## 1 Introduction  
Tectonic discontinuity is the result of geotectonic movement and is widely distributed in rock mass, which leads to uncertainty regarding the stability of engineering geological bodies [1]. There are special engineering geological conditions in the Three Gorges Reservoir (TGR) area [2; 3; 4; 5]. After many years of geological evolution, a large number of engineering rock masses with discontinuities have been formed. Located in the Wushan section of the TGR area, a special filling discontinuity exists in the Banbiyan dangerous rock mass (BDRM), as shown in Figure 1. The filling of this discontinuity is a shear band composed of rock blocks, which gradually formed in the process of rock mass unloadingand structural movement. The failure mode of the BDRM is most likely to be of the slip type [6]. This kind of failure mainly depends on the material properties and structural form of the fillings. Once the BDRM becomes unstable, the safety of the Yangtze River waterway and surrounding people and property will be threatened. Therefore, further research on the shear mechanical behaviour and failure mechanism of the shear band has great significance for the stability control of the BDRM. As common special structures in natural rock mass, the shear mechanical properties of discontinuities have been widely researched. In the field of shear strength, the bilinear shear strength model was first proposed by [PERSON] [7]. Subsequently, [PERSON] et al. [8] developed the JRC-JCS model of discontinuity shear strength based on a large number of shear laboratory tests. Since then, numerous tests on the shear strength of discontinuities have been carried out, and a variety of peak strength models have been put forward, including the JRC-JMC model that considers the structural plane coincidence coefficient [9], the three-parameter hyperbolic nonlinear failure criterion [10], the strength model based on quantified surface descriptions [11], the peak strength criterion that considers morphology [12], and the brittle drop mode damage-based constitutive model [13]. The shape of a discontinuity is a considerable factor used to determine shear strength and failure, so research examining the shear behaviour of discontinuities with different morphological structural planes has received extensive attention. [PERSON] et al. [14] studied the shear mechanical behaviour of rock joints under different morphologies, and the result showed that the peak shear stress approximately linearly increased and the peak shear displacement decreased with the increase in first-order asperities. According to an anisotropic plaster shear test, [PERSON] et al. [15] found that with an increment of the inclination angle of the critical waviness, both the normal displacement and sheared-off asperity mass increased, following power law functions. [PERSON] et al. [16] conducted a sandstone discontinuity shear test under water deterioration and determined that the chemical action of water reduces the structural plane roughness and leads to a decrease in the shear strength. [PERSON] et al. [17] focused on the failure characteristics of rock-like samples with different geometric steps, and the influence of normal stress and step height on peak strength was identified. In addition to the above research, morphology characteristics [18; 19; 20; 21] and lithological differences between upper and lower plates [22; 23; 24] were also considered during research involving discontinuity shear testing. From the aforementioned studies, it is evident that research on the shear mechanical behaviour of discontinuities has primarily focused on joints with natural rough  
Figure 1: Geographical location and characteristics of shear band in Banbiyan dangerous rock mass. ness and rock bridges between joints, as these represent common models of discontinuities found in nature. However, special discontinuities such as the shear band in the Banbiyan dangerous rock mass, which are formed due to unique tectonic movements, are rarely encountered in either engineering or research.