Over the last 10 years argillaceous rocks (mudrocks) have gained acceptance as a potential host formation for geological disposal of nuclear waste. Much of the engineering experience with soft argillaceous rocks has been developed over the past 50 years from the civil engineering construction of dams and slopes, all of which take place at relatively shallow depths. The construction of a nuclear waste repository, however, will take place at depths greater than 400 m. With that in mind, it is essential to develop a strong understanding of the engineering behaviour of this material.

In Switzerland, the Jurassic claystone Opalinus Clay is being considered as a potential repository host formation. Observations from tunnels in Opalinus Clay at the Mont Terri rock laboratory, Switzerland, suggest that the excavation-induced response is not linear-elastic in the classic sense, and therefore, the use of linear-elasticity can lead to significant misrepresentation of rockmass deformations and yield mechanisms around underground excavations.

The fundamental nature of the Opalinus Clay's micro-structure has been identified as a source for its unique behaviour. A conceptual model of the micro-structure, including the effects of diagenetic processes, has been developed as a framework to interpret the mechanical and hydromechanical behaviour of the Opalinus Clay. This rock-mass response has been captured by a phenomenological-based model, known as the stress-dependent modulus (SDM) model, and a piece-wise pore pressure formulation. This allows engineers to capture a significant portion of the observed behaviour without undue modelling complexities using data from simple unconfined and triaxial compression tests.

A comparison of the two models with geophysical measurements and piezometer readings in the vicinity of excavations at the Mont Terri rock laboratory provides encouraging agreement. Furthermore, numerical back-analysis of the ED-B mine-by test, conducted at Mont Terri in 1997-98, demonstrated that the unique pore pressure response and unloading-induced large deformations could be accounted for by these models.