Engineering Wonders of the World
Volume I

THE STORY OF THE SEVERN TUNNEL. 87 than a couple of miles, and completely filled the great cutting of the Gloucestershire ap- proach. Had a heading existed right through from the Marsh Shaft nothing could have saved the men below. As it was, the position of affairs looked deplorable enough—the tunnel partly, and the cutting quite, full of water, great damage done, and long delay inevitable. The con- tractor was already some £100,000 out of pocket over the contract. The worst had been passed, nevertheless. There were, indeed, periodical breakdowns of the pumps and minor troubles, but work went ahead so steadily that by the end of 1883 the larger part of the tunnel lining had been put in. The method of tunnelling employed con- sisted of driving a heading, or gallery, at the level of the bottom of the tunnel, making “ break ups ” from this at Method of short intervals, and from the Tunnelling- top of these working top employed. headings in both directions, at the level of the crown. These upper headings were enlarged and tim- bered where necessary ; then the ground between them and the lower headings was broken away vertically ; and finally the two big side benches of stuff were removed. When a length of tunnel had been thus enlarged to full section and timbered, the masons got to work, and under cover of the wooden cage built first the invert and then the side walls. Semicircular wooden “ centres ” were then arranged athwart the upper part of the cavity, parallel to one another, and from 3 to 4 feet apart. Beginning at the top of the side walls, the masons laid a “ lagging ” of stout boards 3 inches thick horizontally along the outside of the centres, which it touched, and con- tinued the brickwork upwards in courses, care- fully filling in all spaces between the brick- work and the ground. Fresh laggings were added on either side as required, and the brickwork brought up to the crown, or top- most part of the arch. For the last 18 inches the mason used short laggings resting on only two centres, and worked endways, completing the arch a few feet at a time, until only a small hole at one end of the “ length ” re- mained. Through this lie made his exit, and filled the cavity afterwards from below. The stout horizontal wooden “ crown bars ” and the “ poling boards ” behind them, which hold up the ground, are either built in or gradually drawn out endways as the brick- work proceeds and they are no longer needed. In the latter case the way must be prepared for them by excavating an adjacent length, the timbering of which is simplified by the fact that the crown bars rest on the completed brickwork at one end. The employment of advance headings and “ break ups ” makes it possible to excavate and finish several lengths simultaneously; whereas if none are used, and the tunnel is excavated to full size at once, excavation and masonry must proceed alternately. The whole of the tunnel on the Glouces- tershire side, from the Shoots to the open cutting at the eastern extremity, was com- pleted by August 1884 ; and by the end of the same month the section from near the Great Spring to the western entrance was also finished. There remained only a short section in the vicinity of the Great Spring, requiring special precautions. In order to divert the water, which could not be confined, Sir John Hawkshaw decided to take the spring in flank. A side heading was therefore driven parallel to the centre line of the tunnel Taking the and 40 feet north of it, on a _ Great Spring- in much slighter gradient than Flank, that of the tunnel itself, so as to tap the spring at a point below the invert. While this was being done the loose bed of the little river Nedern, which was suspected as being the source of the spring, received attention in the form of a concrete invert