American Mining Congress, MINExpo International ’88, Chicago, Illinois, 24–28 April 1988
Authors: J. F. T. Agapito and H. N. Maleki (AAI), and M. Moon (Utah Power & Light Company)
Severe pillar bursting at the Utah Power & Light Company’s Deer Creek Mine, Utah, has caused serious safety and ground control problems. The bursts occurred in pillars 40- to 80-ft wide in three- and multi-entry systems as a result of high abutment stresses from longwall mining. Some of the major bursts have been 300 to 400 ft long, and have completely filled the entries. High pillar stresses have also caused a massive roof fall in a three-entry development 60 ft below in a different seam. To reduce the burst-prone stress levels, a 30-ft-wide yield pillar was introduced in a two-entry gate road system in 1979. During the last eight years, no bursting or any other significant ground control problem has been encountered in more than 60,000 ft of two-entry development. Back analysis of two major burst events indicates that burst-prone stress levels can be developed easily in pillars 40-ft wide or wider, and that 30-ft-wide pillars yield just before the burst-prone stress levels are reached. There is a lack of understanding about pillar loading and yielding, and as a consequence, there is no rational criterion for yield pillar design. Yield pillar systems should be used within a framework of comparative experience after careful geotechnical evaluation and under controlled test mining conditions. The Deer Creek experience is a positive case study of a two-entry, yield pillar system where significant safety and productivity benefits were obtained.