. Shield and compressed air tunneling . p;. ^-^^=- Sfcq S 2 250 SHIELD TUNNELING bottom heading and thus reduce the delaj^ to the heading workcaused by the nuick from the main enlargement. 15. Center Heading.—On the East River tunnels of thePennsylvania Kailroad (A-19) it was found best to drive the head-ing approximately on the axis of the tunnel with the lower com-partment of the shield cut away to allow the muck cars to passthrough the shield. The rock below the heading was taken outas a bench in an ordinary rock tunnel. A heavy timber platformwas built out from the floors of the middl
. Shield and compressed air tunneling . p;. ^-^^=- Sfcq S 2 250 SHIELD TUNNELING bottom heading and thus reduce the delaj^ to the heading workcaused by the nuick from the main enlargement. 15. Center Heading.—On the East River tunnels of thePennsylvania Kailroad (A-19) it was found best to drive the head-ing approximately on the axis of the tunnel with the lower com-partment of the shield cut away to allow the muck cars to passthrough the shield. The rock below the heading was taken outas a bench in an ordinary rock tunnel. A heavy timber platformwas built out from the floors of the middle level pockets to catchthe muck shot from the upper pockets and thus prevent blockingthe tracks below. Most of the top muck was caught on thisplatform. The method is shown in Fig. 107. As the timberplatform did not prove entirely satisfactory, the drills in theheading were turned upward and a top bench drilled and excavation left in the top was so little that it was allowed tofall on the tracks and was quickly cleaned away. Th
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