Experimental study on mechanical properties and microscopic mechanism of expansive soil in a project in north Xinjiang

A project of north Xinjiang has encountered multiple sliding failures since running which across expansive soil area. Our primary aim is to explore the sliding failure mechanism. Therefore, the indoor direct shear test, compression test, seepage test and electron microscope scanning test of soil are carried out separately, the results of mechanical properties of soil are investigated and their affecting mechanism is discussed. The cohesion decreases with the increase of water content. The internal friction angle decreases slowly when it is less than the optimal water content, and then decreases significantly. Due to dry density increasing, the cohesion increases significantly and the internal friction angle increases more slowly. According to the analysis result, the stable void ratio decreases with increasing water content, and the compressibility of soil increases. With increasing dry density, the initial void ratio decreases, and the stable void ratio tends to be constant. With increasing consolidation pressure, the structure type of soil gradually evolves from flocculation structure to turbulence and laminar flow structure, the effect of particle aggregation is obvious, the number and size of pores decrease, and the compressibility of expansive soil decreases. All the above results are obtained based on the result of scanning electron microscopy experimental test (SEM). The permeability of expansive soil is strong under low consolidation pressure, and it is small under high pressure (200~1 600 kPa), with an order of 10⁻⁶~10⁻⁸. The permeability coefficient is positively correlated with the void ratio, which can be expressed in the form of power function. The results of scanning electron microscopy (SEM) show that the loose accumulation structure changes into a closely combined laminar flow structure with increasing consolidation pressure, and the area of pores decreases, which provides the conditions for the permeability coefficient to decrease significantly.