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2 <strong>ISRM</strong> (India) Journal FROM THE EDITOR’S DESK I thank all the readers of the inaugural issue of the journal for their feedback about the journal. The feeback from all the quarters has given us the encouragement to our initiative and to bring out a quality journal. The Society initiated its activities with a Seminar on "Geotechnical Challenges in Water Resources Projects" in January 2012 in Dehradun. The objective of the proposed Seminar was to provide a forum for the design and construction engineers to discuss the solutions to various geological and geotechnical surprises encountered during the execution of various River Valley Projects and use the knowledge for safe and economical design and construction of such projects. There was participation from 26 organisations, including from Bhutan, in the Seminar. The activities planned during the second half of the year, include Seminars on "Ground Control and Improvement", in September 2012 and "Slope Stabilization Challenges in Infrastructure Projects" in November 2012. The objectives of the proposed Seminar are to provide a forum for the design and construction engineers to discuss the potentially applicable ground improvement methods for civil works structures and the solutions to various geological and geotechnical surprises encountered during the execution of various River Valley Projects and use the knowledge for safe and economical design and construction of such projects. From the year 2013, the Society has planned for one major activity on Rock Mechanics to be held annually, in addition to other courses and workshops on the subject. The first national event is planned shortly. The formal announcement will be made soon. I thank all the authors for their contributions to this issue. I thank the members of the Editorial Board for sparing their valuable time to review the papers and offer their comments/suggestions to improve the contents of the articles. One of the suggestions received from the readers of the journal is to take up more case studies related contributions. I request all the readers and their colleagues/fellow professionals to contribute the case studies to further improve the utility of the Journal. V.K. Kanjlia Member Secretary Indian National Group of <strong>ISRM</strong> Volume 1 No. 2 July 2012
3 SUPPORTING COLLAPSED AND HIGHLY DEFORMED AREAS OF BLOCK CAVE EXTRACTION LEVELS D. Beck E. Villaescusa Beck Engineering Pty Ltd, Australia CRC Mining, Australia ABSTRACT Large deformations in deep mines occur due to the exposure of extensively damaged rock mass adjacent to an excavation. In order to properly simulate the support response in such conditions, the ground deformation must be captured accurately. The mechanisms of rock mass damage, dilation and deterioration must first be correctly simulated to produce a realistic tunnel deformation in three dimensions. Furthermore, the physical response of the support elements must be realistic. In this paper, a multi-scale approach to mine deformation modelling has been described to improve the simulation similitude of ground support capacity and demand. Case studies were used to demonstrate the behaviour of several heavy support systems using this approach. Some support designs for squeezing ground were tested and the limitations and vulnerabilities of the support systems described. The modelling methods used to simulate the mine deformation, drive behaviour and support system response were discussed and some sufficiency requirements for similar analysis were highlighted. 1. INTRODUCTION The purpose of ground support is to ensure that excavations remain safe and open for their intended life span. The effectiveness of a ground support strategy is important for two main reasons, namely safety to personnel and equipment and to achieve the most economical access to extract ore. Ground support consists of rock support and rock reinforcement (Windsor & Thompson, 1993) which is installed into a deformed, discontinuous rock mass. The loads that develop in the support system are a function of displacement and distortion due to damage in the rock mass and the resultant forces generated by blocks and wedges that would otherwise be kinematically free to fall or slide into the excavation (Beck et al, 2010). To evaluate the likely performance and behaviour of a support system, the evolution and timing of the discontinuous deformation within a rock mass must be captured. This is a complex problem - if the simulated ‘loading system’ displacements, or the simulated support system load-displacement response is incorrect, the evaluation will not be sufficient. 2. CAUSES OF SIGNIFICANT EXTRACTION LEVEL DEFORMATION Deformation in undercut and extraction level development occurs in stages. Damage to the walls of excavations prior to extreme closure can be significant but is usually managed with modest support or rehabilitation, while extreme damage - drive closure and collapse - occurs when pillar cores fail. Pillar core failure is the most extreme case of extraction level collapse, as opposed to damage and differentiates ‘normal’ caving ground control problems from extreme failure. By definition, pillar instability is reached when the strain in the pillar core reaches a critical level, either due to failure of the rock mass in the pillar or due to yield and dislocation of structure. The effect is the same; at the critical level of strain in the pillar core, pillar deformation will continue to occur at constant (or reducing) stress. Large scale structures contribute by either weakening the pillar or by concentrating loads in susceptible areas of the footprint. Interpreting pillar core conditions using plastic strain is consistent with standard geotechnical practice. Conventional theorems of plastic collapse for limit analysis are well documented (Yu, 2006; Hodge, 1958; Davis, 1968; Lubliner, 1990; Drucker et al., 1952; Hill, 1951). The only possible interpretation of significant plastic strain in pillar cores is pillar collapse, while moderate plastic strain should be associated with a step change in adjacent tunnel deformation if not collapse. For block caves, the estimation of pillar core damage is complex. The extraction horizon is a closely coupled system of small, diamond shaped pillars - a heavily extracted slice 4-5 or so metres high within which virtually all of the vertical cave loads acting at the base of the cave must be carried and an equilibrium attained. An example relation between crosscut spacing and extraction ratio (assuming all other dimensions are fixed) for a particular block cave extraction level design is shown in Figure 1. As crosscut spacing decreases, the extraction 3 Volume 1 No. 2 July 2012
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