## 6 Conclusions  
1. The main reasons for foundation pit inrush accidents are that the water-resisting curtain is not fully extended into the aquiclude and that the aquiclude at the bottom of the elevator shaft is too thin. The maximum seepage velocity at the defect position is 40 times greater than that at the other positions because of the insufficient length of the water-resisting curtain, which seriously threatens the safety of the foundation pit. The water-resisting layer at the bottom of the elevator shaft is thin. After the steel sheet pile is removed, the seepage path is rapidly shortened, and the seepage velocity is increased by approximately 120%. Groundwater can easily break through the water-resisting layer and cushion, and inrush accidents can occur. 2. With increasing excavation depth of the foundation pit, the difference between the head pressures inside and outside the pit increases, and the direction, velocity and flow of groundwater seepage strongly change. When the excavation depth is 9.2 m, the seepage direction of the water-resisting curtain defect changes to seepage inside the foundation pit, and the seepage velocity changes from 6.4 mm/day outside the pit to 78.8 mm/day inside the pit, posing a threat to the safety of the foundation pit. 3. The construction quality defects of the three-axis mixing method pile in the silty sand layer at depths of 9-12 m on the east and south sides of the foundation pit cause the maximum horizontal displacement position of the supporting structure to shift downwards. When an inrush accident occurs, the horizontal displacement near the defect increases by 63%. In the emergency disposal of sudden surge accidents, an unreasonable pumping sequence further causes a large amount of soil erosion, resulting in the collapse of the bottom plate, destruction of the sidewall and other damage. 4. The piled soil in the pit reduces the maximum deformation near this position by approximately 33.2%, and the seepage velocity at the bottom of the lower pile decreases by approximately 9.8%, leading to the effect of back pressure soil. Owing to the long distance, the displacement of the elevator shaft is reduced by only approximately 6.6%, and the increase in seepage velocity is less than 1%. The piled soil did not have a significant effect on accidents but did exert a significant reinforcement effect on the supporting structure and the lower soil, limiting groundwater seepage to some extent. 5. The waterproof curtain of the foundation pit should be fully extended to the water-resisting layer to ensure that the confined water inside and outside the foundation pit is separated. In the thicker fine soil granular soil layer, the construction quality of the three-axis mixing pile should be strictly controlled, and the lap length should be appropriately increased. Giving full play to the back pressure effect of excavation earthwork on the construction site can effectively improve the stability of foundation pit. Figure 17: Comparison of the displacement and seepage flow in the elevator shaft: (**a**) displacement comparison; (**b**) seepage flow comparison. Future research can be carried out on fine control measures for inrush accidents and the application of back pressure soil in deep foundation pits in tropical water-rich sandy strata. Conceptualization, P.L. and J.H. ; methodology, P.L. ; software, Z.S. ; validation, Z.S. ; formal analysis, Z.S. ; investigation, P.L. and Y.W. ; resources, P.L. and Y.W. ; data curation, Z.S. ; writing--original draft preparation, Z.S. ; writing--review and editing, P.L. and J.H. ; visualization, Z.S. ; supervision, J.H. ; project administration, J.H. and Y.W. ; funding acquisition, J.H. All authors have read and agreed to the published version of the manuscript. This research was funded by Hainan Provincial Natural Science Foundation of China (Grant number 623 RC451), Technology Project of Hainan Exploration and Design Institute of Non-Ferrous Engineering (Grant number HD-KYH-2024321). Not applicable.