The micro–composition of the soil is closely related to its engineering properties. In Northeast China, seasonal freezing and thawing can induce deterioration of the physicomechanical characteristics of saline soil, which can ultimately lead to engineering failures and geological disasters. To investigate the micro composition evolution including the microstructure and grain size distribution of saline soil under the influence of freeze–thaw cycles, mercury intrusion porosimetry (MIP) and laser particle size analysis (LPSA) were used to test undisturbed saline soil specimens undergoing different freeze–thaw cycles. The pore size distribution curves showed a double–peak shape and the grain size distribution curves showed a bidirectional change. Overall, the pores >0.4 μm increased while the pores smaller than 0.4 μm decreased with increasing freeze–thaw cycles. Moreover, the grain size distribution showed a different changing trend at a pore diameter of approximately 10 μm. During the whole freeze–thaw cycle process, the fragmentation of coarse particles (> 10 μm) and the aggregation of fine particles (< 10 μm) were evident, causing the transformation of different pore sizes, and the bidirectional change existed synchronously in the whole process. The freeze–thaw cycles for saline soil caused the lyotropic salt precipitation and dissolution and the ice volumetric expansion and ice segregation to change the grain size distribution leading to the evolution of pore distribution, ultimately changing the soil micro–composition and resulting in the change of macro engineering characteristics. The study is useful for understanding the change in the engineering properties of saline soil under freeze–thaw cycles from a microscale perspective.
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