## 2 Test Materials and Methods
### Sample Preparation and Experimental Equipment  
Loess is a kind of soil which is sensitive to pressure, water, and vibration, and is easily disturbed. In order to make loess undisturbed as much as possible, we excavated two exploratory wells on the left side of the sliding source. The location of the exploratory well is shown in Figure 2, with a diameter of 0.8 m. Then, large pieces of the loess samples are cut from the side of the well and brought to the ground to be cut. A cylindrical soil block with dimensions of 160 mm \(\times\) 300 mm was cut carefully along the vertical direction from the large pieces using the method of manual trimming. The gap between the sample and the cylinder container is filled with the loose soil left after cutting. Immediately, containers containing the loess samples were sealed to prevent water loss. In order to prevent vibration interference during transportation, a sponge is used between the cylinder containers.  
The soil profiles of the landslide source and deposition are shown in Figure 3 with the corresponding sample points and shear wave velocity (Vs). The top elevation of S1 is 1871 m with 34 m of the soil. Apart from a thickness of 1.6 m for the top cultivated soil layer, a thickness of uniform loess is about 32 m, which is commonly designated as Q\({}_{3}\) loess (Malan loess), based on the formation age and physical properties. The lower part is mudstone, which has low permeability and high strength. The stable groundwater level is 30 m below the ground. We sampled at the depths of 4 m, 6 m, 10 m, 16 m, 20 m, 24 m, and 30 m, respectively, and the number is S04, S06, S10, S16, S20, S24, and S30. The upper part of the S2 is cultivated soil with a thickness of 1.8 m, and the lower part is loess-like soil with a thickness of 35 m. Similarly, the stable groundwater level is 30 m below the ground. The depths of the 2 m, 4 m, 7 m, 10 m, 13 m,18 m, 20 m, 24 m, and 30 m soils were sampled respectively, and the number is D02, D04, D07, D10, D13, D18, D20, D24, and D30. In addition, shear wave velocity tests of two pits were conducted via the signal hole method. The results, as shown in Figure 3, present the shear wave velocity as increasing rapidly from the ground to a depth of 7 m, and the oscillation increases slowly below the depth of 7 m in S1. The average shear wave velocity in the upper 30 m (Vs30) is 306 m/s. And, the shear wave velocity increases from the ground to a depth of 14 m, and the oscillation increases slowly below the depth of 14 m in S12. The average shear wave velocity in the upper 30 m (Vs30) is 288 m/s.  
Figure 2: Sampling locations of undisturbed loess (S1) and landslide deposit soil samples (S2).  
In this study, a series of laboratory tests were conducted. Table 1 shows the summary of laboratory tests and standards for soil properties. Due to the unique macro pore and the sub-stable microstructure of loess, the disturbance may have a serious impact on the mechanical properties of the soil mass. Therefore, the sample preparation should be conducted carefully in strict accordance with the established sample preparation standards to ensure that high-quality loess samples are obtained. The resonant column test adopts the GDS-RCA resonant column test system [29], where the test sample specifications are 50 mm in diameter and 100 mm in height.  
Figure 3: Soil profile of sampling site (Vs values are measured via down-hole tests).