TIMBERING AND LINING. The purpose of tim bering or strutting in tunnel work is to prevent the caving in of the roof and side walls of the excavation previous to the construction of the 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 lining 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 by transverse beams extending across the trench and abutting against longitudinal timbers laid 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.