. Official proceedings . ary American method oftaking out and timbering the top third of the excavation and thebenches were removed by the use of the steam shovel. The ma-terial through which the tunnel penetrated was exceedingly un-stable, being a disintegrated soft shale rock. Here slips were 72 Proceedings Railway Club of Pittsburgh. frequent and heavy timbering was used to overcome the ten-dency of the whole mountain to slide into the excavation. Whenheavy timbers proved too light, 12 lb. beams bent to thecurve of the tunnel and erected in three sections spliced togetherwere tried. Th
. Official proceedings . ary American method oftaking out and timbering the top third of the excavation and thebenches were removed by the use of the steam shovel. The ma-terial through which the tunnel penetrated was exceedingly un-stable, being a disintegrated soft shale rock. Here slips were 72 Proceedings Railway Club of Pittsburgh. frequent and heavy timbering was used to overcome the ten-dency of the whole mountain to slide into the excavation. Whenheavy timbers proved too light, 12 lb. beams bent to thecurve of the tunnel and erected in three sections spliced togetherwere tried. These also proved too light for the work and manyof them were seriously bent and twisted under the tremendousearth pressure, as shown in Fig. 3. The settlement was finallystopped and the tunnel completed by the use of 12 55 lb. I-beamrings spaced two feet center to center and filled in solid by con-crete, as shown in Fig. 4. Inasmuch as steel mine timbers, inmore or less similar form, have been found highlv satisfactorv in. Figure 4. coal mining ii]x-rations in this countr_\- and abroad, there seemsto be no reason whatever to prevent their similar use in standardrailroad tunnel construction where heavy loads are to be sus-tained and where the ordinary methods of timbering appear likelyto prove inadequate. Sorr CRouxu tuxxels: The essential differences betweena rock tunnel and a soft ground tunnel is that the former isdrilled and blasted wliile the latter is bored and dug; machine Tunnels. Particularly Subaqueous. 73 drills give place to augers, and timbering common in stableground is replaced by the fore-poling method of the ground contains no water and danger of subsidence issmall, the timbering necessary is reducible practically to theamount required properly to support the excavation during theprocess of lining. If, however, the material contains water andthe excavation is carried on at shallow depth in close proximityto buildings, the use of a shield may be fo
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