Coal mining Underground mining | Britannica
Roof support at the face (the area where coal is actively mined) is intended to hold the immediate roof above the coal face. In modern mechanized mines, roof bolting is the most common method employed. Steel bolts, usually to 2 metres long and 15 to 25 millimetres in diameter, are inserted in holes drilled into the roof by an electric rotary drill and are secured by either friction or resin.