[Joralemon Street Tunnel: IRT Lexington Avenue Line]
Photograph of a sandhog exiting the 'man-lock' between Furman and Joralemon Streets, Brooklyn during the construction of the IRT Lexington Avenue Line. A 'man-lock' is a pressurized chamber. One of the occupational hazards of tunnel work was a condition known as 'the bends,' caused by a build-up of gas bubbles in tissue. The highest pressure in a tunnel was 100 pounds per square inch. Men could work under these conditions only for half hour shifts.
Underwater Tunneling, the third type of construction, occurs underneath the river. This photo shows construction of the Joralemon Street tunnel. Subway water tunnels are built approximately 30 feet below the riverbed. Because of the immense pressure exerted by 30 feet of rock, sand, and silt, plus 50 feet of water, such tunnels were in danger of collapsing. To counteract this force, tunnels underwater were pumped full of pressurized air, which pressed out against the walls of the tunnel and kept it in place. However, this pressurized air made for difficult working conditions for sandhogs. When working in pressurized air, workers were in danger of developing decompression sickness, similar to what divers might experience. Because of this, workers could only spend three hours under water before needing to spend several hours above ground. Before exiting or entering the tunnel, sandhogs would adjust to the pressurized air in a decompression chamber, also called an air-lock or a man-lock.
