WASTE ROCK DUMP DECOMMISSIONING AT MINES IN NORTHERN WESTERN AUSTRALIA
Gordon McPhail Daniel Wilkinson
Metago Environmental Engineers (Australia) Pty Ltd
15 Bowman Street
South Perth
WA6151

ABSTRACT
Mine waste rock dumps in northern Western Australia are frequently some 400 m high. The dumps are located in a region susceptible to tropical cyclones and regularly experience high intensity rainfall events over the wet season. The majority of the dumps are highly erodable. Moreover, frequently, minerals such aspyrite, talc and chlorite present in the waste rock materials release hydroxides and inorganic salts such as Ca SO4 and MgSO4 which are mobilised by infiltrating water and are released as seepage. The resulting contamination, together with large scale erosion issues, poses significant long term decommissioning challenges. A range of decommissioning alternatives that address infiltration, erosion, storm control issues to varying degrees have been conceptualised and evaluated on the basis of practicality, effectiveness, cost and reliability. This paper describes the evaluations with an emphasis on the key issue of erosion management and control.

Decommissioning of mine waste structures in northern Western Australia, a region susceptible to cyclone events, presents specific problems associated with erosion control as well as dispersion of leached salts through infiltration and seepage. Even where the waste comprises blasted waste rock the intensity of rainfall and associated runoff is sufficient to erode significant tonnages of rock from the slopes of the dump. Slope erosion, and the subsequent deposition of eroded material on benched or terraced areas of waste rock dumps, results in re-profiling of the benches or terraces.
Mining economics frequently dictate that waste rock be dumped in a series of terraces so as to minimise the haul distances and gradients. More specifically, where, waste stripping is carried out at higher elevations, the tendency is for waste rock to be dumped along the natural slope in a series of benches. The benches are tied in to lower terraced areas and frequently result in effective waste rock dump heights of 400 m.

Figure 1 below indicates a typical waste rock dump in the Kimberley region ofWestern Australia. High elevation benches are evident together with lower elevation terracedareas

Figure 2: View of the waste rock dump as at the end of mining

This paper describes assessments carried out on a number concepts for a dump such as indicated in Figure 2. Each concept has been developed to reduce erosion and infiltration. The advantages and disadvantages are set out and a conclusions drawn as to the merits of a novel approach that would take advantage of specific waste rock types to simultaneously address both the infiltration as well the erosion issues.

CONCEPTS EVALUATED
 Stepped profile concept

It has been well established by Willgoose and other researchers [1] and [2] that over the long term slopes erode to a concave profile where the eroded slope gradient is steeper at the upper reaches of the slope and flatter at the lower. It is therefore logical that if the slope profile is constructed as close as practical to the anticipated long term concave profile the total erosion would be minimised.

A number of drawbacks to the stepped profile concept become evident the moment the concept is considered in detail. These are:

• The profile can be costly to construct in as the bulk of the waste rock is placed in the wider terraces which are at low elevation and are also furthest from the source.

• The concept does little to reduce infiltration and therefore does not address leaching problems that may be associated with acid rock drainage products and the like.

  Surface drainage concept

Controlled discharge from the waste rock dump is a concept frequently considered. Notable features of the concept as illustrated are:

• Provision of discharge channels to transfer the water from one terrace level to the next as well as the provision of stilling basins with each discharge channel.

• Formation of drainage trenches on the surface to direct runoff to the discharge channels. These would be lined to reduce infiltration

• Flow rates down the drainage channels range from 10 to 15m3/s and velocities are above 4m/s making it necessary to line the channels and thereby increase infiltration

Sub-surface drainage concept

 Typically zones of a benign hard rock such as quartzite could be interbedded with other materials and could be used to provide preferential drainage routes through the rock dump.

Notable features of the concept are:

• Placement, as part of routine mining operations, of zones of quartzitic material 200 m wide and 30 m high aligned with natural drainage channels
• The formation of sumps which connect the quartzitic zones. The sumps would capture surface runoff directed to the sumps via drainage trenches as per the surface drainage concept described above and then allow infiltration to the toe area via the quartzitic zone. This is illustrated in Figure 5 below. Drainage in the sumps would be predominantly from the side slopes as the base of the pit would be sealed by sediments drawn into the sumps.

• The reliability of the sumps in terms of long term drainage rate and rate of filling with sediments
• Blinding of the quartzitic zone by sediments drawn from the sumps
• Differential settlement of the terraces which may cause changes in surface drainage and thereby increase infiltration

Store and release concept

A typical store and release concept as applied to waste rock dumps would entail the following :

• Paddocking off of the dumps as in the surface and sub-surface concepts
• Placement of a 0.5m compacted clay layer or layer containing sufficient clay materials to form a plastic seal to the rock surface as well as sufficient clay to prevent migration of the clay during seepage
•  Placement of a 2m zone of waste rock with fines. This zone is paddock dumped and flattened without introducing compaction. Vegetation is established in this zone (hence the reason for fines).

The store and release concept operates by trapping and storing rainfall runoff in a perched zone above the clay liner and within the loose waste rock zone during the wet season. Over the year this water is evapo-transpirated by the vegetation established in the uncompacted waste rock zone. The clay layer reduces infiltration to a minimum during the store and release cycle.

Issues of concern in regard to this concept are:

• Availability of suitable clay materials. These would need to come from disturbed areas and would have to be stockpiled until the dump is completed.
•  Substitution of the clay materials with low permeability silty materials or even benign tailings would increase the risk of erosion of the layer into the waste rock and subsequent rat-holing.