Deformation and failure mechanism of phyllite under the effects of THM coupling and unloading

Although the study of TM (Thermo -Mechanics), HM (Hydraulic — Mechanics) and THM (Thermo — Hydraulic — Mechanics) coupling under a loading test have been under development, rock failure analysis under THM coupling and unloading is an emerging topic. Based on a high temperature triaxial unloading seep test for phyllite, this paper discusses the deformation and failure mechanism of phyllites under the “H↔M, T→H, T→M” incomplete coupling model with unloading conditions. The results indicate that the elastic modulus and initial permeability decrease and the Poisson’s ratio increases with increasing temperature; the elastic modulus decreases and the Poisson’s ratio and initial permeability increase with increasing water pressure. During the unloading process, rock penetrability is small at the initial elastic deformation phase, but the penetrability increases near the end of the elastic deformation phase; mechanisms involving temperature and water pressure affect penetrability differently. Phyllite failure occurs from the initial thermal damage of the rock materials, splitting and softening (which is caused by pore water pressure), and the pressure difference which is formed from the loading axial pressure and unloading confining pressure. The phyllite failure mechanism is a transtensional (tension — shearing) failure.