A guide to leading practice sustainable development in mining

Landform construction
In general, erosion of constructed landforms on mine sites is dominated by
gullying—a direct consequence of concentration of run-off by the berms and
discharge of concentrated flows onto batter slopes once the berms fail. The reasons
for berm failure include inaccurate construction, tunnel erosion and overtopping due
to deposition of sediment. Where erosion rates remain significant (commonly in arid
areas where surface vegetation cover is too low to provide erosion control) outer
batter profiles that include berms will require regular maintenance (de-silting) as
long as the erosion continues, or else they will fill with sediment and overtop, causing
gullying. For this reason, some sites have adopted a practice of using berms or some
form of across-slope bank during initial rehabilitation, then removing the berms once
vegetation has established and stabilised the slope.
Other sites have incorporated rock into the surface of outer batter slopes to
reduce erosion potential and enable them to construct relatively long, high, slopes
without berms. Another option is to create concave slope profiles to reduce erosion
potential; usually by a factor of two or three. To develop a new approach to waste
dump construction, the Murrin Murrin nickel mine had the erodibility of a range of
wastes and topsoils assessed using both laboratory and field measurements. Using
that data and long-term rainfall and climate data for the site, computer simulations
of run-off and erosion were used to compare a range of options for outer batter
slopes. Concave slope profiles were developed that had relatively low erosion risk,
though addition of tree debris and laterite gravel was recommended for segments
of the slope that the simulations showed to have highest erosion potential (see LP
Monitoring p.45).
Figure 5.26 – Concave waste dump slope at Murrin Murrin
Surface roughness is an important consideration in rehabilitation of mine site
landforms. Roughness tends to trap water and seed, and there is general acceptance
that a rough surface will provide better vegetation establishment than a smooth one.
However, while the creation of large surface roughness via rip lines or moonscaping
may give benefits in the short term, in the longer term it may lead to increased
erosion and instability of the landform. The value of surface roughness is closely
linked to its persistence through time, which is largely controlled by the particle size
distribution of the material in which the roughness is created.
A GUIDE TO LEADING PRACTICE SUSTAINABLE DEVELOPMENT IN MINING 163