page_title: Chicago Tunnel Company
section_title: Design and construction
hierarchical_section_title: Chicago Tunnel Company / Design and construction
caption_attribution_description: English: Original captions: Mines and Minerals: "Lateral Tunnel Under Construction." Page's: "LATERAL CONDUIT UNDER CONSTRUCTION." Western: "Lateral Conduit Under Construction" and inscribed on the photo, "[Illegible] 9-15-02 Ill. Telephone &amp; Telegraph Co -- Construction Methods --". This image is a composite from 3 published sources in order to eliminate artifacts from both printing and scanning. For scale, note that the track in the tunnel is 14-inch gauge.
context_page_description: The Chicago Tunnel Company was the builder and operator of a 2 ft narrow-gauge railway freight tunnel network under downtown Chicago, Illinois. This was regulated by the Interstate Commerce Commission as an interurban even though it operated entirely under central Chicago, did not carry passengers, and was entirely underground. It inspired the construction of the London Post Office Railway.
context_section_description: The standard tunnel was egg-shaped, 7 feet 6 inches (2.29 m) high and 6 feet (1.83 m) wide, with tunnel walls 10 inches (25 cm) thick and a floor 14 inches (36 cm) thick.  Some trunk-line tunnel segments were built larger, 14 feet (4.27 m) high by 12 feet 9 inches (3.89 m) 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 pounds per square inch (69 kPa) 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 ¹⁄₂ feet (6.86 m) below the pavement in order to allow room for a future streetcar subway.
  During construction, temporary 14 in (356 mm) gauge tracks were laid.  The 6-foot (1.83 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 m³), 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 ft (610 mm) 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 (3.2 km) of tunnel per year per heading during the first few years of development
  The 2 ft (610 mm) gauge track was laid in the tunnels, using rails 4 ¹⁄₄ inches (110 millimetres) high (56 pounds per yard or 27.8 kilograms per meter).  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 little as 16-foot (4.88 m) radius, and the grand unions under street intersections were built to a 20-foot (6.1 m) 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 (12.19 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.
caption: Tunnel under construction in 1902. The freshly cut clay ahead of the concrete work shows clear knife marks.
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