The Role of Capillary Suction and Dilatancy on the Interpretation of the Confined Strength of Clay Shales

Peak laboratory strength data obtained from low permeability clay shales/rocks often display a bilinear or non-linear failure envelope. In many of these tests pore pressures are not measured because of issues associated with back-saturation prior to confined compressive loading. In such tests the effective state of stress at failure remains unknown and provides room for speculation. The non-linear shape of the failure envelope may have several reasons including the influence of capillary forces or the tendency of a saturated clay shale to dilate as the confining stress is changed. The former stems from sample preservation, preparation or core unloading effects and represents a partly saturated state (i.e., not representative for the in-situ state); the latter represents the response of a saturated material. This contribution discusses these issues based on a data set obtained from Opalinus Clay specimens, and concludes that without a careful back-saturation processes knowledge of the underpinning reasons for the non-linear failure envelope cannot be obtained.