Engineering geology properties and creeping strength characteristics of the silty mudstone in Gongjue County in Tibet of China

The Triassic silty mudstone is encountered in the Sichuan-Tibet Railway, when it passes through Gongjue County in Tibet of Chian, and the rock is prone to cause large deformation under the condition of high geo-stress. In this study, we carried out the triaxial rock compression and triaxial creep tests under different confining pressures. Combined with the PFC numerical simulation, we have also studied the creeping characteristics and long-term strength of the silty mudstone under different confining pressures. The results show that the rheology of the Gongjue silty mudstone is characterized by the Nishihara creep model. With the continuous increase of the confining pressure, under the conditions of the creep and conventional triaxial tests, the silty mudstone specimens all undergo transition from tensile-shear failure to single-shear failure. The angle between the shear fracture surface and the horizontal line gradually decreases, and the number of microcracks decreases. Compared with the conventional triaxial test, the creep test has a wider range of influence on the crush zone caused by tensile stress. Under the conditions of high confining pressure, the silty mudstone is more prone to rheology. The numerical simulation results show that when the confining pressure increases, the axial strain, lateral strain and volumetric strain will increase, while the number of microcracks decrease. There is a linear increasing relationship between the instantaneous elastic modulus and the viscoelastic coefficient with the confining pressure, the viscoelastic modulus has a logarithmic growth relationship with the confining pressure, and the viscoplastic coefficient has an exponential growth relationship with the confining pressure. Under long-term loading, the long-term strength phase of the rock is lower than the instantaneous strength.