spill

In advancing a mine level through soft or broken ground, a spill is a horizontal wooden pole or lath driven ahead of the main timbering frame to support the immediate roof and walls. It acts as a temporary buttress, stopping loose material from collapsing into the working area before permanent supports can be set. Spills are typically 4 to 6 inches in diameter and pushed or wedged at an angle, forming part of the props and lagging system that makes underground work safer as the face advances.

The work happens in sequence: the miner or timberman drives spills ahead into the exposed ground, often using a sledge or maul to seat them firmly. Once several spills are in place and the worst of the loose material is caught, heavier timbering can be installed, such as sets (full rectangular frames) or cap timbers that will remain in place. In very soft clay, sand, or fractured strata, spills may be driven almost as fast as the face advances, creating a moving shield of wood ahead of the crew.

Materials and placement

Spills are generally cut from softwood or hardwood pit timbers, whatever is available locally. They differ from props mainly in being driven ahead rather than set vertically after the space is cleared. In some operations they are called sprags or sprogs. The angle and density of spill placement depend on ground quality: weak ground might call for spills every foot or two horizontally, with secondary spills cross-laced to catch smaller falls. Experienced timbermen read the rock and adjust their pattern accordingly.

Failure of spill systems shows itself as material bulging or squeezing past the timbers, or as spills themselves splitting under load. Once the main props are set and lagging is in place, the spills are often left in position or drawn out if they obstruct further work. The term is most common in British and European mining; North American mines sometimes use the terms prop, cog, or simply timber bar for similar purposes.

Spills represent a low-tech but essential link in the chain of underground support. They are labour-intensive and demand constant attention as conditions change, but they allow one or two men to make headway into poor ground that would otherwise be unworkable. Their presence in a mine plan is a sign that the ground is difficult enough to demand active, skilled timbering rather than reliance on bolts or resin anchors alone.