## 1 Introduction
Therefore, laboratory direct shear tests are more suitable for studying the strength and deformation characteristics of SRM. However, there has been less research on the shear deformation behavior of SRM by laboratory tests, particularly on the formation and evolution of the shear zone of SRM during the shearing  
Figure 1: Sand–pebble mixture slope along national highway 318 in Xigaze, Tibet. process. Traditional methods such as strain gauges and CT are difficult to apply for large-scale discrete materials like SRM. Owing to the limitations of the test size and observation of the rock block movement, numerical simulations are popular methods. The discrete element method (DEM) was used to investigate the effects of the stone content, stone strength, and stone shape on SRM strength by introducing a shape factor to form a polygonal fracture module [42]. DEM was also used to analyze shear banding in shear tests [43]. Particle flow code (PFC) models were used to simulate stress-strain relationships in SRM biaxial compression tests [44] and shear tests with large dimensions of \(500\times 500\times 400\) mm [45]. The finite element method was used to analyze the direct strength of direct shear box tests [46] and the mesostructure and mesomechanical characteristics of SRM [47]. Based on DEM, a 3D random mesostructure modeling system of SRM was developed and used to generate a mesostructural model of SRM to simulate shear stress-strain curves [48]. However, these numerical simulation results are only qualitative or semi-quantitative. Table 1 gives the comparison between the sliding zone thickness and sliding body thickness in landslides. The aim of this study is to investigate the deformation characteristics and failure mode of the shear zone as well as the movement rule for the block stones in SRM from an experimental perspective. This differs from previous studies, which have often focused on changes in the strength properties of SRM. Some deformation studies were conducted using numerical simulations, which largely depended on the selection of the constitutive model, physical parameters, and boundary conditions. Thus, deformation tests of SRM were carried out through large-sized direct shear tests controlled by a motor servo in this study. The deformation characteristics of the shear zone were monitored by placing aluminum wires and dry ash in the specimen holes. Three main structural factors, such as the stone content, overburden vertical pressure, and stone size, were considered to investigate the interaction of the block stones during the deformation process. It is of great importance to study the shear characteristics and deformation evolution of specimens to understand the progressive shear process of the sliding zone and the failure mode of SRM landslides. \begin{table}
\begin{tabular}{c c c c c c c} \hline
**Location** & **Landslide Name** & **Composition** & **Sliding Zone** & **Sliding Body** & **J/W (\%)** & **Reference** \\ \hline Mengla County in Yunnan & H\({}_{1}\) landslide & Crushed stone and clay & 0.4–0.7 & 3.0 & 13.3–23.3 & [49] \\ Ya’an City in Sichuan & Tangonggi landslide & Slity day with pebbles & 0.3–0.5 & 4.0 & 7.5–12.5 & [50] \\ Guisi City in Guangxi & Jiava landslide & Clay, gravel, and weather rock & 0.3–4.0 & 13.3 & 2.3–30.1 & [51] \\ Minjiang in Sichuan & H\({}_{1}\) landslide & Pebibly slip day & 0.1–0.55 & 5.2 & 1–9.10–6.6 & [52] \\ Tonggan County in Shanxi & Fenghuang mountain H\({}_{2}\) landslide & Loess & \(<\)0.5 & 6.5 & \(<\)_7.7_ & [53] \\ Ankang City in Shanxi & Ziyang landslide & Clay with gravel & 0.5–1.0 & 6.0 & 8.3–16.7 & [54] \\ Puge City in Sichuan & Dacao Village landslide & Slit with gravel & 0.4–1.5 & 7.0 & 57–21.4 & [55] \\ Lanzhou City in Gansu & Bao/ensi landslide & Slit, mudstone fragments with & 0.2–1.0 & 16.4 & 1.2–6.1 & [56] \\ Xuanhan County in Sichuan & Panyanazi landslide & Slity day with weatherbed mudstone & 0.5–1.2 & 12.0 & 42–10.0 & [57] \\ \hline \end{tabular}
\end{table}