Experimental Study On The Strength Properties Of Unsaturated Expansive Soil In Cixian On The North Section Of South-to-north Water Diversion Project

Expansive soil is a high-ductility clay, and it mostly consists of montmorillonite illite and gaolin which are strongly hydrophilic clay minerals. Its typical features are water-swelling and dehydration-shrinking, whose strength changes greatly with the change of water content. In addition, it also has multi-fissure and intensity attenuation. In consequence of these particularities, the destruction of the buildings caused by expansive soil often has repeatabilities and long-term potential dangers. The problems of expansive soil become a severe disaster which threatens human life and property. In this thesis, GDS unsaturated triaxial instrument、Whatman NO.42 filter and FLAC 3D numerical simulation software were used to have a serial of research to unsaturated strength properties and water holding capacity and slope stability of expansive soil of South-to North Water Diversion in Cixian. The main achievements in this thesisi can be summarized as follows:(1) The unsaturated properties of expansive soils were researched by using GDS unsaturated three axle test instrument. The test results manifest that: the shearing strength of the unsaturated expansive soil is related to its moisture content and dry density. Along with the increase of moisture content, the shearing strength of soil lowers in turn. And the total cohesion and the total internal friction angle reduce with the enlargement of water content. While along with the dry density increasing, the shear strengths of soil are enhanced dramatically, and the increase of the cohesion is far greater than the increase of the internal friction angle.(2) The shear strength of the unsaturated expansive soil is also closely related to the size of confining pressure and the matric suction. The strength increases as the the confining pressure rises, in addition, soil sample shows strain softening phenomenon in the case of the lower confining pressure. In the drying process, the strength is improved by the growth of the matric suction, and the cohesion is larger nonlinearly with the larger of matric suction, while the effective angle of internal friction is nothing to do with the matric suction.(3) The soil-water characteristic curve of expansive soils was tested by using Whatman 42 filter paper. The test results manifest that: as the matric suction is smaller, the curve between saturation degree and matric suction is more better than the rcurve between moisture content and matric suction, because the former can indicate the influence of the dry density on the soil-water characteristic curve more clearly. As the moisture content is lower, the matric suction increases with the moisture content; while the water content is higher, the increasing trend is not obvious. The relationship between matric suction and moisture content shows a nonlinear relationship, and with the enlargement of moisture content, matric suction decreases sharply. The results of different dry densities of soil samples are fitted with Van Genuchten soil-water characteristic curve and the effects are great.(4)FLAC 3D software was used to simulate the stability of expansive soil slope under different conditions, which adopts the strength reduction method. The results indicate that: the differences of dry density and moisture content influence the safety coefficients and the displacements of slope. In this thesis’ simulation conditions, with the diminish of dry density and the enlargement of moisture content, the safety coefficiernts reduce and the displacements enlarge.