Effect of Bedding Angle and Mineral Composition on Mechanical Properties and Fracture Behavior of Phyllite Under Unloading Confining Pressures

Studying the mechanical properties of rock mass plays a critical role in reducing the frequency of accidents such as large deformation and rock bursts that often occur in tunnels. In this study, a series of phyllites with different bedding structures and mineral compositions were drilled from different tunnels firstly, then the effects of different bedding angles and mineral compositions on the mechanical properties and failure modes of phyllites under unloading confining pressure conditions were studied. The results show that the deformation characteristics of phyllite have complex relations with confining pressure, bedding angle and mineral composition, and the influence of other parameters cannot be ignored to quantitatively analyze the influence of a certain parameter. The strength of phyllite shows an increases trend as the increase of confining pressure, while the strength shows a trend of first decreases and then increases as the increase of bedding angle. Under the same confining pressure and bedding angle, the strength of phyllite with more quartz is generally the largest, followed by phyllite with more chlorite. With the constant release of confining pressure (unloading), the elastic modulus of phyllite decreases and the Poisson's ratio increases, and the rate of decrease and increase varies with the tunnel bedding angle and mineral composition. The bedding angle, mineral composition and confining pressure have great influence on the unloading failure mode of phyllite, but the influence degree is different. The bedding angle has the greatest influence on the failure mode of phyllite, followed by the mineral composition, and finally the mechanical environment.