Deterioration Mechanism of Shear Strength of Badong Formation Soft Rocks in the Hydro-Fluctuation Belt of Three Gorges Reservoir

With the periodic impounding and draining of the Three Gorges Project,the rocks and soils in the hydro-fluctuation belt would undergo "wetting and drying" cycles under the influence of reservoir water fluctuation.The mechanical strength of rocks and soils would deteriorate due to the effects of dryingwetting,and it would cause great damage to the bank slope stability,which might further induce a catastrophic geo-hazard.This study was performed to investigate the changes in the mechanical parameters of soft rocks from Badong Formation under the influence of wetting and drying cycles.For this purpose,the shearing test under different wetting and drying cycles was conducted to simulate the process of water-rock interaction on bank slopes during fluctuation of water level with Badong Formation soft rocks,and to explore the deteriorated shear strength of soft rocks under drying-wetting cycles.The mineral compositions of selected samples of soft rocks were measured to investigate the weathering and slaking mechanism of soft rocks in the hydro-fluctuation by X-ray diffraction experiment.The test results show that the greatest deterioration in values of shear strength for T₂b² and T₂b⁴ was observed after 1and 3-5cycles,respectively.The deterioration mechanism was different in T₂b² and T₂b⁴,the shear strength deterioration in T₂b² was due to the expansibility of montmorillonite,and that in T₂b⁴ was due to the development of fracture pressure.Research results may have an important reference for stability analysis of reservoir bank slopes.