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 Al 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.