# Role of Bimodal Water Retention Curve on the Unsaturated Shear Strength
## 2 Investigated Soil and Methodology
### Methodology  
Index properties tests were performed based on the ASTM standards (Table 1) [35; 36; 37; 38; 39]. The compacted soil specimens underwent a saturation process using Tempe cell until the mass equalizes before WRC tests and shrinkage tests were conducted. WRC tests were carried out using Tempe cell setup for matric suction from 1 kPa to 100 kPa and pressure plate setup for matric suction from 100 kPa to 900 kPa, in accordance with proceduresexplained in ASTM D6836-16 [40] and [PERSON] et al. [41]. WRC tests using both Tempe cell and pressure plate were conducted using the axis-translation method [42]. The ceramic disks in Tempe cell and pressure plate setups were saturated with distilled water prior to the test and regularly flushed to ensure they were saturated. The WRC measurement followed a small increment of matric suction to obtain WRC with greater accuracy. Moreover, the low matric suction (1 to 5 kPa) at the beginning of the test was controlled using a manometer set up to ensure the accuracy of the matric suction. The specimens were weighed every 15 min for the first 1 h at the beginning of each suction and twice a day thereafter. The matric suction level was increased only when the mass of the specimens reached a constant value. There were two types of shear strength tests, namely, saturated and unsaturated shear strength tests carried out in this study. Both tests were started with saturation of the specimens in order to have a uniform initial condition. Saturation was performed by applying cell pressure (\(\sigma_{3}\)) and back pressure (\(u_{w}\)) to the specimens. To avoid significant swelling of specimens, a net confining pressure (\(\sigma_{3}\)-\(u_{w}\)) of 10 kPa was retained until pore-water pressure parameter (\(B\)) was larger than 0.95, as recommended by [PERSON] and [PERSON] [28]. Multistage testing was performed by loading and unloading procedure on each specimen. Compared to single stage testing, multistage testing consumes less time to obtain comprehensive data. However, it should be expected that the measured shear strength would be slightly lower than the actual, due to cumulative strain in the specimen. So, the maximum allowable axial strain in this study is limited to 20% according to [PERSON] et al. [43]. The cohesion intercept (\(c^{\prime}\)) and friction angle (\(\phi^{\prime}\)) of the specimens were obtained from the multistage consolidated undrained triaxial tests by plotting Mohr-Coulomb failure envelope using the peak deviator stresses acquired from the stress versus strain curves. Firstly, the specimens were isotropically consolidated until there was no volume change in the specimens. The specimens were then sheared under three effective confining pressures (i.e., 25, 50, and 75 kPa) with pore-water pressure measurements at the base of specimens. The shearing rate used was 0.05 mm/min according to [PERSON] et al. [43]. A modified triaxial apparatus was used for multistage consolidated drained triaxial testing to investigate shear strength of an unsaturated soil, as described by [PERSON] and [PERSON] [28]. The setup procedure was started by placing a high air-entry ceramic disk with diameter of 50 mm on the pedestal. The specimens were then put directly on the saturated ceramic disk. Filter paper, porous stone and top cap were positioned to the top of the specimen while a rubber membrane was used to enclose the specimen. Eventually, O-rings secured the membrane at the top cap and pedestal. After saturation process, the specimen was isotropically consolidated and the consolidation was stopped when pore-water volume change reached equilibrium. Pore-water, pore-air (\(u_{a}\)), volume changes and cell pressures were set and controlled by Digital Pressure and Volume Controller. Next, the shearing stage was started with a strain rate of 0.0009 mm/min, as suggested by [PERSON] et al. [43], in order to ensure excess pore-water pressure was zero, which implied that the pore-air and pore-water pressures remained the same as those before the shearing.