Correctly designed and cost-effective highwall bunds are essential for safety in large open cut coal mines
Highwall and low wall stability are essential during open cut mining. Geotechnical investigations are required to quantify the mass and substance properties of soils and rocks within the mining horizons. The required minimum safety factor for open cut batters is 1.2. The effect of highwall mining and augering needs to be considered for long-term highwall performance.
Temporary highwall bunds are required for safety during open cut mining prior to the infilling of pits, and permanent highwall safety bunds are necessary to prevent public access to abandoned open cut final voids. Safety bunds should be located at adequate distances from highwall crest edges to ensure long-term integrity. There are a number of geotechnical features that affect highwall stability, including:
- depth of surface clay and weathered rock
- shear strength of soil and rock units
- batter angles for material types and the presence or absence of presplit blasting
- attitudes of structural defects such as faults and dykes
- the location of the water table and the influence of pore pressure
- subsidence caused by longwall undermining, highwall mining and highwall augering
- the presence of highwall spoil.
Material properties of overburden
Shear strength values have been determined for tertiary clay; completely, highly and moderately weathered mudstone; highly weathered claystone; completely weathered siltstone; highly weathered sandstone; fresh siltstone and fresh sandstone for use in slope stability analyses. The results listed in Table 1 are indicative of stable batter angles and can be used to calculate highwall safety factors.
Geological structures such as faults, intense jointing, bedding, folds, rolls and weathered dykes can affect slope integrity and require investigation.
For improved stability, highwalls are generally excavated at one (horizontal) to one (vertical) in clay and completely to highly weathered rock and 0.5 (horizontal) to one (vertical) in fresh rock. Presplit blasting facilitates highwall integrity. Safety benches are sometimes constructed at the base of weathering. Where there is a substantial surface thickness of tertiary clay and/or highly to completely weathered rock, advance benches are commonly excavated to maintain practical highwall geometries. Efficient drainage prevents highwall scour and degradation.
Highwall monitoring is recommended where highwall mining and augering have been completed to ensure ongoing stability.
Figure 1 shows a typical highwall after open cut and highwall mining. In Figure 2, spoil that has been dumped on the highwall is to be rehabilitated. Spoil is dumped on the highwall during dragline mining of the final strip if low wall placement becomes spoil bound.
Locating highwall safety bunds
Geotechnical assessments of the highwalls and underground mining subsidence effects are required prior to locating sites for bund construction. The bunds need to be constructed outside of the potentially unstable zones adjacent to the highwall crests. A guideline for safety bund walls around abandoned open pit mines was prepared by the Western Australian Department of Industry and Resources in 1997. No recommendations are available for highwall safety bunds adjacent to open cut coal mines in Queensland or New South Wales.
The extent of the potentially unstable highwall edge is generally related to the depth of surface clay and weathering. The following angles from the horizontal plus 10 m should be used as a guide for locating highwall safety bunds away from potentially unstable edges. Offset details (Figure 3) are as follows:
- an angle of 30° for clay and completely weathered rock
- an angle of 45° for highly weathered to slightly weathered rock
- an angle of 60° or greater for fresh rock.
Where highwall mining or augering has occurred, geotechnical monitoring is required to assess highwall stability and estimate the extent of long-term highwall edge deterioration. According to Geotechnical Consulting Services Pty Ltd (2016), a standoff distance of 10 m is recommended for highwall augering. Mining-induced subsidence is not considered deleterious to the integrity of offset highwall safety bunds. Where such bunds are also used as levees to divert water flows, any subsidence cracks require remediation to prevent leakage.
Specifications for highwall safety bunds
Safety bunds should be 2 m high with 2 m wide crests. The crests need to slope away from the highwall at one per cent. They are constructed as homogeneous embankments comprising either dozed or compacted earthfill or dumped rockfill. Bunds constructed from dumped rockfill are cheaper and easier to build. Fresh Permian spoil comprising sandstone and siltstone is a suitable rockfill. Bunds are primarily built for safety, but compacted earthfill bunds divert run-off water away from the highwall, particularly where the topographic surface downdip of the highwall slopes towards the highwall or there is a creek adjacent to the highwall.
Temporary highwall safety bunds need to be cost-effective, easy to construct and stable until final voids are infilled. Permanent highwall safety bunds should be durable, erosion resistant and properly constructed.
Specification for rockfill safety bunds
Foundations for rockfill safety bunds need to be stripped of topsoil, which is stockpiled. Fresh Permian spoil or other suitable rockfill is to be used. Fill is dumped and trimmed using an excavator. Batter angles are one (vertical) on 1.5 (horizontal; Figure 4).
Spoil safety bunds can be constructed on the crests of rehabilitated, geotechnically stable highwall spoil piles. These bunds are constructed using local spoil and have the same dimensions as permanent rockfill bunds.
Specifications for earthfill safety bunds
Temporary and permanent earthfill bunds can be constructed. Temporary earthfill bunds are only required during mining until final voids are infilled. Permanent earthfill bunds are constructed for safety and management of surface water flows.
Temporary earthfill bunds are constructed by a dozer pushing local earthfill after topsoil has been removed and stockpiled. Batters are generally one (vertical) on 1.5 (horizontal). Although earthfill quality control is not essential, erosion resistance can be improved by using non-dispersive soils. After final void infilling, temporary earthfill bunds need to be rehabilitated.
Permanent earthfill bunds require one (vertical) on three (horizontal) batters (Figure 4). Topsoil needs to be removed and a cut-off trench is required with excavation down to impervious clay. Good-quality, non-dispersive, impervious material is required for the permanent bund. The material should be a well-graded, sandy, silty clay with the following properties:
- Liquid Limit WL 30-60 per cent Plasticity Index Ip 15-45 per cent.
The first 1000 mm placed in contact with natural foundations should have a minimum of 20 per cent passing the 75 µm sieve. The Emerson dispersion class, as defined in Table 2, should be Class 4 or higher.
The minimum required density ratio is 100 per cent standard compaction at optimum moisture content ±2 per cent. The maximum dry density should be determined in accordance with Test No 5.1.1 (Standard Compaction) of AS 1289 for cohesive materials and in accordance with Test No 5.5.1 and 5.6.1 of AS 1289 for cohesionless materials.
The batters and crests need to be topsoiled and seeded for long-term erosion resistance.
Open cut coal mines cover extensive areas, and cost-effective highwall bunds are required for safety. Durable rockfill bunds are constructed using fresh Permian spoil consisting of sandstone and siltstone. Temporary earthfill bunds can be constructed using local clayey material. Permanent earthfill bunds are needed where surface water management from run-off and creeks is required adjacent to highwalls. Suitable clayey material is required for construction.
Department of Industry and Resources, 1997. Safety bund walls around abandoned open pit mines: guideline [online], Government of Western Australia. Available from: www.dmp.wa.gov.au/Documents/Safety/MSH_G_SafetyBundWallsAroundAbandoned Mines.pdf
Geotechnical Consulting Services Pty Ltd, 2016. Stability of undermined highwalls (unpublished).
Klenowski G, Ward B and Xaykhamphoune P, 1992. An economically viable method of dragline mining in weak unstable ground at German Creek Mine, Central Queensland, in Proceedings Third Large Open Pit Mining Conference (The Australasian Institute of Mining and Metallurgy: Melbourne).