Study of Microscopic Pores in Sulfate Saline Soils under Freeze-Thaw-Dry-Wet Cycles

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.