New Technology for Longwall Ground Control. Proceedings: U.S. Bureau of Mines Technology Transfer Seminar. Pittsburgh, PA: U.S. Bureau of Mines, 1994 Aug; :5-17
Successful longwall mining requires a stable tailgate entry. Gate entry performance is influenced by a number of geotechnical and design factors, including: 1. Pillar size and pillar loading; 2. Roof quality; 3. Floor quality; 4. Entry width; 5. Artificial support (primary and secondary). This paper describes a comprehensive, practical design methodology based on statistical analysis of a nationwide database of longwall ground control experience. Geotechnical surveys were conducted by U.S. Bureau of Mines (USBM) researchers at 44 U.S. longwall mines, and underground observations of site geology, entry conditions, and support design were recorded at each mine. The observations were combined with discussions with mine personnel to identify 69 longwall gate entry designs as satisfactory, unsatisfactory, or borderline. Only conventional longwall designs, in which the pillars are expected to carry the full abutment loads, were included in the database. Designs that employed yield pillars only were excluded. The case histories were characterized using five descriptive parameters, pillar design was described by the Analysis of Longwall Pillar Stability Factor (ALPS SF). A major new contribution is the Coal Mine Roof Rating (CMRR), a rock mass classification system that quantifies the structural competence of bolted mine roof. Other rating scales were developed for primary support, secondary support, and entry width. Statistical analyses indicated that in 84% of the case histories the tailgate performance could be predicted correctly using only ALPS and the CMRR. Most of the misclassified cases fell within a very narrow borderline region. The analyses also confirmed that primary support and gate entry width are essential elements in successful gate entry design. The relative importance of the floor and of secondary support could not be determined from the data. Based on these results, a simple equation was developed to guide the design of longwall pillars and gate entries: ALPS SFR = 1.76 - 0.014 CMRR, where ALPS SFR = ALPS SF suggested for design. Guidelines for entry width and primary support density, as related to the CMRR, are also provided.