Study On Modified MSD Method And Parameters Analysis For Deformation Of Supported Foundation Pit

With the development and utilization of underground space entering a period of vigorous development,the basements of high-rise buildings,subways,highway tunnels,underground commercial streets,and common trenches have produced a large number of "deep,long,large" foundation pits and asymmetric foundation pit projects.The theoretical research of these foundation pits lags behind the actual engineering,and the design calculation of these foundation pits lacks the corresponding standard guidance,which is not conducive to the safety of foundation pit engineering.In view of the large number of deep and large asymmetric pits appearing in actual engineering,the theoretical study of complex pits is one of the most important research topics in foundation pit engineering.This paper uses MSD method,a new method for calculating the deformation of foundation pits,to conduct research in this area.First of all,this paper improves the existing MSD method,including optimizing the plastic deformation mechanism,improving the influence wavelength of deformation,improving the undrained shear strength of the soil,improving the transfer coefficient of soil shear strength as a function of soil shear strain,and using an iterative method to get the MSD solution of foundation pit deformation.Secondly,this paper analyzes the parameter sensitivity of the modified MSD method.The parameters include the virtual fixed point position coefficient,the influence range factor of the excavation depth,the bending stiffness and supporting stiffness of the retaining pile,the friction angle in the soil,and the shear strength of the soil.Thirdly,this paper improves the load asymmetric foundation pit model,uses the elastic foundation beam method to obtain the asymmetric deformation of the foundation pit caused by the load asymmetry,and then obtains the modified MSD method solution of the load asymmetric foundation pit deformation.Finally,this paper analyzes the influencing factors of asymmetric foundation pits under load,and analyzes the influencing factors of asymmetric deformation caused by different forms of overload.The parameters include the size of the overload,the distance from the edge of the pit,the distribution range of the overload,and the operation of the overload Timing.Through research,the following conclusions and research results are mainly obtained.1.Compared with the traditional MSD method,the modified MSD method used in this paper to calculate the lateral movement of the retaining structure and the ground settlement caused by excavation of the foundation pit is more in line with the measured data.This is mainly due to the improved plastic deformation mechanism and the incremental deformation function.Compared with the traditional MSD method,the modified MSD method used in this paper to calculate the lateral movement of the retaining structure and the ground settlement caused by excavation of the foundation pit is more consistent with the measured data,mainly due to the following reasons: The improved plastic deformation mechanism and incremental deformation function are more suitable to reflect the actual incremental deformation of the foundation pit.Considering the deformation-affected wavelength affected by the excavation depth is helpful to truly reflect the actual deformation of the foundation pit.The introduction of undrained shear strength of the soil at different angles of the slip line can improve the accuracy of the MSD solution.The iterative method is used to solve the elimination problem in the solution of the energy conservation formula.2.Among the various parameters of the modified MSD method,the internal parameters,including the virtual fixed point position coefficient and the influence range coefficient of the excavation depth.The internal parameter can be deformed as a function of the form of the deformation function and the maximum horizontal displacement of the retaining pile.The compressive stiffness of the internal support has a stronger inhibitory effect on the deformation of the foundation pit than the retaining pile stiffness.Changing the compressive stiffness of the internal support or setting up the support in time can significantly control the deformation.Internal friction angle and soil shear strength parameters have significant effects on the deformation of the foundation pit.3.The traditional analysis method of symmetric foundation pit is not suitable for the calculation and analysis of deformation of foundation pit with asymmetric load.The improved load structure calculation model in this paper is suitable for the analysis of deformation of asymmetric foundation pit.The external load adopts the "three-segment polyline method" simplified based on the Flaman integral solution,which can better reflect the distribution of horizontal additional stress caused by different load forms in the depth direction of the foundation pit retaining structure.4.The displacement of the retaining structure caused by the existing overload is mainly concentrated in the excavation process with a shallow excavation depth.The magnitude of the overload and the distance from the edge of the pit can significantly affect the asymmetric deformation of the asymmetric foundation pit.The asymmetric deformation of the foundation pit caused by the increase of the overload range is not obvious.The temporary overload caused by the construction pit side due to construction and other reasons is more significant than the asymmetric deformation of the foundation pit caused by the existing overload.The innovations of this paper are: improving the existing MSD method,theoretically deriving the modified MSD analytical solution of the symmetrical foundation pit,and the improvement measures include adopting an improved plastic deformation mechanism that introduces an internal friction angle,using a deformation wavelength that can consider the effect of the excavation depth,adopting soil undrained shear strength that can take into account the soil depth and the stress-spinning rotation of the soil mass,using Duncan-Chang model based on hyperbolic model to determine the transfer coefficient of soil shear strength-soil shear strain function,and using the iterative method to solve the elimination problem in the solution of the energy conservation formula.In this paper,the asymmetric foundation pit model of load is improved,and the asymmetric deformation of the foundation pit caused by the asymmetric load is obtained by using the elastic foundation beam method,and then the modified MSD method solution of the asymmetric foundation pit deformation is obtained.Then,a new idea is proposed for calculating the deformation of the asymmetric foundation pit.