Comparative geomechanical investigation of empirical, analytical, and numerical methods utilized in designing flat-roof excavations in discontinuous and laminated rockmasses

Abstract

Simplification of complex geologic systems has been a necessary hallmark of geological engineering research and design to date. However, oversimplification and subsequent over-application of existing methods leaves room for significant improvement in our understanding of rockmass response to excavation. Although indisputable advancements have been made in increasing the safety of underground workings, falls of ground continue to injure or kill personnel and delay production. The overapplication of existing simplified methods is particularly problematic in discontinuous and laminated systems, where the response to excavation can be anisotropic and significantly impacted by the orientation, intensity, and condition of discontinuities. With the advancement of computational power and numerical modeling techniques, more of the mechanical complexities associated with discontinuous systems can be explicitly considered. Therefore, the goal of this research is to identify the geomechanical considerations for a wide range of discontinuous and laminated geologic conditions that should be incorporated into analytical and empirical methods to increase the safety and productivity of mining and civil works. This thesis focuses on addressing and overcoming two of the most significant simplifications often employed in the design of flat-roof excavations: assuming that the overburden has no self-supporting capacity, and representing discontinuous systems as continua. To that end, this research utilizes the explicit discrete element method (DEM) to identify and account for the relevant geologic and mining conditions that control local and global stability. Model complexity and scale is increased incrementally, and model results are compared to existing, well-established analytical and empirical methods to validate, confirm, or frame the implications of the numerical results and their relationship with “reality”. The first objective of this thesis is to evaluate roof self-stability and stress arching capacity through application and enhancement of the voussoir beam analog. Gaps in existing analytical calculations are identified and addressed through the methodical variation of geometry, material properties, and boundary conditions in explicit DEM voussoir beam numerical models. An adjusted voussoir beam analog is developed that can account for novel aspects of complexity such as post-peak material behavior, horizontal stress, and layered roofs that are passively bolted. The adjusted voussoir beam analytical method is then applied to a case study of the Bondi Pumping Chamber excavation in Sydney, New South Wales, Australia. The second objective of this thesis is to analyze roof self-supporting capacity and bolted stability through a parametric sensitivity analysis of 8,640 unique explicit DEM models of hypothetical coal-mine entries conducted with a particular focus on discontinuity properties. Additional considerations include in-situ stress magnitude and horizontal stress ratio, as well as material stiffness, strength, and anisotropy. Model inputs are utilized to assign a Coal Mine Roof Rating (CMRR) value to each model case, and the Analysis of Roof Bolt Systems (ARBS) is subsequently used to assess the reliability of the model results and focus future statistical analysis. Multivariate binary logistic regression is used to identify the statistically significant parameter inputs that determine the probability of a stable roof condition in unsupported and bolted models. Recommendations such as adjusting the cohesion-roughness rating and consideration of joint orientation in CMRR, as well as accounting for in-situ horizontal stress ratio in ARBS, are posited. The last objective of this thesis is to identify how excavation roofs and pillars are mechanically linked. A calibrated, confinement-dependent coal pillar constitutive model is combined with the significant controls on roof stability identified through the course of this study to assess pillar-overburden interaction in single-entry and multi-entry models. Entry-scale models are used to identify the interaction between roof stress arching capacity and pillar confinement, and panel-scale models are subsequently developed to incorporate in-situ complexities such as panel width-to-height ratio, lithologic heterogeneity, and depillaring to assess overburden stress arching capacity and pillar response. Lastly, the panel-scale model results are compared to state-of-practice analytical and empirical methods such as tributary area theory (TAT), the Analysis of Retreat Mining Pillar Stability (ARMPS), the abutment angle concept, and the Mark-Bieniawski pillar strength equation. Results confirm that properties that increase stress arching in the overburden tend to decrease pillar loads and increase pillar strength. The results of this study identify that increasing both the accuracy and applicability of existing analytical and empirical methods, as well as our holistic understanding of flat-roof excavation stability requires mechanically coupling the pillars to the roof and floor. Without this explicit consideration, state-of-practice and state-of-knowledge cannot advance towards both safer and more efficient excavations.