Underground limestone mines typically use the room-and-pillar method of mining in the generally flat-lying limestone formations. In some cases the dip may exceed 5° which can result in unique roof instability problems. Stability may be further exacerbated by the presence of horizontal tectonic stresses. Field studies and numerical model analyses are combined to assess the affect of the dip in various stress conditions. The results showed that where the horizontal tectonic stresses are sufficient to cause intact rock failure, increasing dip results in increased failure potential. The effect of the dip is greater when the maximum horizontal stress is parallel to the dip. Two case studies are presented and evaluated against the findings of stress analysis results. The study showed that the empirical rules of orienting headings parallel to the maximum stress are generally applicable to dipping excavations, except when mining in the up-dip direction. The importance of accurately determining the orientation of the maximum horizontal stress is emphasized because the window of favorable orientations relative to the stress is small.
BACKGROUND
There are 118 operating underground stone mines listed in the Mine Safety and Health Administration?s list of stone mine workings [1]. All of these mines may not be operational at any particular point in time. The mines generally use room and pillar methods to extract limestone formations at depths of cover that range from outcrop to approximately 900 m (3,000 ft). About 80% of the mines operate at a depth of cover of less than 90 m (300 ft). The formations are usually flat dipping, but some mines that are located along synclinal or anticlinal structures can operate at moderate dips of between 5° and 20°. A few cases exist where mines operate at dips of between 20° and 90°.
1.1. Horizontal Stress
Horizontal tectonic stresses have been identified as one of the causes of roof instability in stone mines [2] and can be associated with microseismic activity [3]. The mines are located in the mid-North American plate where elevated horizontal stresses have been measured [2] in the limestone formations and in many of the area?s coal mines [4]. Latest research has shown that the horizontal stress may be explained by the effect of plate tectonics [5, 6]. Tectonic loading is related to the movement of the North American plate as it is pushed away from the mid-Atlantic ridge. Elevated stresses are not necessarily present in all the limestone formations because local features such as outcropping and folding may have relieved the stresses over geological time [7]. The tectonic stresses can cause roof failure by progressive shearing of rock beams in the roof of the excavations. Failure of the intact rock has been observed in dipping excavations, where tectonic stresses were present [8].
In lower stress environments the dip of the workings can also affect the stability of the roof beam by mobilizing shearing along specific joint sets, without failing the intact rock. Failure in low stress conditions is characterized by movement of large blocks that are bounded by joint surfaces.
