Kettlebottoms in mine roofs
Definition and formation: “Kettlebottom” is a term used by miners to describe isolated rock masses in mine roofs, which tend to be semicircular when viewed from below and have a rounded or flat bottom like a kettle. Most kettlebottoms are in-place, fossil tree stumps. More precisely, they are hollowed-out trees filled with sediment. Kettlebottoms are tubular, conical, or bell-shaped. In some cases, roots are preserved laterally at the base of the trees, although most kettlebottoms lack infilled roots.
Discontinuities and obstacles: Kettlebottoms occur in the roof above coal seams, so are not discontinuities or obstacles within the seam.
Potential roof-fall hazards: When in-place, standing fossil tree trunks (kettlebottoms) are positioned above coal seams, removal of the underlying seam can allow the kettlebottoms to fall out of the mine roof. Kettlebottoms have been the source of many small roof falls and were the cause of many historical fatalities in coal mines. Understanding what kettlebottoms are and what they look like in underground mine roofs helps miners spot potential kettlebottoms before they fall so that supplemental roof support can be installed and potentially hazardous roof falls can be avoided.
Kettlebottoms near the top of a coal seam in an underground mine roof are unstable because:
1). They are often composed of different roof rock (the stump fill) than the surrounding rock, and are not well bonded to the surrounding rock. Also, as vertical features in the roof rock, they cut across horizontal layering. Most roof bolting practices use tension to create beam strength in horizontal layers. Objects that cut through those layers weaken the beam and create potential roof problems.
2). They are often slickensided (slick shiny surfaces caused by compaction) along their outer edge. Some may have a coal rind that is slickensided along the outer edge. The slickensided surface has the potential for slippage. Since most standing fossil tree trunks are thicker at the base than at the top, they can fall out of the roof along these slick surfaces.
3). They may initially be invisible to miners beneath, because they are only a few inches above the level of the excavated seam and draw rock: through time they may fall from the roof if not noticed and supported.
4). Their fills may be layered differently than the surrounding roof rock, and may break along those layers into pieces, falling out of the mine roof up to the top of the preserved stump, even after the initial fall.
Trends: Fossil trees are most common in gray shale roofs and in stackrock roofs. They are uncommon in sandstone channels and black shale and marine roofs. In some mines, increased frequency of kettlebottoms occurs along the margins of paleochannels in stackrock conditions formed by ancient splays and levees.
Where one kettlebottom is found, many more will likely be encountered, because they were usually part of an ancient swamp forest. Similar to trees in a forest, it is difficult to predict the position of individual fossil trees (kettlebottoms) after initial discovery. Just know that more are likely to be encountered. In some cases, circular bulges in shale roof may indicate hidden fossil trees in the roof.
Known Kentucky occurrences: Kettlebottoms have been reported above many of the mined coals in eastern Kentucky. Published reports give examples above the Blue Gem (Greb, 1991), Pond Creek (Lower Elkhorn) (Iannacchione, 1989; Greb and Popp, 1999), Upper Elkhorn No. 3 (Jellico, Darby) (Hylbert, 1980), Hazard No. 4 (Fire Clay) (Greb, 1991; Greb, 2003), Hazard No. 7 (Cobb and others, 1981), and Hazard No. 8 (Coalburg) coals (Hylbert, 1980; Greb, 1989). Kettlebottoms do not occur, or are less common, above the mined Carbondale coals of western Kentucky, which tend to have marine shale roofs.
Planning and mitigation: Kettlebottoms are small compared to mine area, and cannot be detected in cores. During mining, however, where one kettlebottom is found, more will occur. Whole forests have been found above some coal beds.
Roof support: Chase and Sames (1983) showed the best support plans for kettlebottoms. The former U.S. Bureau of Mines recommended bolting on either side of the kettlebottom with a header or strap between the bolts to catch the kettelbottom if it comes loose from the roof. Bolting up the center of a kettlebottom is not recommended, because the height of the kettle in the roof is not always known, and therefore the bolt may just anchor into the kettle.