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.