. Shield and compressed air tunneling . Bridge Track.—At the doors of the material lock itis necessary to use short lengths of movable tracks spanning overthe gap or well in which the lock door swings. This may be apiece of track in the form of a solid bridge which is picked upbodily and replaced by the lock tender, or hinged to swing upand allow the door to open and shut. 129. Lock Tender.—A specially reliable man of some force ofcharacter and strength of arm who can stop men from going inand out on the muck tap or interfering with the orderly work ofthe locks, must be on duty at all times an
. Shield and compressed air tunneling . Bridge Track.—At the doors of the material lock itis necessary to use short lengths of movable tracks spanning overthe gap or well in which the lock door swings. This may be apiece of track in the form of a solid bridge which is picked upbodily and replaced by the lock tender, or hinged to swing upand allow the door to open and shut. 129. Lock Tender.—A specially reliable man of some force ofcharacter and strength of arm who can stop men from going inand out on the muck tap or interfering with the orderly work ofthe locks, must be on duty at all times and held to account forthe operation and maintenance of the locks. It must be remem-bered that the men in the air chamber are helpless if all the locksare open to the normal air side. The lock tender should beprovided with telephone connections both with the heading andthe surface, and gauges should be provided so that he can tell atall times what the pressure is in the air chamber as well as in allthe locks. PLANT AND EQUIPMENT 223. 224 SHIELD TUNNELING 130. Location of Locks in Parallel Tunnels.—It is worth notingthat if two tunnels are driven parallel to each other and closetogether it is advisable where the ground is open, to place thelocks on the same transverse line, because otherwise the air mayleak through the lining and the ground from the tunnel with thehigher pressure into that with the lower pressure. SAFETY APPLIANCES FOR USE IN COMPRESSED AIR 131. Necessity for Safety Equipment.—In compressed airtunnel work under water there come moments when the menworking in the compressed air chamber are in serious danger oftheir lives, and certain appliances are provided to give the mena chance to escape when the time comes that they must abandonthe tunnel. 132. Safety Appliances.—The safety appliances are (a) The emergency runway. (6) The emergency air lock, (c) The safety screen. {d) The lock extension. 133. The Emergency Runway.—In most modern specificationsfor co
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