## 1 Introduction
In particular, interfaces with rock steps have not been a focus of discontinuity research. As a result, their mechanical properties are often overlooked. In this context, there is a scarcity of in-depth studies on how the size of prominent steps affects the mechanical properties of such interfaces, especially regarding the size effect. Hence, it is clear that novel research on step-like band-bedrock interfaces needs to be conducted. Understanding crack propagation during the shear process may help to identify the shear failure path in engineering rock mass. However, due to limitations in test conditions, it is difficult to clearly and accurately record some shear failure processes. In recent years, the development of numerical methods has solved this problem to some extent. The main advantage of the discrete element method (DEM) is to obtain information on the dynamic failure process and crack propagation of the specimen, and to monitor information that cannot be obtained from a variety of laboratory tests. Therefore, the DEM has been widely used in geotechnical engineering when numerical simulation is required. [PERSON] et al. put forward the shear box genesis approach for generating the upper and lower plates in direct shear simulation [25], solving the problem of particle interlocking in the bond removal method. Cement-rock interface shear simulation tests were conducted by [PERSON] et al. [26], and the evolution law of the sample force chain during the shear process was presented. [PERSON] et al. [27] explored the shear behaviour of 3D nonpersistent jointed rock-like specimens using PFC software (Version 5.0) and obtained the structural plane stress variation law, which cannot be observed in experimental testing. The energy evolution process and the change in the crack number of samples with nonpersistent joints were clarified by [PERSON] et al. [28] through simulation. Moreover, [PERSON] et al. [29] summarised the crack types in rock containing nonpersistent joints under uniaxial compression by analysing the particle relative displacement. [PERSON] et al. [30] utilised the discrete element method to simulate the mechanical properties of sandstone under multi-level cyclic loading, revealing that models with higher heterogeneity in tensile strength and cohesion can better reproduce the stress-strain relationship patterns. In addition to reproducing the experimental results, numerical simulation is often used for extended learning due to its convenience. Observational studies have been carried out based on the DEM that examined the effect of size [31; 32; 33; 34], random discrete fracture [35; 36; 37], fracture numbers, and geometry size [38; 39] on the mechanical behaviour of rock or rock-like samples. It follows that the discrete element method may be helpful for the study of the micro-mechanical properties of shear bands in the BDRM. At the same time, the challenge of large workloads in size effect laboratory tests will be effectively solved through numerical simulation. In summary, with the help of existing survey data and related references on the BDRM, a series of laboratory experiments and numerical simulations were carried out on the shear mechanical behaviour of the band-bedrock interface generalised from the BDRM. This paper is structured as follows. In Section 2, the geological conditions of the BDRM are briefly described. In Section 3, the establishment and schemes of the experiment and numerical simulation tests are introduced. Then, in Section 4, the shear coalescence mechanism, stress evolution, and crack-type characteristics of the band-bedrock interface are analysed. The effect of the sample and step geometry size on the shear mechanical properties of the band-bedrock interface is discussed in Section 5. Finally, the conclusions are presented in Section 6. It is hoped that this paper may provide some help in understanding the instability and failure mechanisms of the BDRM, contributing to the mitigation of geohazards in regions such as the Three Gorges Reservoir area.