of the westbound expressway. Design and construction The standard tunnel was egg-shaped, high and wide, with tunnel walls thick and a floor thick. Some trunk-line tunnel segments were built larger, high by 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 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 below the pavement in order to allow room for a future streetcar subway. During construction, temporary gauge tracks were laid. The 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 , 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 gauge cars for haulage to the Grant Park disposal station. Tunneling work continued around the clock, 24 hours a day, completing an average of of tunnel per year per heading during the first few years of development The gauge track was laid in the tunnels, using rails high (). 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 radius, and the grand unions under street intersections were built to a 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, 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