## 1 Introduction  
Discontinuous interfaces are widely distributed in all types of geological environments, including rock mass structural planes, bedding, faults, and slip zones, and are the main factor controlling the properties of rock that contains cohesion and friction angle. The discontinuous interfaces are inevitably encountered in engineering because of its wide distribution and properties. The anisotropy of a rock mass has always been an important and difficult issue in rock and soil research. Most research on the anisotropy of interface strength focuses on the discontinuity. The anisotropy of rock and soil mass is mainly controlled by the orientation of the interface, and the anisotropy can be effectively characterized by the strength of the rock and soil mass at interfaces with different dip angles [1; 2; 3; 4]. Furthermore, as a typical engineering geological model, rock and soil mass contact in tunnel engineering has a more obvious response to anisotropy because of the differences between the two materials which contain the morphology of the interface and the plasticity behavior. The interfaces affecting tunnel engineering mainly include the bedding, structural surface, and sliding zone. As a particular type of interface, the slide belt is divided into the primary slide belt and secondary slide belt. The primary slip zone mainly includes the rock mass structural plane, bedding, and soil-rock interface, and is closely related to the topographic structure. The secondary slip zone is the interface formed under the action of human engineering activitiesor geological forces, and the slide band is curved into the soil and straight into the rock. When the tunnel passes through an interface with different orientations, different forms of deformation and failure may occur, resulting in the instability of the surrounding rock and a series of failures in the tunnel chamber [5]. The tunnel deformation is closely related to the movement mode of the interface, which is controlled by the tunnel's orientation. Therefore, the orientation of the strata is an important factor affecting stability (Figure 1). Presently, the methods used in tunnel-landslide system research mainly include numerical simulation [6; 7; 8], field monitoring [9; 10; 11], and laboratory testing [12; 13; 14]. Subsequently, the influence of landslides on tunnels was investigated through a series of mechanical analyses [15; 16]. This is a very broad research area, and the spatial location that includes the landslide and tunnel, the stratigraphic structure and lithology, the landslide starting mechanism, and the tunnel support are important considerations with wide coverage by many studies. Many factors, such as the rock and soil characteristics, shape of the interface, water content state, and stress history, affect the strength of the interface. From the perspective of influencing factors, the interface is actually a special structural surface of a rock mass. Therefore, the study on the mechanical properties of the structural surface is also applicable to the interface; the difference lies in that there are different materials on both sides of the interface. [PERSON] proposed the shear failure condition of the structural plane to investigate the strength characteristics [17]. [PERSON] investigated the strength characteristics of the structural plane based on the earliest Mohr-Coulomb strength criterion, and proposed the rock mass structure strength criterion [18]. [PERSON] proposed the concept of structural plane roughness based on [PERSON]'s results [19]. The structural plane empirical strength criterion was proposed based on the strength of the sidewall of the structural plane and the friction angle of the rock. The strength criterion was constantly supplemented and improved [20; 21; 22; 23; 24; 25; 26] in the next years. Subsequent studies on the structural plane strength were mostly based on the empirical strength criterion proposed by [PERSON] [27; 28; 29; 30].