The Effect Of Stress Paths And Stress Increment Direction On Mechanical Properties Of Sand

Sand is a kind of common granular medium material,which we have to treat with frequently in engineering practice.Its mechanical properties have been one of the key topics in geotechnical engineering research.The deformation of sandy soil has obvious stress path dependence.The deformation and strength of sandy soil are obviously different under different stress paths.At present,studies on the dilatancy,strength and plastic flow law of sandy soil under different loading direction stress paths are relatively few.In this thesis,a dynamic triaxial test system controlled by GDS servo motor is used to carry out drainage tests of saturated sand under different initial stress states and loading directions.The strength,dilatancy and plastic flow law were systematically studied.The main results this thesis are as summarized follows.(1)In this thesis,the strength of sand materials under different initial stress states and stress paths in different loading directions are studied.It is found that in the p-q coordinate system,the stress increment ratio dq/dp changes and the corresponding peak shear stress decreases with the counterclockwise rotation of the loading direction.The peak stress ratio and internal friction angle of parameters describing shear strength of soil are not affected by stress path,initial stress state and loading mode.Shear strength of soil only depend on the effective normal stress on the failure surface.(2)The variation law of dilatancy of sandy soil is studied.The test results show that both stress path and initial stress state parameters(Ratio of initial stress ratio to peak stress ratio)have influence on dilatancy of sandy soil.With the counterclockwise rotation of stress path,the dilatancy of sand gradually increase.Under the same loading direction,the shear dilatancy for stress paths in the direction of stress increment less than 90 degrees increase gradually with the increase of stress state parameters.The dilatancy for stress path in the direction of stress increment greater than 120 degrees decreases gradually with the increase of stress state parameters.(3)The plastic flow law with changing loading direction is studied numerically.The relationship between stress increment direction and plastic flow direction is numerically simulated and the polynomial expression describing the relationship between them is given.The influence of generalized shear stress and average normal stress on plastic deformation is explained.(4)The effect of initial stress state on the direction of plastic flow is studied.The effect of stress state on the direction of plastic flow is not obvious when the loading direction is less than 0 degrees,but obvious when the loading direction is greater than 0 degrees.When the loading direction is greater than 0 degrees,with the increase of stress state parameters,the stress state-plastic flow direction curve changes from upward trend to downward trend.