Semester

Spring

Date of Graduation

2009

Document Type

Dissertation

Degree Type

PhD

College

Statler College of Engineering and Mineral Resources

Department

Mining Engineering

Committee Chair

Syd S. Peng.

Abstract

Roof falls are among the most serious safety hazards faced by underground coal mines worldwide. Due to the stringent safety measures and development of the innovative support systems in the past few decades, their numbers were drastically reduced but have not been eliminated. Underground observations reveal that a number of larger roof falls are preceded by the development of shear failures near pillar ribs, termed cutters or guttering. In the past, many factors were identified as responsible for the development of cutters and ultimately roof falls. These factors can be broadly classified as stress related and non stress related. Although some useful work on the stress related aspects was conducted in the past, in this dissertation the cutter instability is investigated in more details while including some finer aspects of the mining process in particular the cutting sequence, which were not given due consideration before.;Three dimensional finite difference modeling has been carried out to accomplish the research objectives in this dissertation. The strain softening material behavior with cutting sequence has been used to realistically simulate the cutter formation as suggested by Gadde and Peng, 2005. A few cutting sequences employed by some U.S. coal mines have been considered in this study. This was done to understand if the cutting sequence has any significant influence over cutter formation. Apart from cutting sequence, factors such as the cut length, step cutting and the turning direction of crosscut into and away from major and minor horizontal stress are also examined for their effect on cutter development. Further, in contrast to past work, the effect of change in the immediate roof rock properties and horizontal stress directions are studied in several multiple entry models while simulating some realistic cutting sequences.;Field observations show that some cutters develop after a significant amount of time is elapsed since the area has been mined. While such time-dependent effects could be simulated with numerical modeling by using appropriate creep laws, due to the lack of knowledge on creep properties of coal measures rocks, realistic analysis is difficult at this stage. However, to consider the time effect on development of roof instability, a simple methodology has been suggested in this dissertation. The effect of different parameters like, entry and pillar width, intersection geometry has been correlated with the standup time for the observed roof falls at an IL Basin coal mine.;The combination of weak immediate roof and high horizontal stresses could have a devastating effect on roof stability. It's very difficult to completely avoid roof instability in such conditions. In this research, based on the understanding of cutter development and roof falls, a few simple and practicable recommendations are made to minimize such instabilities. While these suggestions may not completely eliminate the roof failures, they may enhance the standup time to levels that will allow safer extraction of the reserve.

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