Variable elastic anisotropy controls stress in shallow crown pillars

As easily accessible natural resources become depleted, it is necessary to extract material from deeper levels and so mines may opt to develop a process of transition from open-pit to underground mining methods. In some cases, however, the process develops in the opposite direction where shallower resources from historic underground districts are mined by surface extraction methods. In both cases, it is necessary to maintain a crown pillar to ensure the stability of the pit and underground infrastructure. The dimensions of these crown pillars are typically designed using a combination of empirical methods and numerical modelling. In both methodologies, rocks are often treated as elastic and isotropic materials, even when they exhibit a clear direction of anisotropy caused by bedding planes, foliation, or closely spaced joints. To explore the role of this anisotropy in the stress state surrounding and within crown pillars, a series of two-dimensional finite element models were built using the code FEniCS. The results of this study show that tectonic loading leads to significantly higher compressive stresses, 2 to 4 times greater than gravitational loads alone. Tensile stress also increases notably, with values reaching almost −11 MPa compared to −1 ​MPa under gravitational loads. Therefore, the degrees of anisotropy and its orientation is likely to play a significant role in stress distribution. Our findings highlight the importance of constraining the in-situ stress, the geology of the host rock and the degree of anisotropy at laboratory scale for adequately addressing the risk of crown pillar failure and mining subsidence.