The Anatomy of a Mine - Mining and the Environment: From Ore to Metal - page 203

Most national jurisdictions put
limits on open pit mining in
forest areas.

Environmental concerns are especially widespread if the ore deposit lies in wilderness
areas and/or forest. Forest plays an essential role in global biochemical cycles such as those
for carbon and nitrogen, as well as maintaining biodiversity and essential wildlife habitat.
Today most national jurisdictions put limits on open pit mining in forest areas.
A common perception of mining is that there is always large-scale disturbance in wilder-
ness areas with signii cant downstream pollution. The reality is that direct mining impacts
are isolated and coni ned to relatively small areas. It is estimated that no more than 0.02
percent of Australia's land surface area (Farrell 1996), less than 0.001 percent of Indonesia's
land surface (Clive Aspinall Report 2002), and about 0.1 percent of Thailand's land sur-
face are directly affected by mining. Similar i gures hold true for most countries. Small is,
however, a relative term. What seems to be a small impact on a national or regional scale
may appear very large at the local scale. It is also the case that, as is often true in Western
Australia, even a small area may encompass a unique and irreplaceable environment.
Ore bodies mined by open pit methods are typically three-dimensional. In contrast, ore
bodies mined by open cast mining predominantly spread in two-dimensions horizontally,
and often there is more need for extensive land clearing. A fair comparison of course, needs
also to take into account the area required for mine waste placement. While mined areas are
larger in surface cast mining, the in-pit placement of waste rock and, sometimes, tailings as
well, eliminates the need for off-pit disposal sites, a key requirement of open pit mining.

Removal of Topsoil and Subsoil

After land clearing, the next step is topsoil removal. Soil is a thin layer of the Earth's crust
that has been modii ed by the natural actions of agents such as water, plant-life, and micro-
organisms. Most terrestrial plants could not survive without soil, so topsoil management
plays an important role in mine reclamation planning. Prior to mining, topsoil is carefully
removed and stored at separate areas to be used for subsequent mine site rehabilitation.
Timing and storage of topsoil removal are important. Topsoil which is too wet or too dry
can result in compaction and loss of soil structure, and long-term stockpiling will reduce
its quality. Where practical, the mine's environmental manager aims for an early return of
the topsoil for rehabilitation of the reconstructed landforms.
In open cast mining (most often seen in coal mining) the rate of rehabilitation is similar
to the rate of mining, so to a large extent, direct return of topsoil is possible, eliminating
the need for stockpiling. Nevertheless, topsoil management can be a demanding process,
requiring substantial resources in terms of expertise, stafi ng, equipment, and i nances.
Further discussion of the value of topsoil in rehabilitation is provided in Chapter Twenty
One - Mine Closure.

The mine's environmental
manager aims for an early return
of the topsoil for rehabilitation of
the reconstructed landforms.

Waste Rock Removal
From the standpoint of physically opening a mine, the major purpose of initial mine devel-
opment is to provide access to the ore deposit. In a surface mine, this means removing suf-
i cient overlying material to expose the uppermost part of the ore body. This is referred
to as 'pre-stripping' and sometimes forms part of the construction contract. Subsequently,
the mine is usually enlarged as a series of 'cut-backs' each of which extends outwards and
downwards, one bench at a time. Each of these cut-backs requires mining of ore and waste
rock, but the ratio between them will vary depending on the shape of the ore body. Apart
from tailings management, handling and relocation of waste rock continues to remain one
of the main environmental challenges in the mining industry, and is considered in detail in
Chapter Nineteen.