A stopping in which the floor and the sidewalls of the passage are built of sandbags, and the roof may be the roof of the roadway or covering boards used between the webs of steel arches, or preferably, corrugated steel sheeting used as lagging behind steel arches. The plughole or passage is generally tapered from the inby end from 3 to 3.5 ft (0.9 to 1.1 m) square to 2.5 ft (0.76 m) square so that, in the event of an explosion, the plug of sandbags in the passage is subjected to a wedging action assisting to retain the plug in place. The plughole may be placed in the most convenient position and although this is often at the top, it is sometimes placed to the side and reasonably near the floor.