Researches On The Stability And Deformation Of Foundation Pit Based On Reliability Theory

The purpose of foundation pit’s stability checking and deformation checking is to make an evaluation on the safety of foundation pit’s support design scheme to avoid the accident due to improper design. At present, that the design accounts for 30%~45% of foundation pit accidents causes shows that the existing stability and deformation checking method has some shortcomings. In this paper, reliability theory was used in the checking calculation of foundation pit’s stability and deformation, the variability of soil, support structure and other factors were fully considered, and the calculation method of foundation pit’s stability and deformation reliability was researched. The main contents and conclusion are as follows:(1)Three kinds of reliability’s expression were analyzed: reliability probability, failure probability, reliability index; three kinds of uncertain factors’ dealing method were discussed: χ2 test method, K-S test method, A2 test method; several kinds of reliability’s calculation methods were concluded: A two order moment method, Monte Carlo method, stochastic finite element method, optimization algorithm; and the system reliability’s calculation method was researched.(2)The calculation method of foundation pit’s stability reliability was studied: on the basis of limit balance method, the difference of foundation pit’s resistance effect and load effect was regarded as the reliability function and the probability that reliability function is bigger than zero is the stable reliability. Due to the different force of different support structures, on the basis of stress analysis, soil-nailed wall’s whole stability function, anti-sliding function, anti-overturning function, cantilever supporting structure’s anti-overturning function, pile strut bracing structure’s anti-overturning function, gravity shoring structure’s anti-overturning function, anti-sliding function were established; each layer soil’s cohesion and internal friction angle were considered as random variables, and a two order moment method were chosen to calculate the stability reliability, according to the characteristics of function.(3)The calculation method of foundation pit’s deformation reliability was studied: according to the reliability theory, it proposed that the difference of foundation pit’s deep soil displacement, bottom heave, peripheral subsidence value’s warning value and the corresponding actual maximum deformation value were regarded as function and the probability that reliability function is bigger than zero is the deformation reliability. As to the characteristics that deformation is hard to be expressed with explicit equation, The stochastic finite element method was chosen to calculate the deformation reliability; Each layer soil’s cohesion and internal friction angle were considered as random variables, and the maximum value of deep soil displacement, bottom heave, and peripheral subsidence value were considered as the output random variable. The way to calculate deformation reliability using ANSYS-PDS module was studied.(4)The foundation pit’s stability and deformation evaluation method based on reliability theory was applied to a foundation pit engineering whose support structure is two support and piles, to verify this method’s applicability. On the base of force analysis, the anti-overturning function was established, a two order moment method was chosen to calculate the stability reliability, and it compiled the corresponding calculation function with MATLAB; then, it compiled the analysis file of foundation excavation with APDL, used the PDS to calculate the deep soil displacement, bottom heave, peripheral subsidence’s reliability, and calculate the deformation reliability according to system reliability theory. The result showed that both the stability reliability and the deformation reliability were bigger than the target reliability, which means the foundation pit is safe, and it’s consistent with the reality. The research proved the foundation pit’s stability and deformation evaluation method based on reliability theory is applicable.