Evolution characteristics of microscopic pore structure of saline soil profile in Qian’an country, Northeastern China

The pore characteristics of saline soil greatly influence its engineering properties, such as deformation, strength, and permeability. To explore the microscopic pore characteristics of the saline soil profile in Qian’an country, Northeastern China, a study based on the mercury injection test (MIP) and the scanning electron microscope (SEM) test combined with fractal theory was carried out. The results reveal that the pore size of undisturbed saline soil at the sampling point presents the characteristic of trimodal distribution and shows different microscopic characteristics with a depth of 60 cm as the boundary. The shallow soil (surface—40 cm) has poor compactness, high porosity, and macropores (4–40 μm) dominate, while the deep soil (60–100 cm) has higher compactness, smaller porosity, and more uniform pore size distribution. In addition, the pore morphology of the saline soil profile is closer to the sub-long strip or oblate, which indicates the pore has a poor roundness, and the pore edge lines are relatively smooth. The geostatic stress, freeze–thaw cycle, clay content, and soluble salt affect the pore microstructure together. Furthermore, the pore characteristics were evaluated based on the fractal dimension ( D ) and surface fractal dimension ( Dr ). D has the strongest correlation with the content of small pores (0.04–0.4 μm), and Dr has the strongest correlation with the shape coefficient. D and Dr can jointly reflect the microscopic pore structure characteristics with a good corresponding relationship.