Definition and formation: Clay veins, also called clay dikes, are irregular, generally vertical to subvertical, clay-rich dikes (intrusions) in a coal seam. They may thicken upward, thicken downward, or show no preferred thickening trend. Clay veins commonly have jagged contacts with the coal, and pieces of coal are mixed in with the clay vein; laminae or thin beds of clay may also be laterally intruded into the coal bed.
Clay veins appear to form from clays from roof or floor sediment being mobilized into tension fractures and tears in the seam. They may form from compactional or tectonic stresses during or after coalification, generally long after the coal was buried (Donaldson, 1979; Chase and Ulery, 1987).
Discontinuities and obstacles: Clay veins can completely or partially replace a seam with clay, causing a vertical discontinuity in the coal seam. Clay veins are also commonly associated with small offsets (clay-vein faults), which offset the coal inches to several feet. Individual clay veins may be laterally continuous for hundreds of feet to as much as a mile (see, for example, Ellenberger, 1979). Clay veins are commonly several feet in width. Clay veins with apparent widths of more than 10 feet are usually oriented obliquely or subparallel to the entry, causing their width to appear wider than it actually is. Clay veins can be mined through, but lead to increased reject (noncoal rock that is mined with the coal and must be disposed of).
Potential roof-fall hazards: Roof quality is almost always deteriorated above clay veins. Slickensides and small-offset clay-vein faults (miners’ “slips”) are common in clay veins, seams adjacent to clay veins, and fine-grained roof rocks penetrated by clay veins (Ellenberger, 1979; Krausse and others, 1979; Nelson, 1983; Chase and Ulery, 1987). Clay veins are also commonly associated with cutter-style fractures and roof failures along the ribs of entries (Krausse and others, 1979; Hill and Bauer, 1984; Iannacchione and others, 1984; Hill, 1986).
Trends: Clay veins generally occur in clusters or groups. Where one is encountered, more will occur. In the Northern Appalachian Coal Field (Pennsylvania, northern West Virginia), clay veins commonly follow distinct joint trends (see, for example, Chase and Ulery, 1987). In some cases, clay veins may parallel sandstone roof rolls and cutouts (see, for example, Moebs, 1981). In much of the Illinois Basin (including western Kentucky), however, clay veins appear to related to pinching and swelling of black shales and gray shales around limestone roof strata (Krausse and others, 1979; Damberger and others, 1980; Nelson, 1983), or have relatively random patterns of occurrence, and therefore are difficult to predict.
Known Kentucky occurrences: Clay veins are most commonly reported from upper Middle and Upper Pennsylvanian coal beds in the northern Appalachian Basin and Illinois Basin, but have also been reported in many other coal basins (Chase and Ulery, 1987). They are locally common in the Springfield (W. Ky. No. 9) and Herrin (W. Ky. No. 11) coals in western Kentucky. They are rare in the Eastern Kentucky Coal Field, although a few have been reported in the Pond Creek (Lower Elkhorn) coal and High Splint and Harlan coals in southeastern Kentucky (Chase and Ulery, 1987).
Planning and mitigation: Where clay veins are encountered, their longitudinal orientations should be measured. If orientations show consistent trends, mine plans can be adjusted accordingly. Where trends appear random, check to see if there are dominant and minor trends, in which case mine headings might be adjusted for the dominant trend and miners made aware of minor trends. In the Herrin coal, clay veins may be associated with thickness changes in overlying limestones and black shales. Changes in floor elevation (crest of floor highs and lows) may also be associated with veins. Detailed mapping of roof lithologies (height to black shale, height to limestone, thickness of black shale, thickness of limestone) can be compared with known occurrences of clay veins to see if there are any associations. Similarly, the position of clay veins can be compared to known floor and roof rolls. If comparisons are positive, then mine and safety plans can be projected in advance of mining.
Roof support: Weaknesses in roof rocks above the clay veins may extend to 12 feet above the top of the coal bed. Slickensides in the clay require immediate support, such as bolting, blocking, and strapping (Moebs and Ellenberger, 1982). Roof support will depend on orientation relative to ribs, density, height of slips and fractures, and frequency and continuity of individual veins, but full-column grouted resin bolts in conjunction with crossbars and steel mats, angle bolting, and strapping on either side of a major vein fracture or slip may be needed. Cribbing is often required where clay veins occur along a rib line (Chase and Ulery, 1987). Chase and Ulery (1987) provided examples of support and monitoring. Because clays are weak and susceptible to moisture effects, continued deterioration, sagging, and spalling is possible.
Where roof cantilevering (tilting) occurs because of dips of clay veins in the roof, bolting and strapping the roof on each side of major slips or fractures associated with the clay vein may help to support the roof. Proper bolt length and angle of installation will depend on the height and dip of the clay vein and fractures (Chase and Ulery, 1987).