Analysis on soil structural damage induced by synergistic effects of upward-downward seepage and drying-wetting cycle

The periodic fluctuations of reservoir water level can often cause the soil in the hydro-fluctuation belt to the upward-downward seepage and dry-wet cycles. The soil structure damage can also be induced by erosion and dry cracking. This study aims to investigate the evolution and synergistic effect of suffusion and dry cracking during soil erosion in the reservoir area. A simulation experimental system was self-developed to separately perform the multiple cycles of upward-downward seepage and dry wet cycles. Their alternating combined effects were also utilized to obtain the poorly graded soils. A systematic investigation was carried out to monitor the fine particle loss in the soil and the development of surface cracks during simulation. An analysis was also made to explore the influence of surface dry cracking on the suffusion and the influence of suffusion during dry shrinkage cracking. A summary was finally given on the synergistic effect of suffusion and dry cracking on soil structure damage. The results showed that there was the random and persistent development of suffusion channels in soil under the individual action of upward-downward seepage. Multiple periodic upward-downward seepage was used to wash away the originally blocked or deposited fine particles, indicating the "repeated washing" behavior. A stronger degree of particle loss was also found than that in the persistence of one-way seepage. The suffusion damage was attributed to the overall pore structure of the soil. The development of soil surface cracks shared the memory and gradual increase under the single action of dry-wet cycles. Once new cracks occurred at the old cracks, there was a slow increase in the width and gradual blunting of openings. There was an increase in the loss ratio of fine particles in the soil and the ratio of soil surface cracks, as the number of cycles gradually stabilized. The cracking damage was characterized by the destruction of soil integrity. The suffusion in soil was induced to form the seepage holes, and then disrupt the soil continuity under the alternating combined action of the two. Thereby, there was an increase in the content of fine particles in the surface layer, leading to the intensified development of cracks. Soil surface cracking was accelerated to detach the fine particles and the expansion of dominant channels, leading to the high particle loss ratio in subsequent cycles. The location and quantity of crack development were attributed to the non-uniform distribution of fine particles on the soil surface. A crack ratio was only stabilized after three cycles. At the same time, the presence of cracks also expanded the advantageous seepage channels, leading to an increase in the differences between the upper and lower layers of the soil sample and the degree of fine particle loss. As such, there was the coupled suffusion and shrinkage cracking. Furthermore, the cumulative particle loss ratio under the combined action was 116.67% higher than that under the single action of upward-downward seepage after five cycles. The surface crack ratio was 73.33% higher than that under the single action of the dry-wet cycle, indicating the significant synergistic effect on the soil structure damage. The finding can provide a strong reference for the mechanism of soil structure damage in the hydro-fluctuation belt. The evolution of soil erosion and shoreline retreat can also offer important theoretical support to soil erosion prevention and ecological governance in the Three Gorges Reservoir.