Splits
Definition: Splits are broadly wedge-shaped rock bodies that cause parts of the seam (benches) above and below the rock wedge to separate or split from each other. Depending on the direction of mining advance, these splits can also sometimes be formed by two different seams merging. Most splits are formed from flood deposits from ancient channels, including fan-shaped features called crevasse splays. Splits commonly thin laterally into a persistent and relatively thick parting in the coal.
Discontinuities and obstacles: Splitting causes major obstacles to mining and often permanent lateral barriers to mining in the direction of splitting. Splitting commonly results in a change in seam height because part of the bed separates from the other.Individual benches of the coal above and below the split may thin causing decreased seam thickness. Relative to the total seam, a split also increases the amount of reject (noncoal rock) in the mined coal. Mining toward splits presents a series of options, which must be carefully considered and depend on individual situations. In some cases, the seam and parting/split are mined together until the amount of reject is too great. At that point, mining may stop or continue in only one of the coal benches. Advancing in either the lower or upper coal benches can be problematic and depends on the specific situation (bench thicknesses, dip, roof-rock strength, composition of split, etc.). In general, advancing in the lower seam leaves a coal rider in the roof (and increased roof-fall potential). Depending on the situation, either bench may be cut out or thin laterally (sometimes the upper, sometimes the lower). Also, increased floor elevation or dip is a possibility for either the upper or lower split, depending on the specific situation.
Potential roof-fall hazards: Other than the scenario discussed above, in which the lower bench of the coal becomes the roof, splits (on their own) are not associated with any specific geologic roof problems.
Trends: Most splits form lateral to paleochannels. They represent flood deposits or levees of ancient channels. Map-view geometry of splits can be variable, but tends to be lobate or fan-shaped, radiating outward from the channel. In cross section, splits are wedge-shaped, thickening (sometimes rapidly, and other times gradually) toward the channel. Some splits can be irregular in shape, or elongate and parallel to the bounding channel.
Known Kentucky occurrences: Splitting is much more common in coals mined in eastern Kentucky than in western Kentucky. Most of the major mined coal beds in eastern Kentucky are prone to splitting. Of the mined major coals in western Kentucky, only the Baker (W. Ky. No. 13) coal bed commonly exhibits splitting. Several older coal beds of the Tradewater Formation also split locally (e.g., Amos coal bed, Foster coal bed), but are not currently mined.
Planning and mitigation: Splits can generally be detected using exploratory core data through correlation of coal benches and intervening rock partings. If unexpected splitting is encountered, increasing separation of coal benches above and below the rock parting generally follow directional trends. Additional drilling may be needed in the direction of splitting to determine the rate of splitting and the ultimate thickness of intervening rock between coal benches, as well as the possibility of lateral cutouts.