INVESTIGATING THE STRENGTH AND FAILURE MECHANISM OF HIGHLY INTERLOCKED JOINTED PILLARS USING 2D CONTINUUM MODELS

Abstract

Pillars are commonly used in underground mines to maintain the stability and integrity of the openings. An optimum design of mine pillars dictates that the pillars should be as small as possible and meet the load-bearing requirements. Therefore, a proper estimation of the rock mass strength is of paramount importance for a reliable design of mine pillars. It is known that the Hoek-Brown failure criterion, with its strength parameters obtained based on the Geological Strength Index, tends to underestimate the confined strength of well interlocked jointed hard rock masses. Therefore, pillar designs based on this approach could lead to oversized pillars due to underestimated strength of the pillar core. The central objective of this research is to better understand the strength and failure mechanisms of highly interlocked jointed pillars. For this purpose, a grain-based model is developed using the continuum numerical program RS2 to reproduce the laboratory behavior of intact and heat-treated Wombeyan marble. The heat-treated Wombeyan marble is considered to serve as an analogue for a highly interlocked jointed rock mass. An iterative calibration procedure is utilized to match the macro-properties of RS2-GBM to those of marble. It is found that the calibrated RS2-GBM captures some of the most important characteristics of brittle rocks, including the non-linear strength envelope and the change in the failure mode with increasing confinement. Next, the calibrated RS2-GBM of granulated marble is upscaled to simulate jointed pillars of various width-to-height ratios. The results of numerical simulations inferred that the slope of the pillar stability curve obtained from this approach is comparatively steeper than those of existing continuum and discontinuum models of jointed pillars. This is attributed to the high degree of block interlock leading to higher rock mass strength at the pillar core. It is demonstrated that this modeling approach provides more realistic results in terms of pillar failure processes compared to other continuum models, in which the rock mass is simulated as a homogeneous medium. The advantage of the continuum over the discontinuum GBM is its shorter computation time. Therefore, the proposed modeling approach can be used as a practical tool for stability analysis and design of mine pillars in jointed rock masses.