Sulfate saline soil has obvious characteristics of soil structure changes with seasonal temperature changes, which causes great damage to the project. In order to study the thawing characteristics of sulfate saline soil under the condition of single temperature rise, the indoor compaction degree is 90%, 95% and 98%, and the salt content is 0%, 2%, 5%, 8% and 10%. The sulfate saline soil sample is subjected to a single heating test in a high and low temperature test box. The results show that the thawing settlement process of sulfate saline soil can be roughly divided into four stages under the condition of single temperature rise, the temperature in the range of -15 ℃ ~ -10 ℃ (±5) is the “structural thawing” stage, and the interval of -10 ℃ ~ 0 ℃ is the stage of “melting sedimentation adjustment”, and the interval of 0 ℃ ~ 15 ℃ is the stage of “ice ablation sedimentation”, and the interval of 15 ℃ ~ 25 ℃ is the stage of “crystal thawing sedimentation”. When the salt content is 0%, the thawing settlement rate decrease with the increase of the degree of compaction. The thawing settlement rate was greater than that under the condition of 0% of salinity. When the degree of compaction varied within the range of 90% ~ 95%, the thawing settlement rate and the degree of compaction presented an inversely proportional change trend. The thawing settlement rate of 98% compacted soil decreases generally when the salt content is 2% ~ 8%, and increases sharply when the salt content is 10%. In addition, when the compaction degree is 95% and 98%, the increment of thawing settlement rate reached the maximum in the range of 15 ℃ ~ 20 ℃ and reached the maximum at 20 ℃. And when the compaction degree is 90%, the temperature reached this state is 25 ℃. Comprehensive analysis considers that the compactness of the saline soil foundation should be 95%. This study can provide a certain reference for engineering construction in sulfate saline soil area.

In order to study the effect of freeze-thaw-dry-wet cycles on the microscopic pore structure of sulphate saline soil in Ningxia, SEM and MIP tests were carried out on the samples after six freeze-thaw-dry-wet cycles. With the help of ImageJ2X image processing technology, the microscopic pore structure was analyzed from the apparent porosity, pore distribution and pore fractal dimension. The results show that: (1) After six freeze-thaw cycles and dry-wet cycles, for lower salt content (S≤1%), the apparent porosity of sulfate saline soil increased first and then decreased with the increase of salt content under different compaction degrees, and reached the peak at 0.5%, and was at the trough at 1%. When the salt content is higher 1%, the apparent porosity of the lower compaction soil (K<95%) continued to increase with the increasing of salt content, while the higher compaction soil (K≥95%) showed a change trend of slightly increasing first and then decreasing. Compared with the unfreeze-thaw-dry-wet cycle, the final apparent porosity has increased. In order to improve the durability of the project construction, it is recommended that the compaction degree of sulphate saline soil foundation should not be less than 95%. (2) With the increase of salt content, the change of pore size is mainly concentrated in the range of 3~40 um, and the changes in pore distribution are mainly reflected in large pores and medium pores, and the apparent porosity is basically consistent with the change of the percentage content of large pore volume. (3) Based on MIP data, the fractal dimension of soil pores fluctuates up and down with the increase of salt content, and it is contrary to the change trend of apparent porosity and macropore volume percentage. This study is helpful to enrich the microscopic theory of sulfate saline soil and provide theoretical support for the construction of projects in sulfate saline soil areas.