Room and Pillar is one of the most common underground mining methods, where excavated spaces (rooms) are separated by pillars of rocks retained to support overburden. The engineering discipline referred to as geotechnical span design represents the process of finding out the maximum safe span of the excavated room and the size of its supporting pillars. The right methodology aims at optimizing resource extraction and securing structural safety of the mine in the long run. This task requires profound knowledge about rock mechanics, geological peculiarities and operational restrictions that may trigger local collapses and a chain of catastrophic pillar failures.
The solid approach to designing geotechnical span is based on thorough geotechnical study of the site. It is important to estimate the characteristics of the rock mass, such as uniaxial compressive strength, elastic modulus and Geological Strength Index (GSI) of the orebody and the surrounding rock host (Mehra & Budi, 2024). Also, the careful mapping of the structural discontinuities, including joints, faults and bedding planes, is required, since they define the failure modes of the roof span. Without reliable in-situ stress and core logging, further mathematical modeling will result in false stability factors.
Traditionally, the size of spans and pillars was determined based on empiric equations and tributary area analysis methods, which allow equally distributing the load from overburden over the specified simple geometric shapes of pillars. Modern geotechnical approaches involve more sophisticated numerical modeling methods. The application of three-dimensional non-linear numerical models based on failure criteria, like Mohr-Coulomb failure criterion, allows simulating complicated redistributions of stresses and yielding zones at different working depths (Mehra & Budi, 2024). As a result, it is possible to determine precise combinations of parameters in which there will be tensile stresses in the roof span, thus allowing for dynamic changes in the width of rooms and thickness of pillars.
The accurate estimation approach should also consider various environmental and hydrogeological factors that lead to a continuous weakening of rock mass. Changes in water level and pore water pressures considerably affect the stability of immediate roof span and underlying pillars. For example, recharge of groundwater and changes in the position of phreatic line may cause tensile failure and bending of the roof, which finally results in its instability (Zevgolis et al., 2022). Thus, hydrogeological modeling is a necessary stage of estimation of maximal span of underground workings.
The heterogeneity of rock masses makes deterministic design techniques, which depend on one safety factor, less and less popular and more often replaceable with probabilistic analysis of risks and machine learning. The modern design of spans uses stochastic continuous and discontinuous modeling to determine the probability of the failure of roof or pillars based on the study of variability of input parameters (Monsalve et al., 2025). Moreover, predictive models, developed using the algorithms of decision trees, can predict the pillar stability depending on the width/height ratio and stress variability (Ahmad et al., 2020).
To sum up, the right geotechnical methodology for span design in the context of room-and-pillar mining is a multidisciplinary and iterative process. This process starts with detailed geological investigation and continues with sophisticated three-dimensional numerical simulation and hydrogeological investigations and probabilistic risk analysis. Instead of applying simplified empirical equations, it allows engineers to predict the rock mass behavior under complex loads. Therefore, this holistic approach guarantees maximum extraction of minerals along with ensuring people’s safety and avoiding catastrophic failures.
References
Ahmad, M., Al-Shayea, N. A., Tang, X.-W., et al. (2020). Predicting the Pillar Stability of Underground Mines with Random Trees and C4.5 Decision Trees. Applied Sciences, 10(18), 6486. https://doi.org/10.3390/app10186486
Mehra, A., & Budi, G. (2024). 3D Modelling approach to identify parametric configurations for pillar stability in underground metal mine: a case study. Geomatics, Natural Hazards and Risk, 15. https://doi.org/10.1080/19475705.2024.2367630
Monsalve, J. J., Soni, A., Bishop, R., et al. (2025). A Risk-Based Pillar Design Approach Combining Stochastic Continuous and Discontinuous Modeling in an Underground Stone Mine. Mining, Metallurgy & Exploration. https://doi.org/10.1007/s42461-025-01210-7
Zevgolis, I. E., Theocharis, A. I., Deliveris, A. V., & Koukouzas, N. C. (2022). Numerical Analysis of Groundwater Effects on the Stability of an Abandoned Shallow Underground Coal Mine. Sustainability, 15(1), 529. https://doi.org/10.3390/su15010529
