Strength distribution for mudstone absorbing water and its softened statistical damage constitution model

The mechanical strength of mudstone after absorbing water significantly decreases, seriously affecting the stability of tunnel projects. It is very important to study the laws and mechanism of this strength deterioration of mudstone. In this study, the main causes of the diseases in mudstone tunnels were determined. Through a series of the uniaxial compression and scanning electron microscopy tests, the strength deterioration mechanisms were studied and a statistical damage constitutive model was established. The results show that softening and expansion of mudstone after absorbing water were the main causes for the mudstone tunnel diseases. The probability of disease occurrence in mudstone tunnels from high to low was the inverted arch, vault, side wall and haunch. The softening and expansion of the mudstone were mutually reinforcing, and their mutual interaction formed a vicious cycle of water erosion. The stress–strain curve of mudstone showed a plastic–elastic–plastic curve. As the water content increased, the failure mode shifted from splitting failure to shear failure, while the degree of plastic deformation increased. Based on the Weibull distribution model and the Mohr–Coulomb failure criterion, a statistical damage constitutive model for mudstone absorbing water was established; the model can depict the failure process of softening mudstone.