in Figure 3.14. The friction angle of the test sand is 37˚.
Figure 3.12 Apparatus of the triaxial test
Figure 3.13 Specimen preparation
(a) Deviatoric stress versus axial strain
(b) Volume change versus axial strain
(c) Mohr circles
3.3.4 Consolidated Undrained Triaxial Test
For testing soil with relatively low permeability (i.e., Vietnam quartz sand with 20%
Kaolinite), consolidated undrained triaxial test should be done to evaluate the shearing properties of the testing soil. The testing apparatus and specimen preparation are the same as that used in the consolidated drained triaxial test, as shown in Figure 3.12 and Figure 3.13. What is worth noticing is that during the shearing phase, the valve for drainage remained closed to prevent drainage. Pore water pressure was monitored throughout the test. Mohr circles together with the friction angle of the testing soil can be determined from the tests, and the results are presented in Figure 3.15 and summarized in Table 3.5. The effective cohesion and friction angle of the testing soil is 17.3 kPa and 32.6˚, respectively.
Table 3.5 Result of the consolidated undrained triaxial test
Total stress Effective stress
Cohesion, c (kPa) Friction angle, ϕ (˚) Cohesion, c’ (kPa) Friction angle, ϕ’ (˚)
48.7 26.6 17.3 32.6
(a) Deviatoric stress versus axial strain
(b) Excess pore water pressure versus axial strain
(c) Total and effective stress Mohr circles
Figure 3.15 Results of the consolidated undrained triaxial tests
3.4 Material Properties of Reinforcement
The model test performed in this study had been scaled down to secure a consistent behavior between the model and the prototype. Therefore, the dimensions and the strength of the reinforcement had also been scaled down. Section 2.3 thoroughly introduced the principle of scaling down. Wide width tensile strength tests were carried out to determine the material for reinforcement. A mosquito net is used in this research as the geogrid reinforcement because of its accordance with the requirements of the scaled-down strength.
Table 3.6 summarizes the basic properties of the reinforcement. Figure 3.16 shows the picture of the reinforcement and the facing.
Table 3.6 Basic properties of the geogrid reinforcement and geotextile facing Reinforcement
Type Geogrid
Mass per unit area (g/m2) 13.48
Facing
Type Non-woven geotextile
Mass per unit area (g/m2) 29.60
Figure 3.16 Testing materials: (a) Geotextile facing; (b) Geogrid reinforcement
3.4.1 Wide Width Tensile Strength Test
In order to investigate the mechanical properties of the testing material, wide width tensile strength was conducted to get the maximum tensile strength and the allowable strain of the testing material. According to ASTM D4595, the size of the specimen should be at least 200 mm in both length and width, and the pulling rate should be 10±3 %/min. The test was performed in Tamkang University using the universal testing machine from Geotech Testing Machine Incorporation, as shown in Figure 3.17. The specimen is cut to 200200 mm in length and width. Regarding the clamps, roller clamps and sanders clamp are recommended in the ASTM D4595; sanders clamps were used in the tests. Since the testing material is anisotropic, the test was carried out in both longitudinal and transverse direction. However, based on the test results, only longitudinal direction was used in the model test. Figure 3.18 shows the results of the test in longitudinal direction and Table 3.7 summarizes the test results. The maximum tensile strength of the testing material is 0.5 kN/m and the allowable strain is 6.5 %.
Table 3.7 Test results of the tensile strength test for geogrid along longitudinal direction Test 1 Test 2 Test 3 Average Tensile strength, Tult (kN/m) 0.51 0.48 0.50 0.50 Ultimate tensile strain, εu (%) 5.5 6.4 7.5 6.5 Stiffness at 2% strain, J2 (kN/m) 6.8 7.5 7.3 7.2 Stiffness at 5% strain, J5 (kN/m) 7.4 8.3 7.6 7.8
Figure 3.17 Universal testing machine
Figure 3.18 Results of the tensile strength test along longitudinal direction
3.4.2 Soil-Geogrid Interface Shearing Strength Test
The overburden pressure along with the surcharge is the antecedent of the existence of the friction between the soil-geogrid interface, as mentioned in Chapter 2. The enhancement of the global stability of GRS walls can be attributed to the increase of the shearing strength, which results from the interaction between the tensile strength of the geogrid and the soil-geogrid friction. Hence, the importance of investigating the shearing properties between soil-geogrid interface cannot be underestimated.
