Experimental studies on seepage characteristics during the loading and unloading confining pressure of post-peak fractured rock

In order to study the effect of loading and unloading confining pressure on the seepage characteristics of rock fractures,a self-developed temperature-stress-seepage multi-field coupled rock triaxial test system was used to carry out the seepage tests of post-peak fractured rock under the process of loading and unloading confining pressure. The non- linear seepage characteristics under different paths of loading and unloading confining pressure were investigated. To quantify the nonlinear seepage process of fractured rock,the Forchheimer formula was used to fit the relationship be- tween the hydraulic gradient and seepage flow. Dividing the seepage process of post-peak fractured rock into linear Darcy flow stage and nonlinear Darcy flow stage,a nonlinear influence factor E was introduced as the critical point for distinguishing Darcy flow from non-Darcy flow. The relationship between seepage flow and permeability under the same conditions of the process of loading and unloading confining pressure was compared,and the mechanism behind the seepage difference was analyzed. Results indicate that the seepage was mainly shown up as nonlinear characteristics, and the permeability decreased with the increase of the confining pressure during the process of loading confining pres- sure of post-peak fractured rocks. However,under the action of high confining pressure,although the fracture surface will produce a certain degree of compression closure,a small number of tension cracks will gradually expand and con- nect,which may alter the permeability of the post-peak fractured rock. When the confining pressure was unloaded from a higher level and the osmotic pressure of the fracture surface was at a lower state,the permeability of the fracture did not increase but decrease. This result indicates that the fracture surface had irreversible normal deformation under the combined influence of the loading and unloading confining pressure and overflowing scouring,resulting in the gradual decrease in the opening of the fracture surface. When the confining pressure was unloaded to the same level as the loading process,the seepage flow under the osmotic pressure of each level dropped sharply,indicating that there was a significant hysteresis effect on the recovery of the seepage capacity of the fracture surface.