Model Test Study On Pile Foundation Underpinning And Underground Excavation Of Existing Buildings

The underground excavation of existing buildings will change the bearing behavior of underpinning pile foundation.How to give full play to the bearing potential of underpinning pile foundation and analyze the influence of excavation on the bearing characteristics of underpinning pile foundation is a key problem to be solved urgently.In view of this problem,this paper uses theoretical analysis,model test and numerical simulation to carry out research.Combined with the existing building basement expansion project,the vertical load test of underpinning pile foundation(hereinafter referred to as : load test)and the underground excavation layer test(hereinafter referred to as : excavation test)are carried out respectively.Through the load test,the influence of different grades of load on the bearing characteristics and settlement characteristics of underpinning pile foundation is analyzed.The load transfer model of pile side and pile end in underpinning pile foundation is established,and the variation law of ultimate lateral resistance of pile side along the buried depth of pile is revealed.The influence of underground excavation on the bearing characteristics and settlement characteristics of underpinning pile foundation is analyzed by excavation test.The variation law of bearing capacity of underpinning pile foundation with excavation depth is revealed,and the analysis method of bearing characteristics of underpinning pile foundation under underground excavation is formed.The main work and achievements of this paper are as follows:(1)The vertical load test of the underpinning pile foundation and the underground excavation layer test were carried out.The axial force of the underpinning pile,the bearing capacity of the pile end and the settlement characteristics were obtained through the load test.It was found that the resistance of the pile end showed a trend of work hardening under different grades of dead load.The bilinear hardening model was used to characterize the degradation trend of the stiffness of the pile end,and the relationship between the pile side resistance and the relative displacement of the pile and soil was described by the hyperbolic model.Through the excavation test,the axial force of the underpinning pile,the bearing capacity and settlement characteristics of the pile side pile end under different excavation depths are obtained.The influence of excavation depth on the bearing characteristics of the underpinning pile foundation is analyzed.The function relationship between the pile side resistance and the buried depth of the pile under different excavation depths is established.(2)The theory of normal stress and interface friction coefficient is used as the theoretical model to solve the friction resistance of pile side.Considering that the soil at the bottom of the pit is in the over-consolidated state under the condition of excavation and unloading,and the shear modulus of the soil increases with the depth,the over-consolidated ratio and the shear modulus increase coefficient are introduced to calculate the pile side resistance after excavation.In the calculation relationship of pile-soil relative displacement of load transfer model,soil rebound caused by excavation is considered,and soil rebound is used to correct pile-soil relative displacement.Combined with the load transfer method,an iterative calculation method for the load-settlement of single pile top under underground excavation conditions is proposed.According to the model test results,this analysis method is extended to the underpinning pile foundation.(3)The finite element software ABAQUS is used to simulate and analyze the vertical load characteristics of the underpinning pile,and the results are compared with the model test results.The results show that the numerical simulation results are basically consistent with the load-settlement trend of the model test results,but the settlement value of the underpinning pile calculated by the finite element is obviously larger than the measured value under different grades of load.The ultimate bearing capacity of the underpinning pile calculated by the finite element simulation is 12 % smaller than the measured value of the model test.