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

The periodic fluctuations of reservoir water level will cause the soil in the hydro-fluctuation belt to upward-downward seepage and dry-wet cycles. The soil structure damage induced by erosion and dry cracking is the key cause of soil erosion in the reservoir area. To investigate the evolution process and synergistic effect of suffusion and dry cracking, a self-developed simulation experimental system was used to perform multiple cycles of upward-downward seepage and dry wet cycles separately, as well as their alternating combined effects, for poorly graded soils. The rules of fine particle loss in the soil and the development process of surface cracks were monitored. According to the test results, the influence of surface dry cracking on suffusion and the influence of suffusion on dry shrinkage cracking were analyzed, and the synergistic effect of suffusion and dry cracking on soil structure damage was summarized. The results showed that under the individual action of upward-downward seepage, the development of suffusion channels in soil exhibited randomness and persistence. Multiple periodic upward-downward seepage would wash away the originally blocked or deposited fine particles and exhibit a "repeated washing" effect, resulting in a stronger degree of particle loss than the persistence of one-way seepage. The essence of suffusion damage is the change in the overall pore structure of the soil. Under the single action of dry-wet cycles, the development of soil surface cracks showed memory and gradual increase. The development of cracks mainly occurs when new cracks occur at old cracks, accompanied by a slow increase in width and gradual blunting of openings. The loss ratio of fine particles in the soil and the ratio of soil surface cracks both increase with the number of cycles and gradually stabilize. The essence of cracking damage is the destruction of soil integrity. Under the alternating combined action of the two, suffusion in soil will forming seepage holes and disrupting soil continuity, thereby increase the fine particles content of surface layer, leading to intensified crack development. Soil surface cracking will also accelerate the detachment of fine particles, increase the expansion of dominant channels, and improve the particle loss ratio in subsequent cycles. The location and quantity of crack development are affected by the non-uniform distribution of fine particles on the soil surface, resulting in a crack ratio that only stabilizes after three cycles. At the same time, the presence of cracks also expands 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, reflecting the coupling characteristics of suffusion and shrinkage cracking. After five cycles, the cumulative particle loss ratio under the combined action is 127.16% higher than that under the single action of upward-downward seepage, and the soil surface crack ratio is 74.66% higher than that under the single action of dry-wet cycle, which has a significant synergistic effect on soil structure damage. The research results have important reference value for deepening the understanding of the mechanism of soil structure damage in the hydro-fluctuation belt, revealing the evolution laws of soil erosion and shoreline retreat, and providing important theoretical support for soil erosion prevention and ecological governance in the Three Gorges Reservoir.