Page:Walker (1888) The Severn Tunnel.djvu/139

80 may be supported on short props resting on ledges of the rock, which, from the shape in which they are put in, are called ‘sprag-props,’ i.e., short spreading props. Or the head-trees may require to be supported by ‘side-trees’ the whole height of the heading; and these side-trees may, if the ground is soft in the bottom, require ‘footblocks,’ or ‘sills,’ which latter are timbers laid across the bottom of the heading from side to side.

In driving in clay or chalk, or in the shales or marls at the Severn Tunnel, it is possible to drive a kind of benching in the top, 1 foot in advance of the face, into which the head-trees are rolled and wedged in. If boards are required to support the roof, they are put in at the same time. This head-tree rests upon the ground of the face itself; then if side-trees are required two chambers are cut, one on each side, into which the side-trees are inserted; the weight in the meantime resting on the ground of the face, which has not been disturbed, or a temporary prop in the middle. When the side-trees are in place, the head-trees are wedged up from them, and the small wedges which are used in this and other places in timbering the tunnel are called ‘jacky pages’ (perhaps ‘jack up edges’).

By these means the heading can be advanced in ordinary ground at a speed varying from 12 to 20 yards per week, allowing three or four months from the commencement to get the heading well advanced, and start what are called ‘break-ups;’