Review of design parameters for discontinuous numerical modelling of excavations in the Hawkesbury Sandstone

Within a well-established stress environment and with well-established laboratory properties, Hawkesbury Sandstone in the Sydney region of Australia hosts, and will continue to do-so, a significant proportion of the underground infrastructure. In order to conduct the most rigorous geotechnical design, it is recognized that the impact of scale on the response of the excavated system must be well understood. In this regard, one of the major challenges is to translate the small measured (laboratory) geomechanical properties and those of the joints observed at a larger scale to a scale and stress-dependant strength value for design. This study provides a comprehensive review of existing geomechanical data for the Hawkesbury Sandstone and provides an analysis of strength-scale dependency. Large-scale, Synthetic Rock Mass (SRM) tests have been completed to establish a strength-scale relationship that consider defect spacing. Validation of the SRM sample responses has been completed for both the strength and deformation modulus based on reported and/or commonly accepted characteristics of a tunnel-scale rock mass. The post-peak, strain-softening behaviour of SRM samples has been used to establish the critical plastic shear strain and residual strength of each class of Hawkesbury Standstone at the excavation scale. A correlation between the Geological Strength Index (GSI) and the peak and residual strength has been derived.