Shear strength and damage mechanism of saline intact loess after freeze-thaw cycling

Abstract

Salt accumulation in the surface of soil mass and the structural damage during freeze-thaw cycling are a critical cause for the collapse of loess slopes in northwestern China. A simple method is proposed for preparing intact specimens containing soluble salt, called the leaching method, by which salt infiltrated can be uniformly distributed with the disturbance to the soil structure minimized. Verifications prove that the prepared specimens exhibit evenly distributed water and sodium sulfate. Triaxial shear test results show that the cohesion of specimens decreases after freeze-thaw, and the reduction mainly occurs after 1 to 2 cycles. Specimens with more water or sodium sulfate also manifest lower cohesion. Two characteristic indexes including crack ratio and fractal dimension that quantifies the apparent cracks of specimens indicate that more pronounced cracks appear at higher salt contents. This trend is intensified after more freeze-thaw cycles. The cohesion-based damage coefficients are calculated based on a specific damage path considering both freeze-thaw and salt erosion. The damage coefficient of freeze-thaw decreases at higher salt contents while salt erosion accounts for a major part to the total damage at contents higher than 1.0%. The ratio of freeze-thaw damage coefficient to that by salt erosion decays with freeze-thaw, especially in the initial five cycles, beyond which it stabilizes, like the attenuation of cohesion. The mesoscopic damage due to coupled effect of freeze-thaw and salt erosion is analyzed based on the computed tomography (CT). Results show that the mean CT value (ME) exhibit similar variations to the shear strength after freeze-thaw while the standard deviation (SD) value grows.