Page:Walker (1888) The Severn Tunnel.djvu/138

Rh gangs, and lowered to the required level by other gangs.

But generally in tunnelling it is necessary to drive a heading—first, to ensure the correctness of the line of the work; and secondly, to attain sufficient speed. And without doubt a bottom heading is the best for this purpose. If the tunnel is without an invert, the heading should be driven on the level known as ‘formation level;’ that is, 2 feet below rails. If the tunnel has an invert, the heading should be driven at the level of the top of the invert, as, if it were driven at the bottom of the invert, the brickwork of each length would block the heading.

Driving the heading at the level of the top of the invert necessitates, where many lengths are going on at the same time, bridging over each length as the invert is taken out.

A heading is a small tunnel, and if timbered at all, it is timbered quite differently from the tunnel itself.

A heading 7 feet square is quite large enough for working skips or small trolleys. If it is decided to get ordinary tip-waggons into the works, the heading must be at least 9 feet square.

If the ground is in rock, only occasional timbers may be required to support the roof when stones in the top are loose, or sound, when struck, what the miners call ‘drummy.’ The top in that case may require ‘head-trees,’ which are timbers across the line of the heading, and polling-boards.

The head-trees may be let into the rock itself, or