TUNNEL. 542 TUNNEL. of the ground is restored. Generally the trench is opened in short lengths and each length is completed ready for surface traffic before the ad- jacent length is opened. Another form of longi- tudinal trench method is to open two narrow parallel trenches in which are built the side walls. The soil between these walls is then removed to a sufficient depth to permit the roof to be built and covered over, and the final process is to take out the core of earth inclosed by the side walls and roof. Generally one side wall and one-half of the roof arch are completed and covered over before the other side wall and half arch are begun. By this arrangement of the work one-half of the street is always unobstructed. strengthened and sharpened to form a cutting edge. The rear end extends backward so as to overlap the completed lining of the cast-iron rings. Around its inside hydraulic jacks are at- tached at frequent intervals so that their piston rods bear again.st the front edge of the completed lining. By applying power to the jacks their piston rods pass with great force against the immovable lining and thrust the cylinder forward into the soft earth. Generally the cylinder has a vertical position or diaphragm near the front end. which prevents the soft earth from rushing back into it except as it is allowed by opening suitable doors. In operation the cylinder is thrust forward until its sharp cutting edge pene- FlG. 11. MA80SBY WOHK IN TUNNEL. In the transverse-trench method a trench about 12 feet wide is opened across the street, and in this trench a short length or 'slice' of the subway is built. Owing to the small size of the opening, it is possible to cover it with a timber platform which carries the street traffic during construc- tion. This method was extensively used in con- structing the subway at Boston, ilass., and at New York. SuBM.BiNE 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 maj^ 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 s.ystem or to a combination of the two. In the compressed-air method the forward end of the tunnel where ex- cavation is progi'essing 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 Barlow already referred to was greatly improved by ISlr. J. H. Greathead. and was first used in building the London Tower tun- nel begun in ISfin. 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 jsteel plate cylinder, 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. NoT.BLE Tunnels. The number of tunnels notable because of their length, the difficulties encountered in their construction or for other reasons, has become so great that mention can be made here of only a few of those which are most celebrated. The ilont Cenis Tunnel through the Alps places France in direct communication with Italy. It passes from a point near Modane, in France, under the Col de Frfjus, about 18 miles west of the actual Mont Cenis, into Italian territory, at a point near Bardo- necchia. about 24 miles from Susa. The length of the timnel between extremities is 7.001G miles. It is formed with a rising gi-adient from each end, at the rate of about 1 in 45^^ from Modane and 1 in 2000 from Bardoneechia. the summit or meeting of the gradients being half way through the tunnel. The railway does not enter at the ex- tremities of the tunnel, but joins it by means of special curved sections of tunnel at each end. The total length of tunnel traversed by trains is 7.0806 miles. The following is the geological for- mation of the rock traversed by the direct tunnel advancing from the north end: Carbonaceous schist, 1.3027 miles; quartz. 0.2414 mile; lime- stone, 0.2210 mile; calcareous schist, 5.83G5
