glut

A glut is a short length of wood, typically wedge-shaped or rectangular, driven behind cribbing or tubbing to take up gaps and transfer load evenly. In mine shaft sinking and support, cribbing consists of squared timber frames stacked horizontally; tubbing is permanent lining made of cast iron or concrete rings. Neither fits perfectly against the surrounding rock, so gluts fill the voids, preventing movement and ensuring the load from the rock mass bears uniformly on the structure rather than concentrating at high points.

Gluts are driven in with hand tools or compressed air hammers, often in multiple layers. A typical glut might be 3 to 6 inches wide, 1 to 2 feet long, and made from pine, oak, or other structural timber. In old shafts, they are sometimes split from larger blocks on site. The angle of the wedge varies, but shallow wedges (around 5 to 10 degrees) are preferred because they resist backward creep better than steep ones.

Placement and failure modes

Gluts are positioned radially around the shaft perimeter, between the outer surface of the cribbing or tubbing and the rock wall. Poor gluting leaves spaces where water can pool and cause rot, or where load concentrates and splits the timber. Inadequate driving leaves gluts loose, so they settle and create slack as the structure shifts. Conversely, over-enthusiastic driving can split the cribbing itself.

The term appears in mining engineering texts from the 19th century onward and remains standard in shaft sinking practice, though modern deep shafts increasingly use continuous concrete or steel lining, reducing glut work. Where gluts are still used, their quality and placement often determine how well a shaft structure will perform over decades.