The deformation of clays and shales under high pressure conditions

The deformation of mudrocks has previously been extensively investigated in the low (0 to 1 MPa) pressure range but, due to the long consolidation times required, there is a paucity of data on these materials under higher pressures (1 MPa to 70 MPa). Use has been made of state-of-the-art triaxial testing equipment, based on soil mechanics technology, to investigate the mechanical behaviour, under high effective strsesses, of a number of mudrocks from both the UK and overseas. The deformation experiments have demonstrated that mudrocks broadly obey the critical state model of material behaviour, although some modifications are necessary. Rapid changes in the deviatoric component of stress acting upon mudrocks tend to be accompanied by undrained deformation because of their low permeabilities. This condition is simply emulated during laboratory testing using undrained loading following consolidation. Shear deformation experiments conducted at a range of consolidation pressures on a range of mudrocks demonstrated that these materials define a failure envelope, although the gradient of the envelope appears to reduce at very high pressures when the materials are deforming in a ductile manner. At moderate pressures (>4 MPa), the London Clay samples demonstrate a previously unreported bifurcation in behaviour, representing the brittle - ductile transition. As undrained deformation proceeds the samples reach a stable state during which strain accumulates with no increase in stress. This continues until at a critical strain (dependent upon consolidation pressure), sudden weakening occurs, when the stress path drops to a residual value. These residual stresses define a second failure line with similar properties to a residual critical state line. The nature of this behaviour is investigated and deformation is compared with that of other geological materials and artificial substances. Some examples are given of the influence of this transitional behaviour in geological environments