Engineering Wonders of the World
Volume I

THEIR DESIGN AND CONSTRUCTION. BY HARLEY H. DALRYMPLE HAY, M.Inst.C.E. PART II.—(Continued from page 240.) THE tunnels for the stations on the first section of the City and South London Railway were built of brickwork in the old-fashioned way by excavating the ground, supporting it for a short length with heavy timbering, and building Tunnels for brickwork under cover of Stations. . . , _ . „ ,. it. A great deal of the tim- bering was buried behind the brickwork. Subsequently, Greathead proposed that there should be used for constructing the station tunnels large shields similar in principle to those employed for the smaller tunnels in the clay ; and this practice, which is now uni- versal, has been observed in all the London railways constructed since 1893. Before a station tunnel can be built, it is necessary to construct what is known as a “ shield chamber ”—that is, a length of tunnel sufficiently large to enable the shield to be built within it. In some of the early in- stances this chamber was formed of brick- work, built in an excavated space supported by solid baulks of timber. During the con- struction of the Waterloo and City Railway, a better although a more expensive form of construction was adopted by substituting steel beams for the large timbers formerly used. A station tunnel shield has a cutting edge at the forward end, and is divided by vertical and horizontal girders into a number of cells, rigidly connected together, from which the miners attack the face of the excavation. Hydraulic rams placed under the floors of the cells support the face; and when the shield is being moved forward, the pressure in these rams has to be overcome by the pressure in the rams located all round the circumference of the machine, which thrust against the wooden grouting rib placed between the ram heads and the leading face of the last ring of iron segment. The segments of a large tunnel being some- what heavy, they are best handled by means of hydraulic erectors, which have a circular motion as well as a motion of extension or contraction. The station tunnels on the Central London and on the more recent Tubes have an internal diameter of 21 feet 2| inches, but on the Waterloo and City and the Great Northern and City Rail- Particulars wavs this is increased to 23 feet. .. A tunnel oi 21 feet 2j inches Tunnels, diameter contains a platform and one line of way. On the latest extension of the City and South London Railway, although