TUNNEL. 541 TUNNEL. lining. Timber is nearly always employed for strutting in tunnel work, but iron strutting of special form is sometimes employed. The method of strutting employed depends upon the nature of the material penetrated. (Fig. 7.) In the hard, firm rock no strutting is employed: when the rock is cracked or shelly in spots only, an occasional prop and beam is all that is neces- sary; in soft and broken rock and in soft ground the framing is continuous, and in very soft ma- terial often forms practically a water-tight lin- ing. Tunnels are lined with timber (Fig. 8), with masonry (Figs. 9 and 10) , and with cast-iron cylinders. Timber lining is used only in America and is intended to be replaced by masonry at some future date. The most common form of masonry lining is brickwork or a brickwork roof arch with stone masonry side walls. In recent years monolithic linings of concrete have been considerably used. Iron linuig is practically limited to submarine tunnels driven by the shield system. In masonry- lined tunnels niches (Fig. 11) are built into the side walls at intervals to serve as places of refuge and in some cases for the storage of tools masonry. This trench is strutted by means of side struttings of vertical planks held in place Fl8. 9. CONCRETE LINING FOR TONNELB. by transverse beams extending across the trench and abutting against longitudinal timbers laid Cross Section. Longitudinal Section. Fig. 8. A TYPICAL FORM OF TIMBER LINING FOR TUNNELS. and supplies. The ends of the tunnel lining near- ly always terminate in a portal designed and con- structed with some pretensions to architectural beauty. Open-Cut Tiinneiing. When a tunnel or rapid transit subway has to be constructed at a small depth below the surface, the excavation is generally performed more economically by mak- ing an open cut. building the lining inside it, and filling in the space outside the lining, than it is by subterranean tunneling proper. The neces- sary condition of small depth which makes open- cut tunneling desirable is most generally found in constructing rapid transit tunnels under city streets. This fact introduces the chief difficulties encountered in such work, since the surface traffic makes it necessary to obstruct the streets as little as possible. The two methods of open- cut tunneling commonly practiced may be classed as the longitudinal-trench method and the trans- verse-trench method. The simplest manner by which to construct open-cut tunnels is to open a single cut or trench the full width of the tunnel against the strutting planks. The lining is built in this trench and then filled around and above with well-rammed earth, after which the surface FlQ. 10. TYPICAL MA80NBY LINING FOR TCNNKLB.
