Chapter 4 Model Tests and Test Program
4.1 Model test
Figure 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
In this study, the model GRS walls were compacted to 90% of the maximum dry density at optimum water content (ω = 10.7%). Regarding wall models with granular soil for improved methods, the friction angle and the permeability of the soil barely varies with the increase of relative density if the relative density is larger than 60% according to Fong (2010). Thus, the model GRS walls with granular backfill in this study were compacted to 70% relative density. In addition, an initial water content is needed to develop apparent adhesion for the soil to remain stable. It was recommended that an initial water content of 5-10% be used in model GRS walls with granular backfill. However, an initial water content of 10% would make it hard to observe the progression of the wetting front. Hence, 5% initial water content is used in this research for easy investigation of the wetting front while maintaining the initial wall stability.
Blue Styrofoam boards were used as the formwork during the construction of the specimen, as shown in Figure 4.6. The Styrofoam boards were cut according to the designated length, width, and height and were piled up along with the construction of the specimen layer by layer. Moreover, interface treatment to reduce the effect of boundary friction is adopted according Liu et al., (2014), in which they suggested that lubricant sandwiched between PE sheets can reduce up to 67% of the boundary friction. Thus, water-based lubricant sandwiched between two PE sheets were applied to both side of the sand box to reduce the effect of boundary friction in this research. Thorough description of the procedure is enunciated subsequently.
Figure 4.6 Blue Styrofoam mold used for construction
Figure 4.7 presents the picture of the tools used for construction. First, the sandbox was cleaned by a brush. The perforated holes for the pore water pressure transducer were also brushed. The water-based lubricant sandwiched between two PE sheets were applied subsequently. Notably, holes were cut in the PE sheets at the location of the sensors in order to allow contact between the sensors and the specimen. Several 3 cm-thick Styrofoam was put into the sandbox as the mold of the first layer, depended on the spacing (i.e., Sv = 6 cm, 9 cm, or 12 cm). A geogrid was then placed at the bottom of the first layer.
Furthermore, black colored coarse sand was applied near the surface with an angle sleeker using an acrylic mold with dimensions of 30 cm in length, 0.5 cm in width, and 0.5 cm in height, forming a dashed line with 2.5 cm spacing. The dashed line formed by colored sand was used to observe soil deformation and to analyze the reinforcement strain.
Displacement would be found in the dashed lines when the reinforcement was under tension. Specifically, the grain size of the colored sand is slightly larger than the backfill materials, making it easier to separate them by sieving after the experiment.
Figure 4.7 Tools used for construction: (a) Angle sleeker; (b) Concrete trowel
Figure 4.8 (a) Hammer; (b) Filter layer; (c) Scarifying between layers
Afterwards, the construction of the first layer started by compacting the backfill soil with a hammer with a diameter of 15 cm, as shown in Figure 4.8 (a). The backfill soil was uniformly mixed in a large bowl in advance. However, a 2 cm thickness near the wall face
was blocked by Styrofoam boards. After compaction of each layer, the two Styrofoam boards were removed and Quartz sand was used as the facing filter layer to prevent the erosion of fines contents in the backfill during rainfall. This approach is also to model the sandbags typically adopted as the facing elements in practical design of wrapped-around GRS walls. The facing filter layer was also uniformly mixed in advance with a 5% initial water content, allowing the apparent adhesion to develop. Once the filter layer was constructed, the geogrid was wrapped around into the backfill layer, as shown in Figure 4.8 (b). Notably, a pre-tension was given to the geogrid to ensure the mobilization of the tensile strength upon the start of the test. In addition, the surface and facing of the layer is trimmed with a concrete trowel. Next, the specimen was scarified between each layer to increase the friction between two compacted soil interface, which is critical to the progression of the failure surface. Figure 4.8 (c) shows the scarifying between the layers.
The construction procedure was repeated until reaching the design wall height. A picture of the model GRS wall is shown in Figure 4.9.
Figure 4.9 Model GRS wall after construction
4.1.3 Improved design measure
Three improved design measures were proposed in this study, namely, reduction of the reinforcement spacing, selection of a better quality backfill, and the adoption of sand cushions. The inclusion of sand cushions can enhance the stability of the GRS walls with marginal backfill. It is often applied to accelerate pore water pressure dissipation and reduce the surficial intrusion and long-term clogging in the nonwoven geotextiles by fine-grained soil. Moreover, sand cushions provide drainage to the GRS wall since the permeability of the sand is larger than that of the marginal backfill. Additionally, the strength and the deformation characteristics were improved because the efficiency factor at the interface between the geogrid and the sand is much larger than that at the interface between the geogrid and the marginal backfill. According to Yang et al. (2018), replacing around 20% of the marginal backfill with sand is the optimization. Thus, in this research, the geogrid was sandwiched between two layers of 2 cm-thick sand cushions, as shown in Figure 4.10.