Mining the coal reserves under the Cataract Reservoir

Mining beneath ground water, reservoirs, rivers, seas and harbours has been successfully carried out in numerous countries for many decades. The risks and engineering considerations have been refined over a long time to enable safety and geotechnical outcomes to be defined and adequately controlled. The experience in Australia has been a prolonged debate over three decades between the mining industry and numerous sectionalised Government Departments. The first major step forward was the Reynolds Inquiry recommendations in the 1970\u27s which determined the limitations to which mining could proceed under stored waters. In 1978, the NSW Government enacted the Dam Safety Act and established a Dams Safety Committee to oversee applications of mining within these areas. Over 3 years from 1990 - 1992 South Bulli worked with the Dam Safety Committee to gain approval for mining the restricted zone beneath Cataract Reservoir for the ongoing viability of its operations. This was the first real test of the recommendations that were handed down by Justice Reynolds Inquiry, and the entire area was divided into zones to determine more accurately some of the engineering factors of subsidence, stress, strain and how the geometry of mine design interacted. These zones would be approved depending on the results of monitoring these parameters in a controlled manner. This thesis analyses the results to date from Stage I and partial Stage II extraction zones and has been used to supplement reports to the Dam Safety Committee. Mining to date has consisted of the extraction of 2.5 million tonnes of raw coal, via development of 30 km of underground roadways within the Cataract area (six longwalls developed), and the extraction of four longwall panels, each of 110m width, separated by 66m wide coal pillars which provide support to the overlying strata. A wide-ranging monitoring programme has been developed and implemented on the surface, in the overburden, and the mine workings, which includes;- • Surface Geological Mapping • Underground Mapping • Geophysical Surveys • In-Seam Longhole Drilling • Surface Subsidence • Surface and Sub-Surface Strains • Groundwater • Mine Water Inflow/Outflow . Pillar Stability The results from these programs confirm that the strata response to mining operations has been very small, which is very close to original expectations. This is significant in that greater confidence can now be placed in the Stage II design. Since the initial predictions were made, various additional work has been carried out. The most significant of these investigations have been reported and discussed in this thesis and contribute to the high confidence in the Stage II mine design. Stage II Application was to develop and extract the next seven mini-longwall blocks, from Longwall 508 up to and including Longwall 514. The mine layout is essentially the same as the Stage I Application, with longwalls retreating approximately from East to West, and development of panels progressing northward. The main change in layout is an increase in extraction width from the current 110m to 120m from Longwalls 511 onwards, and a decrease in chain pillar width from an existing value of 66m to 60m. The - reduction in pillar width is in line with the reduction of cover depth, consistent with the Reynold\u27s Inquiry guidelines. Work described gives a high level of confidence in the assertion that the proposed layout is still conservative and will not threaten the integrity of the Reservoir. This will be confirmed by continuing the existing monitoring and investigation programs, and supplementing them by additional programs to cater for the requirements of the Stage II. The proposed Stage II Monitoring programs are detailed elsewhere in this thesis. The predicted values of surface subsidence and strain for both Stage I and II are detailed below where: a) maximum subsidence will go from 200 mm (Stage I) to 280 mm (Stage II) b) maximum tensile strain will go from 0.6 mm/ m (Stage I) to 0.8 mm/m (Stage II) c) maximum compressive strain will go from 0.6 mm/m (Stage I) to 1.2 mm/m (Stage II). The most important of these parameters is tensile strain. Test work carried out has indicated that the tensile strain failure of the base of the reservoir (in the Hawkesbury Sandstone) is between 3.0 and 7.0 mm/m, which is at least 3.5 times greater than the anticipated strain, and is a sufficient factor of safety to protect the reservoir. As all mining at South Bulli is now concentrated in the Cataract area, continuity of production is vital if the viability of the mine is to be maintained. The Stage II layout extends the mine life by 2.5 years, providing an annual export revenue of $65 million