. Shield and compressed air tunneling . Fig. 124.—Work in mixed face with sliding hood. Pennsylvania Railroad EastRiver Tunnels (A-19). (FromJ^rans. Am. Soc. C. E., vol. 68.) 286 SHIELD TUNNELING supported by a cantilever attached to the hood (see Fig. 125).In this way all the soft ground was removed down to the rocksurface and the roof, sides and face sheeted with timber. Theroof and side polings were lost and left behind. The fixed hoodmade it possible to set the face from 7 to 8 ft. ahead of thecutting edge. This was enough for two shoves and was thestandard method, although many faces were
. Shield and compressed air tunneling . Fig. 124.—Work in mixed face with sliding hood. Pennsylvania Railroad EastRiver Tunnels (A-19). (FromJ^rans. Am. Soc. C. E., vol. 68.) 286 SHIELD TUNNELING supported by a cantilever attached to the hood (see Fig. 125).In this way all the soft ground was removed down to the rocksurface and the roof, sides and face sheeted with timber. Theroof and side polings were lost and left behind. The fixed hoodmade it possible to set the face from 7 to 8 ft. ahead of thecutting edge. This was enough for two shoves and was thestandard method, although many faces were set for one shoveonly. 92. Blasting.—When the rock part of the face was soundenough and high enough a bottom heading was driven 20 or 30ft. ahead of the shield, and a concrete cradle placed. This workwas carried out independently of the work in the soft groundabove. The rest of the rock was taken out by firing top and siderounds into the bottom heading after the soft ground had beenexcavated. Gi-eat care was used to prevent distur
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