Study on the mechanical properties and permeability evolution of sandstone under cyclic loading and unloading stress-seepage coupling

This study investigates the evolution of sandstone seepage characteristics under coupled multi-field effects in deep rock engineering. Specifically, cyclic loading-unloading stress-seepage coupling tests were conducted on sandstone specimens using a French triaxial multi-field coupling test system. Through systematically controlling confining pressure and hydraulic pressure conditions, the dynamic response mechanisms of sandstone's mechanical properties and permeability under complex stress paths were revealed. Experimental results indicated that, at a constant water pressure of 1 MPa, increasing the confining pressure from 10 to 12 MPa resulted in an elevation of peak stress from 59.5 to 64.3 MPa, constituting an 8.1% increase. This elevation in confining pressure significantly enhanced the sandstone's compressive strength and stiffness, simultaneously suppressing volume expansion and microcrack propagation. Conversely, at a constant confining pressure of 10 MPa, an increase in water pressure from 1 to 3 MPa led to a substantial decrease in peak stress from 59.5 to 49.2 MPa, representing a 17.3% reduction. Such elevated water pressure was observed to reduce rock strength by promoting microfracture propagation, potentially triggering a transition from brittle to ductile failure under high water pressure conditions.Regarding permeability, it generally increased with the number of loading-unloading cycles, consistently exhibiting lower values during the loading phase compared to the unloading phase. Illustratively, during the second cycle at 10 MPa confining pressure, when axial pressure reached 60 MPa, the permeability during loading was measured at 73.1 × 10^-18 m², subsequently increasing to 120.4 × 10^-18 m² during unloading. Moreover, the relative influence of confining pressure and hydraulic pressure dynamically shifted across cycles: hydraulic pressure exerted a dominant effect initially, whereas confining pressure emerged as the primary controlling factor in later stages. Complementary numerical simulations, when compared with experimental data, further confirmed that confining pressure effectively slows sandstone seepage velocity under fluid-solid coupling conditions. Collectively, these findings provide critical theoretical support for the safe design and risk mitigation of deep rock engineering projects operating under complex geological conditions involving high stress and high hydraulic pressure. Crucially, they significantly deepen our understanding of the mechanisms governing rock damage accumulation and the evolution of seepage characteristics under multi-field coupling effects.