SYNOPSIS:

It has been shown from case studies that the occurrences of coal bumps in deep coal mines in Hokkaido, Japan, was restricted to the zone around coal pillars which had a sandstone roof measuring more than four times the thickness of the coal seam. In order to clarify the mechanism of coal bumps, laboratory tests, field measurements and numerical analyses on the coal seam behaviour were performed. As a result, it was ascertained that, when coal fails, a roof composed of sandstone supplies a large amount of energy to the coal seam compared to that composed of shale. Furthermore, both the mode of energy release and the patterns of stress changes are remarkably disturbed when an irregular distribution of mechanical strength of coal appears in the coal seam. These two conditions are the most important factors causing coal bumps.

RESUME:

Les etudes relatives à de profondes mines souterraines de charbon à Hokkaido, au Japon, nous amènent à conclure que les eclatements du charbon (coal bumps) sont localises autour des stots ayant pour toit des bancs de grès mesurant plus de 4 fois l'epaisseur de la couche du charbon. Afin de pouvoir expliquer le mecanisme de ces eclatements, on a procede à l'execution d'essais en laboratoire, au prelèvement de donnees sur le terrain et ensuite aux analyses numeriques rela-. tives au comportement de la couche de charbon. Comme resultat, il a ete conclu qu'à la rupture, le toit compose de grès fournit beaucoup plus d'energie que celui compose de schiste et que le mode de relachement de l'energie et celui du changement de la contrainte sont bien perturbes si la distribution de la resistance mecanique du charbon s'avère irregulière dans la couche de charbon. Ce sont les deux facteurs les plus importants parmi ceux qui provoquent les eclatements du charbon.

ZUSAMMENFASSUNG:

Es zeigte sich in Fallstudien, daß das Auftreten yon Kohlebergschlagen in tiefen Kohlegruben Hokkaidos, Japan, auf Zonen um Kohlepfeiler beschrankt war, die ein Sandsteinhangendes mit mindestens der vierfachen Machtigkeit des Flözes aufwiesen. Um den Mechanismus der Kohlebergschlage aufzuklaren, wurden Laboratoriumsuntersuchungen, Feldmessungen und numerische Analysen durchgefuehrt. Es ergab sich, daß mit dem Bruch der Kohle ein Sandsteinhangendes eine im Vergleich zum. Schiefertonhangenden vergleichsweise große Energiemenge an die Kohle abgibt. Ferner zeigte sich, daß die Art der Energiefreisetzung und die Änderung des Gebirgsdruckes stark von Unregelmaßigkeiten der Kohlefestigkeiten abhangen. Diese zwei Faktoren sind fuer Kohlebergschlage von entscheidender Bedeutung.

1. INTRODUCTION

In deep coal mines in Hokkaido there are frequent occurences of the brittle fracture of coal seam in or near working areas, being accompanied with fracture noise and seismic activity. The present paper is the summary of the investigations which have been conducted as a part of the research program in order to clarify the cause and mechanism of coal bumps. Though most of the informations attained in this study concern brittle fractures, they will be available to understand the mechanism of coal bumps and rock burst.

2. COAL BUMPS OCCURED IN HOKKAIDO

The largest one of the nine fatal coal bumps occured in the past was that of Bibai Coal Mine in 1968. The coal bumps took place three times in different places of the same protecting pillars left in each side of the roadway along the upper gate of longwall pannel. The sites of the bumps are illustrated in Fig.l. The pillars situated at 650m below surface and had a dimension of about 25m in width and 3m in height. The first bump occured at the part of the pillar of small and complex shape 25m ahead of working face as designated by (1) in Fig.1. With further advancing of the face, the second and third bumps happened sequently at the place of (2) and (3) behind the face front. All the three bumps were so severe that an earth tremor could be observed at the seismogram station of the meteorological observatory, l20km distant from the coal mine. The seismic shock was estimated at 2.2 in magnitude for the first bump, 2.7 for the third bump and less than 1.9 for the second bump. The damage of roadway attained to 100m long in the first bump, 170m in the third bump. In the severest case the part of roadway was completely closed due to the floor heave and the extrusion of wall. The coal is hard and its uniaxial compression strength is 20 to 30MPa. The presence of a strong and thick overlying sandstone measuring more than four times the thickness of the coal seam is a common factor found in the nine bumps experienced in Hokkaido.

3. DEFORMATION BEHAVIOR OF DOUBLE LAYERED MODEL

In order to investigate how sandstone roof exert influences on the deformation behavior of coal, uniaxial compression tests were performed using the double layered models of sandstone and coal connected in series.

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