Experimental Study on Pore Structure Evolution of Unloaded Rock Mass during Excavation of Reservoir Slope under Dry–Wet Cycle

There is a long sequence of periodic characteristics of reservoir water storage and discharge in large hydropower stations. The unloaded rock mass formed by blasting and excavation in the reservoir slope of the reservoir fluctuation zone is not only subjected to the penetration erosion caused by the change of the water level of the reservoir slope, but also the dry–wet cycle caused by the reservoir water storage and discharge. There is an obvious process of crack derivation and pore structure expansion, and the subsequent strength degradation breeds reservoir slope risks, which is one of the important factors restricting the operation safety of power stations. To study the pore structure evolution law of unloaded rock mass in reservoir slope excavation of reservoir fluctuation zones, the dry–wet cycle test simulating the periodic storage and discharge environment was carried out with samples of equal unloading amount obtained by indoor triaxial unloading test. The variation law of mesoscopic parameters such as wave velocity, mass, and nuclear magnetic resonance spectrum under dry–wet cycle was compared and analyzed, and the physical and mechanical mechanism of the pore structure evolution of the unloaded specimen under dry–wet cycles was explored. The results show that: (1) With the increase of dry–wet cycles, the evolution of wave velocity and dry mass of unloaded samples has obvious stage characteristics, which generally presents a rapid change in the early stage, moderate in the middle stage, and gradually stable in the late stage; (2) nuclear magnetic resonance (NMR) shows that the number of macropore structures in unloaded samples increases gradually with the dry–wet cycles; (3) the smaller the initial confining pressure, the larger the first peak area and the peak value of unloaded samples, and the spectral area corresponding to each peak under low confining pressure is significantly larger than that under medium and high confining pressure; (4) the unloading amount affects the overall proportion of macropores in the sample, which determines the deterioration process and evolution law of the mesostructure of the sample under dry–wet cycles.