The bedding interface has a significant influence on the mechanical properties of layered rock. However, the mechanism of this influence on rock strength and failure characteristics under true triaxial stress, is not yet understood. In this study, the numerical simulation is applied to investigate the influence of angle and mechanical characteristics of the bedding interface on rock strength and failure characteristics. The results show that when the bedding interface angle is between 45° and 75°, the strength and deformation of layered rock are significantly affected by the bedding interface. Rock failure is mainly caused by the destruction of the bedding interface, and rock strength is mainly influenced by the bedding interface. The stiffness and internal friction angle of the bedding interface are the most important factors affecting the strength and failure characteristics of rock. With the increase of the intermediate principal stress, the strength of the rock increases significantly under true triaxial stress state. The influences of intermediate principal stress on the failure process of layered rocks are different for different bedding interface angles.
Sedimentary rocks with bedding interfaces are ubiquitous in nature. The bedding interfaces in these rocks are mainly composed of clay minerals. There is very little adhesion between mineral grains. Therefore, the strength of the bedding interface is very low, which leads to the anisotropic mechanical properties of layered rocks (Mao and Yang, 2005). The anisotropy of layered rocks presents technical problems with respect to the design and operation of rock engineering, such as civil and petroleum engineering.
Many researchers have investigated various types of layered rocks, such as shale, schist, and slate, to explore their anisotropic strength behavior and failure characteristics (Donath, 1964; Chenevert and Gatlin, 1965; Mclamore and Gray, 1967; Niandou et al., 1997; Ajalloeian and Lashkaripour, 2000; Nasseri et al., 2003; Yong et al., 2006; Mao and Yang, 2005; Cho et al., 2012; Shuai et al. 2015). These tests show that the bedding interface significantly influences the mechanical properties of layered rocks. With increasing bedding interface angle (β, the angle between the normal of the bedding interface and the maximum principal stress), rock strength will first decrease, and then increase. The maximum strength is observed at β = 0° or 90°, and the minimum strength is measured at around β = 60°. Additionally, when β is close to 0° or 90°, the failure of layered rocks is mainly shearing failure of the rock matrix. On the other hand, when β is in the middle of this range, the rocks are mainly characterized by slipping failure of the bedding interface. The experimental test method currently being for layered rocks is the conventional triaxial test. Therefore, the compression strength obtained from the test is based on the assumption that the intermediate principal stress σ2 is equal to the minimum principal stress σ3.