Abstract: The first phase of water supply project in northern Xinjiang crossed the expansive soil area, and the mechanical properties of expansive soil are seriously weakened after repeated drying-wetting-freezing-thawing cycles, which cause local shallow landslide and frost heave damage of the canal slope. To further explore the deterioration mechanism of expansive soil canal slope, the changes of compression and permeability indexes are analyzed from macro-, meso-, and micro-perspectives through the compression test, the permeability test, and the SEM microscopic scanning test under drying-wetting-freezing-thawing cycles. The overall compressibility of expansive soil increases with the increase of drying-wetting-freezing-thawing cycles, and its compression curve can be divided into pseudo-elastic section and pseudo-plastic section. With the increase in the number of cycles, the rebound index shows a fluctuation tendency. The compression index is exponentially positively correlated with the number of cycles, and it is linearly correlated with the meso-micro cracks. Under the action of drying-wetting-freezing-thawing cycle, clay particles form a loose temporary structure of 'aggregates-pores-filled particles', flocculation structure increases, and anisotropy decreases. When the soil sample is subjected to vertical pressure, the pore spacing of expansive soil decreases, and the compressibility is large. When the pressure exceeds the consolidation yield stress, the aggregate particles become flat, the polar angular frequency increases, the pores are compacted, and the compressibility is gradually stabilized. Three stages i.e., slow, rapid, and stable stages, are identified in variation of permeability coefficient in the cycle process. The permeability coefficient changes greatly in the fifth cycle, and gradually stabilizes after 7 cycles, which is positively correlated with the number of cycles and surface fracture rate. The grey correlation degrees between the permeability coefficient and microscopic parameters are greater than 0.65, and microscopic porosity is the most important influencing factor. Under cyclic action, the microscopic pores develop obviously and form new seepage channels. The permeability coefficient is linearly and positively correlated with the microscopic porosity.

Key words: drying-wetting-freezing-thawing cycles, expansive soil, compression test, permeability test, micro mechanism