Cross-Section.
—The cross-section selected for the tunnels had vertical side walls and a three-centered roof arch with the flattest curve at the crown. The interior dimensions were 25 ft. wide and 16 ft. high. The selected cross-section was not the best suited for a tunnel to be driven through rock, where the sharpest curve should be at the top, but in this case the flattened curve was chosen because of local conditions; chiefly, the presence of the existing tunnel and the consequent necessity of leaving a certain thickness of rock between it and the new tunnel, without depressing very much the grade of the subway.
Fig. 58.—Sequence of Excavation in the Murray Hill Tunnel.
Excavation.
—The two parallel tunnels were driven exclusively from the ends reached by shafts; thus the tunnels were attacked at four parts. It was in these tunnels that a comparative test was made of the different methods of driving tunnels through rock. The contractor applied the heading and drift method at the southern ends of the tunnels, the eastern tunnel being driven by means of a drift while in the western tunnel the usual heading method was followed. This latter method is illustrated in the [chapter following] and the eastern tunnel at 33rd Street, excavated by means of a drift, is here considered.
[Fig. 58] shows the sequence of cuts adopted for this tunnel. It was begun by a bottom drift, about 10 ft. high, 8 ft. wide and 7 ft. deep, which was located at one side of the axis of the tunnel, as indicated in the figure. This drift was immediately widened by removing the portions marked 2. About 50 ft. in the rear the part marked 3 was taken away, thus clearing the entire lower portion of the tunnel. Section 4, about 50 ft. to the rear of section 3, was then broken down and removed.
The methods of drilling and blasting were as follows: In taking out the original drift, a wedge-shaped center cut was made and then enlarged to the full size of the drift by drilling parallel holes. The succeeding sections, 2 and 3, were removed by driving parallel holes, while the top section, 4, was taken away by a center cut and parallel holes. The drills were mounted on columns, two drills to a column, and the holes were usually drilled about 7 ft. deep, starting with a diameter of 23⁄4 in. and ending with a diameter of 13⁄4 in. They were blasted with 40% dynamite in light charges, only a few holes being fired at a time, usually not more than three or four.
Fig. 59.—Traveling Platform for the Excavation of the Upper Side of the Murray Hill Tunnel.