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Pillar rib failure in a limestone mine

Background

The room-and-pillar method is used in all operating underground stone mines in the United States. The dimensions of the pillars and roof spans in these mines are largely chosen on the basis of past experience without consideration of the local geological conditions that affect stability. A consequence is that unexpected rock falls from the roof and ribs of the workings can lead to injuries and fatalities among mine workers, and, in fact, have been the largest single cause of fatalities in underground stone mines over the past decade. This project will provide guidelines for roof spans and pillar layouts so that mine designers and operators can proactively design stable excavations to reduce the hazards associated with uncontrolled rock falls.

The project is being carried out using empirical methods in which case histories of stable and unstable mine workings (roof spans and pillars) in limestone mines in the eastern United States are evaluated to establish relationships between rock characteristics and stability. At each site, data on rock mass properties causes of instability, support techniques, and mining practices are being collected. These case histories will form a database for the development of design guidelines.

The case histories are being supplemented by numerical models and laboratory tests on rock samples from the mines to provide additional information on rock strength. The numerical modeling is being conducted to better understand the failure mechanics of pillars in hard, brittle rock and as part of the evaluation of the case studies.

Potential Outcome

The research will reduce the risk of rock falls in the more than 100 operating underground limestone mines. Project impacts will be achieved following 4 years of research by providing:

1. A methodology for assessing rock mass strength in underground stone mines based on current rock classification methods. In situ stress issues will also be addressed.
2. Design guidelines for pillar dimensions and pillar layouts based on local rock mass characteristics. Emphasis will be on slender pillars, which are more likely to be unstable than wider pillars.
3. Guidelines for designing stable roof spans to suit local rock mass and rock stress conditions.

Outputs