Experimental study on shear strength characteristics of sulfate saline soil in Ningxia region under long-term freeze-thaw cycles

Sulfate saline soils are widely distributed in Ningxia region of Northwest China. The soils experience long-term freeze-thaw cycles during the spring time due to the large temperature difference between day and night. To study the effects of freeze-thaw cycles and salt content on the shear strength of sulfate saline soils, a series of unconsolidated undrained (UU) triaxial tests were carried out. The soil specimens were prepared at three different salt contents (0%, 2% and 5%), experiencing a series of freeze-thaw cycles (up to 50 cycles). The Scanning Electron Microscopy (SEM) and Laser Diffraction Analysis (LDA) were carried out under different freeze-thaw cycles to study the strength degradation mechanism of the sulfate saline soil. The triaxial test results showed that the stress-strain curves of the soil gradually changed from a strain-softening behaviour to a strain-hardening behaviour, with increasing freeze-thaw cycles, salt content, and confining pressure. It is found that the shear strength for the soil specimens decreases with the increase of salt content. The higher the salt content, the greater the strength decreases. Moreover, The SEM and LDA results suggest that the long-term freeze-thaw cycles could break down the soil particles and change the soil structures, resulting in more fine particles and particle rearrangement. Thus, the strength parameters of the saline soil get further deteriorated.