Modelling a mine-by test at the Mont Terri rock laboratory, Switzerland

An instrumented mine-by test was conducted at the Mont Terri rock laboratory, Switzerland in 1997–1998 to assess issues associated with tunnel excavation in a middle Jurassic claystone known as the Opalinus Clay. Excavation-induced stresses resulted in unusually large deformations and the development of an excavation disturbed zone (EDZ) around the tunnel where the observed dominant mode of yielding consisted of extensional fracturing. From field and laboratory observations, it has been observed that Opalinus Clay exhibits responses that often are not represented adequately by linear-elastic or elasto-plastic models. In particular, strong non-linear elastic behaviour at low stresses was observed in laboratory tests. This behaviour has been captured by a phenomenological-based model, known as the stress-dependent modulus (SDM) model. The concepts of the SDM model have been extended to a piece-wise pore pressure formulation that captures the hydromechanical rockmass response. These models were implemented into a finite difference method numerical code and used to simulate the short-term deformations and pore pressure response of the ED-B mine-by test. These simply calibrated models provided a reasonable fit to the field data, particularly in regions of unloading where rockmass deformations were not dominated by dilation.