Extensive experimental investigation on the effect of thermal treatment and lateral pressure on the shear behavior of intact mudstone

The coupling environment of temperature (T) and lateral pressure at great depths promotes intact rocks to shear failure, posing a serious threat to underground engineering. Temperature effect on shear behaviour is of particular importance due to the possible mineralogical alterations in mineral composition, especially in clay-rich rocks such as mudstone that has a great affinity for water. Accordingly, the effect of thermal treatment on the shear behaviour of intact mudstone was investigated, in this study, using the Short Core in Compression (SSC) method. Three temperatures of RT, 250 and 500 °C, and four lateral pressures of 0.0, 0.5, 2.0, and 4.0 MPa were adopted. Numerical and experimental observations showed that the resulting fractures in SCC samples are shear and by increasing the lateral pressure, shear failure is promoted. Compared with other rock types such as granite and sandstone, shear properties in mudstone have only one positive trend with temperature increase up to 500 °C, by increasing T from RT to 500 °C, mode II fracture toughness, peak friction angle, and the cohesion increased by about 15 to 47%, 4.9%, and 47.7%, respectively. The bilinear Mohr–Coulomb failure criterion can be used to model the peak shear strength behaviour of intact mudstone before and after thermal treatment.