Engineering Wonders of the World
Volume I

238 ENGINEERING WONDERS OF THE WORLD. Fortunately, in none of the London Tube shafts has the influx of water been so great as to require the application of compressed air to exclude water during construction. TUNNELLING. A number of different methods of tunnelling were necessary under different parts of Lon- lengths of tunnel, in which the work was executed without a shield at all, to save the expense of putting in a shield to work for but a short time. In water-bearing strata two methods of tun- nelling were employed—namely, the Great- head “ assisted shield ” method and the “ clay pocket ” method. Each of these STATION SHIELD AT WORK ON CITY AND SOUTH LONDON RAILWAY. {Photo, F. Milner.') Observe the long wooden shoot for excavated material, and the hydraulic segment erectors s s. don to cope with the varying nature of the ground. For instance, although the bulk of the tunnelling was executed in the London clay, water-bearing gravels or sands were encountered, at certain points on each, of the railways. „ Where the tunnels were in London clay, the Greathead shield and the rotary shield were adopted, except for some very short methods will therefore be described separ- ately. The London clay is almost an ideal material in which, to construct a tube railway, as it contains no water and is easy to excavate. The work being executed very quickly, the clay has no time to swell and bring undue pressure on the finished tunnel, which, unlike a brick tunnel of the old type, attains its full