Stackrock mine roofs
Definition and formation: Miners refer to alternating horizontal or slightly inclined flat beds of sandstone, shale, siltstone, and sometimes coal as “stackrock.” In general, the term is used when individual beds are inches to less than 3 feet thick. The stacking of alternating beds of sandstone and shale in coal-bearing strata is common in rocks deposited in levee and channel-margin crevasse-splay deposits (Moebs, 1977; Horne and others, 1978; Moebs and Ellenberger, 1982; Moebs and Ferm, 1982; Nelson, 1983; Hylbert, 1984) and in mouth-bar deposits (Horne and Ferm, 1978). Interlaminated, thin alternations of sandstone and shale, which could be called “stackrock,” also occur in tidal deposits (see, for example, Greb and Popp, 1999).
Potential roof-fall hazards: The interbedding of different rock types (usually of contrasting grain size) causes rocks with different mechanical properties to be stacked, often with sharp bedding planes that lack cohesion between beds. Thin interbeds of different rock types cause natural weaknesses in the rock, which makes forming a competent beam in the roof using conventional bolting more difficult (Moebs, 1977; Hill, 1986; Sames and Moebs, 1991). Horne and others (1978) noted that bed thicknesses of less than 2 feet in horizontally or planar-bedded sandstones were more problematic than situations in which beds were more than 3 feet thick. In stackrock, concentrations of mica along bedding surfaces in sandstone, and concentrations of carbonaceous plant debris in shales, siltstones, and sandstones may cause weakness along bedding planes (see, for example, Hylbert, 1984). These weaknesses can be accentuated where forces (natural and mine-induced) cause horizontal stress in mine entries or crosscuts. Thin shales in stackrock roofs may contain the same weaknesses discussed for weak-shale mine roofs. Stackrock with discontinuous coals also contains the same weaknesses discussed for coal-rider and claystone roofs.
Trends: Stackrock situations formed from channel flooding have more predictable trends than those formed in other situations. In cross section, both levee and splay deposits thicken and become sandier as broad wedges toward channels. Likewise, both levee and splay deposits tend to thin away from channels, either pinching out into or interfingering with gray shales, which were deposited in lakes or bays between channels. In plan view, levee deposits will form thin strips along paleochannels. Crevasse splays form semicircular fan shapes or irregular shapes (conforming to paleotopography) away from the source channel. Unfortunately, stackrock formed from interlaminated tidal deposits not associated with channels, and thin interbeds of sandstone in shales farther away from channels, do not necessarily follow recognizable trends.
Known Kentucky occurrences: Stackrock is common above many, if not most, coal beds in eastern Kentucky. Examples above mined coals are found above the Lower Elkhorn (Pond Creek) (Greb and Popp, 1999; Greb and Weisenfluh, 2000), Upper Elkhorn No. 3 (Jellico) (Hylbert, 1980), Hazard No. 4 (Fire Clay) (Greb, 2003), and Hazard No. 8 coals (Greb, 1989).
Planning and mitigation: Coring can identify stackrock situations in advance of mining so that adequate plans can be made for roof support. Careful attention should be paid to how well interlaminated or interbedded rock types that will be in the immediate roof stay together in core. Slaking tests of shales between sandstone and siltstone beds may also help to determine if shales are moisture sensitive. If clay and thin coals occur within stacked beds, vertical disruptions in bedding caused by rooting and slickensides are possible, as are expandable clays (see claystone and rider roofs).
Roof support: Roof falls on tapering edges of splay stackrock require increasing bolt length to bolt into competent strata above the thinning stackrock (Weisenfluh and Ferm, 1991a). Several reports have noted that block falls of stackrock strata often occur up to the height of bolting, suggesting that bolts were anchored along a weak bedding plane and contributed to the fall (see, for example, Krausse and others, 1979). Moebs and Ellenberger (1982) suggested staggering bolt lengths (star pinning) in stackrock strata to lessen the potential of anchoring all support in a single weak horizon. Resin bolts and truss bolts have also been successful in bad stackrock roofs (Krausse and others, 1979).