lath

A lath is a thick, sharp-edged wooden plank hammered horizontally into the ground ahead of advancing tunnel work to hold back loose soil, mud, or weak strata. Unlike the thin laths used in building, mining laths are substantial timbers, typically 3 to 6 inches thick and 8 to 12 feet long, with a beveled or sharpened leading edge to ease driving into compacted earth. The worker or machine drives the lath forward incrementally as excavation progresses, creating a temporary retaining wall that prevents cave-in during heading work.

Laths work by bearing against the soil mass and distributing pressure across a wider area than a simple prop would manage. As the tunnel advances, each successive lath overlaps slightly with the one behind it, forming an interlocking shield. The laths are typically set at a slight angle, steeper at the top than bottom, so gravity and soil weight help keep them in place. Miners sometimes coat the driven face with clay or use a separate backing of boards or wattle to seal gaps and stop small soil particles from trickling through.

Regional and situational variation

The term spill (or spile in some coalfields) is used interchangeably with lath, particularly in British mining. The choice of timber matters: softwoods like pine are easier to drive but wear faster, while harder woods resist splitting under hammer blows. In very wet ground, laths can absorb water and swell, creating unexpected pressure; in clay, they may jam and become difficult to remove or advance further. Old laths left in abandoned workings can become a hazard to later excavation if their position is forgotten.

Lath work is labor-intensive and has been partially displaced by steel sheet piling in modern tunneling, especially in deep or long headings where hydraulic extraction allows faster advance. However, in shallow coal mines, old slate workings, and rescue or exploratory tunneling where speed matters less than adaptability, timber laths remain practical because they are simple to modify, salvageable, and require no specialist equipment beyond wedges and a maul. The practice is old enough that its etymology is unclear; it may derive from German Latte or Dutch lat, both meaning a thin board or batten.