SYNOPSIS:

A detailed study has been made of the U.S. practice and design needs for room-and-pillar coal mining. Although some 90% of underground coal mining in the United States is by the room-and-pillar method, no comprehensive design procedure is available for this purpose. Following a survey of over 200 coal mines, typical mining conditions and room-and-pillar configurations were analyzed during a three-year research project. Specific investigations were conducted to improve the methods of span selection, roof support and pillar design in U.S. coal mines. In particular, pillar strength formulae for coal mining were studied. Based on the results obtained, an improved procedure for room-and-pillar coal mining in the United States was proposed.

RESUME:

La pratique americaine d'abattage par chambres et depilage a ete etudiee en detail en ce qui concerne la necessite de developper des methodes de dimensionnement. Bien que 90% de l'extraction souterraine de charbon aux Etats Unis soient sous forme d'abattage par chambres et depilage aucun procede general de dimensionnement n'existe. Dans le cadre d'un projet de recherche (d'une duree de trois annees) plus de 200 houillères ont ete inventoriees pour'analyser les conditions typiques de minage et les configurations des chambres et piliers. Des recherches detaillees ont ete effectuees pour ameliorer les methodes de dimensionnement pour les portees des chambres, pour le soutènement du toit et pour la grandeur des piliers. En particulier les "formules de resistance" pour les piliers ont ete examinees. Les resultats de ces etudes permettent de proposer dès maintenant un procede ameliore pour le dimensionnement de l'abattage par chambres et depilage.

ZUSAMMENFASSUNG:

Die Praxis des Kammer- und Pfeilerbaus im amerikanischen Kohlenbergbau wurde im Detail untersucht, vor allem bezueglich notwendiger Entwicklungen von Dimensionierungsmethoden. Obschon Raum- und Pfeilerbau ca. 90% des bergmannischen Kohlen- abbaus in den USA ausmachen, besteht kein umfassendes Entwurfsverfahren fuer diese Abbaumethode. Im Rahmen eines dreijahrigen Forschungsprojektes wurden deshalb Aufnahmen in ueber 200 Kohlengruben gemacht, und typische Abbauverhaltnisse und Raum- und Pfeilerabmessungen aufgezeichnet. Spezifische Untersuchungen wurden durchgefuehrt, um Dimensionierungs methoden fuer Kammer-Tragweiten, fuer Firstenausbau und fuer Pfeilerabmessungen zu verbessern. Vor allem wurden die "Pfeiler- Widerstandsformeln" untersucht. Die Ergebnisse dieser Forschung ermöglichen es nun, ein verbessertes Verfahren fuer Raum- und Pfeilerbau vorzuschlagen.

INTRODUCTION

The room-and-pillar mining method is used in the United States in some 90% of underground coal mining. Yet, systematic design guidelines are lacking both for roof control and pillar sizing. Since this method of mining will continue to be prominent in U.S. coal mining in the next two decades, in spite of an interest in longwall mining, it is obvious that rational room-and-pillar design guidelines are needed. Detailed studies conducted at Penn State during the last few years have revealed that enough scientific and practical data are available to enable consolidating all this information together for the purpose of a design code for roan-and-pillar mining. The aim of such a code would be to facilitate selection of roof spans in the coal mines and the sizing of coal pillars in accordance with the coal mining conditions in the United States. This means that the established mining practice and experience would be utilized in preparing the design guidelines. In addition, the objective would be to incorporate as much up-to-date scientific knowledge as is compatible with practical-applications in coal mining to facilitate improved planning of new mines as well as optimizing the mining operations already in progress.

1.1. Room-and-Pillar Mining in the U.S.A.

The room-and-pillar method of coal mining as used in the United States involves a system known as retreat mining. In this method the rooms (openings) and pillars are developed as mining advances but subsequently the pillars are recovered "on retreat" which can result in a high percentage of coal extraction, e.g., 70%. However, room-and-pillar retreat mining techniques vary widely in the United States, depending on specific local conditions. The average coal extraction in the U.S. is just aver 50%but the average percentage of mines practicing pillar extraction is only 26%,although in some areas (western Pennsylvania and northern West Virginia) this percentage is 60%to 70% (Kauffman et al., 1981). Clearly, there is much scope for improvement. While pillar stability is of much importance in the room-and- pillar mining method, the stability of mine roofs is a crucial aspect of strata control in the United States. Hence, selection of roof spans and their reinforcement must be addressed in any design code. There, the pillars receive the main attention as they are generally not recovered on retreat but left permanently in place.

2. 0.SURVEY OF U.S. MINING CONDITIONS

In 1979, the Pennsylvania State University initiated a national survey of room-and-pillar design practice and mining dimensions in the United States. The survey included a comprehensive study of such parameters as the depth below surface, seam thickness, roof spans, pillar height, pillar width, pillar length, width-to-height ratios, percentage extraction and method of design.

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