Systems Approach to Pillar Design

The Australian coal mining industry, through AMIRA, JCB and ACARP have funded research into pillar design methods for over 10 years with several million dollars worth of industry funds expended.

As a result of the research projects, several approaches to pillar design have evolved (further developments will undoubtedly continue in the future). The end result of previous research -rather than clarifying pillar design issues -has resulted in a degree of confusion and misunderstanding of the differences between approaches used, with misapplication of design methods occurring. This may result in potential pillar instability and mine safety concerns, or overly conservative pillar design with excessive development driveage and coal sterilisation.

The major methods currently in vogue at the time that this project commenced were:

• Formulae-based design, supported by empirical databases.
• Site-specific assessment using measurement and computer modelling.
• Tailgate Stability/Serviceability (ALPS/ALTS-style methods).

The integrated project was developed at the instigation of ACARP who sought both collaboration and a combination of the three separate projects. These projects had geotechnical issues in common that were associated with the design and/or performance of pillars in underground coal mine workings.

OBJECTIVES OF MODULE 1: Pillar Design Procedures ? Unified Approach to Pillar Design

• To identify the range of different pillar design applications and the extent of different geotechnical/mining environments encountered in Australian mining conditions.
• To delineate the applicability, compatibility and range and limits of application of currently used pillar design approaches, relative to the above applications and environments.
• To develop industry guidelines in the form of recommended pillar design procedures for the use of different design approaches based on their application and geotechnical environment.

This Module 1 report provides a summary of the ?state of the art? with respect to coal mine pillar design issues and methodologies in Australia at the present time. It does not attempt to define every individual scenario that might be confronted in the field of pillar design. However, it has been compiled to assist mine design practitioners to recognise the critical issues, and to understand both the areas of application and the areas of limitation with respect to different design approaches.

This Module 1 report should be read as a set of industry guidelines and recommendations. It does not provide every component of technical input or decision-making required, and should not be in any way seen to replace the need for sound geotechnical engineering analysis, judgement and experience by appropriate personnel.

The report clearly identifies that there is no one design methodology that addresses every geotechnical issue. There are a range of methodologies available which, more often than not, should be used in a complementary manner, rather than in isolation.

It is important to note that a significant amount of additional material is provided in the Appendix to this report. This material is essential reading, in conjunction with the main body of the report, since it offers expanded discussion of the issues, plus publications on a number of the major design methodologies. It also contains a summary of the Pillar Design Workshop which was held in 2001 as a part of this project. This workshop summary, prepared by Professor Ted Brown, captures many of the issues of ?application and limitation? in relation to pillar design. The summary is an excellent representation of the consensus views of the workshop participants, who included specialist pillar design consultants and researchers from Australia, South Africa, USA and UK.

OBJECTIVES OF MODULE 2: Long Term Stability of Flooded Mine Workings above Claystone

• To understand and quantify and process and mechanisms of claystone degradation through investigations into the material's response to changes in affecting factors such as stress history or moisture content;
• To characterise the extent of weakened floor under pillars in selected mine sites, especially in areas where water was present and claystone floor deterioration has occurred over time;
• To identify the dimension of an intact core within a pillar required for the long term stability of the pillar as well as the claystone floor under the pillar;
• To develop a procedure for the assessment and prediction of claystone floor deterioration, and
• To identify factors which may be relevant to soft floor performance and to make recommendations for the management of long term stability of flooded mine workings.

In order to capture the data for this and any future work on soft floor areas, an objective of the project was to develop a database of the insitu conditions at the test sites.

OBJECTIVES OF MODULE 3: Strata Displacement about Longwall Chain Pillars

The primary objective of this project was to determine the location of strata movement above chain pillars. At the time of the original ACARP application, the lack of this information was impacting on at least four longwall mines. In its simplest form the problem was that the degree of strata fracturing above a chain pillar had not been measured to date. Quantification of this behaviour has implications for chain pillar stability, surface subsidence prediction and control and isolation pillars between longwall goaves prone to spontaneous combustion.