A Proposed Framework to Estimate Pillar Strength in Room-And-Pillar Hard Rock Mines

ABSTRACT: Pillar strength determination is a fundamental aspect of room-and-pillar mine design. Pillars are left between excavated rooms to maintain stability by restraining the hanging wall. In a typical mining environment, pillars of different rock mass conditions and geological characteristics are cut, resulting in variations in pillar strength. This is because pillars are highly anisotropic and cannot always be formed according to design dimensions. Consequently, unstable pillars may be present, requiring site-specific calibrated methods for ongoing monitoring and strength estimation. Pillar strength determination is typically accomplished using empirical formulae or numerical modelling. Although several numerical codes are available, mine sites still rely on empirical approaches despite their well-known limitations. Empirical methods are popular in the mining industry due to their simplicity. However, it is widely acknowledged that these methods can only be effective within the boundaries of their database and the specific problem scale for which they were designed. In contrast, continual monitoring of pillars and practical experience can enhance pillar strength evaluation. This work briefly describes pillar conditions at a case study mining operation. Additionally, a framework is proposed to estimate pillar strength based on ongoing pillar mapping, risk management decision making, numerical modeling, and analytical hierarchy process. 1. INTRODUCTION The room-and-pillar mining method is one of the most vital underground ore extraction methods in practice today. In this method, pillars are systematically left in situ to support the overburden weight of the overlying rock mass. If the rooms and pillars are not designed carefully, it can compromise safety or lead to sterilization of the ore reserves. Designing non-yield pillar systems for shallow hard rock room-and-pillar mines has always been a challenging task. It is important to know the stress distribution in the pillar and the strength of the pillar so that a sound design be produced. Due to the significance of these parameters, extensive research has been conducted on the strength of pillars in hard rocks. However, despite the numerous research outcomes, pillar failures still occur in underground room-and-pillar mines. The literature contains documented examples of such failures and their consequences (Esterhuizen et al., 2006, 2019; Malan, 2012; Ozbay et al., 1995).