Mine Rehabilitation Handbook - Mining and Blasting

5.2 Landform Design
The re-shaping and grading of a site is an essential aspect of
rehabilitation. Unless slopes are stable, the effectiveness of subsequent
topsoiling and re-vegetation is greatly reduced and maintenance may be
prolonged. When planning the final landform the whole of the mine and
associated infrastructure needs to be considered. The final landform
should be hydrologically and, if possible, visually compatible with the
surrounding area. The following factors should be considered in the
planning stage:
• Will the final landform be stable? The erosion potential of the
material on the site needs to be assessed. A geo-technical engineer’s
report may be required. Important factors are elevation, slope and
drainage density. Steep and long slopes allow surface run-off to
accelerate, resulting in erosion. Shallow (gentle) slopes are less
erosive and vegetation is more easily established. Slope angles,
lengths and shapes within the range of natural slopes in the area are
more likely to be visually compatible with the surrounding area and
are potentially stable under local rainfall patterns and erosion
processes. However, it should be recognised that post-mining
landforms consist of unconsolidated materials. Individual particles in
such materials are generally uncemented which make them more
prone to soil erosion than natural occurring soils.
• The drainage patterns for the overall site must be planned as part of
the overall landscaping. Drainage density (length of natural
watercourses / unit area) of adjacent land areas will provide a guide
to site requirements. However, the density will often need to be
higher on the post mining landform. Changes in land-use and
vegetation may also increase the drainage density requirement.
• Open-cut mines with large volumes of overburden will normally
require a drainage density higher than existed prior to mining to
compensate for the increase in the gradient of slopes and drainage
channels. As a guide, where runoff is being concentrated into drains
or diversion channels individual catchment areas of 1-2 hectares
are appropriate.
• Slopes should be designed so as to reduce the velocity of runoff as
the catchment of the slope increases (refer figure 5.1). Avoid long
straight ridges and sharp angles, as these will look unnatural. Slope
angles should be less than 20 degrees, except for slopes
constructed of non-erosive rock.
17
(a) Ideal slope profile (b) Profile design when
space is limited
Figure 5.1
• Where site limitations prevent the formation of a stable slope profile,
contour benches or similar erosion control methods may be required.
Slopes with an overall convex profile should always be avoided.
• Benches are best located in the middle of the slope. Where long
slopes cannot be avoided, several benches may be required and
their spacing will need to consider slope and run-off characteristics.
Benches need to be “surveyed in” to ensure accurate construction.
Table 1 provides a guide for spacing of benches along the slope.
Benches will need to be spaced closer in areas with high rainfall
intensities and/or very erodible soils.
Slope (degrees) Suggested Maximum
Spacing between
Benches (m)
3
6
7 - 9
9 - 11
11 - 13
13 - 17
Table 1
18
520
220
100
80
50
30