The Study Of Strength In Unsaturated Loess Based On The Soil-water Characteristic Curve

With the development of social economy and increasing of human activities, the problems of unsaturated soil are arising in actual engineering. As an important research object of unsaturated soils, loess has a range of special mechanical properties, which manifest very strong structural and water sensitivity. However, the problems of loess brought by these properties can't be solved by classical theory of soil mechanics. The focusing topic of this thesis is the intensity of loess. Based on the theory of unsaturated soil, this thesis discusses the impact of matric suction on the strength of loess through matric suction determination and shear tests. This thesis also studies the methods of the stability of loess slope. The following results were achieved:(1) The matric suction tests are made on undisturbed and disturbed loess in southern Ningxia and the soil-water characteristic curve was drawn. As is found in the analysis, the change of suction in loess is closely related with occurrence of water in soils (consistency condition) and the force of particle-scale, showing a law that the matric suction increases with the decreasing of water content; moreover, the residual water content in undisturbed loess sample is smaller than that of disturbed loess and the soil suction value is larger in undisturbed loess sample than that of disturbed loess. This indicates that the structure of loess and stress history exerts great impact on the matric suction.(2) Matric suction is one part of suction stress. Based on the shear test of water content in undisturbed and disturbed loess, the shear strength parameters under different matric suction were obtained. The analysis showed that from saturated to dry loess, the matric suction gradually increased and the corresponding cohesion doubled, but the internal friction angle changed very little. Therefore, the impact of matric suction on the strength of loess was shown by cohesion. The traditional cohesion is actually the frictional strength caused by suction stress between soil particles on shear plane. In addition, under the condition of the same moisture content (matric suction), the shear strength increases with the increase of normal stress, ie when the intensity of friction remain the same, the shear strength of loess depends on the net friction strength caused by external load The. Therefore, based on the shear strength of suction stress formula more accurately reflects the soil particles along the shear surface between the friction mechanism of dislocation damage.(3) The method to calculate isotropic tensile strength and uniaxial tensile strength based on the concept of suction stress and Mohr-Coulomb failure criterion was introduced. Based on the mechanics of this method soil-water characteristic curve, the tensile strength characteristics of unsaturated loess was studied, the formula for calculating the depth of soil cracks was proposed.(4) On the basis of suction stress (saturation), Swedish slice method formula of Slope Stability was derived. This formula clearly reflected the mechanism of net friction and adsorption friction of slope failure, providing a theoretical basis for the quantitative analysis of slope stability in case of rainfall infiltration.