Article

Instrumentation and Monitoring of a Large-Scale, Potentially Contaminating Trial Waste Rock Dump

Authors:
  • EMGA Mitchell McLennan
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Abstract

Between 2004 and early 2006 a large-scale, instrumented, potentially contaminating trial waste rock dump was constructed and monitored at Cadia Hill Mine, in NSW, Australia. The trial waste rock dump was instrumented with lysimeters to measure rainfall infiltration and seepage through its base, and temperature sensors and gas sampling tubes to evaluate oxidation of the waste rock, together with three instrumented trial store and release covers on the surface. This thesis describes the construction and instrumentation of the trial waste rock dump and the monitoring results obtained to date, and applies unsaturated soil mechanics principles to understanding the early performance and predicting the future performance of the trial waste rock dump and trial store and release covers. For a given rainfall regime, the rate and quantity of rainfall infiltration into a waste rock dump of a given height, the wetting up of the dump over time, and the occurrence of base seepage will largely be dictated by the particle size distribution of the waste rock delivered to the dump, and the stratigraphy of the dump. The particle size distribution of the waste rock delivered to the dump will depend on the fragmentation of the rock due to blasting and the degree of weathering and hence breakdown on handling of the rock. A waste rock dump constructed by conventional loose end-dumping from haul trucks from a tip-head, as was the case for the trial waste rock dump, consists of a trafficked surface layer extending to a depth of approximately 1 m, underlain by discontinuous alternating coarse and fine-grained layers raveling at the angle of repose of the waste rock, with a base rubble zone of boulders which ravel to the toe of the dump on end-dumping. Trafficking of the surface of the dump by dozers and haul trucks leads to the breaking down, burial and side-casting of the rock to form a well-graded material typically finer than 100 mm in particle size, with a moderate to high water storage capacity. The underlying coarse-grained angle of repose layers serve as air pathways during dry conditions and preferred seepage pathways during and following periods of heavy rainfall resulting in base seepage. The fine-grained angle of repose layers have a moderate to high water storage capacity and largely retain water in storage rather than generating base seepage. The base rubble zone may contain boulders up to 1 m in size, depending on the fragmentation of the rock due to blasting and the degree of weathering and hence breakdown on handling of the rock. It serves largely as a pathway for air during dry conditions, while passing base seepage during and following periods of heavy rainfall. As the dump wets up, partially saturated “fingers” develop and extend into the dump. Partially saturated fine-grained layers, having a medium to high water storage capacity, largely retain their partial saturation, while coarse-grained layers drain resulting, in base seepage. Plugs of water temporarily stored within the dump drain down through the dump, so that the base seepage that emerges is “old” water, not the rainfall infiltration (“new” water) that generated it. The size of the rainfall event required to generate base seepage will decrease as the dump wets up and the partially saturated fingers extend closer to the base of the dump. The residence time of water within the dump that passes along preferred seepage pathways will be relatively short and will become shorter as the dump wets up, while the residence time of water stored within the fine-grained layers will be very long, and possibly indefinite in a dry climate. The ingress of air through the base rubble zone, up the coarse-grained angle of repose layers, through the sides of the dump, and to a lesser extent through the trafficked layer, by the processes of convection, advection and diffusion, respectively, results in the exposure of reactive waste rock to oxidation. The fine-grained reactive waste rock, presenting a far greater surface area per unit volume than the coarse-grained waste rock, and typically having a greater proportion of fresh surfaces, is by far the most reactive. The ingress of air into the fine-grained layers is largely by diffusion from the adjacent coarse-grained layers. The transport of oxidation products from the dump largely occurs during and following periods of heavy rainfall, when preferred pathway flow is mobilised and base seepage occurs. The main exposure to preferred pathway flow is along these pathways, where the surface area per unit volume and hence the proportion of oxidation products are low, with much of the oxidation products formed on the fine-grained particles retained within the dump along with stored water. Due to the discontinuous stratigraphy of a waste rock dump, the preferential pathways for flow are randomly located within the dump. In addition, preferential pathways evolve over time as the waste rock weathers, settles, and as fines are transported with the flow. The trafficked surface of the dump also evolves over time, becoming more heterogeneous as the surface settles differentially, generating internal rainfall runoff and the transport of fines, and the development of “sinkholes” for the preferred entry of ponded rainfall. The principle purpose of cover systems over waste rock dumps is to restrict net percolation into the dump, so that percolation through the reactive waste rock is minimal in the longer term. The approach used to design any cover system is dominated by climate. Semi-arid environments are conducive to store and release cover systems which take advantage of well-graded oxide materials to provide high storage capacities, low percolation and stability. Three trial store and release covers, each comprising a sealing layer overlain by a thick mounded rocky soil mulch layer, were installed at Cadia Hill Mine in 2005-2006 to assess their feasibility to limit net percolation under the climatic conditions encountered at Cadia. This research described in this thesis has demonstrated a number of key issues that should be considered in the management and closure of waste rock dumps: • the initial moisture condition of the end-dumped waste rock will effect its early ability to store incidental rainfall; • the available water storage capacity of the waste rock will affect the size of the triggering rainfall event and the base seepage response time, with the storage capacity being taken up as the dump wets up, reducing both the size of the triggering rainfall event and the response time; • iterative modelling and calculations using HYDRUS-2D suggest that the trial waste rock dump will take between 3 years and 6 years to become sufficiently saturated that it will pass any rainfall infiltration, depending on the extent to which the waste rock weathers over time; and • all three trial store and release covers have demonstrated good performance over the monitoring period, and this has been verified using HYDRUS-2D, , with any net percolation being the result of an initial high placement moisture content of the cover materials.

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