Paleoslump mine roofs
Definition and formation: Paleoslumps are ancient channel-margin collapse and slide deposits preserved in coal-bearing rocks. Slumps are characterized by beds that have been rotated from their original horizontal position along a curved (listric) glide plane. They are common in paleochannel deposits, so may be associated with roof rolls and cut outs.
Discontinuities and obstacles: Paleoslumps form discontinuities in coal when they occur in paleochannels. In such cases, most of the obstacles associated with cutout channels and roof rolls apply to paleoslumps.
Potential roof-fall hazards: Slumps commonly contain bedding at a variety of angles to the horizontal, which results in natural weaknesses or separation planes in mine roofs. Bedding dip in slumps increases in the direction of a slide from the top, downward toward the toe of the slump. Toes of slumps tend to be very deformed with minor shear faults and vertical, contorted, and overturned bedding. Slumped strata contain slickensides along and at many angles to bedding, isolated blocks of rock surrounded by slickensides and microfaulted strata, small-scale compactional and shear faults, and folding and contorted bedding (Williams and others, 1965; Greb and Weisenfluh, 1996). Slumps can occur as single rotated intervals or blocks, or in successive en echelon sheets of slumps (Greb and Weisenfluh, 1996). Where slumps overlie coals, sand dikes and sediment intrusions also may occur in the underlying coal (Greb and Weisenfluh, 1996). Slumps result in extremely poor roof conditions, because the through-going slickenside planes isolate large rock masses that cannot be supported with conventional bolting systems. Extreme and costly roof-control measures are almost always required in this case and may make further advancement impractical.
Slumps can occur as single rotated intervals or blocks, or in successive, en echelon, sheets of slumps (Greb and Weisenfluh, 1996). Where slumps overlie coals, sand dikes and sediment intrusions also may occur in the underlying coal (Greb and Weisenfluh, 1996). Slumps result in extremely poor roof conditions, because the through-going slickenside planes isolate large rock masses that cannot be supported with conventional bolting systems. Extreme and costly roof-control measures are almost always required and may make further advancement impractical.
Trends: Paleoslumps are most common in channel and channel-margin strata (Horne and others, 1978; McCabe and Pascoe, 1979; Moebs, 1981; Nelson, 1983). As such, they are typically curvilinear deposits 100 to 500 feet in width. En echelon, or imbricated, slumps may have widths in excess of a mile.
Known Kentucky occurrences: Paleoslumps are known above many coal beds in roadcut exposures in eastern Kentucky (Horne and others, 1978; Cobb and others, 1981; Greb and Weisenfluh, 1996). Hester and Brant (1981) listed 18 roadcuts with paleoslumps and gave several examples from eastern Kentucky. Examples of paleoslumps detected in underground mines have been published for the Pond Creek (Lower Elkhorn, Imboden) (Hylbert, 1980; Greb and Weisenfluh, 1996; Greb and Popp, 1999; Greb and Weisenfluh, 2000) and Fire Clay (Hazard No. 4) coals (Greb and Weisenfluh, 1996; Greb, 2003).
Planning and mitigation: Rotated and moderate- to high-angle bedding can sometimes be detected in cores prior to mining. Like paleochannels, however, most are too narrow to be detected during coring. Because paleoslumps tend to occur in paleochannels or down the slopes of levees and marginal channel conditions, the potential for paleoslumps should be planned for in any paleochannel situation. Areas of thinned or over-thickened coal, overlain by variably dipping beds, may be a hint of paleoslump conditions.
Roof support: Paleoslumps are relatively common and in some mines have blocked continued advancement of the mine. As in paleochannels, reorienting headings obliquely to slumps and bounding channels or to the dip of shear planes within slumps will help minimize exposure to the hazard (Greb and Weisenfluh, 1996). Otherwise, supplemental support similar to that needed for channel-margin conditions is needed—angle bolts, cribbing, etc. Some mine operators have resorted to constructing concrete tunnels beneath slumps. Supporting slumped roof is very difficult and costly but will depend on angle and deformation of bedding, the area across which slumped conditions occur, and the amount of time the area needs to remain open.