SUBMARINE TUNNELING. Submarine or suba queous tunnels are tunnels excavated under the bed of a river or other body of water, generally for the purpose of providing land communication between the opposite shores. Inflowing water. which is always a serious difficulty in tunnel work everywhere, is here the most serious prob lem with which the engineer has to deal. Where the tunnel is deep below the stream bed and pene trates a material impervious to water it may be excavated by any of the methods commonly em ployed in subterranean work. When, however, the opposite conditions exist, resort is had to the use of compressed air or to the shield system or to a combination of the two. In the compressed-air method the forward end of the tunnel where ex cavation is progressing is filled with air under sufficient pressure to counterbalance the pressure of the water which seeks to enter. In the shield system the work is carried on by the aid of a rather elaborate mechanical device commonly called a shield. The importance of this invention to the tunnel-builder can hardly be overesti mated.
The shield devised by Birlow already referred to was greatly improved by Mr. J.11. Greathead. and
was first used in building the London Tower tun nel begun in 1869. The remarkable success of this work led to the rapid adoption of the shield system for tunnel work in England and America. Briefly described. the modern tunnel shield is a steel plate with its forward edge trates the earth a little distance. Workmen then withdraw the earth in small quantities through the openings in the diaphragm. After enough material has been thus withdrawn to leave the front of the shield clear, the shield is again thrust ahead and more earth is removed. As fast as the shield moves ahead the lining is erected under the shelter of its tail end. A con stant repetition of these processes completes the tunnel. When the earth is very soft and carries much water. compressed air is often used in con nection with the shield. The shield system of tunneling has been extensively used in England and America and to a less extent in most other civilized countries.