The direct shear apparatus employed in this research was modified from the conventional direct shear apparatus (Tsai, 2011). The lower box was replaced with a steel block, allowing the testing material be fixed on the lower box with screws on the sides, as shown in Figure 3.19. Moreover, sliding was found to occur between the testing material and the steel block if the applied vertical load was too large (Lai, 2018). This will cause the underestimation of the interface friction angle. Hence, the vertical loads of 50, 100, and 150 kPa were applied in this research, for it not only represents the overburden pressure of the model wall in this research but also precisely estimates the interface friction angle.
Mohr-Coulomb failure criteria was used to determine the friction angle. The direct shear test was done according to the ASTM D3080. The results of the direct shear tests are summarized in Table 3.8. Efficiency factor Eϕ was used to understand the difference between the shear strength of soil-soil interface and soil-geogrid interface. The testing material has an efficiency factor of 0.875 and 0.678 in pure sand and silty sand, respectively.
The efficiency factor can be calculated from Equation 3.4:
tan
E tan (3.4)
where ϕ friction angle between the soil-soil interface and δ friction angel between the soil- geogrid interface. The efficiency factor is usually smaller than 1 because during
the shearing, the soil particles will fill into the voids of the geogrid and cause a smaller result of the friction angle.
Figure 3.19 Modified direct shear box
Table 3.8 Results of the soil-geogrid interface direct shear test Vietnam quartz sand
At initial water content
Eϕ
0.88
At saturation 0.86
Silty sand At initial water content
Eϕ
0.68
At saturation 0.50
Table 3.9 summarizes the properties of the test materials (sand, silty sand, and geogrid).
Table 3.9 Material properties of testing soil and reinforcement
Parameter Value
Effective cohesion, c’ (kPa) 17
Effective stress friction angle, ϕ’ (˚) 32
Total cohesion, c (kPa) 49
Maximum tensile strength, Tmax (kN/m) 0.5
Ultimate tensile strain, εu (%) 6.5
Sand-geogrid interface friction angle, δSP (˚) 27.1 Efficiency factor between geogrid and sand, Eϕ,SP 0.875 Silty sand-geogrid interface friction angle, δSM (˚) 17.3 Efficiency factor between geogrid and silty sand, Eϕ,SM 0.678
Chapter 4 Model Tests and Test Program
In this study, a series of reduced scale model tests was conducted to investigate the performance of GRS walls under rainfall conditions. Different backfill materials and different reinforcement layouts were adopted to evaluate different failure mechanisms.
Improved measure using sand cushions was also evaluated. The model wall, model preparation, test procedure, instrumentation, and the digital image analysis are thoroughly introduced in this chapter.
4.1 Model test
4.1.1 Wall modelFigure 4.1 Schematic view of the sandbox and the locations of the sensors
The sandbox used in this research has the dimensions of 100 cm, 30 cm, and 90 cm in length, width, and height, respectively. A Plexiglas window is mounted in the front of the sandbox for visual observation during the tests. Four wheels on the steel frame allows the sandbox be moved to or fixed at the designated location. The backside of the sandbox consists of three steel plates. The top one can be removed for the construction of the wall models. Various holes were chiseled on the steel plates for the installation of the measuring devices: two pore water pressure transducers and three volumetric water content gauges were installed to monitor the hydraulic performance during the tests. The details of the measuring apparatus are discussed later in Section 4.2. Figure 4.1 illustrates the schematic view of the sand box and the locations of the sensors, and Figure 4.2 shows a panorama of the experiment.
