A Model for Pore Pressure Response of a Claystone due to Liberated Residual Stress Dilation

The hydromechanical response of Opalinus Clay is being studied as a potential host formation for the long-term geological storage of nuclear waste. Two separate mine-by field experiments were conducted at Mont Terri rock laboratory, Switzerland: one in 1997–1998 (HD-B) and one in 2008 (MB Niche) to observe the claystone’s response to tunnel excavation. The data from both mine-by tests revealed an undrained pore pressure response dominated by significant pore pressure drop (i.e., a dilational tendency), even in instrumentation locations where stress-induced damage (‘failure’) was not anticipated. This response is not reproducible by conventional hydromechanical models unless very low yield envelopes are used to predict the onset of dilation. An alternative mechanism for the undrained dilational response of Opalinus Clay due primarily to unloading in the low confining stress regime is presented in this paper. The mechanism is derived from observed behaviour of the Opalinus Clay in the field and laboratory, and an understanding of the composition and geological history of the material. The model is based on mechanisms associated with liberation of latent strain energy, and associated residual stress (‘locked-in stress’ or ‘stress memory’) through microcrack formation. Mobilization of this internal stress on the undrained response of Opalinus Clay results in distinct regimes of hydromechanical response, dependent on confining stress levels, that is accounted for in the model. The model was implemented into the continuum finite difference code FLAC3D and produced good agreement with mine-by test observations.