This is a traditional mining term for the geological stratum that forms the floor of a coal working. It is very often composed of underclay or seatearth, representing the ancient fossilized soils where the coal-forming swamp plants originally took root. A soft or swelling floor layer can cause severe ground control problems, as the immense pressure from the pillars pushes the soft material up into the tunnels.
Heard on the job
The seam is solid, but the thill is soft clay that keeps heaving up into the roadways.
Also heard as
- floor
- seatearth
- seat earth
- Spavin
- pavement