Loading and unloading characteristics of mica-quartz schist and constitutive model for its unloading

In order to illuminate the characteristics of transversely isotropic rock under different stress paths, the triaxial compressive full tests and the triaxial unloading tests, elevating axial compression and reducing confining pressure, are carried out on the mica-quartz schist samples from Danba Hydropower Station, which are divided into parallel group and vertical group. The test results show that, for the loading test, the deformation of parallel group is larger than that of the vertical one, and deformation exhibits anisotropic feature. For the vertical group, when damaging, the samples form main cracks firstly and failure surface is connected with secondary cracks, but for the parallel group, failure surface is connected through secondary cracks directly, illustrating different formation ways of failure surface. With the increasing confining pressure, both the peak strength and the residual strength are enhanced, and those of the vertical group are larger than those of the parallel group. Different from those of the triaxial compressive tests, the stress-strain curves of the triaxial unloading tests have obvious drop sections, and the samples show brittle failure. Moreover, the cohesiveness and friction under loading are prominent over those under unloading. Eventually, based on the unloading characteristics of mica-quartz schist, the anisotropic coefficient is used to represent the anisotropic softening during unloading, and a constitutive model for transversely isotropic rock is established. In addition, the rationality of the constitutive model is verified. The research demonstrates that the results of the proposed constitutive model agree with the test data, and it can satisfactorily reflect the unloading characteristics of mica-quartz schist.