Impact of shear induced pore pressures on the ground deformations around underground openings in weak shales

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Countries with argillaceous formations have been investigating these rocks to assess their potential for the disposal of nuclear waste. In Canada, the Nuclear Waste Management Organization (NWMO) has submitted a license application to construct a low and intermediate level nuclear waste repository at a depth of 680 m in clay rich Sedimentary rocks at the Bruce Site in Ontario, and argillaceous formations remains a possible target for used-nuclear fuel disposal. In Switzerland, Nagra has commenced an extensive site investigation program to characterise the Opalinus Clay, and in France, the Callovo-Oxfordian Formation has been selected by Andra for radioactive-waste-disposal characterisation. The nuclear waste industry demands that coupled geomechanical processes are sufficiently well understood to proceed with engineering design. In these argillaceous formations, the geomechanical processes are a function of complex rate and coupled phenomena. To validate our understanding of these coupled processes in situ, field measurements supported with extensive laboratory tests will be carried out to establish the relevant geomechanical processes applicable to repository design. In argillaceous rock, pore pressures must be measured to interpret the fundamental behaviour during the mechanical and thermal loading that will be encountered in a repository underground environment. This environment creates unique boundary conditions that have the potential to generate very large pore pressures on localized zones over large distances. Such pore pressures must be quantified and accounted for in the design of the repository. This research has three specific objectives: 1) Quantify and measure the anisotropic pore pressure response parallel and perpendicular to bedding as shearing progresses using fibre optic scale piezometers installed internally in conventional triaxial tests. 2) Simulate the repository stiffness and stress boundary conditions in a servo controlled direct shear device and quantify the pore pressure response on the shearing plane using internal fibre optic piezometers. 3) Utilize existing and new pore pressure data from a specifically designed test tunnel in the Opalinus Clay at Mont Terri, Switzerland, to re-evaluate the role of shear-induced pore pressures on the underground response. The long-term objective of this research is to establish and describe the appropriate in situ constitutive behaviour of clay shales needed for repository design. In parallel with this work will be the development of a methodology and framework for implementing a performance-based design.