. Shield and compressed air tunneling . Railway (E-8) (see par. 32 and 37)the face had been supported during the shove by stretchersthrough the shield opening. In the Baker Street and Waterlooshield, as modified after the shutters were removed, the stretcherswere carried through special openings. Two pairs of steelstruts were provided. They consisted of steel tubes oj-i in. indiameter and l^? ft. long. The forward end gave a bearingagainst the face walings and the rear end bore an adjustable headby which the strut could be tightened against a cross beam(byatt) wedged against the tunnel lining.
. Shield and compressed air tunneling . Railway (E-8) (see par. 32 and 37)the face had been supported during the shove by stretchersthrough the shield opening. In the Baker Street and Waterlooshield, as modified after the shutters were removed, the stretcherswere carried through special openings. Two pairs of steelstruts were provided. They consisted of steel tubes oj-i in. indiameter and l^? ft. long. The forward end gave a bearingagainst the face walings and the rear end bore an adjustable headby which the strut could be tightened against a cross beam(byatt) wedged against the tunnel lining. The struts passedthrough openings cut in the shield diaphragm. Leather sleeveswere fitted close around the struts to render the joint partly airtiglit. This arrangement made it possible to advance the shield 200 ,S7// /;;/>D T UNNELl NG with the diaphragm closed and increased the safety of the Fig. 04. 51. Method of Supporting Face.—The breast boards were set. sometimes horizontally and sometimes vertically, as might bemost convenient under the varying conditions of the pugged or tempered clay was placed around the shield CONSTRVCTION 2()7 periphery as on the Waterloo and City Railway tunnels. Tlieface boards were pufiged liberally also to keep down the loss of air. 52. Adjustment of Air Pressure.—Several blows occurredalthough care was used to adjust the tunnel air pressure to thevarying heights of the water in the river (the tidal range atLondon Bridge is about 21 ft.). The blow-off of the air receiverwas regulated automatically by a float on the river which carrieda vertical board having an inclined channel bar groove attachedto it. The groove constrained a roller which moved horizontallyas the board rose and fell with the tide and varied by its move-ments the position of the fulcrum of the loaded valve-lever, thusvarying the pressure according to the hydrostatic head { C. E., vol. 150, page 38). 53. Progress.—An
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