Modelling the Failure Behaviour of Mudstones under High Pressures

Understanding the failure behaviour of rocks is very important for many engineering applications. While conceptual models to predict failure behaviour of rocks have been proposed, very few models exist to quantitatively predict the brittle–ductile characteristics of mudstones at high confining pressures. In this study, we performed triaxial compression tests at confining pressures up to 130 MPa on dry indurated mudstones from the Naparima Hill Formation, Trinidad. Experimental results including stress–strain curves, failure modes, and strength parameters of the mudstones were obtained. Analysing the stress–strain curves, shows that the mudstones experienced brittle, brittle–ductile transition, and ductile failure behaviours as the confining pressure increases. The failure behaviour at confining pressures less than 50 MPa is accompanied by axial splitting fractures and shear fractures. At confining pressures greater than 50 MPa, shear band and ductile flow fractures are observed in the mudstones. A failure behaviour model was developed using the failure behaviour and failure strength of the mudstones. This model predicts the brittle, brittle–transition and ductile zones for mudstones at high confining pressures. The brittle to ductile transition is not sudden, but a gradual process that is controlled by the confining pressure and failure strength. The model indicates a narrow brittle-ductile transition zone for weak mudstones as compared to strong mudstones.