Establishing the Strength of Irregular and Rectangular Pillars

The publication in 1995 of the UNSW Pillar Design Methodology provided the Australian coal industry with a tool for designing square shaped coal pillars to a high degree of certainty under Australian conditions. This caused attention to be focused on a range of extended pillar design issues, the principal ones being:

• Rectangular shaped pillars
• Diamond shaped pillars
• Pocketing of overdesigned pillars
• Thick seam hybrid pillars
• Soft and slippery pillar contact conditions

In 1996, an ACARP grant of $21,700 was made to the University of New South Wales to research these issues. The research was intended to be of a preliminary nature to evaluate means by which pillar design procedures could be developed for the above circumstances. A combination of empirical back analysis and the laboratory model testing has been employed for these purposes.

Whilst rectangular shaped pillars are generally considered stronger than square pillars of the same minimum width, this strength increase has not been quantified previously. The early UNSW research outcomes (which were funded by the Joint Coal Board) adopted a conservative approach to this problem by recommending that strength analysis of rectangular pillars be based on the minimum pillar width dimension. Since that time, the UNSW database of failed and unfailed pillars has grown to the extent that it now contains sufficient case histories involving rectangular pillars as to be statistically significant in this regard.

Hence, a rigorous statistical analysis underpinned by extensive field investigations was undertaken of the database using the maximum likelihood method. The strength of this method is that it gives a weighting to unfailed as well as failed cases. Other statistical methods were also evaluated in the course of the research. This has produced fairly rigorous strength relationships for rectangular and irregular parallelogram shaped pillars.

The project has generated a refined suite of pillar strength formulae incorporating rectangular and diamond shaped pillars and based on the power law model that correlates better with Australian field performance than the Preliminary UNSW Pillar Design Formulae of 1995. There is a higher level of confidence associated with its application.

This new knowledge base enables the industry to move forward from both the Preliminary UNSW Pillar Design Formulae and the use of the 'minimum pillar width" in these formulae, to the use of an 'effective pillar width' in the Refined UNSW (Rectangular) Pillar Design Formulae. In a seam height of 2m, this results in an additional 4.5% of coal reserves being recoverable from main development headings and an additional 4.1% of coal reserves being recoverable from lease areas earmarked for longwall extraction. As such, the additional coal reserves available in a single longwall will return to the coal industry the ACARP funds expended on this research. However, the real gains are to had from the safety and productivity improvements associated with the improved mine design knowledge base.