Hydromechanical coupling tests for mechanical and permeability characteristics of fractured limestone in complete stress-strain process

To clarify mechanical and permeability characteristics of fractured limestone in complete stress-strain process, the hydromechanical coupling tests with various differential water pressures and confining pressures were performed. The mechanical characteristics of fractured limestone specimens are sensitive to confining pressure, differential water pressure, and effective stress. The increasing differential water pressure weakens the rock strength and deformation modulus by activating the lateral deformation of fractured limestone, which is attributed to the decrease in the effective minimum principal stress. The experimental results verify the validity of Mohr-Coulomb yield criterion considering the effective stress effect under hydromechanical coupling condition. The permeability values display four stages of decrease-gradual increaserapid increase-small drop in complete stress-strain process, which roughly correspond to volumetric compression stage, elastic deformation stage, yield, and post-peak stage, as well as residual strength stage, respectively. At a low differential water pressure in the range of 2-5 MPa, the corresponding relationship mentioned above is obvious. However, at high differential water pressures up to 8-14 MPa, there is a deviation from the correspondence above, i.e., permeability reduction stage is shorter than the stage of volumetric compression. A cubic polynomial is used to describe the relationship between permeability and volumetric strain at volumetric compression stage. However, it is difficult to describe the relationship between the permeability and volumetric strain by a uniform fitting equation at the dilatancy stage.