Study On The Sudden Damage Mechanism Of Foundation Pit In Confined Water Strata And The Thickness Of Safe Bottom Plat

In recent years,with the rapid development of urban construction,various types of deep excavation projects have increased in coastal cities and in cities with widely distributed overburden water-bearing strata.In the overburden water-bearing strata,when the gravity of the soil layer above the impermeable layer is less than the pressure of the overburden water,water and sand will flow into the bottom of the excavation.Therefore,it is of great practical significance to determine the thickness of the safe bottom plate of the overburden water-bearing strata and to study the mechanism of excavation breakthrough in the deep excavation projects.Based on a typical deep excavation project and using the FLAC 3D finite difference software,20 sets of comparative schemes were designed under the influence of four factors,including the distance from the bottom of the enclosure structure to the overburden water-bearing strata,overburden water pressure,cohesion,and internal friction angle.The stability of the overburden water-bearing strata in the excavation was studied,and a 2D and3 D model for excavation breakthrough was constructed based on the limit analysis theory and the principle of virtual work.A calculation formula for the critical safe water-resistant layer thickness of the bottom plate of the excavation under the nonlinear M-C failure criterion was derived,and the influence of different mechanical factors on the critical safe water-resistant layer thickness was discussed.The specific research contents are as follows:(1)The stability of the excavation was studied under the influence of four factors,including the distance from the bottom of the enclosure structure to the overburden water-bearing strata,overburden water pressure,cohesion,and internal friction angle,using the FLAC 3D finite difference software.The results showed that the plastic zone at the bottom of the excavation had a "trapezoidal" distribution,which was consistent with the 2D and 3D models for excavation breakthrough.The area of the plastic zone at the bottom of the excavation was positively correlated with the overburden water pressure and negatively correlated with the internal friction angle,cohesion,and the distance from the bottom of the enclosure structure to the overburden water-bearing strata.The middle position of the bottom plate of the excavation in the overburden water-bearing strata was the area with the largest uplift displacement,and the uplift displacement gradually decreased from the middle position to both sides.(2)For the two-dimensional strip foundation pit,based on the limit analysis theory and the linear M-C failure criterion,the formula for calculating the critical safe water-resisting layer thickness of the foundation pit floor is obtained,and compared with the simulated numerical results and other literature results,the conclusion is It shows that the calculation results of this paper are between other research results,which proves the rationality of this paper’s theory.At the same time,based on the nonlinear M-C failure criterion,the influence of different foundation pit sizes,different confined water pressures,different cohesion forces and different soil layer weights on the thickness of the critical safe water-resisting layer of the foundation pit floor is revealed.The results show that as the size of the foundation pit and the pressure of the confined water increase,the thickness of the critical safety water-resisting layer of the foundation pit floor gradually increases;As the coefficient increases,the required thickness of the critical safety water-resisting layer on the foundation pit floor gradually decreases.(3)For the three-dimensional circular foundation pit,based on the linear M-C failure criterion and the nonlinear M-C failure criterion,the calculation formulas for the critical safe water-resisting layer thickness of the foundation pit floor were obtained,revealing that different foundation pit sizes and different confined water pressures,Different cohesion and different soil layer weights,different dilation coefficients and different nonlinear coefficients on the thickness of the critical safety water-resisting layer of the foundation pit floor.As the size of the foundation pit and the pressure of the confined water increase,the thickness of the critical safety water-resisting layer on the bottom of the foundation pit gradually increases;The thickness required for the critical safety water-resisting layer of the bottom plate is gradually reduced.Some theoretical references are provided for the prediction of the safety floor thickness of foundation pits in confined water formations.