AN ABSTRACT OF THE THESIS OF Behrooz Abbasi, for the Master of Science Degree in Mining and Mineral Resources Engineering, at Southern Illinois University Carbondale TITLE: A NUMERICAL ANALYSIS OF MINE INTERSECTIONS AND SUPPORT SYSTEMS FOR STABILITY MAJOR PROFESSOR: Dr. Yoginder P. Chugh Back ground: Partial extraction room-and-pillar mining systems provide about 60 % of the underground coal mined in the USA. This mining system develops 3-way and 4-way intersections. Rock falls related to fatal and nonfatal injuries in the USA for the period 2002 to 2007 identified that about 70% of these falls occur at intersections even though they represent only about 20-25 % of the area mined. A recent study on 2004 to 2008 roof-falls data base (RFDB) for Illinois mines, found that over 80% of these falls occurred at intersections. Thus, there is a significant need to improve stability of coal mine intersections if fatal and nonfatal injuries are to be controlled. The overall goal of this research is to develop an improved scientific understanding of stress distribution and instability around a 4-way intersection. An additional goal is to evaluate primary and secondary support layouts for improved stability around 4-way intersections. The following specific tasks were established for this research. * Perform 3-D numerical analyses to develop a better understanding of stress, strain and displacement distributions and associated instabilities around a 4-way intersection to identify ground support requirements. * Identify primary and secondary supports systems that may be suited to meet ground support requirements in (1) above. * Perform numerical analyses with identified primary and secondary supports installed to assess improvements in stability around an intersection. Analyses results show that pillar ribs across an intersection fail first and lead to progressive failure of immediate roof layers. The failure of immediate roof layers extends about 1.8m. In detail, coal ribs around the intersection corners mostly fail due to tensile stress, while roof and floor strata fail due to shear stresses and rib center fail due to combination of shear and tension mechanism. Primary supports significantly improve stability, but they are not adequate to ensure stability of four-way intersections. Secondary supports are needed to further improve stability of intersections. Also, based on the failure analysis a new secondary support system plan was proposed and analyzed.

As hard rock mining progresses into higher stress mining conditions through either late stage extraction or mining at depth, the rock mass is driven not just to the peak strength, but often well into the post-peak until complete ‘failure’ occurs and easier mining conditions become evident. Limited research has been accomplished in identifying the transition of the rock mass and its behaviour into the post-peak and this research investigates this behaviour in detail. As the rock mass progressively fails, fractures are initiated through intact rock and extension and shear failure of these and pre-existing features occurs. Associated with this failure are microseismic events, which can be used to give an indication of the strength state of the rock mass. Based on an analogy to laboratory testing of intact rock and measurement of acoustic emissions, the microseismicity can be used to identify, fracture initiation, coalescence of fractures (yield), localization (peak-strength), accumulation of damage (post-peak) and ultimate failure (residual strength) leading to aseismic behaviour. The case studies presented in this thesis provide an opportunity to examine and analyse rock mass failure into the post-peak, through the regional and confined failures at the Williams and the Golden Giant mines, both in the Hemlo camp in Northern Ontario, Canada. At the Williams mine, the progressive failure of a sill pillar region into the post-peak was analysed; relating the seismic event density, combined with numerical modelling and a spatial and temporal examination of the principal components analysis (PCA), to characterize the extent, trend and state of the yielding zone, which formed a macrofracture shear structure. Observations of conventional displacement instrumentation, indicates regional dilation or shear of the rock mass occurs at or prior to the point of ‘disassociation’ (breakdown of stable PCA trends) when approaching the residual strength. At the Golden Giant mine, the complete process from initiation to aseismic behaviour is monitored in a highly stressed and confined pendent pillar. The PCA technique, numerical modelling and focal mechanism studies are used to define significant stages of the failure process, in which a similar macrofracture structure was formed. Temporal observations of key source parameters show significant changes prior to and at the point of coalescence and localization.

As hard rock mining progresses into higher stress mining conditions through either late stage extraction or mining at depth, the rock mass is driven not just to the peak strength, but often well into the post-peak until complete ‘failure’ occurs and easier mining conditions become evident. Limited research has been accomplished in identifying the transition of the rock mass and its behaviour into the post-peak and this research investigates this behaviour in detail. As the rock mass progressively fails, fractures are initiated through intact rock and extension and shear failure of these and pre-existing features occurs. Associated with this failure are microseismic events, which can be used to give an indication of the strength state of the rock mass. Based on an analogy to laboratory testing of intact rock and measurement of acoustic emissions, the microseismicity can be used to identify, fracture initiation, coalescence of fractures (yield), localization (peak-strength), accumulation of damage (post-peak) and ultimate failure (residual strength) leading to aseismic behaviour. The case studies presented in this thesis provide an opportunity to examine and analyse rock mass failure into the post-peak, through the regional and confined failures at the Williams and the Golden Giant mines, both in the Hemlo camp in Northern Ontario, Canada. At the Williams mine, the progressive failure of a sill pillar region into the post-peak was analysed; relating the seismic event density, combined with numerical modelling and a spatial and temporal examination of the principal components analysis (PCA), to characterize the extent, trend and state of the yielding zone, which formed a macrofracture shear structure. Observations of conventional displacement instrumentation, indicates regional dilation or shear of the rock mass occurs at or prior to the point of ‘disassociation’ (breakdown of stable PCA trends) when approaching the residual strength. At the Golden Giant mine, the complete process from initiation to aseismic behaviour is monitored in a highly stressed and confined pendent pillar. The PCA technique, numerical modelling and focal mechanism studies are used to define significant stages of the failure process, in which a similar macrofracture structure was formed. Temporal observations of key source parameters show significant changes prior to and at the point of coalescence and localization.