Engineering Wonders of the World
Volume I

THE TUBE RAILWAYS OF LONDON. 301 it has tunnels between the stations of a smaller size than any of the other Tube railways of London, the engineers have gone to the other extreme and made the station tunnels 30 feet in diameter, or larger than on any other Tube railway. This was to give an “ island ” plat- form sufficiently wide for incoming and out- going traffic with a line on each side, as can be arranged in a tunnel of this size. The platforms of the older Tubes were built of timber, but in the recently com- pleted Bakerloo, Hampstead, and Pic- cadilly Tubes they are formed of steel joists, concrete, and granolithic paving, and are therefore fireproof. At certain points on the several Tube railways, as well as at the terminal stations, where trains have to pass from the “ up ” to the “ down ” line for the return journey, tunnels ranging from 23 feet 2| inches to 25 feet in diameter have been constructed so that com- plete trains can cross from one line to the other. The method of constructing these is generally similar to that used for the station tunnels, the shield being of slightly larger diameter. TUNNELLING WITH A ROTARY DIGGER. During the construction of the Central London Railway two types of mechanical excavators were tried, with indifferent results —one in the form of a ladder dredger (Fig. 7), and the other as a rotary excavator. The latter machine had a central shaft from which extended a number of radial arms carrying cutting chisels. The shaft being rotated by electrical power, the cutting chisels remove the clay, which, on falling into the invert, is picked up by a number of buckets also fixed to the revolving arms. As the buckets reach the highest point of their journey, the clay falls out of them by gravity on to an in- clined shoot, down which it slides to a con- veyor placed close up to the back of the shield. The conveyor transfers the clay to trucks standing on a small wagon road at one side of the machine. The shield is moved forward in the ordinary way by the hydraulic pressure in the rams, but a very careful manipulation is required to keep the advance properly proportioned to the rate of revolution of the cutter arms. If it be too slow, the cutters will commence to race ; and, conversely, if the “ feed ” be too Fig. 7.—THOMPSON LADDER EXCAVATOR USED ON THE CENTRAL LONDON RAILWAY. fast, the motor driving the cutters will be overloaded. The type of rotary machine used on the Central London Railway was similar to that used recently on the Hampstead and Picca- dilly Tubes, but differed in one important feature from the Principle later machines. In the earlier , machines the driving power Dig-ger was applied at the axis of the shield, whereas in the latest machines the driving power is applied near the circum- ference to a circular rack rigidly attached to and forming a part of the frame in which the radial arms and chisels are fixed. It has been fully proved by experience that this method of driving produces steadier working than is possible if the drive is transmitted through a central shaft at the axis of the shield. The illustrations in the text (Figs. 8 and 9) are front and back views of the latest type of