Sectional Elevation
Horizontal Section
Fig. 139.—Plan and Elevation of First Bulkhead Wall in South Tube Manhattan.
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Excavation.
—In driving these tunnels almost any kind of material was encountered, viz., rock, partly rock, and partly loose soil, sand and gravel, and finally silt.
Rock.
—Much of the rock excavation was made before the shields were erected in order to avoid the handling of rock through the narrow openings of the shield doors. Throughout the cross-section the shield traveled on a cradle of concrete in which 2 or 3 steel rails were imbedded. At the points where the excavation had been made for the full section of the tunnel, it was only necessary to trim off the projecting corners of rock. Where only the bottom heading had been driven the excavation was completed just in front of the shield; the drilling below the axis level being done from the heading itself, and above that from the front sliding platforms of the shield. The holes were placed near together and were drilled short; very light charges of powder were used in order to lessen the chance of knocking the shield about too much.