. Shield and compressed air tunneling . oes not fall within the confines of the tunnelthe curve is laid out in like manner, using one or more chords andintersections. 83. Distance between Monuments.—The distant apart of thejirecise line monuments will vary with the degree of clearness inthe tunnel air. It will be foimd usual that they cannot be muchfurther than 300 ft. apart, and 200 ft. is a common interval. Inorder to obtain the accuracy derived from repetition it is thecustom to use alignment scales arranged as shown in Fig. these scales repeated readings can be taken and the adopted
. Shield and compressed air tunneling . oes not fall within the confines of the tunnelthe curve is laid out in like manner, using one or more chords andintersections. 83. Distance between Monuments.—The distant apart of thejirecise line monuments will vary with the degree of clearness inthe tunnel air. It will be foimd usual that they cannot be muchfurther than 300 ft. apart, and 200 ft. is a common interval. Inorder to obtain the accuracy derived from repetition it is thecustom to use alignment scales arranged as shown in Fig. these scales repeated readings can be taken and the adopted SrRVFY 305 line set as a mean of the vernier readings. These scales not onlyserve to give an accurate point but preserve a record in case ofmovement and give a check on individual runs. 84. Work Not to Interfere with Construction.—The surveywork must be done without interfering with the progress ofconstruction, which in tunnels is usually carried out continuouslyt he by night as well as by day. Consequently it is found best to set. Fig. 156.—A vernier alignment bar in a ca:^t-iron lincil tunnel, showing theplumb-line hanginn from it and a light box of the second or shrouded type {.seetext, Par. 87) behind the plumb-line. Pennsylvania Railroad Hudson RiverTunnels (A-IS). (Courtesy oi Pennsylvania Railroad). instruments on special tables or brackets permanently attachedto the lining of the tunnel for this purpose. By the use of thesetables the tripods of the instruments are not needed and theinstruments can be set up on three-legged stands or trivetsstanding on the tables, which are placed so that the man usingthe instrument is out of the way of the construction and tunnel 396 SHIELD TUNNELING transportation work. The trivets must be heavy castings withwide spread of feet. No spidery-legged trivet is of any use. 85. Platforms on Center Line of Tunnel.—When the tunnelis of 20 ft. diameter or over it is possible to use instrumenttables on the center hne of the tunnel above
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