## 2 Material and Experimentation
### Triaxial Test under Low Confining Pressures  
In the test, the dry density of the soil sample is controlled at 1.2 g/cm\({}^{3}\), and the water contents is set at 4, 8, 12, 16, 20, 24, 28, 32, and 36%, respectively. Based on the soil water characteristic curve (SWCC) measured by the wetting contact filter paper method given in the [PERSON] et al. (2018) test (used the same sample, shown in Figure 2, \(\rho_{\text{d}}\) in the figure represents the dry density of the sample), the suction of the samples in different water content states can be obtained ([PERSON] et al., 2018); the range of suction corresponding to the initial state of the specimen (sample preparation) is 86 to 8248 kPa (corresponding to the water content range from 36 to 4%). The triaxial samples with a diameter and height of 39.1 and 80.0 mm, respectively, are used, and the samples are prepared by implementing the layered compaction method. During the sample preparation process, the prepared wet soil samples with controlled water contents are measured according to the controlled dry density, and they are compacted in eight layers. Each layer is controlled to a height of 10 mm to ensure the uniformity of the sample. The triaxial shear test is conducted by applying GDS (Global Digital Systems Limited) unsaturated triaxial testing of soil (UNSAT), where the volume deformations of the samples are measured by the GDS-HKUST system (Global Digital Systems Limited--Hong Kong University of Science and Technology) ([PERSON] et al., 2018). During the test, a hard plastic film is used to separate the ceramic disk and sample to ensure that  
\begin{table}
\begin{tabular}{l c} \hline \hline  & **Property Index** & **Value** \\ \hline Specific gravity & 2.72 \\ Liquid limit (\%) & 57 \\ Plastic limit (\%) & 38 \\ Plasticity index & 19 \\ Maximum dry density (g/cm\({}^{3}\)) & 1.3 \\ Optimum water content (\%) & 36 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Physical properties of the tested soil.  
Figure 1: Grain-size distribution of sand.  
the test is performed under a constant water content. The top of the sample is connected to the atmosphere through the pore pressure control pipeline, and the pore-air pressure is maintained at 0 (relative atmospheric pressure) during the test. For the samples with different water contents, four sets of triaxial shear tests are performed under different confining pressures set to 0 (unconfined), 25, 50, and 100 kPa, respectively. To simulate the failure characteristics of the soils in a similar shallow environment under a constant water content, combined with the [PERSON] et al. [21] analysis, the confining pressure is applied at a rate of 25 kPa/20 min during the test. Once the confining pressure has reached the target value, it remains stable for 10 min to ensure that the pore-air pressure in the sample can be completely dissipated in time; that is, the compacted sample is fully consolidated and exhausted. Subsequently, the shear test is performed, and the shear rate is set to 0.1%/min during the test [21]. The shear test is terminated when the axial strain of the sample reaches 20%. After the shear test is completed, the sample is removed and its failure form can be observed. The mass change of the sample is weighed, and the water content of the sample is examined layer by layer. The error does not exceed 0.5%.  
Consolidated drained triaxial tests of the saturated samples are also conducted simultaneously for comparison. The dry density and water content are controlled at 1.2 g/cm\({}^{3}\) and 36%, respectively, to prepare the samples in the test. The sample preparation method is the same as that for the unsaturated triaxial test. The tests are performed after vacuum saturation and back pressure saturation. Compared with the unsaturated triaxial tests, two sets of higher confining pressures, 300 and 400 kPa, are added in the saturated state.