In room and pillar mines, many intersections of entries are created, resulting in enlarged roof spans and modified stress distributions. A review of roof fall statistics (2002-2011) shows that the roof over an intersection is about twice as likely to fail as over an entry. Numerical models are used to study the stability of the bedded roof strata in coal mines. Thinly laminated roof beds are first studied as a single thin slab under gravity loading. Fully three-dimensional models are created to evaluate stress redistribution and failure development around intersections. It is shown that under the relatively high horizontal stress environment encountered in coal mines, intersections are less stable than entries, in spite of reduced stress over the intersections. The strategy to rotate the direction of development so that entries favorably intersect the major horizontal stress is shown to provide little benefit to intersection stability. A case study is evaluated in which rotation of the mine layout did not improve intersection stability, and the modeling results demonstrated that failure across the diagonals of the rotated intersection produced increased damage when compared to the entries. The numerical models show further that delamination of bedded strata can occur to much greater heights above intersections than entries, which may further contribute to intersection instability.
Murphy, M.M. (National Institute for Occupational Safety and Health (NIOSH)) | Esterhuizen, G.S. (National Institute for Occupational Safety and Health (NIOSH)) | Tulu, I.B. (National Institute for Occupational Safety and Health (NIOSH))
Fully grouted roof bolts increase the stability of a bedded mine roof by providing resistance to both vertical and horizontal displacements. The bolts provide suspension reinforcement from axial loads and lateral reinforcement from shear resistance effects. The lateral reinforcement provided by a roof bolt is difficult to observe in the field, but is often suggested by observation of roof failure cavities. However, these observations do not indicate whether bolt shear failure precedes or is a result of collapse. This paper highlights the use of a well-calibrated FLAC3D numerical model to investigate the shear resistance provided by a fully grouted bolt. The study first looks at analytical solutions to determine the necessary element size to obtain appropriate deflections of thin beams within FLAC3D. The study then compares different models demonstrating that the shear resistance provided by a fully grouted bolt has a limited impact on the overall stability of the mine roof. The model results indicate that the axial suspension effects of fully grouted bolts are more significant than the lateral reinforcement provided.