This paper discusses a methodology for using discontinuum analysis in the design of deep underground excavations in jointed rock. Discontinuum computer programs, such as distinct element codes, can assist the design process by evaluating the effect of rock discontinuities on the structural behavior of the excavation. Some practical applications are presented to illustrate the approach.
Joint-controlled mechanisms for rock movement are important considerations in design of deep underground excavations for mining, oil and gas production and civil construction. Mining methods such as block caving and blasthole stoping, and techniques for rockburst mitigation are concerned with structural instability associated with movement along discontinuities. Ground falls resulting from blasting or other seismic events such as rockbursts may be enhanced by the orientation and properties of the rock joint structure around the underground opening. Subsidence phenomena resulting from coal mining or oil field depletion are influenced by the blocky structure of stratified rock units above the extraction horizon. Designs to minimize water leakage around pressure tunnels in jointed rock and grouting schemes to maintain integrity of jointed and fractured rock are both concerned with the mechanical and hydraulic behavior of the joint structure. These are just a few examples where engineering design for an underground excavation requires an understanding of the structural behavior of the discontinuities. Discontinuum analysis provides a computational approach to support designs for deep excavations in jointed rock. Discontinuum numerical techniques (i.e., computer programs which model discontinuities in a system explicitly), have evolved for some twenty years and are now suitable for design calculations to evaluate the effect of rock discontinuities on the structural response of the excavation. This paper discusses a methodology for using discontinuum analyses in design calculations, reviews the state of numerical techniques available for discontinuum analysis and summarizes a few practical applications of this approach.
Rock engineering design involves considerations which can impose severe complications on underground design calculations. For example, the rock mass can have high variability in rock types and properties and, because of limitations in gathering data from field investigations, a considerable uncertainty may exist as to the true situation underground. The history of loading of the rock mass can introduce forces on excavations which exceed average pressures due to overburden loading and these forces can be difficult to measure in situ. The rock also can contain extensive geologic features, such as dikes and faults, or sets of discontinuities, such as bedding planes, joints and similar structures. Discontinuities can also be excavation process (e.g., due to blasting). These features generally impose a three-dimensional discontinuous structure around excavations. These considerations limit the ability of computational analyses to support a design in a quantitative manner, as opposed to designs, for example, in mechanical or electrical engineering. For deep underground excavations it must be recognized that the problem setting is "data-limited" --i.e., there is very seldom an unambiguous knowledge about the rock mass and loading conditions when an underground design is developed.
