Construction
The standard tunnel was egg-shaped, 7.5 feet (228 cm) high and 6 feet (182 cm) wide, with tunnel walls 10 inches (25 cm) thick and a floor 14 inches (35 cm) thick. Some trunk-line tunnel segments were built larger, 14 feet (427 cm) high by 12 feet 9 inches (389 cm) wide. The tunnels were built through a layer of soft blue clay, and tunneling was done by cutting the clay with modified draw knives. Parts of the tunnel were pressurized to 10 psi during tunneling, while other parts were tunneled at atmospheric pressure. The tunnel was lined with coarse concrete and then waterproofed with a Portland-cement limestone-dust plaster. George W. Jackson was granted a patent on the system of forms used in making the concrete tunnel lining.
The city asked that the tunnel be built no shallower than 22.5 feet (6.86 meters) below the pavement in order to allow room for a future streetcar subway.
During construction, temporary 14-inch (36 cm) gauge tracks were laid. The 6-foot (1.8 m) tunnel was wide enough for double tracks with this small size. The tunnel company had 900 small cars built specifically to run on this track. The cars had a box with a capacity of only 0.47 cubic yards (0.36 cubic meters), and were pulled by mules from the tunnel headings to hoists that removed the spoil to the surface or later to points where the spoil could be transferred to 2-foot gauge cars for haulage to the Grant Park disposal station. Tunneling work continued around the clock, 24 hours a day, completing an average of 2 miles (3 km) of tunnel per year per heading during the first few years of development
The 2-foot (61 cm) gauge track was laid in the tunnels, using rails 4.25 inches (10.8 cm) high (56 pounds/yard or 27.8 kg/m). Crossties were not used. Instead, rails were mounted on chairs embedded in the concrete tunnel floor. Frogs and crossings were built on steel plates that were then embedded in the concrete floor.
Curves in the tunnels were very tight. Mainline curves were as tight as 16-foot (4.9 meter) radius, and the grand unions under street intersections were built using a 20-foot (6.1 meter) radius. Grades in the tunnel system were limited to 1.75 percent, except for the lines up to the Grant Park disposal station, which climbed at a 12 percent grade.
The tunnel, 40 feet (12 m) below street level, was drained by 71 electric pumps. There was very little seepage into the tunnels, a natural consequence of excavation in clay, but any water that did find its way in was quickly pumped up to the sewers above. Ventilation was natural, relying primarily on the piston effect of trains pushing through the tunnels to circulate the air.
While buildings with deep subbasements could connect directly to the tunnel, connections to surface level and shallow basements were by elevator shafts. George W. Jackson, the contractor who built the tunnel system, received several patents related to building such shafts.
Read more about this topic: Chicago Tunnel Company
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