Weathered granite soil (WGS) is highly water-sensitive and widely distributed across southern China, where the region's rainy climate contributes to geological hazards such as collapsing erosion, landslides, and ground subsidence. This study aims to elucidate the impact of this rainy climate on the deterioration of WGS by investigating the suffosion characteristics of granite residual soil (GRS) and completely weathered granite (CWG) at various stages of weathering. The research explores how suffosion affects their mechanical properties and microstructural features. A series of suffosion tests were conducted under controlled water pressure, followed by one-dimensional consolidation tests, cyclic triaxial tests, scanning electron microscopy, X-ray diffraction, and X-ray fluorescence analyses to analyze the deterioration mechanisms at both macro- and micro-scales. The results show that suffosion leads to the loss of fine particles and overall settlement of the soil samples. Microscopically, Mica is almost entirely lost, iron cementation is disrupted, and clay minerals, along with quartz and feldspar debris, are eroded, causing microstructural damage. The loss of minerals at the micro-scale exacerbates the formation of pores and cracks, increasing WGS porosity and promoting the progression of suffosion. On the macro-scale, suffosion alters the physical properties of WGS, with fine particle migration and loss leading to soil skeleton deformation, reduced stiffness, and decreased compressibility. Furthermore, a suffosion index is proposed, correlating microstructural changes with macroscopic mechanical parameters. This study has practical and theoretical significance for slope stability, collapsing erosion prevention, and surface subsidence mitigation in WGS in southern China.

Studying the effects of weathering on the mechanical properties and microscopic evolution of weathered granite soil (WGS) is essential for connecting microstructure with macroscopic behavior. This study conducts systematic monotonic and cyclic triaxial tests, along with a series of microscopic tests on WGS samples, to explore the influence of weathering on WGS mechanical properties and the mechanism of granite weathering. Results indicate that both effective internal friction angle and effective cohesion decrease progressively with increased weathering. Completely weathered granite (CWG) exhibits greater dynamic strength compared to granite residual soil (GRS). Additionally, as weathering progresses, quartz fragments are lost, while feldspar and biotite weather to form secondary minerals such as kaolinite and illite, leading to an overall enrichment in aluminum and iron in the granite. Weathering causes structural deterioration of WGS. Finally, the mechanical parameters of WGS and their chemical weathering indices show a coefficient of determination ranging from 60 to 99%. This study helps elucidate the fundamental causes of performance changes in WGS, thereby optimizing engineering design and enhancing disaster prediction accuracy, while providing new research perspectives and experimental evidence for WGS.