Shear strength and mesoscopic character of undisturbed loess with sodium sulfate after dry-wet cycling

The deterioration of shear strength of loess under dry-wet cycling is one of the serious threats to natural and artificial slopes in loess terrains in China, especially when soluble salts are involved. Three types of salt weathering spalling were noted from field survey, i.e., strip-, flake-, and concave-shaped spalling. These are closely correlated with the migration of both water and salt as well as the deterioration of shear strength of the soil. A novel saline infiltration method was proposed to prepare undisturbed saline loess specimens by slowly infiltrating sodium sulfate solution into undisturbed loess through a perforated film and medium dense sponge. Conventional triaxial test results prove that the stress-strain curves are all strain hardening. Both the cohesion and internal friction angle decrease after dry-wet cycling, but the magnitude of reduction gradually decreases until a stable value is reached, manifesting as an exponential decay. The two indexes approximately attenuate linearly at higher salt contents. Moreover, the cohesion-based damage ratio was selected to quantitatively estimate the damage caused by dry-wet cycles and salt weathering, respectively. The damage ratio due to dry-wet cycling is higher than that by salt weathering. As dry-wet cycling proceeds, the ratio of dry-wet to salt weathering–induced damage initially increases followed by a decrease, which is approximately parabolic. The dry-wet-based damage ratio reduces at higher salt contents while the proportion by salt weathering grows. By quantifying the apparent surface cracks, the crack ratio grows after more cycles or at higher salt contents. A growing fractal dimension can also be observed. Based on the computed tomography (CT), a damage variable was determined for mesoscopic cracks. Easily, it can be noticed that the changes in crack ratio are synchronized with the attenuation of cohesion of loess as well as the mean CT value (ME).