Openings were made between the headings as follows:
Tunnel D, February 20th, 1908;
Tunnel B, March 3d, 1908;
Tunnel C, March 5th, 1908;
Tunnel A, March 18th, 1908.
It was necessary to cut away the projecting floors of the working compartments before the cutting edges could be shoved together.
Contractor's Organization.—Tunnel operations were carried on continuously for thirteen days out of fourteen, regular work being shut down for repairs on alternate Sundays. When the required pressure was more than 32 lb., four gangs of laborers were employed, each gang working two shifts of 3 hours each, with an intermission of 3 hours between the shifts. When the pressure was less than 32 lb., three gangs were employed, each gang covering 8 hours, but with an intermission of about 1/2 hour in low pressure for lunch.
Air Pressures Required.—During the greater portion of the work in soft ground, pressure was maintained which would about balance the hydrostatic head at the axis of the tunnel. This required a pressure varying from 30 to 34 lb. per sq. in. above that of the atmosphere. In Tunnels B and D, at Manhattan, during the work in soft ground, pressures as high as 37 lb. were maintained for considerable periods of time; in the firm material near the reef 28 lb. was often sufficient. While removing the broken plates, the pressure was raised for a short time to 42 lb., and was maintained between 37-1/2 and 40 lb. for a little more than one month.
Air Supply.—For regular operation the contractor furnished four compressors on each side of the river, each having a rated capacity of 5,000 cu. ft. of free air per minute delivered at 50 lb. above normal, when running at the rate of 100 rev. per min. An additional compressor of the same capacity was supplied on each side of the river, in compliance with the requirement for 25% excess capacity; the additional compressors had also high-pressure air cylinders which could be connected at will, and in which the pressure could be increased to 150 lb., and the air used to supply rock drills, grouting machines, etc. The entire combination on each side of the river, therefore, was rated at 25,000 cu. ft. of free air per minute, or a mean of 6,250 cu. ft. per heading. Its safe working capacity was not far from 20,000 cu. ft. per min.
The shields broke through rock surface in Tunnels B, C, and D, at Manhattan, in November and December, 1905. The consumption of air in the four tunnels soon exceeded 15,000 cu. ft. for 24 hours, and in Tunnel D, on several occasions, it exceeded 7,000 cu. ft. for a like period. Blows had become frequent, and it was evident that the air plant was inadequate for driving four tunnels at once in the open material east of the Manhattan rock. Work in Tunnel A, therefore, was not resumed, after the suspension on December 29th, for about ten months, and Tunnel C was also closed down for more than four months of the time between December, 1905, and July, 1906. During this period the capacity of the plant was increased from the rated 25,000 cu. ft. of free air per minute, to 35,000. In Tunnel D the material had gradually become firmer, with more clay and less escape of air, as the Blackwell's Island Reef was approached, and, at the end of the period, the rock surface was within 3 ft. of the top of the shield; in Tunnel B, the rock of the reef was still a little below the shield, but the overlying material contained a large proportion of clay and held air very well. Tunnel C was still in open material, but, with two lines safe and with the increased air plant, it was deemed best to resume work in Tunnel A, which was done on October 23d, 1906. Thenceforward work was continuous in all headings until the meeting points with the Long Island shields were reached.
This period, January to October, 1906, inclusive, was the most strenuous of the entire work, particularly the first six months. With one and, at times, two tunnels closed down, the consumption of air in the headings from Manhattan was an average of more than 20,000 cu. ft. per min. for periods of from 30 to 60 days; it was often more than 25,000 cu. ft. for 24 hours, with a maximum of nearly 29,000 cu. ft., and doubtless this was exceeded considerably for shorter periods. On several occasions the quantity supplied to a single tunnel averaged more than 15,000 cu. ft. per min. for 24 hours. The greatest averages for 24 hours were obtained later in Tunnel A, after the resumption of work there, and exceeded 19,000 cu. ft., but the conditions in the headings of the other lines were then so favorable that the work was carried on continuously in all.
The deficiency in the original plant at Manhattan was so marked, and the need of driving all headings from Long Island simultaneously so clear, that it was decided to increase the rated capacity of the Long Island compressor plant to 45,400 cu. ft. of free air per minute, which was 10,400 cu. ft. greater than the capacity of the Manhattan plant after the latter had been augmented.