Strength deterioration of a Shaly sandstone under dry–wet cycles: a case study from the Three Gorges Reservoir in China

We selected the shaly sandstone at the rock slope of the Three Gorges Reservoir area in China where water level varies (the water level-fluctuating zone) in order to study the effect of dry–wet cycles on strength deterioration and micro-structure changes of shaly sandstone. After n dry–wet cycles (n being the number of cycles, with a value of 0, 1, 5, 10, 15, and 20), the shaly sandstone samples were exposed to a uniaxial compression test, triaxial compression test, and Scanning Electron Microscope observation. The strength deterioration and micro-structural changes of the samples are then analyzed with PFC2D (particle flow code in 2D). We found that as n increases, the micro-structure of the shaly sandstone changes from a well-organized dense structure stage to a porous stage and then to a cracking stage, the particles within the shaly sandstone bear and transfer a larger load, strength deterioration is more obvious, and particles are more prone to yield failure. The dry–wet cycles lower the micro-structure strength of the particles contact network, at the macro level, which is reflected in the strength decrease of shaly sandstone. At peak stress state, the shaly sandstone mainly shows the characteristics of shear failure after dry–wet cycles. With the loading process and n increasing, the cracks propagation process of the shaly sandstone samples can be divided into four stages: the elastic deformation stage, cracks generation and stable cracks growth stage, cracks damage and unstable cracks growth stage, and post-peak failure stage.