Capillary imbibition-driven deterioration of micaceous residual soils: Slope stability implications for tropical dry-season environments

Micaceous residual soil (MRS) is widely distributed across (sub)tropical regions worldwide. It is typically associated with engineering failures and geohazards, due to its inferior engineering properties and high susceptibility to environmentally induced degradation. While the effect of typical subtropical climate, i.e., rainfall-induced wetting, has been extensively studied, the role of capillary imbibition (CI) on the hydro-mechanical properties of soil during dry seasons remains poorly understood, particularly for mica-rich geomaterials. This study investigates the deterioration of MRS subjected to successive capillary imbibition–drying (CID) cycles through a multi-scale experimental program that combines capillary-rise tests, swelling measurements, unconfined compression tests, cyclic triaxial testing, and SEM observations. The results indicate that increasing mica content markedly alters the physical properties of MRS, lowering the compaction efficiency and accelerating CI process. CID cycling further enhances imbibition efficiency by shortening the transitional time of capillary rise. Swelling deformation intensifies nonlinearly with both mica content and CID repetitions, evolving from local cracking to structural collapse. Strength and stiffness reduced substantially at early cycles, while the degradation rate gradually moderates as mica breaks and shifts from elastic rebound to particle rearrangement. SEM observations reveal progressive microstructural evolutions of mica, including interlayer dilation, surface roughening and fragmentation, which collectively drive porosity increase, structure loosening, and crack development within MRS. This study reveals new insights into the essential role of capillary-driven wetting on mechanical properties of MRS and the underlying implications for engineering geological stability in tropical environments.