Effect Of Dry Density And Matrix Suction On Strength And Deformation Characteristics Of Unsaturated Expansive Soil

Expansive soils are widely distributed in China.According to statistics,about a quarter of the population in China live in the expansive soils distribution area.The economic losses caused by expansive soils are incalculable every year.Under natural conditions,expansive soils exist mostly in unsaturated state.Therefore,expansive soils are a special kind of unsaturated soils.The deformation characteristics of expansive soil under water swelling and shrinkage can not be explained by classical soil mechanics,so it is scientific to use unsaturated soil theory to explain the mechanical properties of expansive soil.Based on the mechanics of unsaturated soils,DGS unsaturated triaxial apparatus was used to conduct drainage shear tests of expansive soils with different dry densities and matrices suction consolidation in Hefei,Anhui Province.The effects of dry density and matrix suction on the strength and deformation characteristics of unsaturated expansive soils are analyzed through experimental results and calculation of related parameters.(1)When dry density and matrix suction are constant,the higher net confining pressure is,the more obvious the strain hardening phenomenon of stress-strain curve is;when dry density and net confining pressure are constant,the peak strength of unsaturated expansive soil increases with the increase of matrix suction,but the contribution of matrix suction to strength decreases with the increase of matrix suction.(2)Dry density can significantly enhance the strength of unsaturated expansive soil,and with the increase of dry density,the stress-strain curve changes from strain softening to strain hardening.(3)According to the analysis of strength parameters,when the dry density is constant,the total cohesion c increases with the increase of the matrix suction,and the matrix suction has little effect on the effective internal friction angle;the effective cohesion and the effective internal friction angle increase linearly with the increase of the dry density,and the adsorption internal friction angle increases first and then decreases with the increase of the dry density,and the relationship is approximately quadratic polynomial.(4)Based on Fredlund's two-stress state variable strength formula,the shear strength formula of unsaturated expansive soils related to dry density and matrix suction is proposed by establishing the relationship between strength parameters and dry density.(5)By analyzing the shear volume strain curve,it can be seen that under the same matrix suction,with the increase of dry density,the specimens gradually show remarkable dilatancy characteristics;when the dry density is constant,with the increase of matrix suction,the dilatancy characteristics of the specimens increase.(6)By analyzing the parameters of the non-linear model,it can be seen that the initial tangent modulus of deformation and tangent bulk modulus increase with the increase of dry density and matrix suction,and a modified formula considering dry density and matrix suction is proposed through linear fitting.