Page:Walker (1888) The Severn Tunnel.djvu/142

Rh the tunnel itself can be completed at the same speed, the number of break-ups and the distances between them being regulated with a view to this.

A ‘break-up’ is a hole driven in a slanting direction from the top of the heading to the top of the tunnel itself, or, if the ground is bad, to a height of 18 inches or 2 feet above that. When the break-up is completed to the top of the tunnel, a top-heading is commenced similar to the bottom heading, but smaller in dimensions.

It may be taken to be about 3 or 4 feet wide at the top and 5 to 6 feet wide at the bottom, and 6 feet high.

The nature of the ground will determine the length to be taken out at once; whatever is decided upon is called a ‘length.’ In very bad ground this may be only 6 or 9 feet; but generally it will not be less than 12 feet, and, if in very strong ground, 20 feet or even 24 feet.

When the top heading is completed to the required length, a benching is cut in the top of the heading at one side, sufficiently large to receive the timbers that have to carry the roof. These timbers should be of good, fresh, round larch, varying from 12 inches to 16 inches in diameter, and are known among miners as ‘bars;’ and the bars of the length are divided into ‘crown-bars’ and ‘side-bars.’

When one bar has been rolled on to the benching prepared for it, another benching is cut upon the other side of the heading, and a second bar placed