Geotech approaches to be put under magnifying glass

AUSTRALIA'S leading geotechnical gurus are due to meet in April for a two-day workshop in an attempt to arrive at a hybrid approach to pillar design for Australia’s underground coal mines that draws on the strengths of the various methods currently available.

Staff Reporter

It is hoped the outcome will be the development of an agreed set of guidelines that will help coal mine operators better understand the "black box" of geotechnical design.


The subject of the ACARP-funded workshop is developing a unified approach to pillar design. The ACARP project leaders are Bruce Hebblewhite (University of New South Wales UNSW) and Winton Gale (SCT), who will co-chair the technical session at UNSW.


An important international perspective will be brought by Chris Mark, of NIOSH in the United States, John Cassie of the UK-based RMT group, and South African Nielen van der Merwe. Mark is one of the key developers of the Coal Mine Roof Rating (CMRR), the forerunner of the ALPS method developed for Australia (see related stories). Those invited include all interested specialist geotechnical consultants, researchers and operational professionals.


The workshop forms part of a three part ACARP-funded project called “Systems Approach to Pillar Design”. This umbrella project is being run by four participant organisations: the University of New South Wales, SCT Operations, Strata Engineering and Coffey Geosciences. Under the umbrella of this project are two other parallel ACARP projects: long-term stability of flooded mine workings above claystone floors; and, strata displacement about longwall chain pillars. The findings of these two studies will eventually form part of the guidelines.


The impetus to develop a unified approach to pillar design has been driven by the perception that the industry is generally confused about the different approaches to pillar design on offer. Broadly speaking these are: formulae-based empirical methods; numerical measurement and modeling methods; and tailgate stability/serviceability methods.


The approaches have evolved in various ways, in different geological settings and with different mining methods. Industry investment into research for the past ten years has also helped generate several different approaches.


For the operator, the difference between approaches is not always well understood, with occasional misapplication of design methods. In the past this misunderstanding has resulted in problems such as pillar instability or overly conservative design that has meant excessive development drivage and coal sterilisation.


In most instances, the desired outcomes described by the various practitioners are not that different. The differences lie in the methodology and its application. The inherent strengths, weaknesses and unknowns inherent in each approach have never really been discussed in a technical forum in Australia before. It is these issues the workshop hopes to tackle.


“The specific objectives of the workshop are to gather the key pillar design specialists in Australia (and overseas, where available) in a forum for a professional, but in-depth technical review of the various pillar design methodologies,” Hebblewhite said.


Internationally, differences in approach to pillar design have been the subject of technical workshops for the last ten years, according to Winton Gale of SCT. In that time he said the divergence in approaches has narrowed. Gale said there was now better understanding of the limitations of the various methods as well as a better database of information. He said there was now a general acknowledgement that UK-derived formula-based approaches cannot be applied in every Australian situation.


Gale said the workshop would examine each design approach in relation to a specific design issue to assess how each deals with the various challenges. The geotech professionals will be given the opportunity to describe their particular approach as well as the theoretical underpinning driving the approach.


The outcome of the workshop will be a set of peer-reviewed guidelines and recommendations. What is expected to emerge is the development of a hybrid approach to pillar design that draws from and combines the best elements of the various methods.


Hebblewhite said the outcomes of the workshop will be used to formulate the industry guidelines and will incorporate a series of design case studies using a range of appropriate methods.


“The overall intention is not to provide any one method with any special endorsement, but to help the industry (and legislators) recognise which method(s) are most appropriate in each circumstance of pillar type, application and environment,” he said.


The guidelines will delineate the limitations of each method and outline the necessary input data requirements needed to deliver the required results. The guidelines would also identify areas where particular methods should not be applied, or applied with lower confidence expectations or under certain qualifications.


Once the guidelines have been developed, expected to be finalised in mid to late 2002, a series of seminars will be presented to the industry outlining the proposed pillar design approach. Against the backdrop of changing legislation, which is moving towards self-regulation, the use of the guidelines is unlikely to be legislatively driven. But for the first time operators will have access to an informed overview of what is on offer.