Experimental study on the mechanical and permeability behaviors of limestone under hydro-mechanical-coupled conditions

In situ stress and underground water are known to significantly influence the deformation and stability of deep underground caverns. These factors should be considered in the design of excavation and supporting schemes. However, the rock failure mechanism and permeability characteristics under the coupling condition remain largely unexplained. Based on the Xianglushan diversion tunnel in Central-Yunnan Water Diversion Project, we conducted triaxial compression experiments on both intact rock specimens and cracked rock specimens with three confining pressures (11 MPa, 18 MPa, and 25 MPa) and three osmotic pressures (2 MPa, 6 MPa, and 10 MPa). The experimental results show that rock strength can be expressed as a function of confining pressure and osmotic pressure. Osmotic pressure can reduce rock strength, while the effect of confining pressure is opposite. With the increase of osmotic pressure, the initial permeability and the peak permeability increase. Compared with the stress-strain curve, the variation of permeability-strain curve is “hysteretic.” For cracked specimens, the stress-strain curve after failure presents a “step down” trend, and the permeability-strain curve appears a “bimodal shape.” The failure pattern changes from shear failure to tensile failure as the osmotic pressure increases. With the increase of osmotic pressure, the crack increases gradually and becomes tortuous and rough. The generation and propagation of cracks gradually develop towards vertical load.