Study On Deformation Characteristics Of A Foundation Pit In Ningbo Considering The Effects Of Stress Path And Unloading Rate

With the acceleration of urbanization in China,foundation pit engineering has achieved unprecedented development,and the requirement of the deformation control of foundation pit is further improved.However,for a long time,the soil strength index determined by conventional triaxial test is often used in the support design and deformation calculation of foundation pit.In fact,this is completely different from the unloading stress path of soil in the process of foundation pit excavation.If the influence of stress path on soil deformation characteristics is ignored,it will result in a great difference between the prediction results of numerical simulation of foundation pit excavation and the site monitoring data after construction.In this paper,the stress path of soil mass in different areas during excavation is analyzed theoretically,and the test method that conforms to the stress change process of soil mass in passive area during excavation is selected.In addition,the lateral unloading test with different unloading rates(0.2,1,5k Pa/min)is designed considering the influence of soil mass excavation speed(i.e.unloading rate)on the mechanical properties of soil mass.Compared with conventional triaxial tests,the following conclusions are drawn: in lateral unloading tests,the stress-strain curves of soil samples tend to be ideal elastic-plastic type in the early and middle stages,and strain softening type in the late stages.The shear softening phenomenon is the most obvious when the unloading rate is 5k Pa/min,while the soil samples in conventional triaxial tests always show strain hardening characteristics;As the unloading rate increases,the cohesion decreases continuously,while the internal friction angle increases first and then decreases.The strength index of soil samples at the unloading rate of 0.2k Pa/min is basically consistent with the results of conventional triaxial tests.By comparing the soil characteristics and applicable range reflected by the constitutive model commonly used in engineering analysis,combined with the deformation characteristics of soil in the lateral unloading stress path,It is believed that the Hardening-Soil model(HS model)that can take into account the shear hardening(softening)and compression hardening characteristics of the soil and reflect the stress path dependence of the soil should be preferentially selected during the deformation analysis of foundation pit excavation.Sectioned modulus of HS model is obtained by using different unloading rates,and the proportional relationship between stiffness parameters of HS model and compression modulus of soil is sorted out,in order to provide reference for the determination of HS model parameters in the deformation analysis of deep foundation excavation of soft soil in coastal area.The soil parameters obtained under different test conditions are applied to the numerical simulation of foundation pit excavation.It is found that the soil parameters obtained under different stress paths and unloading rates have little influence on the horizontal displacement of the ground diaphragm wall,but have great influence on the surface settlement outside the pit.Then by comparing the simulation results with the field monitoring data,the soil parameters obtained under the test conditions are most consistent with the actual strength of foundation soil during foundation pit excavation.