Abstract: To explore the influence of acid or alkali contamination on the shear strength of natural loess, the intact loess samples were first immersed in various concentrations of HCl and NaOH solutions. Then, triaxial shear tests, scanning electron microscopy tests, and mercury intrusion tests were carried out, and the soil chemical composition as well as liquid and plastic limits were measured, so as to assess the influence of acid or alkali contamination on the shear strength, microstructure, chemical composition and plasticity of loess. The results indicate that with the increase of the acid concentration, the peak of soil stress-strain curve attenuates, the shear strength deteriorates, the cohesion decreases exponentially, and the internal friction angle is relatively stable. As the alkali concentration increases, both the peak of soil stress-strain curve and the shear strength increase, the cohesion enhances markedly, and the internal friction angle enlarges slightly. Acid contamination breaks the soil particles, dissolves the cementing material, blurs the boundary between particles and pores, and increases the number and size of pores between skeleton particles and those between clay particles. Alkali contamination leads to the scaffold pore collapse of the soil, and the secondary cementation balances the local damage and strengthens the structural connection. Besides, the content and size of pores between skeleton particles decrease while more pores with a smaller size form between the clay particles. After soaking in acid solutions, the cation content in the soil increases notably, the calcium carbonate content decreases sharply, and the liquid and plastic limits are reduced. After soaking in alkaline solutions, both Al³⁺ and calcium carbonate in soil increase slightly, other cations decrease gently, and the liquid and plastic limits increase. Based on the analysis of test results, the microscopic mechanism of the evolution of the shear strength of loess contaminated by acid or alkali was summarized. Acid or alkali contamination resulted in mineral dissolution, ion exchange, and adjustment of particle and pore structure in the soil, destroying the initial structure of the soil and facilitating the formation of new structures. It is the damage of the initial structure or the new structure formation taking the dominance that decides the comprehensive effect improving or impairing the shear strength of the soil.

Key words: soil mechanics, intact loess, acid- or alkali-contaminated soil, shear strength, microscopic mechanism