Progress of the work—1881. the whole tunnel is completed to the top of the invert.
Two sills may not always be necessary, but the above description deals with ground for two sills; the working down from the upper sill to the lower one being exactly similar to that required for working to the first sill.
If the ground is good the crown-bars are placed entirely inside the brickwork of the tunnel, and they should not be so large as to equal half the space to be occupied by the brickwork. If the ground requires larger timbers than half the thickness of the brickwork, the bars must be worked as what are called ‘drawing-bars,’ that is, bars to be drawn on end as the brickwork progresses; or must be placed entirely above the brickwork of the tunnel and built in.
When the length is completed, and the invert taken out, two profiles of boards—that is, light frames representing the exact shape of the invert—are set at the right level and to the right lines by the engineers; the bricklayers then commence and build the invert of the tunnel. When that is completed, what are called side-frames are set to guide the bricklayers in laying the bricks for the side-walls of the tunnel up to the springing of the arch. When the side-walls of the tunnel are completed to springing, the ‘centres’ are set.
The centres adopted at the Severn Tunnel were wholly what are known as ‘skeleton centres;’ that