Roof Control, Pillar Stability, and Ground Control Issues in Underground Stone Mines

"The U.S. stone industry is in a transition from surface quarries to single and multiple level underground stone mines. The factors driving this transition include surface quarries; ? reaching their economic stripping ratio, ? encroaching on the mineral reserve boundary where the remaining reserves are below the active pit, ? dust, noise, and blast vibration concerns as suburban neighborhoods and development move into formerly rural areas. Ground control is critical to the success of a safe and productive, mining operation. The primary focus of the ground control engineer in an underground stone mine is the stability of the roof or back, development and benched pillar stability, stability of the sill pillar between vertically adjacent mining levels, and floor stability. Roof stability begins with the selection of a stable back horizon based upon the presence of a thin shale parting, stylolite, or other discontinuities from which the production shot can easily ""peel"" away leaving a stable back stratum. Room width and the decision of whether to bolt the roof, the bolt length, bolt type, bolt diameter, and spacing of the support is controlled by the thickness, fracture spacing, jointing, and material properties of the back strata. Horizontal stress controls the header and crosscut azimuth as to avoid driving perpendicular to the maximum horizontal stress direction. Stable pillar dimensions depend upon the interaction of the; ? vertical stress resulting from overburden thickness and multiple level interaction of overlying level(s), ? mining height during development and the final mining height after floor shots are taken while benching, ? presence of faults, folds, or joints passing through the pillar, and ? stone lithology recognizing that the presence of oolitic bands, shale partings, and bentonite beds create zones of weakness that can destabilize an otherwise competent pillar. Floor stability is typically not a significant issue for underground stone mines because the bearing capacity of the stone exceeds the vertical pressure transferred through the pillar. However, floor heave can be a ground control issue where shale partings, moisture sensitive strata, weak stone bands, or horizontal stress is present. Roof stability and pillar stability problems can be initiated as the floor heave enables the pillar and roof to settle. The ground control issues for underground stone mines are most clearly presented through a series of case histories. The case histories presented in this paper are a mix of ground failures and successful mining. The focus is on the geotechnical data required for underground stone mine ground control for roof, pillar, and floor design and stability analysis."