The Occurrence And Characterization Of Geological Anomalies And Cutter Roof Failure: Their Effect On Gateroad Stability

Anomalous geological features in the coal and surrounding strata of a mine in northern West Virginia have fractured and deformed an inherently weak roof rock prior to mining. Horizontal stresses, as much as six times greater than overburden related horizontal stresses, have induced cutter roof failure. This failure was distinguishable from rock deformation associated with geological anomalies because it was induced by mining. The characterization of these features has allowed for the prediction of zones of unstable roof rock and the determination of failure trends on gateroad stability. Geological anomalies included clastic dikes and a narrow zone of sandstone roof. Clastic dikes, representing intrusions of shale and claystone, were invariably accompanied by local roof rock instability, and often formed the boundaries of roof falls. Shale was the dominant roof lithology except where a small sandstone occurred in the northwest portion of the longwall mining area. The sandstone occurs in two phases: a sandstone-filled channel, 5-m- thick and 60-m-wide, which eroded and replaced the shale roof and the upper part of the coalbed; and a sheet sandstone unit which was crossbedded to interbedded with the underlying shale. Horizontal stresses are naturally relieved through the initiation of cutter roof failure. This failure may extend from a few meters to several tens of meters horizontally, and from a few centimeters to several meters vertically. Orientations of entries and crosscuts has a greater influence on the direction and total length of cutter roof failure at the rib-roof intersection than does the principal directions of the in situ horizontal stresses. Seventy-four percent of the cutter roof failures occurred along the rib-roof intersection. Orientations of cutter roof failure away from the rib- roof intersection were clustered in the north-south direction and may have been influenced by the horizontal stress field.