Coal riders and claystone roofs
Definition and formation: Coal riders are coal beds that occur above (generally within 30 feet) of a major mined or named coal seam. Coal also occurs in underground mine roofs in carbonaceous shale or rash, and as isolated mats and stringers. Coal mats are discontinuous, sometimes large, fragments of coal, which are generally found in the base of sandstones. Coal stringers are thin discontinuous layers (generally inches in thickness) in mine roofs. Coal stringers can occur in any rock type. Rash or coal rash is interlaminated coal streaks or coaly shale and shale. Where any of these occur in the immediate roof of an underground coal mine, they can weaken the roof (McCulloch and others, 1975; Moebs, 1975; Horne and others, 1978; Hylbert, 1984).
Rider coals commonly are underlain by rooted claystones, called fire clays (also spelled fireclays), seat earths, and underclays. Rooted fire clays can also occur in mine roofs without overlying coal riders. Coal mats and stringers generally lack underlying clays but in some cases, may be underlain by thin claystones.
Coal beds and stringers underlain by rooted clays in mine roofs formed in the same way as the underlying coal seam (for more information, see How is coal formed). In general, claystones beneath coals represent ancient soils and coals represent the accumulation of peat-forming wetlands. Coal mats and stringers in mine roofs without underlying rooting or claystones can represent transported and compacted logs and ripped-up fragments of peat.
Discontinuities and obstacles: Coal riders and clay roofs are not discontinuities. In some cases, riders may drop in elevation and merge, or come near to merging with the underlying coal bed. In these cases, the relation of the rider to the merged coal seam is similar to that of a split, which is treated separately on this website.
Potential roof-fall hazards: Roof quality is poor where coal riders and underclays (seat earths) are within 10 feet of the top of a mined coal bed (see, for example, Weisenfluh and Ferm, 1991b). Riders are especially problematic where they occur at the same height as the anchor point of roof bolts. Coal-shale and coal-underclay contacts can concentrate moisture, and claystones are susceptible to swelling and deterioration. Even where rider coals are not well developed above clays in the roof, claystone paleosols (ancient soils) commonly have slickensides (curved, glassy planes of movement, sometimes called “slips” by miners) of varying scale, vertical peds (which are closely spaced vertical fractures formed from soil processes), and rooting, all of which cause vertical disruptions in bedding (see, for example, Horne and others, 1978; Greb and others, 1999a; Greb and Popp, 1999). Because of these weaknesses, claystones (fire clays) beneath riders typically have low compressive strength (less than 2,500 psi) (Sames and Moebs, 1991). Coals and carbonaceous laminae can also be zones along which shear zones and bedding-plane faults may occur.
Trends: Coals that occur in zones or are prone to splitting commonly are associated with riders. Some rider coals are widespread, whereas others are very local. Where riders occur, they tend to have variable dips, generally conforming to the surface upon which they were deposited. In some cases, this surface is broadly wedge-shaped (see stackrock), and thickening toward a paleochannel,, so the rider rises in elevation above the underlying coal toward the paleochannel. In other cases, riders are relatively level in the roof, or variable in height above the underlying coal. Riders are more common in eastern Kentucky than in the mined coals of the Carbondale Formation in western Kentucky. The only major mined coal in western Kentucky with common rider coals is the Baker (W. Ky. No. 13).
Known Kentucky occurrences: Riders are common above many eastern Kentucky coals. Various authors have discussed their occurrence, including riders above the Lower Elkhorn (Pond Creek) seam (Hylbert, 1980; Iannacchione, 1989; Nelson and others, 1991; Greb and Popp, 1999), the Upper Elkhorn No. 3 (Jellico) coal (Hylbert, 1980), Hazard No. 4 (Fire Clay) coal (Greb and others, 1999a; Greb, 2003), Hazard No. 8 coal (Greb and Cobb, 1989; Greb, 1991,; and Stockton coal (Hylbert, 1980).
Planning and mitigation: Coal streaks and riders in mine roofs can be recognized in cores, from roof-bolt drilling rates, through the use of bore scopes, or by observing rock- and coal-dust layering in the dust box of the roof-bolting machine after a test hole is drilled (Greb, 1991). In Kentucky, 7.5-minute geologic quadrangle maps of the mined area should be checked during planning to see if riders were encountered above the mined seam during mapping. In areas of known rider development, roof bolters should regularly monitor the height of the rider seam and modify the length of the roof bolts to avoid anchoring near the rider seam. Making maps of the height to the rider may help in projecting expected heights in advance of mining.
Roof support: Claystone and coal roof rock must be supported quickly after exposure to prevent small, continual falls. Bolted headers and straps, and full-column resin bolts may be needed to support low-strength underclays in the roof (Sames and Moebs, 1991).Most of the roof-support issues of weak shales are compounded for claystones, and as moisture absorption progresses, bolt headers, straps, wire mesh, cribs, and timbers may be needed for long-term support. Horne and others (1978) reported that mine entries with root-penetrated and slickensided claystone roofs commonly had to be abandoned, or the weak roof material had to be removed.