FIGURE 2 Mortar block sample preparation process (a) Material preparation, (b) Mixing, (c) Pouring into the mold and vibrating/smoothing, (d) Curing, (e) Demolding, (f) Measuring dimensions

2.3 Interfacial sample

After naturally air-drying the soil samples collected on site and passing them through a 2mm sieve, they were thoroughly mixed to ensure homogeneity. The initial water content was determined using the oven-drying method, and then a certain amount of water was added to achieve wet soil samples with water contents of 10%, 13%, and 16%. These samples were placed in sealed bags and allowed to stand for 24 hours to ensure uniform water content distribution. A mortar block was placed on the lower side of the sample mold, and wet soil with a compaction of 95% was weighed and compacted in three layers into the mold until level with the top edge of the mold box. The size of the test sample was 100mm×100mm×50mm (Figure 3a). The test sample consisted of two layers, with the mortar block being 25mm tall, and the soil layer on top and the mortar block below. This configuration placed the shearing interface exactly in the middle and aligned with the upper and lower shear box interfaces of the direct shear apparatus, enabling better study of the mechanical properties of the interface during the shearing process. After preparing the direct shear samples, they were wrapped in plastic wrap, and the bottom and sides were further wrapped with insulating material and placed in a constant temperature chamber (Figure 3b). The temperature of the chamber was adjusted to the target test temperature and maintained for 24 hours to complete the freezing of the direct shear samples.