Engineering Wonders of the World
Volume I

THE NEW YORK SUBWAY. 347 Fig. 5.—EXCAVATION IN ROCK UNDER A BUSY STREET. Only half the full width excavated at once. the soil to be excavated. Narrow transverse trenches were first dug under the tracks, at intervals of 40 feet, down to the Subway floor level. In these trenches, on a footing of concrete, were erected frames of timber posts and bracing, carrying at street-level on either side of the tracks a pair of longitudinal steel girders (see sketch, Fig. 6). The girders sup- ported hangers, which held up short transverse beams drawn in through drifts under the tracks. Then wedges were driven be- tween these beams and the track structure, so as to throw the weight of the latter entirely on to the system of beams, girders, and trestles. The excavating and building operations were thus carried out; in some cases first on one side to completion, then on the other ; in other cases on the full width simultaneously. Cableways were used for hoisting out the excavated earth, or rock. Broadway in its upper part is unusually wide, and has a parked strip, 22 feet wide, extending along the middle, between the car tracks. The Subway was to extend Fig. 6. about 10 feet beyond either track, leaving some width of undisturbed pavement along the Rock lay from 5 to below the surface on this required blasting. The contractor’s method carve away the parked strip only, as he had also to undermine the car tracks laterally, it was necessary to support these tracks by timbering. For this purpose a very short length, say 40 feet, was the basis of operations. Strong timber or steel trusses were set longitudinally on either side of each track over the 40 feet working length, and excavation progressed on this length, from the middle toward the sides. As soon as a cavity had been made under the track at one point, a crossbeam was pushed through, hung by bolts to the trusses, and drawn up to a bearing against the track. Thus in time the whole 40 feet length of track was carried by the trusses, and the soil could be taken out to the bottom for the full width required (Fig 7). Then posts were set up under the curbs. 3. Along the Boulevard. 10 feet much. of this length. ; was simply to and dig down ; Fig. 7.—timber truss supports used along BROADWAY. track, and the load transferred to them from the trusses. The trusses being thus relieved, they could bo shifted along to the next length, and the operation repeated. In Fig. 8 several sets of these trusses are seen in use. The posts continued to support the tracks during the erection of the steelwork and con- crete, until other supports could be provided on the first of the roof beams ; and, finally, stone filling was laid over the finished roof as a new bedding for the track. 1 While the preceding is a fair picture of how the problem of building the Subway was