Numerical Analysis for the Design Assessment of Underground Mine Operations

ABSTRACT: In the design of room-and-pillar mine systems, the loading capacity of the pillar is equally as important as the stability of the roof and walls to preserve safe working conditions and commercial ore productivity. The optimized design of pillars requires an assessment of the generated stress and available strength which depend on the mechanical behaviour of the rock mass. Moreover, the actual stress level imposed on a pillar depends also on the location of the pillars within the mine layout. These aspects cannot be properly considered by employing empirical and simplified approaches, while numerical methods provide the required accuracy in the determination of stress variations across the pillars as well as the assessment of the pillar strength. In this paper, we adopt numerical modelling to better constrain the implication of employing simplified approaches for a proper definition of the safety margins of a room-and-mine pillar layout. The results highlight the advantage of adopting advanced constitutive laws for the rock mass and discontinuities resulting in fully optimized designed solutions in terms of safe and, at the same time, economical designs. 1. INTRODUCTION In mining excavations, rock pillars are commonly used in most underground mining methods, either temporary or permanent, to support adjacent underground openings and thus guarantee the stability of the rooms for extracting the ore safely. In the design of room-and-pillar systems a suitable factor of safety that relates the pillar strength, defined as the maximum resistance to the axial compression, to the pillar stress is required. With this regard, empirical methods for estimating pillar strength and simplified methods for evaluating the pillar stress (i.e., tributary area method) have found wide acceptance in the mining industry. This has led to the formulation of several strength equations (e.g., Hedley and Grant 1972; Von Kimmelmann et al. 1984; Krauland and Soder 1987; Potvin et al. 1989; Sjöberg 1992; Lunder and Pakalnis 1997; Walton and Sinha 2021) which in some cases also consider account the effect of the length for rectangular pillars (e.g., Wagner 1992; Mark and Chase 1997; Dolinar et al. 2007) as well as the impact of large angular discontinuities (e.g., Esterhuizen et al. 2011). However, these methods should be used with caution (e.g., Dressel and Diederichs 2023) because they fail to consider specific failure mechanisms and do not account for the actual stress level imposed on the pillar due to the location of the pillars within the mine layout.