TUNNEL. 540 TUNNEL. English, Austrian, and Italian methods, the dif- ference consists simply in the different order in which the drifts and headings arc driven, in the difference in the number and size of these advance galleries, and in the different forms of strutting 3 3 2 6? Fig. 6. DIAOBAM SHOWING SEQUENCE OF GALLERIES IN GERMAN METHOD OF SOFT-GROUND TUNNELING. framework employed. The shield system is con- sidered separately under the succeeding heading of Submarine Tunnels. The other methods may be described briefly as follows: The Belgian method of tunneling is the one most commonly employed in soft soils. It consists in excavating the upper part of the section first ; then building the masonry arch which is supported upon the unexcavated ground ; and finally in excavating the lower portion of the section and building the side walls and floor or invert. The sequence of galleries is shown by Fig. 5. In the German method two drifts are first opened at each side of the tunnel near the bottom. Other galleries are excavated one above the other on each side to extend or heighten the first two until all the perimeter is open except across the bottom. The masonry lining is then built from the bottinn up- ward on each side of the crown of the arch, and then the centre core is removed and the invert is built. The sequence of galleries is shown by Fig. 6. In the English and Austrian methods short lengths of the whole sectional profile are ex- cavated before beginning the masonry. In the English method the lengths of section excavated vary from 10 to 25 feet. The masonry invert is built first, then the side walls, and finally the roof arch. The excavators and masons Avork al- ternately, the excavation being stopped while the masonry is being built and vice versa. The Austrian method differs in two particulars from the English ; the length of section opened is made great enough to allow the excavators to continue work ahead of the masons, and the side walls and roof are built before the invert. The Italian method is A-ery seldom employed because of its cost, but it can often be vised successfully in very soft ground when other methods fail. It consists in excavating the lower lialf of the section and buiUfing the invert and side walls and then filling the space between the walls in again except for a narrow passageway for the cars for handling structural material and removing the waste : next the upper half of the section is excavated, as in the Belgian method, and the arch is fiuilt : and finally the filling in the lower part is permanent- ly removed. Tunnels through quicksand are driven by one of the previously described soft- ground methods after draining away the water, or else as submarine tunnels. In short tunnels or tunnels of great depth be- neath the surface the excavation is carried on from the two ends only, but in long tunnels when the depth is not too great shafts are sunk at in- tervals along the tunnel line, and excavation is carried forward in both directions from the foot of each shaft as well as from the two ends. Tim- nel shafts are merely vertical walls, and they may be either temporary or permanent. Tem- porary shafts are usually made rectangular in section and are filled in after the tunnel is com- pleted: permanent shafts are usually made cir- cular, lined with brickwork, and left open to sup- ply ventilation. When shafts are used, elevators operated by hoisting engines have to be employed to hoist out the cars loaded with excavated ma- terial. Timbering a^'d Lining. The purpose of tim- bering or strutting in tunnel work is to prevent the caAang in of the roof and side walls of the excavation previous to the construction of the Fro. 7. SYSTEM OF STRUTTING PRACTICED IN ENGLISH METHOD OF SOFT-GROUND TUNNELING.
