Water and sand leakage disasters are likely to occur during construction in water-rich sand layer areas, resulting in ground collapse. The stress-strain action characteristics of discontinuous graded sand under different internal erosion degrees, and the evolution mechanism of water and sand leakage disasters caused by the internal erosion need to be further explored. Therefore, this paper takes the discontinuous graded sand in a water rich sand layer area in Nanchang City of China as the research object. Considering the influence of different fine particle losses (0, 10%, 20% and 30%) under the internal erosion of sand, the salt solution method is used to realize the specified loss of fine particles in the internal erosion. The stress-strain behavior after the loss of fine particles due to internal erosion is studied by triaxial shear test. Meanwhile, the physical model test and PFC-CFD method are both used to study the evolution rules of water and sand leakage disaster considered the influence of internal erosion degrees. Results show that: (1) under the same confining pressure, the peak failure strength of sand samples decreases along with the increase of fine particle loss. (2) In the water and sand leakage test of saturated sand, a natural filter channel is formed above the observed soil arch. The greater the loss of fine particles, the steeper and wider the collapse settlement area. (3) The relationship between the cumulative amount of water and sand leakage and time is nonlinear. The total mass loss of sand increases along with the increase of internal erosion degree. (4) After the soil arch is formed around the damaged opening, the sand continues to converge above the soil arch under the action of water flow, resulting in the dense convergence of contact force chains.

The existence of defects in the enclosure structure is the primary cause of water and sand leakage in foundation pits, as well as being a significant source of danger in pit construction, but current research lacks an in-depth investigation of the generation mechanism and gestation process. In this paper, which comprehensively considers the microscopic particles and macroscopic level, the development mechanism of a water and sand leakage disaster in a foundation pit with a water-rich sand layer was studied using the principle of computational fluid dynamics and discrete element method coupled analysis (CFD-DEM); moreover, based on the anisotropy of the particle force and fluid energy analysis, the deformation of the stratum and ground stress field were analyzed. The results show that the stress field will produce a plugging effect at a certain distance from the defect, and the strata exhibit a dominant displacement tendency in the vertical direction, resulting in the emergence of a gradually concave stress relaxation zone and an elliptical contour in the strata displacement map near the defect. The fluid energy describes the displacement of the sand layer very well, and it is separated into the sand layer's centralized loss region and the major loss area based on the high and low levels of the fluid energy class. The impact of fluid at the defect reaches the maximum kinetic energy, which penetrates the structural weakness and causes the loss of sand particles, and the cross- of the water influx near the defect gradually expands with the loss of particles, indicating that there is a danger of further expansion of the defect under the impact of water flow. These results have technical implications for the management of water and sand leakage disasters in foundation pit engineering.