Figure 4.2 Panorama of the experiment
The model GRS wall was designed with a scaling factor N equals to 10 based on the reduced scale designation which was elaborated in Section 2.3. Table 4.1 provides the parameters in this research and its corresponding value in the prototype. A GRS wall with dimensions of 50 cm, 30 cm, and 60cm in length, width, and height, respectively, was constructed in the sandbox. The reinforced zone is 35 cm while the retained zone is 15 cm. An 8 cm-thick foam board was sealed at the bottom of the sandbox to serve as an impermeable foundation to simulate the RC slab usually used in the field to increase the bearing capacity. Notably, sandpaper was stuck to the foundation to increase the friction between the GRS wall model and the foundation. The Styrofoam foundation was sealed to the sandbox with silicon to prevent soil particles and water leaking into the foundation.
Table 4.1 Reduced-scale model parameters
An irrigation system hanging over the sandbox was used to simulate rainfall. The tube at the left side of the sandbox served as the outlet of the rainfall and surface runoff.
The irrigation system consists of two series of nozzles, 8 nozzles on each series was hung 160 cm above the sandbox to simulate rainfall conditions. The picture of the irrigation system is presented in Figure 4.3 (a). A pressurized motor connected to a faucet was used to pump the water up to the nozzles (Figure 4.3 (b)). The rainfall sprayed out from the nozzles was smaller than 0.1 mm. The fine sprays were ensured to achieve the terminal velocity upon falling on the GRS wall and do not erode the crest.
Figure 4.3 (a) Irrigation system; (b) Nozzles; (c) Transparent boxes
Both rainfall intensity and uniformity was ensured before the experiments. 30 transparent boxes (Figure 4.3 (c)) having the dimensions of 10 cm, 10 cm, and 3 cm in length, width, and height, respectively was placed inside the sandbox to collect water, for the purpose of determining the rainfall intensity and uniformity. Regarding the rainfall intensity, the intensity can be calculated from Equation 4.1 given by Technical Regulations for Soil and Water Conservation:
t A
I 600 Q (4.1)
where I = intensity of the rainfall, (mm/hr), Q = accumulated volume of rainfall in the testing period, (cm3), A = area that collects rainfall, (cm2), and t = time (min). The rainfall intensity can be controlled by either adjusting the nozzles or the faucet. The irrigation system can achieve a maximum intensity of 135 mm/hr; lower intensities can be reached by closing part of the nozzles and decreasing the pressure of the pump. The uniformity of the rainfall is determined by Equation 4.2:
average value measured from all the boxes. The uniformity of the rainfall (Table 4.2) was within the range of 82% to 92%, which is considered reasonable compared to preceding studies.Table 4.2 Rainfall uniformity in preceding studies Uniformity (%)
Filter layer is often adopted in GRS walls with marginal backfill to enhance drainage and prevent erosion. The thickness of the filter layer has no particular specifications;
nevertheless, the filter layer cannot be too thick because the pore water pressure would then be dissipated too fast, subverting the intention of investigating the performance of a GRS wall with marginal backfill. A 2 cm-thick filter layer was adopted in this research.
Figure 4.4 is a schematic view of the relationship between the filter and the protected soil. The design of the filter layer should satisfy the two criteria proposed by Terzaghi and Peck (1948). First, the grain size of the smaller particles of the filter material should be smaller than the grain size of the larger particles of the backfill material to prevent internal erosion of the soil and piping, which is referred to as the retention criteria. Second, the grain size of the filter material should be larger than that of the backfill material to ensure the permeability of the filter is large enough to prevent pore water pressure accumulating in the GRS wall. The above-mentioned two criteria can be expressed in Equation 4.4 and 4.5:
where subscript F denotes the grain size of the filter layer and subscript S represents the grain size of the backfill soil. The design of the filter layer in this research is shown in Figure 4.5.
Figure 4.4 Schematic view of the function of filter layer
Figure 4.5 Design of the filter layer 4.1.2 Model Preparation
The wall was constructed layer by layer by compacting the soil to the designated height. A reinforcement spacing of 9 cm was used while a larger spacing (Sv = 12 cm) was used as a control group and a smaller spacing (Sv = 6 cm) was used as one of the
improved methods. The amount of soil needed per layer is calculated by Equation 4.3, the
improved methods. The amount of soil needed per layer is calculated by Equation 4.3, the