Observations on non-linear rock mass strength from the Nantlle Valley, Gwynedd, Wales

Accurate prediction of non-linear rock mass strength is required to model the ground as a mechanical continuum to aid design and analysis of slopes and underground excavations. Two failure criteria are discussed: The Generalised Hoek-Brown criterion defined in terms of maximum and minimum principal stress, and Barton’s criterion in terms of normal and shear stress. The Generalised Hoek-Brown criterion defines non-linearity and the frictional properties of the rock by way of a petrographic constant mi. Barton’s joint roughness coefficient controls the curvature of the failure envelope and gives more pronounced non-linearity. Numerical simulations back analysing failure of a large man-made near vertical rock slope identified combinations of Hoek-Brown parameters resulting in failure. Neither criterion was found to provide a satisfactory explanation for the observed failure, several decades after the cessation of quarrying. The collapse was attributed to failure of rock bridges after delayed fracture propagation, potentially from the weathering of minerals promoted by water penetration into the rock mass from joints formed or opened in response to significant horizontal stress relief from original quarry excavation. To support the back analysis process and the potential development of the quarries for a hydropower scheme several strength envelopes based on Barton’s criterion were determined for the principal geological subdivisions of the area. Failure envelopes were determined for various late Neoproterozoic, Cambrian and Ordovician, tuffs, metapelites and metapsammites as well acid and basic intrusions, beyond the extent of the proposed hydroelectric scheme. From the failure envelopes derived instantaneous Mohr-Coulomb shear strength parameters are provided for various stress increments