Dissolution decomposition and mesoscopic characteristics of weakly cemented pebble-cobble mixed soil

Weakly cemented pebble-cobble mixed soils, widely distributed in southwestern China, have become a major hidden danger in slope engineering and seriously threaten engineering safety due to their low degree of diagenesis, poor cementation, and tendency to soften and disintegrate when exposed to water. Taking six typical types of weakly cemented pebble-cobble mixed soils from the project area of Zhala Hydropower Station in Xizang as the research objects, this paper systematically investigated their chemical characteristics, dissolution and migration laws, disintegration characteristics, and microstructure evolution mechanisms under water-soil interactions using a combination of chemical analysis, dissolution and decomposition tests, dry-wet cycle disintegration tests, and scanning electron microscopy (SEM) observations. The results showed that: ①Among the six types of weakly alkaline soils, all are rich in soluble salts dominated by SO₄^2- and Cl^- except calcareous-cemented types. Immersion in surface water and groundwater can accelerate the loss of soluble salts (especially SO₄^2- and Ca²⁺), while calcareous-cemented soils exhibit excellent dissolution resistance. ② Dry-wet cycles significantly affect the disintegration characteristics of the soils, with the most severe damage occurring in the initial cycles. Soils with large porosity have the highest disintegration risk. Cementation type and integrity are the key factors controlling the degree of disintegration, and their unfavorable spatial combination leads to synergistic deterioration, resulting in a doubling of the disintegration amount. ③ Water saturation causes systematic deterioration of the soil microstructure. Macroscopically, the stable dense structure (face-to-face contact) transforms into a loose state (edge-to-edge or edge-to-corner contact), accompanied by a series of changes such as particle rounding, crack expansion, increased porosity, and enhanced connectivity. Among them, the soil samples with loose cementation and locally developed pores show the most significant deterioration. The research results deepen the understanding of water-soil interactions at the microscopic level and can provide references for slope stability analysis and design optimization of related projects.