ABSTRACT: Recent improvements are described in several techniques for ground control in underground mining. Promising methods for analysis and mitigation of rockbursts are discussed. Methods of assessing damage to underground excavations from repetitive seismic loading are reviewed. Analysis of mining-induced surface subsidence is discussed.
INTRODUCTION
The purpose of this paper is to assess the current status of rock mechanics practice in mining and excavation engineering, and to identify those topics which need further attention to improve the reliability of particular aspects of engineering design in rock. The review is by no means comprehensive. It is intended instead to indicate some areas where substantial progress has been made recently, in both engineering principles and field practice, and to consider issues of topical significance in both mining practice and engineering construction in rock.
Mining practice involves maintenance or deliberate modification of the properties of the host rock mass. In this regard, research in techniques to preserve rock mass integrity has improved the technology of large-scale rock reinforcement and engineered backfills. Improved practices for design of grout curtains are developing for reduction of fissure permeability around shafts and similar facilities where groundwater flow must be restricted. For mine settings prone to induced seismicity and rockbursts, there has been considerable improvement in understanding of rockburst mechanics, with the prospect that control and mitigation measures may be developed.
There are several matters for which the state of theory and practice needs to improved to satisfy demands for a more reliable capacity to predict rock mass response to engineering activity. These include surface subsidence over longwall mines in areas with faulted cover and irregular surface topography, the dynamic performance of underground excavations subject to repetitive seismic loading and joint-controlled creep around excavations in hard rock.
1 GROUND CONTROL TECHNIQUES
Recent developments in ground control practice have been concerned with improvement of rock mass capacity to sustain induced loads and to maintain integrity while resisting displacements. Techniques which have improved substantially, in either operational function or design principles, include large-scale rock reinforcement, backfill design and grouting.
1.1 Rock reinforcement
Several recent well-executed investigations demonstrate the performance and benefit of cable reinforcement of stope boundaries. These include test stopes at the Cart Forks Mine (Pariseau et al., 1984), and the Mount Isa Mine, Australia (Greenelsh, 1985). The evaluation of several cable bolt reinforcement patterns in stopes at the Pyhasalmi Mine, Finland, is reported by Lappalainen and Antikainen (1987).
Field investigations of rock reinforcement show that the grouted steel tendons are effective when loads are mobilized in the reinforcement elements by inelastic strains in the host rock. This indicates that appropriate methods of design analysis for cable bolt reinforcement must provide for large strain in the constitutive model of the rock mass. A finite difference scheme simulating the interaction of deforming rock with grouted tendons has been described by Cundall and Board (1988), based on the conceptual model of reinforcement presented by St.John and Van Dillen (1983).
