Study On The Strength Characteristic And Slope Stability Of Unsaturated Expansive Soil Under The Effect Of Matrix Suction

Unsaturated soil is a kind of soil with solid phase, liquid phase and gas phase which is different from saturated soil. Expansive soil exists all over the world. In the engineering construction, some accidents happened because saturated soil theory is used to solve the problems of expansive soil slope and subgrade and so on. The main reason is that unsaturated soil has the factors such as the matrix suction which is different from saturated soil. So it is very necessary to use unsaturated soil theory to solve the problem of expansive soil. The expansive soil in the foundation pit of a construction site in Hefei was taken as the research object in this paper. Experimental study on the influence of matrix suction on the unsaturated expansive soil was carried out. On the basis of the test results, the stability of slope considering the influence of matrix suction was studied.Firstly, compression tests under equal vertical load or equal air pressure were completed by FGJ-20 SDSWCC. The relationship between overburden pressure, water content(or saturation) and matrix suction in unsaturated soil is discussed. It indicates that different vertical loads lead to soil-water characteristic curve’s difference; vertical loads’ influence on the soil water characteristic curve will be reduced gradually with the enhancement of suction; with the increase of vertical load, the parameter a and parameter m will increase while the parameter n will decrease, a, m, n are parameters in Fredlund-Xing three-parameter model; vertical loads’influence on soil moisture content related to matric suction, influence of different suction ranges need to be considered in actual analysis process; in the condition of equal soil suction, the variation of the water content of soil tended to raise at first and then reduce gradually with the increase of vertical loads.Secondly, direct shear tests of controlling suction and controlling water content were carried out respectively by unsaturated soil direct shear apparatus and conventional direct shear apparatus. Both stress-strain curves under different suction, water content and the shear strength parameters value of c and φ were obtained from the tests. The effects of suction and water content on shear strength of unsaturated soil were discussed, and the relation between shear strength and water content was proposed.Finally, according to the parameters obtained from the direct shear test, the typical slope examples were established. Then slope stability was analyzed by ABAQUS software. The stability safety factors of the slope under different matrix suction and water content were obtained. The influence of matrix suction and water content on slope stability was discussed. Based on the research results of soil-water characteristic curve, a view that the effect of matrix suction on slope stability can be obtained by analyzing the effect of water content on slope stability in practical engineering was proposed.