The dam safety minefield16 June 2010
Eight percent of coal production in New South Wales’ booming mining industry was mined near dams and reservoirs last year. David Hilyard explains the balancing act which has to be maintained between dam safety and the efficient utilisation of coal resources in Australia
New South Wales in Australia has a booming mining industry, with both coal and metal mines prominent in the state’s economy. Over 100 large dams overseen by the New South Wales Dam Safety Committee (DSC) are located on or near mine sites, or are affected by mining activities. NSW is notable for having extensive coal deposits – and the associated mine workings – adjacent to several large water supply dams which supply a large proportion of the water for Sydney, Australia’s largest city with a population of 4M. The state government moved to protect dams from the impacts of mining in 1978, creating a system with a high level of regulation. The DSC manages this system, regulating actions by both the mining companies and dam owners.
Mining could have serious impacts on dams and the community, ranging from potential environmental damage – release of toxic or unsightly tailings, or loss of saline mine water, through to loss of life in dam failures, and damage to major water supply infrastructure, resulting in a loss of drinking water for large populations.
Table 1 is a list of potential risks to dams resulting from mining activities, and typical management procedures used to deal with the risks. Although the DSC’s main aim is to ensure dam safety is not compromised, it is also aware that available coal resources should be efficiently utilised where possible. The consequence of not allowing mining would be the loss of a valuable resource to the state, and of the economic activity generated by exploiting mineral resources near dams. In 2008/09, 13.5M tonnes of coal were mined near dams and reservoirs, which was about 8% of total coal production.
Mine subsidence is caused by underground mining, particularly longwall mining where staged mine roof collapse is expected. Blast impacts mainly result from open cut mining. The coalfields south of Sydney, surrounded by drinking water infrastructure (see Figure 1), are worked by underground mining, including large modern longwall mines. These are extremely efficient but, if not managed properly, may result in extensive mine subsidence impacts to water supply and other infrastructure.
The DSC oversees the safety of all prescribed dams in NSW, including water dams (public drinking water supply, irrigation, mining, and power station dams), mine tailings and power station ash dams, and dry detention basins. The DSC has powers under NSW state legislation, mainly the Mining Act (1992) and Dams Safety Act (1978). These powers allow the DSC to define areas around dams where mining activities can be overseen, and to amend mining leases so that mining companies are required to comply with the DSC’s requirements.
Mining currently impacts water supply dams, tailings dams, and ash dams, mainly in the Sydney Basin coalfields. When mining companies apply to mine in defined areas, the DSC assesses applications using a risk-based approach. The requirements for mining in a high-risk environment – say longwall mining near a public water supply dam wall and reservoir – will be substantially higher than in a low risk situation, such as open cut mining distant from a small dam retaining stabilised inert tailings with no significant infrastructure downstream. The application process requires the company to present a convincing argument that mining will not reduce safety of the dam or storage below limits acceptable to the DSC. Good understanding of site geology, hydrogeology, mining engineering, and mine subsidence are critical. Site-specific investigations by independent experts typically form a major part of the application.
Risks are assessed from a dam engineering perspective, with generally lower tolerance of risk, over longer time frames, than may be common in the mining industry. The review process includes an opportunity for the dam owner to make comments on the application. The DSC has developed a coherent risk assessment procedure for assessing applications (Reid 2007). Acceptance criteria are based on frequency and consequence of the hazard, and also reflect a civil engineering perspective of tolerable risk (see Figure 2). Risks to the dam wall and of storage loss are reviewed using a structured process. Risks falling outside tolerable criteria require further input from the company to develop additional controls and to demonstrate that the modified proposal meets the criteria. Modifications may include review of mine layouts and practices, increased monitoring, and trigger-action-response plans (TARPs).
For assessing cases where loss of storage is a potential risk, a systematic set of possible flow paths and mechanisms was developed. Tolerable storage losses are defined in risk terms, with very low levels of tolerability for large losses. Modelling of flows along identified flow paths can indicate whether losses are regarded as tolerable or not (see Figure 3). Uncertainties in input parameters are handled using Monte Carlo methods.
Approvals to mine usually include a programme of monitoring, tailored to suit the application. Typical monitoring streams may include:
• Dam surveillance.
• Inspection of mine workings — to ensure that pillar and roof strata are behaving according to predictions, and to note changes in seepage into the mine.
• Survey measurement of surface movements resulting from mine subsidence.
• Mine blast vibration at the dam wall.
• Mine water balance.
• Mine water chemical fingerprinting to determine the source of inflow water.
• Groundwater monitoring.
Adequate dam surveillance is a fundamental monitoring tool in these management plans. It should be robust, high-frequency, and multi-stream. Visual inspections of the dam, crest movement surveys, seepage, piezometers, and crack monitoring may all be useful. During mining activities, frequencies of inspection and monitoring are usually increased to intervals much shorter than those for conventional dam surveillance programmes. Drawing down supply levels whilst mining is nearby may also be a useful strategy, if this is feasible. Monitoring is carried out by the mining company and dam owner, with regular oversight and inspections by DSC staff.
In addition to monitoring programmes, for high-risk mining proposals, the DSC requires contingency and mine closure plans, and may make recommendations on security deposits to the state mining agency.
In recent years, successful mining projects have included:
• Repeated subsidence of a Hunter Valley coal tailings embankment by successive longwalls. The tailings in this 40m high homogenous earth and rockfill dam were not fully consolidated. An open cut and an underground mine entrance were located immediately downstream, increasing the consequence of dam failure by placing five to ten identifiable lives at risk. The embankment was subjected to open cut mine blasts at the same time as longwall mining at depths of 150-170m subsided sections of the dam by up to 1.5m. A comprehensive programme of dam wall inspections, crest subsidence surveys, and monitoring of blast vibrations, piezometers, and extensometers was carried out. The result was successful mining of four longwalls under the dam, and open cut mining downstream, with no significant impact on its stability.
• Longwall mining under a major Sydney water supply reservoir. The hazard of leakage of reservoir waters into mine workings posed a threat to a reservoir forming part of Sydney’s drinking water supply. The coal seam was 300-350m deep, and in the area of the reservoir, longwall mining was modified by reducing the amount of coal extracted by using narrower longwall panels and wider pillars. The revised mine layout reduced mine subsidence impacts on the reservoir. Longwall mining was not permitted near the dam wall, a 56m high rigid concrete gravity dam. Extensive monitoring included observing stability of mine workings, geological mapping, mine water flows, subsidence, strain, and dam wall movements. The result was, again, successful mining of 15 longwalls from a coal resource that otherwise would have been sterilised.
• Open cut mining of a pit located immediately downstream of a mine water dam north of Newcastle. A crew of five to ten workers in the pit were subject to the risk of immediate inundation in the case of catastrophic dam failure, as the pit was about 50m downstream of the toe of an 18m high zoned earth and rockfill dam. The dam was threatened by mine blast vibrations, and failure of the pit wall destabilising the embankment. A pit wall stability review was required. Strict blasting limits were imposed, and monitoring included blast vibration measurements, regular dam inspections, crest surveys, pit wall inspections, and piezometer readings. Mining was completed successfully below the dam, without any significant impact on the stability of the structure.
David Hilyard is a geologist currently employed as a senior consultant in mining and dams safety by Aurecon in Sydney, Australia. From 2003-10 he worked for the NSW Dams Safety Committee, and was Manager of Mining Impacts. Email: [email protected]