Strength Characteristics And Engineering Application Of Unsaturated Granite Residual Soil

With the continuous development of cities,the situation of traffic congestion has been aggravated.Medium and large cities vigorously develop underground rail transit.As an important part of the construction of underground rail transit,deep foundation pit of Metro has been paid more and more attention.In the southeastern coastal area of Fujian Province,because the distribution of granite residual soil is widespread,and the granite residual soil is defined as "special" soil by academia.Most of the soils encountered are unsaturated soils,which causes more engineering problems.How to correctly grasp the physical properties,strength variation law and engineering characteristics of unsaturated granite residual soil is the key to solve engineering problems.The main contents of this paper are as follows:(1)The origin and composition of granite residual soil are introduced.Experiments on physical parameters such as moisture content,density and particle gradation were carried out.The disintegration characteristics,engineering characteristics and suction effect are also described.(2)Soil-water characteristic curves of granite residual soils under different densities are experimented with unsaturated soil triaxial apparatus.The expression of soil-water characteristic curve of granite residual soil under van Genuchten model is obtained by using MATLAB fitting function curve.The fitting degree is analyzed to find the optimal fitting function,and the specific values of four parameters of van Genuchten model under this function are obtained.Combining the relationship curve between saturation and matrix suction in the range of Engineering influence,a more concise mathematical expression of soil-water characteristic curve is obtained.(3)Five groups of shear tests of granite residual soil were carried out by saturated triaxial apparatus and unsaturated triaxial apparatus respectively.The stress-strain relationship curve and Mohr-Coulomb envelope diagram were analyzed by strength comparison between unsaturated and saturated soil samples,and the shear failure states of different types were explained.Based on Bishop and Fredlund unsaturated strength formulas,the relationship between total cohesion,net normal stress angle,suction angle,contribution coefficient and matrix suction is obtained,and the formula of shear strength of granite residual soil related to saturation is obtained.(4)Taking the deep foundation pit of unsaturated granite residual soil as the research object,this paper numerically simulates the variation of foundation pit under different saturation degrees when the granite residual soil is affected by rainfall,explores the variation law of horizontal displacement,vertical displacement and supporting axial force,and discusses the influence process of granite residual soil on the deformation of foundation pit when the saturation increases.The control measures and monitoring suggestions for foundation pit deformation under unsaturated condition are put forward.