ABSTRACT

Underground mechanized coal mining in the United States has transitioned significantly from room and pillar mining using cutting machines and continuous miners to mining a high percentage of reserves with longwall mining methods. However, as the shallower coal reserves were mined out, operators pursued the mining of deeper coal reserves thus coping with increased ground stresses. Sometimes the release of strain energy was not gradual and sudden violent failures of coal occurred. In order to cope with the coal bumps, two basic plans of attack have been historically used: 1) work with the design or layout of the longwall face, orientation and entry configuration and 2) use various operational methods to reduce the incidence or severity of the bumps.

This work included an extensive survey of public domain documents to identify location of coal bump events as well as information on existing or past techniques to enhance or ensure acceptable ground conditions on the longwall face. This included a review of new innovative mining techniques for extraction of deep reserves including advancing headgate-retreating longwall concept and use of high capacity support.

Although de-stressing has historically been used in many operations with some success, a rock mechanics evaluation of its effectiveness and any adverse effects on neighboring structures has not been completed. Using a recent case study dealing with sudden failure of mine floors in western US mines, the first author has completed detailed FLAC3D modeling of one new floor heave event to both improve the understanding of floor heave mechanisms and the effectiveness of destress blasting in the mine floor. Modeling results are in agreement with blasting experiments at the cooperative mine, confirming minor changes in stability of strata using Mohr-Coulomb failure criteria before and after blasting.

INTRODUCTION

Underground mechanized coal mining in the United States has transitioned significantly from room and pillar mining using cutting machines and continuous miners to mining large percentages of reserves with longwall mining methods. One of the first longwall operations documented in the literature was that of the Sunnyside Mine in Utah. Sunnyside Mine started their first longwall in 1961 [1]. Over the ensuing years many longwall operations were developed throughout the United States and other countries. Longwall operations offered higher productivity at an overall lower cost. Longwall methods also offered a safer work environment. However, as the shallower coal reserves were mined out, operators were forced to pursue the mining of deeper coal reserves at 3,000-ft of cover. Doing so required operators to cope with increased ground stresses. Sometimes the ground stresses could not be controlled and sudden unplanned violent failures of coal occurred. These failures were typically referred to as coal bumps. In a longwall mining operation, these violent failures of coal or bumps normally occur in the entries close to the face or on the face itself.

In order to cope with the coal bumps, two basic plans of attack have been used: 1) work with the design or layout of the longwall face, orientation and entry configuration and 2) use various operational methods to reduce the incidence or severity of the bumps.

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