ABSTRACT
Cutter roof failure is a specific type of ground control problem which frequently results in massive roof failure. It is a common occurrence in coal mines of the Northern Appalachian Coal Basin, causing delays in production and posing a safety hazard to mine personnel. The Bureau of Mines is conducting research on the causes of cutter roof failure to gain a basis from which to prevent its occurrence and to support such roof when failure does occur. Research conducted in a coal mine of central Pennsylvania has revealed a correlation between the occurrence of clastic dikes and formation of cutter roof failure. In-mine mapping of ground conditions showed an increase in roof failure in areas of high frequencies of clastic dikes. Rock pressure monitoring around clastic dikes registered the greatest amount of roof loading near the intersection of dikes with the rib. Load cells measuring horizontal pressure changes in the roof indicated that the greatest pressure changes were occurring perpendicular to entry headings when clastic dikes were present. Analysis of rock pressure monitoring shows that the roof behaved as two cantilever beams when severed by a clastic dike. Additional roof supports such as trusses and cribbing were found to effectively support the roof in areas of clastic dikes and prevent cutter roof failure. However, these methods were only successful when employed shortly after mining.
INTRODUCTION
Occurrences of cutter roof failure during development mining have been observed in increasing numbers. In response to this, the Bureau of Mines is investigating the causes of this type of roof failure, which can deteriorate to a major roof fall if additional support is not applied. The uniqueness of this problem has limited the techniques available for its treatment. The approach of trial and error treatment has proven unsatisfactory in many cases, as the causes are often different from mine to mine. Cutter roof failure initially begins as a fracture along one or both roof-rib lines of an entry and propagates nearly vertically into the roof (fig. 1). When the fracture breaks to a height above the anchor horizon, or along a weak bedding plane, massive roof failure may occur. Figure 2 is an example of severe cutter failure and figure 3 illustrates the end result of such cutter development. Previous research by the Bureau has shown a correlation between cutter roof failure and high horizontal stress fields. Aggson (1979) and Kripakov (1982) conducted studies addressing the particular stress state that would initiate cutter roof using finite element analysis. Research conducted by Thomas (1950) approached cutter development from a purely practical perspective, without any instrumentation. His work is believed to be the earliest attempt to understand cutter development. The influences of the direction of mining and occurrence of clastic dikes on cutter roof failure formation have been observed through detailed mapping by Iannacchione, et.al., (1984). The occurrence of cutter roof has had a major effect on entry stability at the Greenwich Collieries North Mine of Indiana County, Pennsylvania. The investigation conducted there was comprised of two basic phases which contribute to an understanding of the pres- sure dynamics surrounding cutter development and the influence of geologic anomalies.
