Research On Deformation Characteristics Of Foundation Pit Considering Unloading

Foundation pit has obvious characteristics of the excavation unloading.The mechanical properties for a specified soil under unloading condition are the basis of determining parameters required to simulating analysis and design of foundation pit,which can provide feedback to optimize design and ensure the safety of the excavation of stability during construction.Based on a real project case,triaxial unloading tests for undisturbed soil samples in project site are carried out in this paper,and combined with numerical simulation and field monitoring data analysis to study on deformation characteristics of foundation pit considering the effects of unloading condition.The main work and results are as follows:(1)The basic physical properties and compression-swelling characteristics of the soils in engineering site which are 5 m and 7 m deep were explored.Then two series of triaxial tests under loading and unloading stress paths for undisturbed soils were conducted on GDS triaxial apparatus.The differences in deviator stress against axial strain relationship and excess pore water pressure during shear under loading and unloading stress path were pointed out.This paper discussed the influence of different shear mode on the effective stress path and persented initial tangent modulus under loading and unloading conditions.(2)Based on the results from sensitivity analysis of modulus in the numerical simulation analysis,the main computing parameters for Hardening Soil Model—reference tangent modulus and reference secant modulus were determined through oedometer primary loading test and triaxial test.Simultaneously,the affects of unloading stress path on secant modulus were researched and the results indicate that the secant modulus of soils under unloading stress path is greater than that under loading stress path.The paper respectively established the relationship of the compression modulus,the tangent modulus and the secant modulus of soils under loading and unloading conditions.(3)Monitoring data statical analysis was conducted to study the characteristics of wall deflection and ground surface settlement caused by excavation.Based on a database of 33 case histories reported in literatures,this paper presented the maximum lateral wall deflection ranging from 0.24%H to 2.22%H,with an average value of 0.89%H.Then the construction quality of retaining wall was evaluated through the comparative analysis with practical case in this paper.The finite element calculation results compared with the monitoring data,show that the numerical simulation adopting the conventional loading stiffiness parameters significantly overestimate excavation deformation,while the numerical simulation with stiffness parameters considering the effect of unloading is closer to the measured values.