Engineering Wonders of the World
Volume I

THE UNDERGROUND RAILWAYS OF CHICAGO. 363 intelligence by means of electricity or other- wise.” The distribution of newspapers or letters, for instance, comes under the head of “ intelligence,” but one would not expend millions in building a tunnel system for the purpose of handling one paper or one letter. Papers and letters must be treated in bulk and as freight. All that the authorities stated at the time was that the tunnels were to be made high and wide enough for a man to work in comfortably, with ample space for the suspension of the telephone wires from the roof and side walls, and planned to meet the future growth of the system. They also stipulated that the subways were to be some 40 feet underground. This was an exceed- ingly wise condition, as it permits the con- struction, should it ever be needed, of a tunnel for passenger trains between the street level and the subways, the floor of the latter being 46 feet below the surface. Having obtained permission, the engineers commenced operations. The first thing was to survey the line of route. Every street in the business section of the Surveying cify was surveyed thoroughly, Streets enS^neers refusing ac~ cept the existing maps, many of which were found to be faulty. As observations could not be made on week-days, when the s’treets were choked with traffic of every description, the work had to be done on Sundays. For the sake of speed and economy, it was necessary to operate from several widely separated points simultaneously. At each of the points selected a shaft was sunk to the tunnel level, and drifted out to the centre of the street, whence excavation was begun in both directions on the axis of the tunnel. Most of these shafts were sunk in the basements of buildings rented by the Tunnel Company, and fitted up as workshops. Here concrete was mixed and lowered to the tunnels, and air-compressing machinery was installed. METHOD OF TUNNEL CONSTRUCTION. This figure shows the steel channel ribs supporting the 2-by-6-inch lagging boards, between which and the ground concrete is rammed tightly in. As soon as the concrete is set, the centres and ribs are removed. The tunnels are of two distinct types— trunk tunnels and lateral conduits. The former follow the main routes of freight traffic. The latter run out to the less im- portant parts of the city, and in time will reach, the suburbs, and end in small conduits adapted only to accommodate telephone and other wires. The main or trunk line tunnels are 14 feet 6 inches from crown to floor, and 12 feet 9 inches wide at the base ; while the laterals are 7| feet high and 6 feet wide. The trunk subways have 18-inch. cement walls and 21-incli cement floors ; the smaller tunnels 10-inch walls and 13-inch floors. At the top and sides of these subways are strong cables containing telephone wires, while the floors are left free for freight traffic. Active excavation began on September 1, 1901, and by November 1904 twenty miles of track had been laid and made ready for use. An ingenious system of excavation was