Abstract: Under freeze-thaw-dry-wet cycles, the main cause of strength deterioration in sulphate saline soils is the microscopic pore structure of the soil. In this study, we investigated the macro and micro response relationships of strength deterioration using unconfined compressive strength tests, mercury compression tests, electron microscopy scanning tests, universal global optimization (UGO) analysis of data, and ImageJ2X processing of SEM images. The results demonstrate that: (1) The unconfined compressive strength tends to increase and then decrease with the salt content under freeze-thaw-dry-wet cycles, and the peak strength corresponds to a low salt content when the compaction is low. Additionally, the percentage of pores between 1-10 μm is the threshold for strength deterioration, which is irreversible, occurs when the percentage is higher than 50%. (2) Under freeze-thaw-dry-wet cycles, the unconfined compressive strength is correlated with the skewness and structural merit of the microscopic parameters. Unconfined compressive strength shows a positive correlation with skewness and a negative correlation with structural merit, with structural merit having a more significant effect on strength than skewness does. (3) Structural merit, skewness, sorting factor, and mean pore throat radius, in descending order, are the microscopic parameters that affect the deterioration degree of macroscopic indicators of unconfined compressive strength. This study provides a reference for further research on the engineering properties of saline soils by exploring the macro-and-micro-response relationships of sulphate saline soils at the quantitative level.

Key words: degradation of strength, macro-and-micro responses, sulfate saline soil, freeze-thaw and dry-wet cycles, unconfined compressive strength test, mercury compression test, electron microscopy scanning test, universal global optimization