Engineering Wonders of the World
Volume I

346 ENGINEERING WONDERS OF THE WORLD. the foundation pedestals for the columns. Next, the steel frame of the half width— columns, wall beams, and roof beams—was set up and riveted. The side-wall concrete with its waterproofing course, and, finally, the con- crete filling of the roof, were placed with the use of moulds. Fig. 4.—SUBWAY WORK ALONG ELM STREET, JUST NORTH OF THE CITY HALL. The steel roof beams of the subway framework are clearly visible. This finished one half of the structure, except for backfilling the earth over its roof and repaving. Then the second half of the width was built in precisely the same way, joining the structure to the completed half. At a few special points, however, the street was opened for its full width, and the Subway structure built complete in one operation. Where rock was met (as in Fig. 5), and under busy streets, the work was more in- tricate. But one of the most troublesome complications was found in street car tracks. 2. Complica= tion^by Car Tracks. In Fourth Avenue, between 9th and 33rd Streets, for example, a double track surface railway of the underground conductor type occupied the middle of the street, and it carried a very heavy traffic, which could not at all be interfered with. One expedient for dealing with the diffi- culty was to shift the car tracks to one side, close to the curb, on a length of 500 to 800 feet, and carry on the trenching and building operations in the clear half of the street ; then to shift the tracks over to the finished work, and construct the other half of the Subway. The most difficuit length was handled by this convenient but expensive method. For other parts of this section, the car tracks were left in position, and trestle sup- ports were inserted under thorn, to permit