A failure criterion for foliation and its application for strength estimation of foliated metamorphic rock

Foliation is a ubiquitous, sheet-like planar structure formed by differential pressure during the regional metamorphism history. Foliation causes metamorphic rocks to exhibit high anisotropy and heterogeneity, which strongly influence the mechanical properties. This study samples three types of slate and two types of schist to investigate the mechanical properties of foliation. A series of pull-off tests and direct shear tests are conducted on the foliations to obtain the tensile and shear strengths, and a failure criterion for foliation is proposed. The proposed foliation failure criterion exhibits a nonlinear trend in the low normal stress range, considers both tensile and shear strengths, and has three material parameters. The proposed criterion is extended to establish a failure criterion for foliated metamorphic rocks, considering various types of rocks under different orientation angles and confining pressures. For the foliated metamorphic rock failure criterion, a total of eight material parameters are required, which can be determined using the foliation pull-off tests, direct shear tests, and triaxial tests. The results show that the proposed criterion can accurately describe the failure envelope of a single foliation on various anisotropic rock, and its application can reasonably predict the strength of metamorphic rock under various conditions.