Characteristics and mechanisms of soil structure damage under salt weathering

Abstract

The structural damage caused by salt weathering in loess soil is a crucial factor leading to soil erosion and geological disasters in the Loess Plateau region of China. However, the impact of salt weathering on the structural damage and strength degradation of soil under unidirectional dehumidification conditions remains unclear. To gain a comprehensive understanding of the structural damage process and strength characteristics of soil under salt weathering, this study focuses on Q2 loess (silt loam, a loose aeolian deposit) in Fugu County, Shaanxi Province. Multiscale observations tests and direct shear tests were conducted on samples with varying sodium sulfate contents. The research findings indicate that at the macroscopic scale, The 3.0 % salt content sample experienced the most severe salt weathering, exhibiting characteristics of powdering and disintegration. Moreover, it exhibited the highest expansion displacement, reaching 22 mm. In addition, all samples go through three stages during the entire salt weathering process, namely budding, growth, and stable stages. At the mesoscopic scale, the displacements and expansions caused by salt crystallization growth on soil particles and pores were captured in real-time. Furthermore, the crystallization behavior of sodium sulfate differs on the surface and near-surface of samples as water content decreases, resulting in four distinct layering structures. At a microscopic scale, salt weathering leads to the formation of aggregates and the generation of numerous expansion pores within samples. Not only that, the shear behavior of samples transitioned from strain-softening to strain-hardening after salt weathering, with the peak strength significantly weakened compared to the residual strength. Additionally, before a salt content of 1.5 %, the cohesion of samples experiences the greatest decline, gradually slowing down thereafter, ultimately decreasing by 16 kPa. However, the decrease in the friction angle is less significant, with only a decrease of 4.8°. In summary, an increase in sodium sulfate content exacerbates the occurrence of "salt crystallization-induced expansion" and "soil drying-induced coagulation" phenomena in saline soils, resulting in severe damage to soil structure and strength. This study will provide valuable insights for soil and water conservation as well as disaster prevention in the Loess Plateau region, serving as a crucial reference for future research and engineering practices in the region.