Abstract

Whereas models for the mechanical behaviour of many types of sedimentary rocks at high mean effective stresses (2–50 MPa) are now reasonably well developed, few attempts have been made to generate models for mudrocks in the same way. This paper combines a new set of undrained shear experiments on London Clay with a summary of other mudrock datasets as described in the literature, including Kimmeridge Clay, London Clay, Boom Clay and clay from mud volcanoes. It has been demonstrated previously that at low mean effective stresses, most undisturbed mudrocks behave in a brittle manner, showing a distinct peak strength before undergoing failure and strain weakening to a residual strength. At very high mean effective stresses they tend to behave in a ductile manner, with the maintenance of peak strength to large strains. This paper shows for the first time that a ‘transitional’ regime can be defined for most mudrocks. In this transitional regime, undrained shear deformation leads to the maintenance of peak strength to a given axial strain in a manner that is similar to ductile deformation, before the initiation of strain weakening to a residual strength. Detailed analysis has demonstrated that during the maintenance of peak strength the sample is undergoing pervasive micro-fracturing and is thus behaving in a ductile manner on the macro-scale and in a brittle manner on the micro-scale. As a result of this study it has been possible to construct a model for the form of the failure envelopes of most mudrocks, with two fundamentally important envelopes being defined: (1) a linear residual strength envelope: at low and medium mean effective stresses the stress path will tend towards this envelope after brittle failure; (2) a peak strength envelope: at low mean effective stresses this envelope has a linear form, but once the transitional regime is attained it reduces in gradient, eventually intercepting the residual strength envelope at the boundary of the ductile regime. This new framework for the behaviour of mudrocks during undrained shear deformation is generally consistent with those developed for other sedimentary rocks.