Compressed Air Work And Diving 1909

HANGING SCREENS. tunnel lining on each side, so that the workers can escape out of reach of the incoming water. Such gangways were used at the Blackwall Tunnel,* and have also been employed at many other tunnels since, both here and abroad. Additional safety is obtained by a hanging screen which comes half-way down the tunnel and acts in a similar manner to the top half of the safety trap. That used at the Blackwall Tunnel was fitted with a lock which was always kept with the inner cloor open towarcls the working face to provide a means of escape shoulcl the water rise to the bottom of it before it could be reached. The hanging screens and gangways were introduced at Blackwall by Mr Moir. Behind the screen again is the bulkhead which divides the front portion of the tunnel off from that portion which is kept at a lower or at atmospheric pressure. This bulkhead is usually made of brick or concrete, and has locks for men and material built into it. These locks are boiler-shaped, and fixed in a horizontal position. The doors open inwards towards the working face, and are lineel with rubber gaskets and kept closed by the pressure of the air. They are fitted with large air valves, so that the pressure can be quickly raiseel and lowered when material is inside, and with smaller ones for men. For tunnels of 23 ft. diameter and over, two parallel locks are used for material, about 7 ft. or so in diameter, and above them is placed the emergency lock for men in the centre, and as high up as possible. This should always be kept open towarcls the working face, but shoulcl be used occasionally to see that it is in working * Pruc. lust. C. E., vol. cxxx.