Page:The New International Encyclopædia 1st ed. v. 19.djvu/619

* TUNJSTEL. 539 TUNNEL. of men can be worked without interfering with each other; the first gallery, being driven far ahead, serves to drain the earth and to disclose in advance tlie nature of the material that will be 2 Z 4-5 5 is indicated by Fig. 3, When the excavation is made by a single wide heading and a single other cut for removing the bench, which is the method preferred by American engineers, the sequence of galleries is the simple one indicated by Fig. 4. The Saint Gotthard Tunnel, described farther on, is one of the most notable tunnels excavated by enlarging a heading by means of several cuts; the Cascade Tunnel in Jlontana is a notable e.- aniple of the wide heading and single-bench method. In all rock tunnel work the various galleries are excavated by drilling and blasting the rock. The most dillicult gallery to excavate is the first heading or drift, as the case may be. Based on the mode of blasting employed, there are two methods of driving the advance gallery, known as the circular and the centre-cut methods. In Fig. 3. CIAGBAM SHOWING SEQUENCE OF GALLERIES IN EUROPEAN HABD-ROCK TUNNEL. encountered; in soft material several small gal- leries can be driven and timbered with much less danger of caving than can one large gallery. The number and arrangement of the galleries vary with the materials penetrated and with the sys- tem of tunneling adopted. The first gallery ex- cavated is called a heading when it is situated at the top of the cross-section, and a drift when it Fig. 4. DIAGRAM SHOWING SEQUENCE OF GALLERIES IN A.MERICAN HEADING .METHOD OF HARD ROCK TUNNELING. is situated at the bottom of the cross-section. The excavation of the main sections is termed enlarging the profile. Two general methods of excavating rock tunnels are considered practical by engineers ; in one the profile is enlarged from a drift and in the other it is enlarged from a heading. The sketch Fig. 2 shows the usual sequence of galleries by which the full cross-section is enlarged from a drift. This method of excavation is quite commonly practiced in Europe, but is seldom employed in America. Among the most notable tunnels built by enlarging a drift are the Mont Conis and the Siniplon, both of which are more fully described farther on in this article. The more common method of tunneling through hard rock is to be- gin the work by a heading instead of by a drift. This heading may be of small dimensions and the remainder of the section may be removed in suc- cessive small parts, or it may be the full width of the section and the enlargement completed in one other cut. When the tunnel is excavated by means of several cuts, which is the method usual- ly employed in Europe, the sequence of galleries Vol. XIX.— 35. 3 3 5 4 5 Fig. 5. DIAGRAM WHKWING SEQUENCE OF GALLERIES IN BELGIAN METHOD OF SOFT GROUND TUNNELING. the first method a set of holes are first drilled near the centre of the front in such a manner that they inclose a cone of rock; the holes, starting at the perimeter of the base of the cone, con- verge toward a junction at its apex. Seldom more than four to six holes are comprised in the first set. Around these first holes are driven a ring of holes which inclose a cylinder of rock, and if necessary succeeding rings of holes are driven outside the first ring. These holes are blasted in the order in which they are driven, the first set taking out a cone of rock, the second set enlarging this cone to a cylinder, and the other sets enlarging this cylinder. These holes are seldom driven deeper than four or five feet. In the centre-cut method, which is the one commonly employed in America, the holes are arranged in vertical rows and are driven from 15 feet to 20 feet deep. The two centre rows of holes converge toward each other so as to take out a wedge of rock, but the others are bored 'straight' or par- allel with the vertical plane of the tunnel. In conclusion it may be stated that the present high development of labor-saving machinery for excavating rock makes this material one of the safest and easiest to tunnel, with which the en- gineer ordinarily has to deal. A necessary equip- ment for tunneling through rock requires a steam or a water-power plant for pumping air or water under pressure, and in large tunnels these power plants are often of considerable size. See Ukill; Blasting. There are a variety of methods of tunneling through soft ground. Some of these, like the open-cut method and the shield method, difTer en- tirely, while in others, like the Belgian, German,