Influence of acid rain climate environment on deterioration of shear strength parameters of natural residual expansive soil

Abstract

Shallow landslides often occur on expansive soil cutting slopes especially during continuous rainy, causing traffic disruptions and enormous economic losses. This study experimentally investigated the effects of acid rain climate environment on the shear strength parameters of natural residual expansive soil (NRES). Firstly, the effects of the acid rain (pH = 3, 5, and 7) and wet-dry (W-D) cycles (n = 0, 2, 4, and 6) on the shear strength parameters of the NRES were studied by the direct shear test. Secondly, the changes in the microstructure and mineral composition of the NRES was studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. Finally, the mechanisms behind the shear strength loss of the NRES under acid rain and W-D cycles were considered. The direct shear test results revealed that acid rain and W-D cycles could accelerate the decline of shear strength of NRES, with the most apparent decrease under the first two W-D cycles. After two W-D cycles, the cohesion (c) of the soil samples under the acid rain with pH = 5 and 3 is 15.2 % and 31.2 % lower than that under the solution with pH = 7, respectively, while the internal friction angles (φ) of the soil samples decreased slightly. The results from XRD and SEM indicated the acid rain infiltration could cause different degrees of dissolution and leaching of free cementitious substances in NRES, resulting in the formation of loose structures with larger voids. This trend is more pronounced after the superposition of W-D cycles. Therefore, the integrity of the soil microstructure unit is reduced and the ability to resist vertical stress is weakened. In the direct shear test, the soil structure is more likely to form a shear fracture surface, eventually leading to the decline of shear strength.