New geomechanics design approach applied to mining

The damage and failure processes around underground openings excavated in brittle rock mass under high in situ stresses are not well understood. A fundamental understanding of these stress-driven phenomena is important, for example, as the depths of Canadian hard-rock mines increase and as Canada seeks to store radioactive wastes at depth. The difficulty of quantifying the complete damage and failure processes is due in part to the fact that it is not possible to test representative elemental volumes of these rock mass materials, due to the scale involved. However, the increasing operating depth of some Canadian mines provides unique opportunities for allowing the observation of these actual processes; therefore, the mine openings can be viewed as analogous to large-scale testing machines. Based on field observations on actual damage and failure processes in both massive and moderately jointed brittle rock masses surrounding up to 2200 m deep excavations, Castro and McCreath (1997a) proposed an interpretation for these processes. It indicates that at great depth and in a moderately jointed rock mass, preexisting discontinuities have only a minor influence on the initiation of fracturing or damage inside the rock mass. The stress-induced damage process begins by the nucleation and propagation of extension fractures within the blocks of intact rock inside the rock mass. With this finding, a basic rock mass damage initiation (DI) criterion was suggested for identifying the DI zones around deep openings. The actual rock mass "peak" strength and the macroscopic mode of failure are strongly influenced by the field loading system characteristics. For this reason, the peak strength was defined as the "rock mass system strength", (sigma(c))(sys), and it should be obtained by back-analysis, such as the approach presented here. This paper presents a practical application of this back-analysis approach, and the use of the DI criterion, to compare the design of access drifts with flat versus arched roofs or backs. Numerical results, using the finite element program Phase(2), are compared with field observations at deep underground mines in the Sudbury Basin in Canada. Authors of papers to proceedings have to type these in a form suitable for direct photographic reproduction by the publisher. In order to ensure uniform style throughout the volume, all the papers have to be prepared strictly according to the instructions set below. A laser printer should be used to print the text. The publisher will reduce the camera-ready copy to 75% and print it in black only. For the convenience of the authors template files for MS Word 6.0 (and higher) are provided.