Experimental Study On Strength And Deformation Of Silt Under Lateral Unloading Path In Plane Strain State

With the continuous development of infrastructure construction,a large number of projects related to national economy and people's live have emerged,such as high-speed railway,expressway,urban rail transit,and residential building.In these projects,excavation works such as foundation pit excavation,tunnel excavation,and slope cutting will be involved.This kind of engineering can be simplified to the problem of lateral unloading in the plane strain state where the vertical stress is constant and the lateral stress is decreasing with the excavation.Modern soil mechanics shows that the stress-strain relationship of soil is affected by the stress path.For a long time,people confirm the soil parameters and constitutive model through loading path,then design and calculation.This makes the existing design deviate from the actual situation.It is of certain practical significance to carry out research on the lateral unloading problem.In this paper,a series of plane strain lateral unloading tests are carried out on the isotropic consolidation,anisotropic consolidation and overconsolidation specimens using a new plane strain apparatus to investigate the influence of unloading on the stress-strain relationship,lateral strain and volume strain of the specimens.Based on the test results,the numerical simulation of the foundation pit is carried out to investigate the effects of two different stress paths,vertical loading path and lateral unloading path,on the actual engineering.The research results of this paper are as follows:1.The strength and deformation characteristics of isotropic consolidation silt samples under unloading and the loading path were compared.The research shows:for isotropic consolidation specimens,the vertical strain at failure of the specimen under unloading path is1%⁴%,which is significantly smaller than that?15%?under the loading path.Under unloading condition,the vertical strain and lateral strain increase sharply and the soil body breaks when vertical strain is 1⁴%,while the lateral strain increases with the vertical strain under the loading path.The sample under unloading path shows shringkage,but there is a tendency of shear dilatancy,which is different from the shringage of the sample under the loading path.Compared with that of the loading path,the internal friction angle obtained by the unloading path increases,and the cohesive force decreases.2.The strength and deformation characteristics of anisotropic consolidation silt samples were compared.The research shows:the tendency of deviatoric stress,lateral strain and volume strain with vertical strain is the same forisotropic consolidation and anisotropic consolidation specimens under lateral unloading path.There is a critical unloading ratio under unloading path.When the unloading ratio reaches the critical unloading ratio,the soil is destroyed.At the same time,the destructive stress??₁-?₃?_f decreases linearly with the increase of K as a whole,and the curves of different lateral consolidation stresses remain basically parallel.3.The strength and deformation characteristics of overconsolidation silt samples were compared.The research shows:for overconsolidation specimens,specimen shows shear dilatancy when OCR>1.5 under unloading path,while specimen shows shingkage when OCR>2.5 under unloading path.And the stress at failure decreases exponentially with OCR,which is different from the power function relationship between them under the loading path.The change process of volume strain under unloading path is divided into three stages:stage1 is mainly unloading rebound,stage 2 is mainly shringkage,and stage 3 is mainly shear dilatancy.4.Based on the results of the loading and unloading tests,the FLAC^3D software is used to simulate the excavation and soil nailing support of the foundation pit.The simulation results show:the maximal horizontal displacement of foundation pit edge and the settlement obtained by the unloading parameters are larger than those obtained by the loading parameters;and the axial stress distribution of the soil nails calculated by the parameters obtained by the two paths is different.This provides a reference for the selection of soil parameters in the design of deep foundation pit.