The safe cleaning and making safe of opencast coal mining highwalls were identified for research and the topic was gazetted for the 2000 SIMRAC research programme. The project commenced in June 2000, with the primary outputs of the project as follows:
- Methods for making highwalls safe.
- Methods to clean highwalls.
- A geotechnical database relevant to highwall planning in South Africa.
- A highwall risk assessment procedure.
- An international benchmarking study on monitoring and cleaning procedures for highwalls.
The study found that a rationale for the safe cleaning and making safe of opencast coal mining highwalls did not exist locally. Traditionally, strip mining in South Africa has been conducted under good geotechnical conditions with limited highwall instability occurring. In the past decade, however, with the exhaustion of the better or shallower coal seams and the commencement of pillar extraction by opencast mining methods, deeper seams are now being mined with a resultant increase in geotechnical risks.
Research showed that much work has been conducted internationally into open pit slope stability over the last three decades. Research further highlighted that highwalls are generally designed using standard slope engineering practices, with no specific rationale being available for strip mine highwall design. It was further found that highwall rockfall hazards were reduced either by the implementation of good smooth- wall blasting practices or by mechanical cleaning with the aid of dragline buckets and bulldozers with chains. In terms of highwall cleaning, international practice differs little from that which is practised locally. However, it was found that overseas operators considered that inclined highwalls were effective in rockfall control. In some countries, highwall heights are also restricted to the reach of the primary earth moving equipment.
A review of South African colliery Codes of Practice to combat rock fall accidents showed that little geotechnical design work is being used on the mines locally. A rationale for the safe and effective design and cleaning of strip mining highwalls is therefore proposed. This rationale is divided into two components, as follows:
- Proactive highwall design.
- Reactive highwall cleaning.
The design rationale for the making safe of highwalls consists of the following components:
- A geotechnical assessment of the ground conditions through which the strip mine will advance. This geotechnical assessment requires the collection of information pertinent to the strength characteristics of both the soft and hard overburden and should be carried out in accordance with established soil and rock slope investigative techniques.
- Construction of a geotechnical domain model of the ground conditions through which the strip mine will advance. This requires an analysis of the geotechnical information gathered during the geotechnical assessment. • A mining geotechnical evaluation to take cognisance of and to determine what type of strip mining method should be used.
- Standard geotechnical analyses to determine highwall heights as well as overall geometries and orientations. In this respect, empirical, deterministic, probabilistic and numerical methods of slope analysis should be used. Kinematic techniques should also be used for analysing stability with regard to highwall orientation.
A key aspect of the design rationale is the compilation of a geotechnical domain model of the area through which strip mining will progress. It was found that the majority of data needed for the compilation of such a model already exists on most mines. In addition, research showed that remote sensing techniques can gather much of this data quickly and efficiently. However, it was found that it may be prudent to train mine rock engineers in photo interpretation and remote sensing techniques. During the project, a field trial was also carried out with highwall lazer mappers for the collection of highwall structural data. These trials showed that such systems used in conjunction with the SIROJOINT automated mapping system could rapidly reduce the time needed to gather geotechnical data.
The study further showed that highwalls should, where possible, be orientated at right angles to major geological structures. In cases where complex geotechnical conditions are encountered, changing to smooth wall blasting strategies or inclining the highwall may be beneficial.
Accident analyses, which were carried out as part of the study, showed that a correlation exists between highwall failures and the height of the overburden being mined. It would appear that the majority of failures occurred within the upper 30m of overburden. Failures could also be correlated to the presence of geological structures with trace lengths greater than 5m. Accident statistics furthermore showed that most fatal accidents occurred on night shift where truck and shovel methods are being used, while additional investigation indicated that mining personnel do not understand the importance of the structural geological regime on highwall stability. The accident analyses therefore showed that a need exists for production and engineering supervisors to be trained to recognise potential geotechnical hazards. This training must encompass highwall structural geological appreciation in the recognition of both shallow and deep-seated failures.
The blasting analysis showed the critical influence of rock mass structure on highwall blasting quality. It further showed the need for evaluation of geotechnical data as well as rock mass pre-split design. The need to develop effective smooth wall blasting techniques in weathered and highly jointed rock masses was seen as important in order to improve highwall conditions. It was furthermore recommended that the current format of the Code of Practice to reduce rock related accidents be amended to a Code of Practice to reduce geotechnical risks associated with opencast coal mining.