Williams, DJ 2006, 'Mine Closure as a Driver for Waste Rock Dump Construction', in AB Fourie & M Tibbett (eds), Proceedings of the First International Seminar on Mine Closure
, Australian Centre for Geomechanics, Perth, pp. 697-706, https://doi.org/10.36487/ACG_repo/605_61
Waste rock dump construction is driven largely by the costs associated with hauling the rock. In recent
times, there have been large increases in certain costs associated with conventional haulage; namely, the cost
of diesel and the cost and restricted availability of new tyres. These cost pressures have impacted on the
balance between horizontal and vertical haulage, affecting the areal extent and height of the dump. Dumps
tend to be expanded, both upwards (by additional lifts) and laterally, in stages. There has been little
incentive to progressively cover the waste rock dump during construction, since the dump is kept active to
accommodate expected and possible expansion of the mine and Net Present Value accounting promotes
delayed mine closure.
Increasingly, other drivers are coming to the fore, including mine closure requirements. These include the
need to encapsulate, with benign waste rock, potentially acid forming waste rock; the need to limit
potentially contaminated seepage from the dump; and the need to limit erosion loss off the dump. Since
benign waste rock may only be produced during the early stages of open pit mining, there is the risk that it
will be buried beneath deeper-won potentially acid forming waste rock, leaving no benign material with
which to encapsulate the potentially acid forming waste rock. Potentially acid forming waste rock needs to
be placed beneath the flat top surface of the dump, and the relative proportions of benign and potentially acid
forming waste rock will dictate the dump width to height ratio, and the final angle of the outer batters of the
dump, that makes this possible.
By keeping the dump active throughout the mine life, rainfall and oxygen are free to infiltrate the dump,
leading to the wetting up of the dump and the oxidation of potentially acid forming waste rock. Even after
an effective rainfall percolation-limiting cover has been placed on the dump, the water stored within the
dump during its operation will continue to seep at a diminishing rate for many years, carrying with it any
oxidation products. The conventional practice of placing a fine-grained growth medium on the steep side
slopes of the dump often leads to unacceptably high rates of erosion loss. The paper presents refined
approaches to waste rock dump construction driven by mine closure requirements.
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Mine Closure as a Driver for Waste Rock Dump Construction D.J. Williams
706 Mine Closure 2006, Perth, Australia