Growth of geologic fractures into large-strain populations: review of nomenclature, subcritical crack growth, and some implications for rock engineering

Several aspects of fracture arrays are reviewed briefly and discussed. The terminology applied to progressive or multi-stage brittle deformation in ro ck masses is improved by noting fundamental mechanical differences in fract ure type and the kinematic coupling between dilatant mixed-mode crack displ acements and wing cracks developed at the fracture tips. An array of initia lly mixed-mode (I-II) cracks will evolve under remote tensile least princip al stress and with increasing strain to a dilatant, mode-I crack array orie nted approximately perpendicular to the remote tensile stress. This progres sive fracture growth thus defeats predictions of fracture-set orientation a nd displacement based only on a Mohr circle estimate of initial elastic str ess (valid in the rock mass only at the earliest stages of fracture nucleat ion). Slow, subcritical crack growth in rock is associated with distinctive changes in fracture population geometry, as shown by published numerical s imulations of fracture-network evolution. An increase in the stress corrosi on index promotes joint clustering and significant changes in joint length- frequency that may lead to characteristic differences in the statistics of large-strain fracture populations. These geometric clues can be used to ref ine estimates of strength and deformability of rock masses and to infer cla sses of physico-chemical processes acting at the fracture tips during the d evelopment of the fracture population. (C) 2000 Elsevier Science Ltd. Ail r ights reserved.