New direct shear testing protocols and analyses for fractures and healed intrablock rockmass discontinuities

Modern geotechnical numerical design is limited by conventional characterization and data collection practices, which do not capture detailed parameters of intact rock and rockmass structure that are necessary for input to sophisticated and powerful simulation tools. Furthermore, as modern underground excavations go deeper and enter into more high stress environments with complex excavation geometries and associated stress paths, healed structures within initially intact rock blocks such as sedimentary nodules and hydrothermal veins (termed intrablock structure, where conventional fractures are termed interblock structure) are having an increasing influence on rockmass behaviour and should be included in modern geotechnical design. The role of sedimentary nodular intrablock structure in the Cobourg limestone on rockmass behaviour is an important consideration for the design of Canadian Deep Geological Repositories (DGRs) for the permanent storage of nuclear waste. This direct shear test program provides valuable laboratory data results for mechanical properties of targeted fracture surfaces and intrablock features in the Cobourg limestone for application to the numerical geotechnical design of prospective DGRs. Measurements of normal stiffness (Kn), shear stiffness (Ks), shear strength (in terms of cohesion (c) and friction angle (ϕ) from the Mohr-Coulomb shear strength criterion), and initial dilation angle (ψ) parameters are assessed and critically evaluated. These established parameters for fractures are applied to the tests on intrablock structures to provide a consistent basis for comparison and to enable the use of existing mechanical parameters in numerical model inputs. The implications of normal and shear stiffness property selection are evaluated at the excavation scale using finite element models with explicit rockmass structure. The critical analyses of conventional calculations of parameters results in improved calculation methods with mechanistic reasoning for the ultimate application of the data to sophisticated numerical models with explicit or discrete rockmass structure.