Plant.
—The plants located at each end of the tunnel near the shafts were almost identical. Each consisted of three 500-H.P. Stirtling boilers, which supplied steam at 150 lbs. pressure. Feed water was supplied by three 131⁄2 metropolitan injectors, and two Blake duplex pumps. Two Worthington surface condensers, each of 2250 sq. ft. condensing surface, took care of the exhaust from the engines and compressors. Condensing water was pumped from the river through a 16-in. pipe. The high-pressure air was supplied by a duplex Ingersoll-Sergeant compressor, with cross-compound steam end 14 × 26 × 30 ins. and simple water-jacket air cylinders 131⁄4 × 36 ins. Its capacity at 100 r.p.m. was 1085 cu. ft. free air per minute. The maximum pressure was 130 lbs. per sq. in. The air for the pneumatic working was supplied by three 14 × 26 × 30 in. duplex Ingersoll-Sergeant compressors. The maximum capacity of the three was 12,000 cu. ft. free air per minute at 125 r.p.m. and a discharge pressure of 50 lbs. per sq. in. The suction air was taken from the outside about 10 ft. above the roof of the engine house. Three aftercoolers, 321⁄2 ins. × 11 ft. 4 ins., each having 809 sq. ft. cooling surface of tinned brass tubes, cooled the low-pressure discharge to within 10° F. of the temperature of the cooling-water. From the aftercoolers, the air passed into three steel receivers each 54 × 12 ft., placed outside the engine room and fitted with weighing safety valves. The receivers were connected to two 10-in. mains; one serving the north, the other the south tunnel. A fourth receiver of the same size was built to receive the discharge of the high-pressure compressor, through a 4-in. pipe. The high-pressure water required for the shield was furnished by three Blake direct-acting, duplex pumps with outside packed plungers. The steam end was 16 × 18 ins., the water end 21⁄16 × 18 ins. At 55 r.p.m. pumping against 5000 lbs. per sq. in., the capacity of each pump was 57 gals. per minute. Two of them, one on each tunnel, were sufficient to run the shields and the third was held in reserve. The high-pressure water was conveyed to the front by means of a 2-in. double, extra strong pipe which was buried between the engine room and the shaft, in a trench, to prevent freezing in cold weather. The electric current for light and power was supplied by two 100-K.W. 250-volt G.E. direct-current generators directly connected to Ball & Wood high-speed engines running at 250 r.p.m. The switchboard had two machine panels, two distributing panels and one panel carrying a circuit breaker for the traction circuit.
Illumination.
—The tunnel was lighted by electricity, there being two rows of lamps, one in the crown and one in the south axial fine. The lamps were 16-c.p., 240-volt, two-wire system, and were spaced 35 ft. apart in the crown and 121⁄2 ft. apart on the axial line. In addition, five nests of 5 lamps each were used at the front. Candles were supplied for miscellaneous and emergency uses. The sockets for electric globes were fitted to a wooden reflector, coated with white enamel paint on the inside.
CHAPTER XXI.
SUBMARINE TUNNELING (Continued); TUNNELS AT VERY SHALLOW DEPTH. THE COFFERDAM METHOD. THE PNEUMATIC CAISSON METHOD. THE JOINING TOGETHER SECTIONS OF TUNNELS BUILT ON LAND.
The tunnels on the river bed or at such a shallow depth that only a few feet of material will remain between the bottom of the river and the roof of the tunnel can be built in three different ways, viz., (1) by a cofferdam; (2) by pneumatic caissons; (3) by sinking and joining together whole sections of tunnels that were built on land.
The Cofferdam Method.—The Van Buren Street Tunnel, Chicago River.—
According to the cofferdam method, the work is attacked at one of the shores, and the tunnel built in sections of such length as not to interfere with the flow of water or the navigation of the river. Round the entire exterior line of the first section a double-walled cofferdam is built, and strongly braced transversely, so as to withstand the pressure of the water. When the water is pumped out, a single-walled cofferdam is built within the first, leaving sufficient distance between the two to allow of the construction of the masonry. The soil is then removed within the inner cofferdam, and the tunnel constructed from the foundation. When the end of the tunnel reaches the channel end of the cofferdam, a crib-wall is erected over the end of the completed tunnel. This crib, in turn, forms the end wall of another cofferdam, built in continuation of the first, so as to allow the second section to be proceeded with, and at the same time to facilitate the removal of the cofferdams of the first section. The work goes on continuously in this way until the distant shore is reached.