Study On Back Analysis Method Of Soil Parameter Uncertainty In Excavation Deformation Calculation Of Foundation Pit

Due to the rapid development of urban construction,the demand for underground space has become stronger,and the number and scale of foundation pit projects have continued to expand.Foundation pit projects in urban areas are often in a complex and densely built environment.Excavation and construction of foundation pits are subject to more stringent restrictions.A little carelessness may cause instability or damage to the foundation pit.Predict and evaluate the deformation of the foundation pit excavation.The response is of great significance to the safety and stability of foundation pit engineering.Among them,the uncertainty and variability of soil parameters are one of the important factors in the reasonable prediction and evaluation of the deformation response of foundation pit excavation.This paper relies on the foundation pit project constructed by the top-down construction method of the four lanes in Qinhuai District,Nanjing.The inverse analysis method based on Bayesian theory is feasible and effective in reducing the uncertainty of soil parameters and improving the accuracy of deformation prediction,and combined with BIM information technology to explore its use in foundation pit visualization,safety assessment and dynamic early warning.The application and advantages.The main content and conclusions of this paper are as follows:(1)Based on the key issues of foundation pit construction based on the top-down construction method,the finite element software ABAQUS is used to conduct threedimensional numerical simulation of the top-down construction method foundation pit excavation,and the elastoplastic constitutive model is used to study the vertical support structure,the settlement of the soil behind the wall,and the uplift of the bottom of the pit.And the deformation law of differential settlement in each stage of excavation and construction.The results show that the support structure,soil settlement,and pit bottom uplift deformation gradually increase with the excavation of the foundation pit,and the ratio of the maximum lateral displacement of the vertical support structure to the excavation depth is 0.4%;due to the three-dimensional effect of the foundation pit,The displacement of the support at the corner is significantly smaller than the displacement at the non-corner.The settlement of the soil behind the wall mainly presents a concave curve,and the influence range is about 4 times the depth of the soil excavation;the ratio of the maximum differential settlement between the vertical support structure and the column to the maximum excavation depth is between 0.10% and 0.20%between.(2)The influence of different soil constitutive parameters and structural parameters on the deformation of foundation pit is analyzed.The results show that there are great uncertainties in the values of soil elastic modulus and Poisson’s ratio.With the increase of soil elastic modulus,the lateral displacement of the wall and the settlement of the soil gradually decrease.With the increase of Poisson’s ratio,the shear strength of the soil decreases,the lateral displacement of the wall and the settlement deformation of the soil increase.The friction coefficient of the contact surface,the stiffness of the wall,the soil strength index and the structural parameters have relatively little influence on the deformation of the foundation pit;Parameter sensitivity analysis provides a theoretical basis for parameter back analysis and optimization.(3)Based on the principle and application of Bayesian inverse analysis method.The finite element proxy model is established through orthogonal experiment and the second-order polynomial response surface method without cross terms,and the Markov Chain Monte Carlo(MCMC)algorithm is used to update the parameter distribution of the model.The results of simultaneous update of a single random parameter and multiple random parameters are compared.At the same time,the influence of each stage of foundation pit excavation on the proxy model error and deformation response prediction is studied.The results show that the Bayesian framework can effectively integrate the existing survey information,and based on more effective prior information,the uncertainty of the parameter distribution can be further reduced.The size of the scaling factor recommended in the MCMC algorithm will affect the efficiency of solving the posterior distribution.When the scaling factor is equal to 1,the Markov chain can quickly form a stable distribution.The model error will also change correspondingly with the excavation of the soil.The updated model error distribution can be used to get a more accurate prediction of the deformation response of the foundation pit.(4)Established a data collection and multi-level early warning mechanism based on lightweight BIM technology,and explored the application of BIM technology in foundation pit engineering.On the basis of the BIM model,the conversion method between the model data format and the finite element and WEB system is analyzed.Based on the results of the inverse analysis,it is proposed to study the safety assessment and response prediction of foundation pits with the characterization of failure probability.The results show that: BIM technology effectively solves the problem of three-dimensional visualization of foundation pits;compared with traditional warning methods,the safety assessment method using probabilistic representation takes into account the uncertainty inside the foundation pit.Respond promptly to the risk of foundation pit deformation;as the depth of foundation pit excavation and uncertainty factors increase,the probability of failure of foundation pit deformation increases.