Investigation of potential longwall mining impact on Upper Cordeaux No.2 Dam

The New South Wales Southern Coalfield encompasses a region that consists of a number of dams and reservoirs. In general, substantial mining near a significant dam structure can only be considered upon completing an investigation to assess the potential impact of mining on the dam structure and storage. Such an investigation was performed by WorleyParsons on behalf of BHP Billiton Illawarra Coal to study the potential mining impact of the proposed Dendrobium Area 2 longwall coal extraction on Upper Cordeaux No.2 Dam. This involved estimating the mining-induced ground movement at the dam site, establishing suitable acceptance criteria for the dam, developing a numerical model to analysis a series of load cases and to understand the dam behaviour, carrying out safety assessment, evaluating the sensitivity and uncertainties of the analysis, and finally making recommendations to manage the proposed mining impact. The response of the concrete arch dam, which was built on a crinanite foundation, due to farfield mining-induced ground movements was simulated. The predicted ground deformation caused by valley opening, upsidence and closure was analysed. The results of the investigation became supporting information to the mining application to the NSW Dams Safety Committee, and to the Sydney Catchment Authority, who is the dam owner. This paper describes the methodology used and shows the various stabilising mechanisms the dam will activate as the valley is gradually deformed. A number of monitoring and remedial actions were recommended to ensure that any potential impact to the dam structure can be managed prior to, during and post mining.