The Study On Water Retention Capacity And Shear Strength Of Unsaturated Soil And Its Application

The study of modern soil mechanics can be divided into two large parts which are the saturated soil and unsaturated soil,according to the research object.In actual Geotechnical engineering we can often encounter many varieties soils,the properties do not accord with the concept and principle of classical saturated soil mechanics.Saturated soil mechanics are a subject that combined with the engineering mechanics and the mechanical properties of soil.The functional relation between the matrix suction of unsaturated soil and changes in water content,and the various influences of the soil water characteristic curve,for example,initial dry density,different vertical pressure,initial moisture content,void ratio,etc.,which is a hot research direction of unsaturated soil.The focus of unsaturated soil mechanical study is the shear of strength unsaturated soil.When the rain infiltration the soil can increase the saturation.The moisture content increases,the weight increases,at the same time,the matrix suction becomes less,the shear strength changes.The influence of different moisture content and matrix suction on unsaturated soil slope stability is also an urgent problem in engineering practice.so,in this paper,mainly around the effect of water-holding capacity of unsaturated soil,deep system exploration and research on building the nonlinear shear strength fit the unsaturated soil and the unsaturated soil slope stability impacted by matrix suction.In this paper the main research is divided into four aspects:1.In order to study the soil water characteristics and influencing factors about the unsaturated soil,to understand about the law of unsaturated soil water deeply,by the existing instruments in the laboratory,mainly to study the unsaturated residual soil and the expanding soil,through the soil water characteristic test,measure the soil water characteristic curve of unsaturated residual soil and expansive soil.Through the soil water characteristic test,measure the soil water characteristic curve of unsaturated impacted by moisture content,soil variety,dry density,consolidation pressure and raw soil,etc..The soil water characteristics curves of the residual soil and the expansive soil were simulated through three functions,the soil water characteristic curve expression can be obtained.According to the test results,to analyze the effect of water-holding capacity of residual soil and expansive soil by dry density.According to the test,to analyze the equation parameters which are between dry density and soil water characteristic curve,to obtain the mathematical relationship expression between dry density,matrix suction,saturation is r dS(28)f(u,?),the water retention capacity of different dry density expansive soil can be described by the proposed equation.The relational expression of water retention capacity and dry density of unsaturated residual soil to verify the applicability of the expression established in this paper,which is soil water characteristic curve contains dry density.2.The study of unsaturated soil shear strength theory and deformation characteristics can reveal the mechanical behavior about the three-phase medium of unsaturated soil,to lay the foundation for the establishment of an unsaturated soil constitutive relationship,also can establish the unified soil mechanics theory of saturated soil and unsaturated soil,and make it better apply in engineering practice.The unsaturated soil shear strength mainly consists of three parts,to obtain the unsaturated shear strength which is non-linearity is mainly impacted by matrix suction,the influence of matrix suction on the total strength of unsaturated soil is consistent with the total cohesion of unsaturated soil.So in this paper,to establish a functional expression for the total cohesion and matrix suction of unsaturated soil with two nonlinear correlation coefficients,n and a.It obtains the nonlinear shear strength expression of unsaturated soil.And for the same soil,and is relevant to a,they come in pairs.Finally,by the experimental data about the four different types of soil to analyze and verify the formula in this paper.The results show that the fitting effect is nice,and the formula is not much different from the actual test results.Compared with the linear computing formula of unsaturated double stress state which was Fredlund put forward,the calculated values are more consistent.For a given soil,the two nonlinear correlation coefficients are constant,it doesn’t change because of the matrix suction change,with the addition of the effective cohesion and internal friction angle,the shear strength of unsaturated soil was expressed by these four numerical values.So the applicability is strong.3.Based on the theory of unsaturated soil strength,adopt the GDS unsaturated soil triaxial test system in the laboratory to take a triaxial shear strength test for expansive soil in an unsaturated state,to control different moisture content during the experiment,mainly to control the matrix suction of unsaturated soil to get a certain water content,and to analyze and to study the shear strength of unsaturated expansive soil.Finally,to analyze the experimental results by the unsaturated soil nonlinear shear strength theory formula in this paper,the nonlinear shear strength expression of unsaturated soil is obtained.Based on the formula of unsaturated soil cohesion,the total cohesion and internal friction Angle of unsaturated residual soil with different moisture content rate are obtained by three axle shearing test indoor.4.The relationship between moisture content and shear strength index was obtained by using the soil water characteristic curve;Establish the extreme analysis model of slope calculation;Through the three extreme analysis methods of Bishop,Ordinary and Janbu and the finite element strength reduction method;combined the total cohesion and friction Angle of the unsaturated soil to solve the safety factor of soil slope with different moisture content,to find the most dangerous slide;through the soil water characteristic curve measured to get the influence of the matrix suction on the stability of the slope.The slope safety factor is calculated by the finite element strength reduction method is bigger than extreme balance analysis methods of Bishop,Ordinary and Janbu,the main reason is that the finite element strength reduction method considers the development of plastic zone,And when the slope destroys,the corresponding plastic area development process can be seen,when the slope sliding surface and form under unknown conditions,to calculate the sliding surface is destroyed,the stress strain relations corresponding to the sliding surface can also be obtained.