Periodic water-level fluctuations in the Three Gorges Reservoir (TGR) severely deteriorate bank slope rock masses, threatening navigation safety. While laboratory wetting-drying simulations on fresh rocks are extensive, the actual degradation state of in-situ near-surface rock masses remains severely under-characterized. This study integrates field investigations with multi-scale laboratory testing to quantify the deterioration of typical carbonate and sandstone formations. Our observations reveal a distinct spatial weathering gradient. In the outermost layer (0–100 cm), compressive strength drops by 53.9% for limestone and 67.9% for sandstone, accompanied by macropore-dominated high porosities (10.6% and 15.2%, respectively). At 3 m depth, degradation mitigates, with porosity dropping below 3.4% and mechanical strength significantly recovering. Lithology dictates the coupled physical-chemical deterioration mechanisms. In limestone, the synergy of calcite dissolution and cyclic hydro-mechanical fatigue creates interconnected fractures, causing structural disintegration. Conversely, sandstone degradation is dominated by clay swelling-shrinkage and cement dissolution, leading to granular disintegration. These findings provide a quantitative micro-mechanical basis for predicting slope instability in reservoir hydro-fluctuation zones.

Reservoir bank slope,Hydro-fluctuation zone,Multi-scale deterioration,Near-surface rock masses