Open Cut » Highwall Mining
The success of highwall mining operations depends on
- accurate prediction of roof strata behaviour in unsupported workings,
- safe evaluation of highwall stability and pre-treatment, and
- on a reliable method of pillar design to safeguard against collapse of the working area.
Additional factors influencing the viability of highwall mining are the impact of geological structure on unsupported roof and the consequences of undermining water filled abandoned highwall entries by conventional longwall extraction.
As highwall mining is newly introduced to Australia there is no background experience of highwall mining in Australian conditions upon which to base design parameters or make engineering judgements. In order to ensure safe and successful commercial application it will be necessary to invest in careful site investigation and evaluation of geological conditions together with the application of appropriate geotechnical monitoring for safety and design performance. Much of this will be research oriented until proven design methods based on practical experience can be derived.
In December 1993 Capricorn Coal Management Pty. Ltd. were awarded ACARP funding for a two year research project, C3053, to address the aims outlined below;
To develop a mathematical basis for the design of pillar widths using a combined narrow and wider partition pillar geometry, and for heading widths based on the tensile strengths of the immediate roof beams.
To geotechnically monitor the highwall, pillars and mined headings to ensure that maximum extraction is being achieved while maintaining adequate factors of safety, and to obtain data for back analyses of design criteria.
To monitor rate of in flow of seepage water into mined, sealed headings using piezometers and to address practical dewatering systems to avoid in flow into subsequent underground workings and
To transfer highwall mining technology throughout Australia.
This report specifically documents activities associated with aims 2 and 4 above and summarises work undertaken with regard to aim 1. The activities associated with aim 1 are documented in full in Ward et al 1993 and Moelle and Li 1993.
Unfortunately the full program of activities planned as part of aim 2 could not be completed. Those activities which were able to be completed are described below together with detailed records of operational experience.
The German Creek Mining Lease is operated by Capricorn Coal Management Pty. Ltd. (CAPCOAL) and is located 80 kilometres north-east of Emerald in Central Queensland. Open cut mining commenced in 1981 in the German Creek Formation, extracting coal from 3 individual seams, resulting in the development of 10 open cut pits up to the present day. The majority of these pits have reached their economic limit in terms of stripping ratio, resulting in final highwalls.
These highwalls were generated as part of normal strip mining operations and as a result were not specifically designed or engineered with highwall mining in mind. The German Creek Highwall Mining Project was carried out by CAPCOAL and commenced in 1993 to examine the geological and geotechnical feasibility of highwall mining to extract coal from final highwalls (Ward et al, 1993).
Mechanised highwall mining commenced on site in November 1994, using an Addcar Highwall Mining System operated by Eltin Highwall Mining Pty. Ltd. perations began in Pit R in the Aquila Seam. Operations were moved to Pit A in the German Creek Seam in April 1995.
Highwall mining at German Creek Mine was disappointing for all parties concerned. Many of the reasons for this are beyond the brief of this study, but from a geotechnical point of view many of the problems associated with span instability could have been better predicted.
An early recognition of the in situ condition of the immediate roof in both Pit R and Pit A could have changed the way the whole site was approached. If this had been done it may well have led to the selection of alternative highwall mining technology better suited to the site conditions and operational environment.
The geotechnical design for the coal pillars performed well in Pit R and only experienced problems in Pit A when operational procedures were varied from those planned.
The assumptions made in the early planning of the layout design appeared to be well founded from the results of the back analysis of the pillar instability in Pit A. The cyclic nature of final entry depths would suggest that the incorporation of barrier pillars may well have had a disruptive influence upon entry stability.
However the understanding of the rock mass response to highwall mining is not yet well enough developed to be conclusive as to the relative merits of barrier pillars in all situations. The data from German Creek, the second highwall mining site in Australia, provides significant information to assist in the future development of highwall mining operations in Australia.
A number of conclusions can be drawn from the work presented in this report based on the data gathered from highwall mining operations at German Creek;
- the structural condition of the immediate roof was not fully assessed in advance of operations at German Creek
- the span stability assessment technique employed proved to be insensitive to the ground conditions and designed spans performed poorly
- the inclusion of barrier pillars appeared to detrimentally influence the final depth of entries
- analysis of layout design indicated that barrier pillars may act to reduce overall layout stability and limit recovery
- the mining records would appear to show a pattern to entry depth on two levels, intra-panel and inter-panel
- suitable pre-feasibility and feasibility studies should be conducted at potential future sites in advance of any commitment to a mining technology, to ensure best match between system and ground conditions
- the coal seam should be mapped at future sites to check for the presence of in-seam shearing,and minor scale jointing/folding etc.
Operators should have sufficient information available to be able to select a level of design risk associated with highwall, pillar and span design with which they are comfortable and which matches the reward (i.e. recovery).
This approach can only be based on the identification of the dominant geotechnical factors at any site coupled with an understanding of how they impact upon system selection and operational procedures.
This project has added valuable information to the Australian database of highwall mining knowledge and assisted in improving the level of understanding of the impact of geological features upon operational performance.