Study on the seepage characteristics and degradation mechanism of a single-jointed sandstone under the cyclic dry–wet process in the Three Gorges reservoir

The hydro-fluctuation belt is the most sensitive part of the reservoir bank slope. The rock mass in the hydro-fluctuation belt has been affected by the cyclic dry–wet process. The water infiltrates into the rock mass through its pores and cracks. Through time, the cyclic dry–wet process is posing a significant impact on the evolution and seepage characteristics of jointed sandstone. In this paper, a series of tests such as seepage test, scanning test of the joint surface morphology, and measurement test of ion concentration were conducted to investigate the impact of the cyclic dry–wet condition on a single-jointed sandstone. The following conclusions were made based on the experimental results: (1) There is a positive correlation between the hydraulic conductivity (k_f) and the number of dry–wet cycles (n). Initially, the trend was steep. Gradually, it reached a plateau. (2) The fractal dimension D was slightly decreasing when the number of dry–wet cycles increased. A considerable drop in the fractal dimension was observed within the first eighth dry–wet cycles. (3) A negative correlation between the hydraulic conductivity and the fractal dimension was established by the logistic model with four parameters. (4) On the one hand, calcareous cement was dissolved and disintegrated by the water. On the other hand, feldspar minerals underwent physical and chemical reactions. In the meantime, the water–rock interaction altered the ion concentrations, microstructures, and the permeability of sandstone. The findings of this paper provide a theoretical basis for the stabilization of the reservoir bank slope.