TUNNEL. 538 TUNNEL. finished in 1870. It was from the close study of some of the difficulties, the great length of the tunnel, and the desire of the engineers to finish it quickly, that all the different improvements were developed which marked this work as a notable step in the advance of the art of tunnel- ing. Thus the first power drill ever used in tun- nel work was devised by Sommeiller, one of the engineers. In addition compressed air as a mo- tive power for drills, aspirators to suck the foul air from the excavation, air compressors, tur- bines, etc., found at Mont Cenis their first appli- cation to tunnel construetion. This important rule pla3'ed by the Mont Cenis Tunnel in Europe in in- troducing modern methods had its counterpart in America in the Hoosac Tunnel, completed in 1875. In this work there were used tor the first time in America power rock-drills, air compressors, ni- troglycerin, electricity for firing blasts, etc. There remains now to be noted only the final de- velopment in the art of soft-ground submarine tunneling, namely, the use of shield and metal lining. The shield was invented and first used by Sir Mare Isambard Brunei in excavating the first tunnel under the River Thames at London, which was begun in 1825 and opened in 1843. In 1869 Peter William Barlow used an iron lining in con- nection with a shield in driving the second tun- nel imder the Thames at London. From these inventions has grown up one of the most notable sj'stems in tunneling now practiced, known as the shield system. Construction. Tunnels may be classified ac- cording to the general nature of the materials penetrated, into hard-rock tunnels and soft- ground tunnels. Whatever the material pene- trated, the general methods of construction are the same, but a variety of differences in detail exist. In certain kinds of tunnels, such as sub- aqueous tunnels and tunnels under city streets, other conditions than the hardness of the mate- rial penetrated exercise a controlling influence. For the purpose of description, therefore, it is common to classify tunnels into hard-rock tun- nels, soft-ground tunnels, open-cut tunnels, and submarine tunnels. In open-cut tunnels and sub- probability of water being encountered, etc., all of which exercise an important influence upon the difliculties and cost of the work. The next step is to establish exactly the centre line of the tunnel ; this is done on the surface of the ground and its purpose is to find the exact length of the tunnel and to furnish a reference lino by which the excavation is directed. The determination of the centre line is a simple problem in surveying, requiring only skill and exactness for its success- ful solntion; the longer the tunnel, and the luLiher and more inaccessible the mountain is above it, the more difficult is the work. So per- fect, howoer, has the skill of the surveyor be- come that it is a common thing to work within a small fraction of a foot in alignment in driv- ing a long mount.ain tunnel. The centre line being established, the next task is to establish the form and dimensions of the cross-section of the hole or passageway which it is proposed to excavate. The form and dimen- sions of cross-section adopted are determined by the purpose to which the tunnel is to be put and by the character of the materia! penetrated. The best one for the majority of conditions, and the one most commonly employed, is the polycentric figure (Fig. 1), in which the number of centres and length of the radii are fixed by the engineer Fig. 1. DIAGUAM SHOWING CROSS-SECTION OP POLYCENTIUC FIGURE. marine tunnels the material penetrated may be cither rock or earth. When it has been decided to construct a tunnel the first task is to construct a geological map which will show as accurately as may be determined the character and inclina- tion of the earth's strata to be penetrated, the / 3 2 3 b 4- I 4 FlG. 2. DIAGRAM SBOTMNG SEQUENCE OF GALIjEBIES IN DRIFT METHOD OP HARD-BOCK TUNNELING. to meet the particular conditions which exist. The dimensions to be given to the cross-section of Ji tunnel depend upon the purpose for which it is to be used, as will be observed in the succeed- ing descriptions of prominent tunnels. In all cases the form and the dimensions are those of the inside of the completed tunnel, which is of course the inside of the lining in all cases except those of unlined hard-rock tunnels, where it is the inside of the original excavation. Exc.WATlON. The work so far described is of the nature of engineering investigation and de- sign. The actual construction of work consists in excavating in the earth or rock a passageway which follows the established centre line and conforms in dimensions and shape to the estab- lished cross-section. In performing this work of excavation the whole area of the cross-section is seldom removed at once, but it is subdivided into two or more galleries, which are excavated in a measure independently of each other and which together form the full cross-section. The objects of thus subdividing the worjc are several in number, the more important being as follows: By driving a number of galleries each somewhat in advance of the succeeding one, several gangs
