Page:Walker (1888) The Severn Tunnel.djvu/150

Rh has to fill up, and complete the arch brick by brick, working himself out of it.

When the arch is entirely bricked in a ‘break-up length,’ what is known as a ‘running length’ is commenced at one end of it. For this the top heading has been already driven, as before described, and then the bars are placed in the same manner as in the break-up length, propped with back props, and then with props upon sills; but, as one end of the bars rests upon the brickwork of the finished length, there is only one face to timber down. The face of each break-up and running length requires to be carefully timbered, and if the ground is loose, polled; and the sills require long struts to secure them against any pressure at the face itself.

When the running length is completed at one end of the break-up, another running length should be taken out at the other end, the miners working in one length, while the bricklayers are working in the other.

The advantage of the bottom heading is this: that all the mining done in every length is dropped down into the skips or waggons with but little expense in filling; and as the bottom heading can be pushed forward at the rate of from 12 to 20 yards per week, about every six or seven weeks fresh break-ups can be started at distances of 100 yards apart. Of course the distance between break-ups must be regulated by the speed it is desired to attain in completing the tunnel.

If the ground at the top of the heading or tunnel