Study On Shear Strength And Dynamic Behaviour Of Unsaturated Silt

The actual construction projects,such as most roads,railways and some subway,are mostly related to unsaturated soils,and most of these projects are built directly on unsaturated soils.If we can make full use of their unsaturated properites when excavating and filling the unsaturated soil foundations,it will be of great significance not only for the stability of the structures themselves and the supporting structures,but also for the reduction of construction cost.In addition,when the foundations of the projects are subjected to the dynamic loads such as vehicle vibration and earthquake,they will produce some deformation or even liquefaction.It will cause great safety hazard to the foundations and structures above them.Therefore,the studies on the shear strength and dynamic characteristics of unsaturated soils have important engineering application value.Compared with sand and clay,the engineering properties of silt are special.In this dessertation,a series of soil-water characteristic tests,direct shear tests,dynamic deformation tests and dynamic strength liquefaction tests have been carried out on unsaturated silt by using the pressure plate instrument,direct shear apparatus,the vapor equilibrium method with saturated salt solution and the suction-controllable dynamic triaxial testing apparatus,respectively.The soil-water characteritic,shear strength,dynamic deformation characteristics and liquefaction characteristics of unsaturated silt have been studied.The specific research contents and achievements are as follows:(1)By performing a series of the direct shear tests on compacted silt at different initial saturations from drying to saturation and the soil-water characteristic tests,the soil-water and strength characteristics of silt have been studied,focusing on the relationship between unsaturated shear strength and degree of saturation.The test results show that at the same normal stress,with degree of saturation increasing,the shear strength and cohesion of unsaturated silt increase at first and then decrease.There is a peak in some degree of saturation.The friction angles of unsaturated silt change little with increasing degree of saturation and can be considered as constant Based on the analysis of the existing theories for unsaturtaed soils,an equation for the shear strength of unsaturated soil is proposed.According to measured soil-water characteristic curves,the strength equation with sSr,sSk and s(Sr-Sra)/(1-Sn)(s is suction,Sr is degree of saturation,Src is capillary degree of saturation and Sra is adsorbed degree of saturation)as the suction stress for unsaturated soils are used to predict the shear strength of unsaturated silt and Nanyang expansive soil.The results of the prediction are analyzed and compared well with measured values.(2)By using a suction-controllable dynamic triaxial testing apparatus,a series of dynamic deformation tests have been carried out on unsaturated silt subjected to wide suctions imposed by the axial translate technique and vapor equilibrium method with saturated salt solution.The skeleton curve,dynamic elastic modulus,damping ratio are obtained over a wide suction range.The effect of suction on dynamic deformation characteristics of unsaturated silt is studied.The test results show that at the same net confining pressure the skeleton curve and dynamic elastic modulus increase with increasing suction,but the damping ratio decrease with increasing suction.As the water-retention state of specimens changes from the boundary effect zone to the transition zone and then to the residual zone,the increase rates in the skeleton curve and dynamic elastic modulus decrease gradually,the decrease rate in the damping ratio is still obvious,and the decay rate of dynamic elastic modulus with strain decreases with increasing suction.The test results can be explained by the average skeleton stress,the surface tension effect of meniscus water and the adsorptive effect between soil and water.(3)Dynamic deformation tests have been carried out on unsaturated silt by using a suction-controllable dynamic triaxial testing apparatus for unsaturated soils.The skeleton curve,dynamic elastic modulus,damping ratio are obtained at different net confining pressures and suction paths.The effect of suction history of drying first and then wetting on dynamic deformation characteristics of unsaturated silt is studied.The test results show that at the same net confining pressure and suction,the skeleton curve and dynamic elastic modulus of specimens subjected to drying and then wetting are higher than those of specimens subjected to drying only,but the damping ratio of specimens subjected to drying and then wetting is lower than that of specimens subjected to drying only.With increasing the maximum suction experienced,the skeleton curve and dynamic elastic modulus increase,and the damping ratio decreases.The test results can be explained by the average skeleton stress and the surface tension effect of meniscus water,which are together used to predict the maximum shear modulus of unsaturated soil.(4)Dynamic strength tests have been carried out on unsaturated silt under different initial degrees of saturation by using the dynamic triaxial testing apparatus with a volume change-measurable meter during vibration process for unsaturated soils.The development process of dynamic stress,axial strain,pore water pressure and volume strain of unsaturated silt subjected to dynamic load are obtained.The liquefaction characteristics of unsaturated silt under different degrees of saturation are discussed from the points of suction and volumetric strain.The test results show that during undrained cyclic vibration process,the volume of unsaturated silt specimen contracts.Under the same dynamic loading condition,the higher the initial degree of saturation of unsaturated specimen is,the easier reduction of suction is to approach or reach zero eventually and the closer the maximum volumetric strain is to the maximum of the pore air volumetric strain,the easier it is to reach the level required to trigger the specimen liquefaction.