Managing High Pressure Water Inflows at the Ernest Henry Underground Mine Anna Greve1, Joe Evert2, Daniel Ashton1 1 Ernest Henry Mining, [email protected], [email protected], Cloncurry, QLD, Australia 2 Mineright, [email protected], Cairns, QLD, Australia ABSTRACT In March 2012 large flows of high pressure (5200 kPa) and high temperature (49 degrees) water were intersected during development in the Ernest Henry Underground Mine. To predict and mitigate the risk of water intersects during future development, the present conceptual model of the local fractured aquifer was reviewed and a dewatering strategy targeting the highest water risk zone was developed. In addition to a targeted dewatering strategy, stand piping and grouting methods have been optimized to allow development through formations with high water pressure and high hydraulic conductivity. Combining hydrograph data from 56 Vibrating Wire piezometers with a reviewed structure interpretation allowed identification of eight aquifer zones with different degrees of inter and intra zone fracture connectivity. Targeting the zone with the highest hydraulic conductivity for dewatering allowed dropping the water pressure at the current main development heading from 4200 kPa to 700 kPa within two months. Drilling through valved standpipes mitigated the risk of uncontrolled water inflows through drill holes. Dewatering and optimization of standpiping and grouting methods has allowed a sequential reduction in the cycle time for mine development through high water risk areas. BACKGROUND The Ernest Henry Mine (EHM) is a copper, gold and magnetite mine located 160 km East of Mount Isa. EHM consists of an open pit and an underground mine. Commercial production of the open pit commenced in 1998 and continued until production of the underground operation commenced in 2011. Underground production is expected to continue until at least 2024. The EHM deposit is located in a fault bounded mineralized breccia pipe, intruding mafic and intermediate volcanics of Proterozoic age. Above the igneous Proterozoic complex lies a cover sequence, up to 60m thick, consisting of Mesozoic sedimentary rocks of the Carpentaria Basin and Tertiary sediments of the Karumba Basin. At EHM, the lowest unit of the Mesozoic sequence is the Gilbert River Formation. This unit consists of semi-consolidated, weakly cemented, quartz sand and gravel and has a typical thickness of about 5 m. Water at EHM is encountered in this Gilbert River Formation as well as in fractures in the Proterozoic volcanics. In March 2012 water flow of approximately 80 l/s at high pressure (5200 kPa) and high temperature (49 degrees Celsius) was intersected 825 m below ground level during development of a priority heading. To control the water intersected within the fractured Proterozoic relief holes from higher mine levels had to be drilled. Continuing development for an additional 600 meters past the water intersect was required to meet the life of mine targets. To predict and mitigate the risk of water intersects during this development, the conceptual model of the local fractured aquifer was reviewed and a targeted dewatering strategy was developed. In addition to targeted dewatering, stand piping and grouting methods have been adjusted from tunneling methods to account for the higher water seepage tolerance and steeper declines of the mining environment. CONCEPTUAL MODEL REVIEW AND TARGETED DEWATERING An improved understanding of the fractured aquifer was needed to predict and mitigate the risk of future water intersects. Five Diamond drill holes were drilled to investigate potential water intersects and to install multi level grouted Vibrating Wire piezometers. Hydrograph analysis from 32 old and 24 newly installed piezometers highlighted a strong aquifer compartmentalization. Based on the hydrograph response as well as through the identification of boundary structures a total of eight aquifer zones were identified at EHM. Boundary structures and the resulting hydraulic disconnection between the eight aquifer zones were identified through differences in hydrograph responses. An example of a disconnection through a boundary structure is shown in Figure 1a. Here hydrographs for a multilevel grouted piezometer intersecting one of the major
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