Modern The greatest stimulus of all to the construction of tunnels was given when the construction of railroads began. From that time until the present, tunnel works of great magnitude, including those through sub-aqueous material, have been con stantly constructed. Amotig the earliest of these were two tunnels built on the line of the Liverpool and Manchester Railway in England. The first tunnel of any kind built in the United States was that known as the Auburn Tunnel near Auburn, Pa., built in 1818-21 by the Schuylkill Navigation Company for the water transportation of coal. This tunnel was several hundred feet long and was about 22 feet high by about 15 feet wide. The first railroad tun ne/ in the United States was also in Pennsyl vania on the Allegheny Portage Railroad' and was built 1831-33. It was 901 feet long, 25 feet wide and 21 feet high. What may be properly termed epoch-making tunnel structures in the United States and in Europe are the Hoosac Tunnel in Massachusetts and the Mont Cenis Tunnel which pierces the Alps between Pied mont and Savoy. These are termed epoch 'making structures because the exigencies of their construction first brought into use power drills and high explosives, the two great modern agents in rapid and economical rock tunnel construction. The Hoosac Tunnel was built be tween 1854 and 1876, the progress of the work having been interrupted over long periods. Its total length is about 4.)4 miles and it is a double-track tunnel. The Mont Cenis Tunnel was built between 1857 and 1872 and has a length of 7.6 miles. This is a single-track tunnel. The beginning of the work on the Hoosac Tunnel was quickly followed by the commencement of the double-track Erie Tun nel through Bergen Hill, near Hoboken, N. J. This tunnel was built 1855-61 and has a length of nearly 4,400 feet. Its greatest height is 21 feet and its width 28 feet. Among the more prominent tunnels of this country driven through rock, there may be mentioned the Croton Aqueduct Tunnel, 31 miles long with a horseshoe cross-section in general 13.53 feet high and 13.6 feet wide. The Niagara Falls Power Tunnel, 6,700 feet long, has a horseshoe section 19 feet by 21 feet. The single-track Cascade Tunnel on the North ern Pacific Railway, built in 1886-88, is 9,850 feet long. It is 16.5 feet wide and 22 feet high and is lined with masonry.
General Methods of Operation.—The more simple methods of excavating tunnels may evi dently be employed for rock and firm earth or other materials relatively dry. In such cases it is customary to divide the cross-section of the tunnel into a number of parts and excavate them in such order as will be most conducive to economy and speed of completion. This di vision of the section into those parts successively excavated is illustrated by Figs. 1. 2 and 3. In each of those figures the numbers show the order of excavating the different portions, the part 1 being the first removed in each case. Fig. 1 shows the sequence of removal followed in the Saint Gotthard Tunnel, while Fig. 2 ex hibits that followed in the Mont Cenis Tunnel; . Fig. 3 illustrates the order of excavation in the "German method)) of tunneling. If the first part, numbered 1 in the figures, is in the top of the tunnel it is called a heading, but if it is at the bottom of the section it is called a drift. The heading or drift being first driven, the full tunnel section is reached by enlarge ment in the order or sequence shown. The usual width of heading is about 8 feet, al though it may be but 6 feet. The height is about 7 feet. These dimensions give room for two men to work. Driving the heading is the most difficult and expensive operation of the tunnel excavation. These headings are some times driven 1,000 to 2,000 feet ahead of the full section, although that is not common. The alignment of the heading, which is also the alignment of the completed tunnel section, is transferred with great accuracy from the sur face, either at the ends of the tunnel or down through shafts by various and well-known methods of engineering surveying. Shafts are
the vertical passages sunk from the surface along the centre line of the tunnel or at a short distance on one side of that centre line for the purpose of attacking the excavation at as great a number of points as possible. They enable the work to be extended both ways from the point where the shaft is sunk and also form points at which the excavated materials are raised from the tunnel; they also permit ma terial for lining or other purposes to be lowered into the excavation and put in place. Central shafts are usually employed, although French engineers frequently adopt side shafts having their axes 30 to 40 feet on one side of the centre line of the tunnel. At the present time power elevators are used in shafts for raising and lowering men and material. When the shafts are left open and lined where neces sary, they become permanent features of the completed structures, affording ventilation. Where shafts are filled after the work is com pleted they are called temporary shafts, and they may be circular in section as is usually the case where they are lined, or they may be rectangular, their sides being braced with tim ber to prevent material falling in. Central shafts are more convenient than side shafts. If water flows into the tunnel excavation or is found in shaft sinking, it must usually be pumped to the surface. Tunnels are usually classified in relation to the material in which they are driven, such as tunnels in hard rock, in loose soil, in quicksand, cut-and-cover tun nels, sub-aqueous tunnels. On the whole, hard rock is probably the safest material in which to drive a tunnel and it gives the least difficulty. This is true chiefly in view of the effective ex plosives and covenient power-drills and other machinery now available for the purposes of excavation. Rock tunnels may be driven by using either a heading or a drift, depending upon the local circumstances in choosing the method. Tunnels in soil may involve serious difficulties if the soil is saturated with water. The excavation may be first made near the top, that is, near the soffit (the Belgian method); or along the perimeter (German method) ; or in two halves entirely independent of each other (the Italian method) ; or, finally, the whole sec tion together (the English and Austrian methods). The Belgian method is more fre quently employed in Europe. After excavating the material under the soffit the arched roof of the tunnel is constructed and supported on either side of the excavation until the lower part of the material is removed, when the neces sary side and bottom lining is completed. In the German method two drifts are driven, one on either side of the lower portion of the sec tion, as shown in Fig. 3, then others are opened above them until the completed peri meter except the lower central portion, which is then termed the °bench" (see Fig. 7), has been exposed. The masonry lining is then com pleted above the bottom. After removing the central portion of the material the invert or bottom of the masonry lining is put in place. The Italian method is more expensive than the others and is not often followed, but the Eng lish method is employed by excavating lengths from 10 to 25 feet, the masonry invert lining being completed first, then the side walls, and the top arch last. Tunnels in quicksand must be driven by methods applicable to soft soil saturated with water. Cut-and-cover tunnels have been more used for subways in cities than for other purposes. The most notable cases of these latter tunnels are the recently completed subways in the city of New York and the Boston subway, although a consider able portion of the latter was built without removing the material over the top of the finished arch. Sub-aqueous tunnels are the most difficult of all to build and require the employment of special methods and appli ances which will be described later on.