Abstract: In practical engineering, the rock mass in the hydro-fluctuation zone of the bank slope is not only subjected to wet-dry cycles, but also influenced by overlying stress, especially clastic rocks represented by mudstone. To study the shear mechanical deterioration characteristics of mudstone in the hydro-fluctuation zone under the combined action of stress and dry-wet cycles, typical mudstone from the bank fluctuation zone of the Three Gorges Reservoir Area was selected as the focal point of this study, and water-rock interaction tests considering the combined action of overlying stress and dry-wet cycles were carried out. The findings revealed: (1) During the combined action of stress and dry-wet cycles, the overlying stress accelerated the deterioration of the shear mechanical properties of mudstone. The greater the overlying stress, the more obvious the decline of the shear stress-shear displacement curve and the steeper the decrease in its slope, and the lower the shear strength and residual strength. (2) The shear strength of mudstone demonstrated a degradation pattern characterized by an initial rapid decline, followed by a gradual slowdown, and ultimately stabilization. The degradation observed in the first five cycles accounted for over 70% of the total degradation, and within the same cycle period, higher overlying stress levels corresponded to greater degradation amplitudes. (3) The increase in overlying stress significantly deteriorated the mesoscopic structure of mudstone, promoting fracture expansion, increasing the contact area of water-rock interaction, and accelerating processes such as the dissolution of calcareous cement and the expansion and contraction of illite and montmorillonite, leading to increased porosity and loose structure of mudstone. The research outcomes offer valuable insights for assessing the stability of clastic rock bank slopes in the Three Gorges Reservoir area.

Key words: mudstone in the hydro-fluctuation zone, dry-wet cycles, overlying stress, shear mechanical properties, deterioration effect