Engineering Wonders of the World
Volume I

80 ENGINEERING WONDERS OF THE WORLD. length. ' The Great Western Railway had before this also determined to attack the great natural obstacle to a direct route from London to the South Wales coalfields and seaports, as the company was obliged cither to send its trains circuitously by Gloucester, over a line troubled by some very severe gradients, or to transport them across the river on special “ ground ” between the roof of the proposed tunnel and the bottom of the Shoots, it was necessary to attain a level The some 140 feet below the gen- Gradients of eral level of the rails on each the yunnej side of the river ; and to avoid gradients exceeding 1 in 90, the length of the tunnel was fixed at miles, with approach ferry-boats at a point a couple of miles below the inflow of the Wye—an operation rendered difficult by the strong tides. Mr. Charles Richardson, who built the landing-stages for this ferry, mooted, in 1862, a project for driving a tunnel under the river a little lower down. Various other proposals were submitted to the public and to Parliament, including one or more designs for high-level bridges ; but the Great Western directors ultimately decided to adopt Mr. Richardson’s scheme, and secured the necessary Act for its construction in 1872. To understand the nature of the under- taking, the reader should consult the sectional diagram given above. At the site of the tunnel the river has a breadth of about 2 miles, and its bed is distinguished by a gully, near the eastern bank, known locally as the “ Shoots,” with nearly vertical sides, and a depth about 50 feet greater than the average of the rest of the channel. The existence of the Shoots vastly increased the difficulties and magnitude of the project. In order to keep a sufficient amount of LONGITUDINAL SECTION OF The dark spot marked A in the drainage heading indicates cuttings three-fifths of a mile and 1 mile long on the Monmouthshire and Gloucestershire sides respectively. To transfer the locale of the tunnel to London for the sake of com- parison, one may imagine a train to begin its downward plunge at the Tower, enter dark- ness at the Bank of England, pass below Oxford Circus at a depth of 160 feet, reappear at Royal Oak Station, and regain land-level in Kensal New Town. The inside dimensions of the tunnel were :— Height, 24 j feet ; width, 26 feet. Before attacking the task of driving this by far the longest tunnel in the British Isles, and the longest and largest submarine tunnel in the world, the engineers sunk W' ork a number of trial bores, which beg-un revealed the fact that the tunnel would pass through rock for at least a couple of miles, and through gravel, sand, and clay for the rest of the distance. A shaft, known as the “ Old Shaft,” was then sunk on the Monmouthshire side to a depth of 200 feet on the line of the tunnel, and from the bottom