Water–soil chemical mechanism and soil structural stability of expansive soil under the action of acid rain infiltration

This work discussed the water–soil chemical interaction mechanism of acid rain infiltration into expansive soil and its influence on the stability of soil structure. The natural residual expansive soil (NRES) from Baise City, Guangxi Zhuang Autonomous Region, China, was selected as the research object. A cyclic water device was designed to simulate the rainwater infiltration environment with pH = 3, 5, or 7. The cyclic water–saturated test was carried out. Then, X-ray diffractometry (XRD), X-ray fluorescence (XRF) spectrometry, and inductively coupled plasma optical emission spectrometry (ICP-OES) were performed to explore the evolution law of minerals and chemical components. The water–soil chemical reaction between expansive soil and acid rain was also analyzed using the geochemical modeling program PHREEQC. Finally, the mechanism of water–soil chemical interaction between acid rain and expansive soil and its influence on the stability of soil structure were analyzed. Results show that the cyclic water–saturated environment of acid rain infiltration can promote the dissolution and leaching of minerals in expansive soil, accelerating the destruction of soil structure. The dominant water–soil chemical interaction of expansive soil is mainly manifested in two aspects. The first one is the ion exchange between clay minerals, such as montmorillonite, illite, and kaolinite. The second one is the dissolution of calcite and oxides, such as CaO, Fe2O3, K2O, and MgO, in free state. The illite is partly converted into montmorillonite as the interaction between acid rain and minerals in the soil intensifies, resulting in structural instability. These findings provide theoretical support for the design and treatment of residual expansive soil slope in acid rain areas. Furthermore, these findings can provide a useful reference for the research on the disaster mechanism of other types of geological bodies under the chemical water–soil interaction.