Experimental Study On Mechanical Properties Of Hangzhou Silty Clay Under Different Unloading Stress Paths

As a typical soft soil,silty clay is widely distributed in Hangzhou area.Due to its low bearing capacity and shear strength,there are potential safety hazards for large-scale foundation excavation construction.Many scholars have conducted relevant research on the strength characteristics and deformation laws of soft soil,but currently,the soil parameter indicators obtained through conventional triaxial tests are widely used in practical engineering calculations,which are unsafe.However,the research on drainage and unloading triaxial tests of soft soil is insufficient.This article is based on silty clay samples from Hangzhou area.A fully automated stress path triaxial apparatus was used to conduct indoor triaxial tests on CUCTC（axial stress increase,radial stress remain unchanged）,CDRTC（axial stress remain unchanged,radial stress decrease）,and CDTC（axial stress increase,radial stress decrease）under two consolidation states of equal pressure and K₀,as well as two stress paths of CDRTC and CDTC at different unloading rates,The mechanical properties of soil were studied,and Duncan Chang and its modified model were presented,and relevant parameters were obtained.The main research results are as follows:（1）Under the same consolidation state,the control test type is a single variable.In the CDRTC test,CDTC test,and CUCTC test,the shear strength of the silty clay increases sequentially,while the internal friction angle decreases sequentially in the CDRTC test,CDRTC test,and CUCTC test.Under different consolidation states,the values of shear strength are almost equal in the CDRTC test,while in the CUCTC test and CDTC test,the values of the isobaric consolidation state are greater than the K₀consolidation state,and the difference in internal friction angle is relatively small in the three tests.（2）The deformation laws of silty clay include:the axial strain corresponding to the peak value of deviator stress in the isotropic consolidation state is greater than that in the K₀ consolidation state;There are also CDRTC tests that are smaller than CDTC tests that are smaller than CUCTC tests.According to the volumetric strain axial strain curve,the CDTC test soil mainly exhibits shear dilatancy.The overall performance of the CDRTC test is shear shrinkage.（3）The strength characteristics and deformation laws of silty clay under different unloading rates are as follows:As the unloading rate increases,the strain softening phenomenon of the stress strain curve in the CDRTC and CDTC unloading tests deepens,the peak value of deviator stress decreases,and the axial strain corresponding to the peak value of deviator stress decreases,which is more evident under the K₀consolidation state.According to the volumetric strain axial strain curve,with the increase of the unloading rate,the peak value of shear dilatancy in the CDRTC test will appear earlier,and the peak value of shear dilatancy will decrease,resulting in a smaller corresponding axial strain;The CDRTC test tends to have a peak value of shear volume change earlier,and the peak value of shear volume change decreases.Comparing the peak value of shear expansion and shear shrinkage in the CDRTC and CDTC tests,it can be found that the K₀ consolidation state is smaller than the isotropic consolidation state.（4）Through linear fitting of triaxial test results under three stress paths,hyperbolic model parameters were obtained,and an empirical formula for the initial tangent modulus of Hangzhou silty clay was obtained.A comparative analysis of three common normalization factors found that the corresponding optimal normalization factors for the two unloading paths were different.Finally,based on the unloading test results under special complex stress paths,a normalization analysis was conducted,and the Duncan-Chang model was modified to obtain a tangent modulus model deformation formula that conforms to the unloading characteristics of Hangzhou silty clay foundation pit excavation.