underhole

Underholing is a manual or mechanical process used in underground coal mining to excavate the lower portion of a coal seam, typically removing 18 to 36 inches of coal and stone from the seam's base. This undermined section weakens the structural support of the overlying coal, causing it to fracture and fall to the pit floor, where it can then be loaded and transported out. The term comes directly from the practice of working into the underside, or bottom, of the seam.

The method is most effective in seams where the coal is naturally brittle or where the overlying strata are dense and heavy enough to ensure collapse once support is removed. A miner underholing may use a pick and shovel, a compressed-air hammer, or a powered coal cutter to slice horizontally along the seam's lower boundary. The height of the cut depends on coal strength and the weight of the roof; thicker underhole cuts reduce the risk of incomplete collapse but require more labor and produce more waste rock in the broken coal.

Variants and modern practice

Underholing differs from other coal winning methods such as pillar extraction or longwall mining in its emphasis on gravity-driven collapse rather than continuous mechanical advance. In older pit workings, underholing was often paired with a topping cut (removal from above) to accelerate breakage. Modern mines in thin or difficult seams still use selective underholing, though most large operations have shifted toward powered loader systems and continuous miners that reduce the need for preliminary weakening cuts.

The main hazards include roof fall before the coal is fully undermined, inrush of water or gas from the underlay, and the physical strain of working in confined height. Poor underholing, where the cut is uneven or incomplete, leaves hanging coal that must be brought down manually, adding time and risk. The quality of the underhole directly affects the cleanliness of the broken coal; a well-executed cut minimizes slate and other stone in the product.

Although the term remains standard in mining vocabulary and textbooks, underholing as a primary production method is now rare in mechanized collieries. It survives as a supplementary technique in small-scale or hand-worked seams, and knowledge of the process remains important for understanding both historical mining practice and the structural mechanics of coal breakage in difficult geological conditions.