Geotechnical Design at a Mine Site Level - We Have No Choice

It is not that long ago that strata control was the domain of the Under Manager. When the authors joined the Australian Underground Coal Industry in the late 1980’s, few if any mines or mining companies employed qualified geotechnical engineers and the determination of roof support rules and coal pillar design for example were done largely within mine management. One only has to look at the list of participants at the various UNSW pillar design and geomechanics courses in the early to mid-1990’s to substantiate the above statement. Geotechnical engineering in the coal industry was seen in a research and consulting role at that time, with specialist geotechnical engineers being employed by the likes of ACIRL and CSIRO and used by mine sites to advise on the causes of roof control problems and unplanned events after they had occurred. It was out of this environment that the consulting firm Strata Control Technology was formed. By the mid-1990’s, the formalised Strata Management Plan was beginning to evolve, it being focussed on the use of roof monitoring and mapping combined with the traditional observations of miners in order to improve the reactive ability of mine sites to changing strata conditions. The consulting firm Strata Engineering was at the forefront of developments in this area and still regularly publish today on the use of strata monitoring data for decision-making purposes (Thomas 2006). The current situation is that most mines now employ a person in the defined role of the geotechnical engineer, as formalised strata management is now firmly entrenched as one of the core requirements of underground coal mining in both NSW and QLD. However despite the substantial improvements in strata management practices at coal mines in the last two decades, major strata control losses are still sustained by longwall mines on an occasional basis with consequent large business losses (especially given the high coal prices and longwall production levels compared to 20 years ago). No amount of roof monitoring data, borescope observations, hazard mapping or local mining experience was presumably able to either predict or prevent these losses being sustained. A critical element of the strata control process must at times be missing, which is the subject of this paper, namely effective geotechnical design, particularly as it relates to the role of the mine site geotechnical engineer. In considering the role and importance of geotechnical engineering design in coal mining, several basic questions will be addressed: (i) What are the basic elements of engineering design? (ii) What does the legislation require in this area? (iii) Does it offer benefit to operating mine sites and the industry in general? (iv) Why is the mine site geotechnical engineer so important to the future of the coal industry? (v) Where is the industry up to and what are the potential areas for further development? In answering these questions, the authors will address the title of their paper “Geotechnical Design at a Mine Site Level – We Have No Choice” and provide a general response to the question posed by Ross Seedsman and his co-authors several years ago in the paper entitled “Chain Pillar Design – Can We?” (Seedsman et al 2005), which discussed the design limitations and various required outcomes for chain pillars at that time