Microscopic Evolution and Static Characteristics of Granite Residual Soil under Wet-Dry Cycles Conditions

【Objective】The existing studies mostly focus on short-term dry-wet cycles, which makes it difficult to reveal the long-term cumulative effects and the influence of fine particle migration. This paper takes the granite residual soil in Zhuhai as the research object, and studies the strength characteristics and microstructure evolution laws of soil with different fine particle contents under the long-term dry-wet cycling conditions.【Methods】Prepare reconstituted specimens with fine particle contents of 10%, 30%, and 50%. Conduct 1 to 100 dry-wet cycle tests and saturated unconsolidated undrained triaxial tests. Combine scanning electron microscopy (SEM) and Image-Pro Plus (IPP) aperture statistical methods to analyze the changes in the soil's microstructure.【Results】The results show that short-term wet-dry cycles (≤10 cycles) generally induce structural loosening, pore enlargement, and a shift in pore-size distribution toward larger pores, leading to an overall reduction in shear strength and cohesion. Under long-term wet-dry cycling (≥50 cycles), pore sizes are redistributed, and the strength gradually tends to stabilize, with a certain degree of recovery observed in some cases relative to the level after the first cycle. Shear strength increases with confining pressure and fine particle content, whereas its variation with cycle number is overall non-monotonic and jointly governed by fine particle content and confining pressure. The granite residual soil exhibits stage-dependent strength evolution under long-term wet-dry cycling, which is associated with pore redistribution and local structural reorganization. The variation in cohesion is generally consistent with that of shear strength, and the critical fine particle content is preliminarily estimated to lie between 30% and 50%. The internal friction angle generally increases with fine particle content and remains relatively stable during the long-term cycling stage.【Conclusion】Granite residual soil exhibits distinct staged strength evolution characteristics under the long-term influence of dry-wet cycles. Its essence is closely related to the reorganization of pore structures and the regulatory effect of fine particles. The research results can provide a basis for the long-term stability analysis and the determination of strength parameters of coastal granite residual soil slopes.