SPECIAL TKKA< MKlIorS (iKoTM) MKTHOD 179 



mortar time to set, the braces, ribs, and lagging which support 

 it are removed. In the meantime the excavation, bracing, pilot, 

 and exterior shell have been carried forward, preparing the way 

 for more masonry. The top plates of the shell are first placed 

 in position, the material l>eing excavated in advance and sup- 

 ported by light poling-boards ; then the side-plates are butted 

 to the top and the adjoining side-plates. In the pilot the plates 

 are united continuously around the perimeter of the circle, 

 while in the exterior shell the plates are used for about one- 

 third of the perimeter on top, unless treacherous material is 

 encountered, when the plates are continued down to the spring- 

 ing lines of the arch. This iron lining is left in place. The 

 bottom is excavated so as to conform to the exterior lines of 

 the masonry. The excavation follows so closely to the outer 

 lines of the normal section of the tunnel that very little loss 

 occurs, even in bad material ; and there is no loss where suffi- 

 cient bond exists in the material to hold it in place until the 

 poling-boards are in position. 



In the Brooklyn sewer tunnel work, previously mentioned, 

 the pilot was built of steel plates | in. thick, 12 ins. wide, and 

 37 ins. long, rolled to a radius of 3 ft Steel angles 4 x 4 ins. 

 were riveted along all four sides of each plate, and the plates 

 were bolted together by 2-in. machine-bolts. The plates weighed 

 136 Ibs. each, and six of them were required to make one com- 

 plete ring 6 ft. in diameter. In bolting them together, iron 

 shims were placed between the horizontal joints to form a 

 footing for the wooden braces for the shell, which radiate from 

 the pilot The shell plates of the 1 ">-ft section of the tunnel 

 were of No. 10 steel 12 ins. wide and 37 ins. long, with steel 

 angles 2j x 2j x | ins., riveted around the ed^es the same as for 

 the pilot, and put together with |-in. l>olts. These plates 

 weighed 61 Ibs. each, and eighteen of them were required to 

 make one complete ring 15 ft. in diameter. The plates for the 

 l_!-tt. section were No. 12 steel 12 ins. wide with 2 x 2 x |-in. 

 angles. Seventeen plates were required to make a complete ring. 



