Evolution of the Shear Strength and Microstructure of Undisturbed Lateritic Soil under the Influence of Alkali Contamination

ABSTRACT With the progression of industrialization, alkaline contamination has become an increasingly serious issue in soil environments, significantly impacting the engineering properties and microstructure of soils. Undisturbed lateritic soil samples were immersed in sodium hydroxide solutions with pH values of 8.8 and 11.4, as well as in deionized water with a pH of 7.0, for soaking times of 1 day, 7 days, and 28 days. The influence of alkali contamination on the shear strength of the lateritic soil was quantified through consolidated slow shear tests. Additionally, scanning electron microscopy, thermogravimetric analysis, nitrogen adsorption tests, mercury intrusion porosimetry, and laser particle size analysis were employed to thoroughly explore changes in micromorphology, mineral composition, pore structure, and particle size of samples, aiming to establish a microstructural evolution model. At pH = 8.8, the cohesion and friction angle of samples are higher than that soaked in water and increase with the soaking time. At pH = 11.4, the cohesion significantly increases on the 1st day, relatively decreases by the 7th day, but remains higher than that at pH = 8.8. Subsequently, with an increase in soaking time, the cohesion continues to rise. The alkali solution dissolves the kaolinite minerals causing the pore structure to collapse. The elevated shear strength at pH = 8.8 is attributed to smaller particles tightly locking together to form a more stable structure. At pH 11.4, the significant increase in cohesion after 1 day is not only attributable to the reasons mentioned above but also attributed to the aluminum silicate gel’s bonding effect on particles. By the 7th day, gel dissolution reduces soil cohesion.