## 1 Introduction  
Soil-rock mixture (SRM) refers to a type of uneven loose deposit material mainly composed of stone, fine-grained soil, water, and pores that is between a homogeneous soil and fractured rock mass from the quaternary period [1]. SRM landslides are widely distributed in China. There have been 1736 landslides in the 100 km\({}^{2}\) area within the catchment of the upper Yangtze River, of which approximately 64% were SRM landslides [2]. Slip zones of large landslides in the Three Gorges area are commonly set on fine-grained soil horizons with a significant amount of coarse-grained particles, particularly gravel-sized particles [3]. There are a large number of slopes in a critical stable state composed of sand-pebble mixture (SPM) mainly composed of pebble, sand, partial clay soil, water, and pores, which is a special type of SRM in Tibet, as shown in Figure 1. SRM landslides have produced serious damage to social security and project construction in the midwestern region of China. Thus, it is of great importance to study the deformation and strength mechanisms, the shear deformation process, and disaster prevention in SRM landslides. SRM can also be referred to as bimsoil [4] or block-in-matrix soil, coarse-grained soil [5; 6; 7; 8; 9; 10; 11], granular soil [12; 13; 14; 15; 16; 17], or gravelly soil [18; 19]. Research on coarse-grained soil, granular soil, or gravelly soil emphasizes the mixing state of the coarse and fine grains. In contrast, SRM research emphasizes the material composition and structural characteristics of the soil. The characteristics and difficulties of SRM research include two main aspects. First, SRM is a discontinuous material with a complex composition. Therefore, it is difficult to characterize the mechanical behavior arising from the high heterogeneity and structural randomness of SRM quantitatively. Second, the structural effect of SRM is prominent. As a result, the strength, deformation, and permeability of the SRM are directly affected by its structural state, including the quantity, size [20], arrangement, shape [21], cementation of the block stones, the porosity [12], and the bedrock surface. The factors affecting the deformation and strength characteristics of SRM are extraordinarily complex [22; 23]. To date, SRM studies have mainly concentrated on investigating the strength characteristics experimentally, considering factors such as the stone content, water content, and stone granularity. The effects of the coarse-grained content on the shear strength of SRM have been investigated under saturated and unsaturated conditions [24; 25; 26; 27; 28]. The computed tomography (CT) has been used to obtain information about the cutting surface of SRM to reconstruct a three-dimensional model of gravels [29]. Uniaxial compression tests have been used to obtain the strength and cracking characteristics of SRM by preparing remolded specimens [30; 31]. [PERSON] [32] studied the deformation and strength characteristics of an SRM specimen surface through triaxial tests and a subpixel point identification and measurement system of the sample surface. [PERSON] et al. [33] carried out triaxial tests on three types of SRM samples with the same stone content, but different grain size distributions containing \"oversized rock blocks\" to study the macroscopic mechanical behavior of SRM using CT. Direct shear tests have been used to obtain the strength and dilatancy characteristics of sand-gravel mixtures, coarse-grained soil, or granular soil [34; 35; 36; 37]. A large number of direct shear tests in situ and large-sized shear tests have been carried out to obtain typical stress-strain curves [38; 39]. [PERSON] [40] investigated the strength of SRM with different block rock mass ratios. [PERSON] [41] studied the effects of the block stone content, size, and shape on the engineering properties of compacted silty clay. The distribution and size of block stones in samples in situ are extremely heterogeneous, and thus field shear test data exhibit greater dispersion.