Definition and quantification of long-term stability of coal pillar systems

The stability of pillars is a critical consideration both during and after mining operations. During mining, one of the most important roles of pillars is to ensure a safe working environment for underground workers. Even after mining activities cease, certain pillars may have an ongoing function to control surface subsidence effectively.

Over time, community expectations of the mining industry have evolved regarding safety and environmental impacts. Many catastrophic disasters in mining history are due to inadequate pillar design and foundation failures from various types of mines worldwide. Ensuring the safe design of coal pillars during the operational phase of mining has become a central focus in underground mine planning and geotechnical engineering. More recently, attention has expanded to include the long-term stability of pillars to manage and mitigate mine subsidence. Key contemporary objectives of mine design include managing subsurface and surface impacts on natural and man-made features. These expectations extend not only throughout the operational life of a mine but also "in perpetuity" following the cessation of mining activities.

The objective of this project was to define and quantify the long-term stability of coal pillar systems, including the pillar, roof, floor, and the interfaces between them beyond their operational lifecycle and to develop software that enables easy calculation of stability parameters. While the original UNSW pillar design formulae remain foundational for contemporary mine design, their applicability is limited in scenarios involving shallow cover depths, low pillar width-to-height ratios (w/h), or weak roof and floor conditions. This project extends the work of Salamon et al. (1998) by incorporating time-dependent failure mechanisms, particularly rib, roof, and floor spalling, which were not addressed in the original UNSW methodology. Assessment of long-term pillar stability is enhanced by separately evaluating these spalling mechanisms and validating their impact, thereby providing a more comprehensive framework for ensuring the enduring safety and stability of underground coal mining operations.

This project delivered a comprehensive method of quantifying the long-term survival probabilities of pillars. This method can be used to assess pillar stability in the case of rib spalling, roof spalling, and bearing capacity. In achieving the final outcome, it is proposed that the long-term survival probability of pillars should be considered using the proposed statistical method by considering the rib and roof spalling, and in the final stage, the bearing capacity should be assessed separately using the residual values of cohesion and friction angle for the floor. This research can be utilised to better manage the environmental consequences.

A novel dual-spalling probabilistic framework was introduced, which considers rib and roof failure mechanisms independently yet concurrently. By applying empirical, analytical, and stochastic modelling approaches, the study offers a practical methodology that can inform robust, site-specific pillar system design under uncertainty, making it directly applicable to modern mine planning.

As part of this project, a web-based software tool has been developed to enable practical application of the models presented in this report. The tool allows geotechnical engineers to calculate the long-term probability of survival of coal pillar systems under rib spalling, roof spalling, and floor bearing failure conditions, without requiring specialist programming knowledge. Input parameters including pillar geometry, cover depth, and floor properties can be entered directly, and the tool returns the probability of survival for each failure mode as well as the combined system survival probability. The software is accessible at https://www.mine-geotech.com/ and is intended to support both the design of new pillar systems and the retrospective assessment of legacy pillars.