Study On Unsaturated Mechanical Properties Of Lignin-lime Improved Silt

Silt is widely distributed in China.Shandong Province also has a large area of the Yellow River alluvial silt,and most of the Yellow River alluvial silt belongs to C group railway embankment filler.It must be improved before it can be used.The improved silt embankment is often in an unsaturated state in the actual environment.Due to the presence of the gas phase and gas-liquid interface in the soil,the mechanical properties of the unsaturated soil are quite different from those of the saturated soil.The improved silt may also undergo environmental changes such as drying-wetting cycles during the working years.The influence of dryingwetting cycles on the mechanical properties of the soil cannot be ignored.In this paper,the Yellow River alluvial silt improved by lignin-lime was taken as the research object.Firstly,the basic physical properties of pure silt and improved silt were obtained by laboratory routine soil test.The unsaturated mechanical properties of pure silt and improved silt were mainly studied.The soil-water characteristic curves of pure silt and improved silt were obtained.The unconfined compressive strength of pure silt and improved silt under different moisture content were compared.The direct shear test was used to analyze the variation of shear strength index with saturation.The influence of matrix suction on shear strength index was studied by unsaturated triaxial test.Then the effects of drying-wetting cycles on soil-water characteristic curve,unsaturated shear strength index,compressibility and permeability of lignin-lime improved silt were studied.Finally,the unconfined compressive strength test of pure silt and improved silt was simulated by discrete element software PFC3 D,and the variation of mesoscopic parameters with moisture content was obtained.The main results are as follows:(1)The soil-water characteristic curves of pure silt and improved silt were obtained,and the curves were fitted by Van Genuchten model and Fredlund-Xing model,and the fitting parameters were obtained.Compared with pure silt,the lignin-lime improved silt has lower saturated moisture content and higher air entry value,and the transition curve of the soil-water characteristic curve is gentle.(2)The unconfined compressive strength and elastic modulus of pure silt and improved silt do not change first and then rise quickly with the decrease of moisture content,and the relationship can be described by exponential function.The unconfined compressive strength of the improved silt was significantly higher than that of the pure silt at each moisture content.The direct shear test results show that the cohesion of the pure silt increases first and then decreases with the decrease of saturation,and there is a peak value.However the cohesion of the improved silt increases exponentially with the decrease of saturation.(3)The cohesion of pure silt and improved silt both increase with the increases of matrix suction,but the matric suction has less effect on their internal friction angle corresponding to the net normal stress variable.According to the Fredlund shear strength expression,the unsaturated shear strength expressions of the Yellow River alluvial silt and lignin-lime improved silt were proposed.(4)After the drying-wetting cycles,the air entry value is reduced,the cohesion of the improved silt is reduced,the internal friction angle is slightly increased,and the compressibility and permeability of the improved silt are increased.(5)The unconfined compressive strength test was simulated by PFC3 D,and the relationship between the mesoscopic parameters such as the bond modulus and the bond strength with the moisture content was obtained.