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| Roof failure in a lead mine |
Background
Room-and-pillar mining is the most widely used underground mining method in the world and, in practice, is utilized in one form or another in most other procedures, particularly during mine development. In the United States, various types of room-and-pillar methods have been used to mine a broad range of mineral commodities, including lead and zinc in Missouri, trona in Wyoming, potash and uranium in New Mexico, and limestone, coal, and salt throughout the country. Accurate design of pillars left in place to support the roof is critical to maintaining a safe mine. If the dimensions of these remnant support pillars and the room spans between pillars are not designed properly, the long-term stability of underground workings can be compromised, resulting in catastrophic chain pillar failures and dramatic collapses of large areas underground. Stability problems are often exacerbated through attempts to increase resource recovery by reducing the size of support pillars and/or mining high-grade remnant ore pillars, which can create large unsupported roof spans or spans that depend on backfill to carry the overburden load. During the last 10 years, roof falls have been the fifth leading cause of mining accidents, resulting in over 1,500 accidents per year.
Research is being conducted at underground lead mines to gain a better understanding of the conditions that cause roof and support pillar failures and to develop engineering practices that will reduce these ground fall hazards. Data are currently being collected and analyzed from instruments installed in support pillars in a retreat section. Another section has been targeted for additional instrumentation because a wide roof span will be supported with cemented backfill. In conjunction with instrument data, numerical modeling results are being used to analyze the pre- and post-yield stress-versus-strain behavior of pillars to quantify their capability of supporting the mine roof before and after they fail.
Potential Outcome
Collaborative research with the Doe Run Co. in one of the company’s seven mines in the Missouri New Lead Belt has addressed concerns regarding the safe extraction of support pillars. By using visual observations, numeric modeling, and instruments to evaluate remnant pillar extraction in a test section of the mine, safety guidelines for maximum roof span, required backfill strength, and instrument placement design were established and published in conference proceedings. The mine staff’s goal of no lost-time injuries during pillar extraction was met the first 9 years after guidelines from the test section were implemented.
The future impact from this research will be to use measurements of pillar behavior to develop more precise guidelines for pillar design. This will immediately benefit the seven operating mines in the New Lead Belt, and the methodology can be applied to all pillar mines. This research received the Applied Rock Mechanics award from the American Rock Mechanics Association in 2004.