Anisotropy in jointed rock mass breakage

Abstract

Cracks in a rock material are generally closed and have no volume effect on the rock material's deformational characteristics. These closed cracks, however, markedly effect the rock materials strength. Joints, on the other hand, do have a volume effect on the rock mass's deformational behaviour as well as its strength or stability. In order to define the rock mass's deformational characteristics, the deformational moduli are expressed in terms of equilibrium and/or compatibility models. For complete definition of the rock mass's deformational moduli, both models are required. These models describe the deforming rock mass when there is no stress redistribution (equilibrium model) or when there is stress redistribution (compatibility model). For joints, it is shown that both the equilibrium and compatibility models are required to define the rock mass's deformational characteristics while for cracks only the equilibrium model is required. It is shown, that shear moduli have major affects on the rock mass's deformational behaviour. By incorporating joint shear strength and Griffith crack energy initiation criteria, Griffith strength loci for hard and soft rock masses are obtained. These strength loci are produced for both shear and pseudo-Young's rock mass moduli. For the hard rock, the strength loci are significantly different indicating that the rock mass's failure mode possesses different energy balances. For the soft rock mass, the two strength loci are similar indicating that in the rock mass's failure, from either shear or direct deformation, the energy balances are nearly equal. This suggests that the soft rock mass has minimal size effects.