Experimental Study On Mechanical Properties Of Saturated Silt Under Different Axial Unloading Strength

Due to the different stresses of soil in different regions,the stress paths are also complicated.Therefore,it is defective to analyze and study the soil strength index by simple single stress path.According to the characteristics of the foundation pit excavation process,the paper analyzes the mechanical properties of silty soil under unloading effect of excavation pit.And take the indoor test of triaxial unloading path on the typical saturated silt,which is from Metro No.1 of Xiasha,Hangzhou.The purpose is to study the mechanical characteristics of saturated silty soil in dynamic and static conditions of different unloading strength grades.Two aspects are studied in detail:(1)Based on the stress path triaxial apparatus,the shear strength,stress-strain,pore water pressure,deformation and displacement variation rule of saturated silt under different unloading strengths are studied.At the same time,the stress-strain under different unloading strengths is normalized.(2)Use dynamic triaxial apparatus to study the cumulative dynamic strain and dynamic pore pressure variation of saturated silt from different unloading strengths.Besides,the models of cumulative dynamic strain and dynamic pore pressure under the long-term cyclic load of different unloading stress paths are fitted,which obtain the suitable model and parameters of this kind of silt.The results of the pilot study led to the following conclusions:(1)The results of static triaxial apparatus show that shear strength decreases with the increase of the axial unloading strength grade under different axial unloading strength.Soil unloading can disturb the soil,which will decrease the soil strength.However,Unloading-reloading can restore the strength of some soil,and the damage intensity is greater than that after unloading,which always less than that under conventional consolidation.During the whole shear process,the soil sample is from “shear shrinkage” to “shear dilatancy”.Besides,with the increase of unloading strength,the shear dilatancy has a obvious effect.Thus,it is verified that the stress-strain has the normalization characteristic,and the normalization equation is obtained.(2)Under the long-term dynamic cyclic loading,the test failure mode of soil in the path condition of unloading unit is the axial tensile failure,and cumulative dynamic strain is divided into mutants and stable types.What's more,with the increase of unloading strength grade,liquefaction dynamic load will become smaller.In the condition of unloading-reloading stress path,the test fail mode of soil is compression failure.The cumulative strain is similar to the test of single unloading path,but always less than the strength index of triaxial test under non-unloading path.The test results show that under the unloading path,the change of dynamic pore pressure is also divided into mutants and stable types.Liquefaction failure decreases with the increase of unloading strength.At last,through the sorting and numerical simulation of the experimental data,the fitting model of the dynamic pore pressure under long-term cyclic loading of different unloading strength is obtained.