Going into the air pressure, the door at the heading end is held closed by the pressure of air against it while one is entering the lock, after which the outer door is also closed. A valve is then opened which permits the air to flow from the working chamber into the lock, until the lock becomes filled with air of the same pressure as exists in the heading. As soon as the pressure is thus equalized, the door at the heading end can be opened and the workmen pass into the heading. Going out, the operations are simply reversed. After the heading door is closed, with the workmen in the air-lock, a valve is opened which permits the air in the lock to exhaust into the normal air, until the pressure within the lock reduces to the same as that outside, when the outer door can be opened and persons inside the lock pass out. Both operations must be gradual, as a sudden change from normal to high pressure, or vice versa, would be very dangerous to anyone.
Shield Cutting Edge Breaking Through Wall at Sixth Avenue and Twelfth Street, Looking South, October 23, 1907
In tunneling under the river, nearly every conceivable combination of rocks and soils were met, but for the most part the material was silt. In such material, with a pressure of 5,000 pounds per square inch on the shield jacks, the shield was pushed through the ground as though one pushed a stick into a heap of snow, pushing aside the silt, and thus obviating the necessity of removing any excavated material. Sand or gravel, or any material which would not flow or become displaced by the shield, of course, had to be excavated ahead of the shield, and removed from the heading prior to pushing it forward. In the silt the most satisfactory and economic progress was attained, and a record was made of seventy-two feet of finished tunnel, completely lined with iron, in one day of twenty-four hours.
The most difficult combination that had to be dealt with under the river was when the bottom consisted of rock and the top of silt and wet sand. In such cases, and there were many of them, the upper section of soft ground was first excavated and the exposed face securely supported with timbers ahead of the shield, and the rock underlying then drilled and blasted. This was very tedious and expensive work. Exceedingly small charges of dynamite had to be used and the procedure conducted with the utmost caution.
In the course of their progress, the shields were subjected to the most intense strains and hard usage, as may well be imagined. One of the shields is [illustrated]. It was used to construct the south tunnel of the up-town pair of tubes, and passed from under the Hudson River, through Morton, Greenwich and Christopher Streets, into Sixth Avenue, and north to Twelfth Street, a total distance of 4,525 feet, of which 2,075 feet was through rock overlaid with wet sand. During the progress of this shield, 26,000 sticks of dynamite were exploded in front of the cutting edge, causing great damage to the structure of the shield, so that when it arrived at its destination at Sixth Avenue and Twelfth Street, it was in such a condition of distortion that it was with difficulty that the tunnel lining could be erected behind it.
North Tunnel, Showing Commencement of New Work
In pushing a shield forward with the battery of powerful hydraulic jacks, each advance is of two feet, and must be followed immediately by installation of the permanent lining in the rear. In the early days, brick work was used for lining, and in recent years it has also been used to some extent, but even with the use of quick-setting Portland cement, neither brick work nor concrete has proved successful for subaqueous work, as the cement cannot reach the required strength within the time it is feasible to leave the shield standing before advancing it again.