In mining, the design of optimum pillar strength has benefits ranging from increased extraction ratio (economic benefit) to the stability of the hanging-wall (safety benefit). Pillar strength is traditionally determined by empirical calculations and based on square, or near-square, pillar shapes. This study focuses on the preliminary results of investigation aimed at determining a pillar shape that could provide optimum strength. Pillars of different surface shapes (constant area) and constant height were compared. Four pillar shapes were chosen for analysis: square, triangular, hexagonal, and circular. For the physical models, a concrete mix was used to compare the pillar strengths to an engineering-designed strength parameter. These physically modelled pillars were loaded using a 5000 kN servohydraulic control console and load deformation graphs for each shape were obtained. The preliminary results indicate that the hexagon pillar has the highest peak strength of 76% normalized to the cube strength, while exhibiting stiff support properties. The triangular shaped pillar had 67% normalized strength. The triangle was found to have the highest energy absorption capability compared to the other shapes. It is noted that shapes such as the circle were not considered since they would be impractical. To ensure reliable results, numerical modelling was used along with a theoretical design criterion. The criteria to assess the pillar shapes were based on effective width and hydraulic radius.
Significant research has been done on the effect of pillar shape, with many researchers contributing to this subject. With width-to-height ratio being considered a shape factor by researchers, the UCS of a rock mass is also emphasized as fundamental. Likewise, a considerable amount of research is focused on pillar design methodologies. However, little attention has been given to the influence of cross-sectional area shape on pillar properties, which is an important aspect in pillar design. The majority of the research that has been done to date has focused on the influence of width to height ratio on pillar behaviour and pillar strength.