Study on the effect of water on the shear behavior and microstructure of red mudstone

Red mudstone has significant hydrophilicity, that is hard at low water contents and soft and unstable at high water contents, and understanding the whole range of water content on the shear behavior of red mudstone is critical for evaluating red beds landslide stability. However, the deterioration mechanism of red mudstone in the process of gradual humidification is not clear in the whole range of water content. In this study, a series of mechanical and microscopic tests were carried out in the whole range of water content. A mechanical behavior prediction model for the entire process under the coupling of initial damage and load damage was constructed. The results show that the deterioration of shear strength, elastic modulus and strength parameters conforms to the "logistic model" curve with the increase of water content in the whole range of water content. In addition, the water sensitivity of cohesion was higher than the internal friction angle, and it gradually stabilized as water content increased. The weakening mechanism of the mechanical strength in the accelerated decline stage is the initiation and propagation of microcracks caused by mineral expansion, which leads to the corresponding shear behavior. Finally, the constructed damage model can be transformed into different yield conditions through parameter changes. The introduction of coupling damage enables the model to more accurately describe and predict the accelerated attenuation stage of red mudstone in the process of gradual humidification in the whole range of water content.