Analysis On Stress And Deformation Mechanism And Failure Mode Of Multi-stage Foundation Pit Support

At present,in soft soil areas such as Tianjin,large-area deep foundation pits adopting traditional row piles and internal supports have problems such as high cost,large space occupation,long construction period,and difficult disposal of demolition waste.It is gradually unable to meet the current requirements.Construction and environmental protection requirements have led to rapid development of unsupported support systems.Multi-level support is a form of unsupported support,which adopts corresponding support forms for each level of the multi-level excavation of the foundation pit to achieve the effect of controlling the deformation and stability of the foundation pit.At present,there have been some projects using multi-level support to prove the feasibility of multi-level support in controlling the deformation and stability of foundation pits,but the mechanism of force and deformation of multi-level support,influencing factors,and classification and discrimination methods of failure modes Other theories still lack systematic demonstration and analysis,which makes the application of multi-level support lack sufficient theoretical support.This article first analyzes the field measurement results of the multi-level support foundation pit in Tianjin,obtains the force and deformation mode of the support structure,and uses PLAXIS 2D to numerically simulate the excavation process of the foundation pit to verify the numerical calculation method reliability.A simplified model was established on this basis,and the finite element calculation results of the soil width model between different two-level support structures were compared to analyze the deformation,bending moment,earth pressure of the supporting piles at each level,as well as the displacement field and shear plasticity of the surrounding soil.The growth area changes with the excavation of the foundation pit,the deformation and stress mechanism of the multi-level support is studied,and the index that affects the degree of mutual influence between the multi-level support is proposed.On the basis of the simplified model,the strength reduction method is used to calculate the safety factor of the foundation pit under different supporting structure spacing.Through the analysis of the safety factor and the shear strain increment cloud diagram under the failure state of the foundation pit,the failure mode of the multi-level support It is divided into integral type,related type and separated type damage,and combined with the index of the degree of mutual influence between the multi-level supports mentioned above,a method to distinguish the damage mode is proposed.Subsequent analysis of the influence of the two-level support length L₁,L₂ and the two-level foundation pit excavation depth ratio H₁/H₂ on the deformation of the two-level support using the method of controlled variables,and further proposes corresponding suggestions for the design of the multi-level support and guidance.Finally,in order to optimize the control of multi-level support to the deformation of the foundation pit,the deformation of the multi-level support under different first-level support forms is compared on the basis of the actual engineering numerical model,and the deformation control effect of the foundation pit is better.The analysis of the supporting mechanism and the law of deformation showed that increasing the length of the diagonal pile and increasing the inclination angle of the diagonal pile is beneficial to the diagonal support of the diagonal pile,which reduces the lateral displacement of the supporting pile.But at the same time the maximum positive bending moment of the supporting pile will increase.