Influence of microstructural alteration of loess on shear strength during the seepage process

This study investigates the degradation mechanism of shear strength in overlying loess within the sliding zones of loess–red bed landslides governed by base–cover interfaces in the plateau regions of Northwest China. The Chigou, Luoyugou, and Shuiyanzhai landslides in Gansu Province were selected as representative cases. A series of seepage–corrosion tests, direct shear tests, X-ray diffraction (XRD) analysis, ion chromatography (IC), and scanning electron microscopy (SEM) were conducted to examine the evolution of mineral composition, chemical composition, and microstructure during seepage-induced strength deterioration. The main findings are as follows: (1) Both the internal friction angle and cohesion decrease progressively during seepage, with cohesion exhibiting a more pronounced reduction. (2) Intense chemical reactions—primarily hydrolysis, acid–base exchange, and ion exchange—occur during the water–loess interaction, particularly in the early seepage stages, fundamentally altering the material composition and weakening the soil structure. (3) These reactions result in significant microstructural changes, including increases in pore number, porosity, and pore area, which correspond to macroscopic strength deterioration.