Creep-seepage coupling laws of quartzite under cyclic loading-unloading conditions

The quartzite of Dadongshan Tunnel is taken as the research object to carry out rock creep and seepage coupling experiments under cyclic loading-unloading conditions. The creep characteristics, permeability and seepage-creep coupling mechanism are analyzed. The development laws of volumetric strain in the phase of densification, crack propagation and crack coalescence are acquired. The relationship between permeability and volumetric strain is summarized. It is indicated that the axial loads between 0 and 50 MPa are in compression phase. If the loads increase, the unrecoverable deformation will be generated. When the loads reach up to 160 MPa, the creep curve enters into the accelerating period. With the increase of axial load level, the general tendency of permeability first decreases and then increases, and the minimum value appears at the largest densification points. After the creep process enters into accelerating period, the permeability sharply increases. Using the Cvisc model from FLAC^3D as the basis and by introducing the zone state index (ZSI) into the volumetric strain, the relationship between ZSI and permeability is established. Numerical simulations are conducted to verify experimental results by adopting a nonlinear creep Nishihara model based on the strain softening. The results show that the permeability along with the development of the time in the process of loading near the inlet side of the permeability changes quickly. The damaged area of ZSI contours is coincided basically with the failure mode of the test samples in the "V"-typed shear zone. The theoretical curves are in good agreement with the numerical ones. The proposed model can reflect the creep-seepage coupling characteristics and the local failure rules of rock under loading-unloading conditions.