Engineering Wonders of the World
Volume I

364 ENGINEERING WONDERS OF THE WORLD. Building the Subways. adopted as affording a safeguard against labour troubles. In this system there can be no possibility of “ caving in ” or collapse should a strike occur and workmen be unob- tainable. A description of the plan followed in the building of the 6-by-7^- foot conduits will give an idea of how the excavations were made and the concrete sub- way installed. The bore in the clay was dug considerably larger than the dimensions of the finished subway. The mining was done by men with hand tools, the material being removed subsequently by mule teams. After the exca- vators had finished their labours the cement- ing gang appeared. Their first task was to place in the bottom of the tunnel the required layer of concrete, tamp it thoroughly, and lay on top of it a lagging of boards. This done, they placed at intervals of three feet steel ribs of the size and shape of the inside of the tunnel. When these ribs were in place, a lagging of 2-inch. planks was placed between STEEL RIBS AND LAGGING USED IN FORMING CON- CRETE LINING OF A 12-BY-13-FOOT TUNNEL. them, and concrete packed in behind the lagging in layers of six inches, so as to fill up the entire space between the wood and the walls of the excavation. In this way subsi- dence of the ground was avoided, because, no matter how irregular was the mining, every void was filled. This method was followed to the top or key of the arch, where, to ensure absolute accuracy, the key-blocks were formed of lagging boards only three feet long, the use of the short section ensuring better work. After the ribs and lagging had been removed, the concrete was given a facing of cement to seal all crevices. In the larger, or trunk, lines a somewhat different process was followed, steel lagging plates being used instead of planks, and 5-inch instead of 3-inch channel irons for ribs. The work was done in sections only three feet long, the steel lagging plates being of just sufficient length to span the 3-foot sections of the ribs. The tunnelling was effected by three shifts of men working eight hours each, the first two shifts doing mining work and the third How the Work progressed. shift concreting. About 850 men were engaged in the three shifts. In this way the tunnel was mined and cementedx at the rate of more than 300 feet per working day, or considerably over a mile a month. More than 300,000 cubic feet of material was excavated in a period of three years, and about half that quantity of stone, cement, and gravel was put in place. As many as thirty-eight connections had to be made between different tunnels, and so accurately was the surveying done that in no instance was a junction an inch out of truth. An air pressure of about 8 lbs. to the square foot was maintained throughout the tunnel during the construction period, this being found sufficient to prevent the water from encroaching as the work progressed. The excavated material was removed in a novel manner. At various points along the