Engineering Wonders of the World
Volume I

308 ENGINEERING WONDERS OF THE WORLD. In this manner the necessary grouting space outside the iron lining was maintained. This method was used with great success in both tunnels of the Bakerloo Railway under the Thames at Charing Cross. It enabled five feet of tunnel to be put in every twenty-four hours, although the material pierced was the race on that account. (An illustration of the effect of a “ blow-out ” is given on p. 307.) ERECTING IRON LINING IN SMALL TUNNELS. * The cast-iron lining of running tunnels con- sists of a series of rings, each 20 inches long, WORKING IN A HOODED SHIELD. {Photo, Woodbury type Permanent Photographic Printing Co.) open gravel permeated by water having a 70-foot “ head ”—30 lbs. to the square inch. We may close this section by narrating a curious episode. While one of these tunnels was being driven, a competitor for “ Doggett’s Coat and Badge,” the celebrated Thames watermen’s race, got into difficulties in a whirl of water caused by a " blow-out ” of air from the tunnel, and, as he maintained, lost and composed ot six segments and a key- piece at the top. The segments of a ring and adjacent rings are joined together by bolts. One of the most interesting things to watch during the actual construction of a shield- driven tunnel is the erection of this permanent iron lining. The operation is performed entirely by manual labour in the case of small tunnels, as