Compressed Air Work And Diving 1909

74 COMPRESSED AIR WORK. top of the tunnel and bed of the river a clay blanket is sometimes used. This is deposited on the bed of the river by hoppers to form a covering about 8 or 10 ft. thick and 150 ft. wide. The clay will not wash away, as silt or mucl would, and the air has to pass through the grouncl in a slanting direction and therefore has a better chance of keeping the grouncl clry without churn- ing it up. There is, however, the danger that a pocket of air may form below the tough clay by reason of the grouncl sinking during the excavation. Any sudden lowering of the pressure woulcl then cause the clay to collapse inwards and this might cause a flooding of the tunnel. This is not a danger if the blanket be made thick enough. A combination of soft running material and rock presents the greatest difficulty, as it is necessary to get in front of the cutting eclge in order to drill and biast the rock. Under these circumstances it will be necessary to closely board up the soft material, and the boarded-up portion must then be struttecl with telescopic jacks or tunnel “guns” so that when the shield is “ shovecl ” forward, the boarclecl-up portion will not be interfered with. At the Blackwall I unnel horizontal iron shiitters were used. These in their forward position were a few inches back from the cutting edge. I hey were kept in position by screws passing through lugs with a nut at each side of the lugs. When the shield was “shoved” the front nut was screwecl forwards so that the shutters remained stationary while the shield moved up towarcls them. Before the next “shove ” the material behind each shutter was excavated separately, and the shutters screwecl forward by means of the nuts to their forward position.