=Air Locks.=--Two bulkhead walls, forming the rear closure of the pneumatic sections, were built in each end of each tunnel, one just ahead of the shield chamber, the other about 1200 ft. ahead of the first. The walls were built of Portland concrete 10 ft. thick, and they were grouted with Portland cement, under a pressure of nearly 100 lbs. per sq. in., to make them thoroughly air-tight. Each wall had in it three locks; for man, material and emergency. Each was equipped with hand valves arranged to be operated from either outer end or from within. The floors of the man and material locks were on a level with the working platform of the tunnel, about 3 ft. 6 ins. above the invert; the floor of the emergency lock was about 5 ft. above the horizontal axis of the tunnel. The locks were made of steel plates and shapes, with iron fittings riveted and bolted together. The man lock was 11 ft. long of elliptical cross-section, 6 ft. vertical diameter and 5 ft. horizontal; the material lock was 25 ft. long, with circular cross-section, 7 ft. diameter, and the emergency lock was 20 ft. long, of elliptical cross-section, 4 ft. vertical and 3 ft. horizontal diameters. Fig. 139 shows the elevation of the air lock used in the Pennsylvania tunnel. [Illustration: ~Sectional Elevation~ ~Horizontal Section~ FIG. 139.--Plan and Elevation of First Bulkhead Wall in South Tube Manhattan.] =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.