Assessing the conditions at the boundary of excavations in bedded or foliated rocks

Abstract

Routine ground support design requires a rapid analysis tool to allow sensitivity studies on the impact of variations in the nature of bedded or foliated rock masses. A heuristic technique based on the adoption of an equivalent transversely isotropic continuum has been developed that includes a shear modulus that reflects the spacing of bedding partings or foliations . This results in the prediction of much higher stresses around an excavation when compared to an isotropic assumption. A unique relationship between a continuum shear modulus and spacing is not possible because the shear stiffness of bedding partings or foliations increases with increasing vertical stress. The heuristic can be applied to sectors of an excavation where the boundary is sub-parallel to the partings being considered. By adopting an anisotropic stress model, rock mass failure can be modelled using an isotropic brittle strength model. A re-examination of published data on overbreak in jointed quartzites and a moderately foliated rock mass at high depths collaborates the use of the technique. The heuristic technique produces similar failure zones to those predicted using an isotropic stress/ubiquitous joint approach. • Adopting transverse isotropy significantly increases the estimated stress changes near the excavation boundary compared to an isotropic assumption. • Combining a transversely isotropic material model with brittle rock failure concepts results in a useable heuristic for analysing excavation in bedded rock. • A shear stiffness of 3 GPa/m has been adopted for bedding partings or foliations under low normal stress. • Simple elastic anisotropic stress models can be used, but implementation requires the use of design sectors for different excavation faces.