Fig. 63.—Sketch Showing Arrangement of Car Tracks, St. Gothard Tunnel.
FORT GEORGE TUNNEL.[10]
From a point north of 157th Street and Broadway almost to Dyckman Street, that is, a distance of nearly two miles, the New York Subway passes under an elevation known as Fort Washington Heights, which almost bounds Manhattan Island at its upper end near the Harlem Ship Canal. Under this elevation the rapid transit railroad was constructed in tunnel. The tunnel was driven from two intermediate shafts over 110 ft. deep, located one at 169th Street and the other at 181st Street and Broadway. Both shafts were sunk at one side of the center line of the tunnel. After these shafts had been utilized for working purposes during the construction of the tunnel, they were equipped with electric elevators to carry passengers from the streets to the deep station.
[10] Condensed from a paper by Stephen W. Hopkins in Harvard Engineering Journal, April, ’08.
Material.
—The material encountered in the excavation of the Fort George tunnel was the usual mica schist met everywhere on Manhattan Island. It was full of seams with strata running in every direction to such an extent that at many points the roof of the tunnel had to be supported by timbers; at other parts along the line the rock was so disintegrated that it was considered a very loose and treacherous soil. Two serious accidents, each accompanied by loss of life, occurred during the construction of this tunnel. Both of them were caused by the sudden fall of a large ledge of rock which, after the tunnel had been excavated to the full section, remained hanging on the roof, deprived of any support and held in place by the little cohesion of the material packing the seams.
Excavation.
—The tunnel was excavated by the heading method in only two cuts, viz., the heading and bench as indicated in the [Fig. 65]. The heading, almost as wide as the upper portion of the tunnel section, was excavated in the manner explained on [page 91]. After the heading was removed, the enlargement of the entire upper section of the tunnel was accomplished by driving three inclined holes at each side of the heading. They were driven at different depths and inclinations, as shown in the [figure] and were called trimming holes. At the same time the bench was removed by means of five holes—three vertical and two inclined. The line of subgrade was reached by means of five grading holes driven almost horizontal with a slight inclination downward. The air drills for the heading were mounted on columns, all the others on tripods. The blasting was done in the following order: the grading holes were blasted in the first round, the bench and trimming in the second, the center cut of the heading in the third, the sides in the fourth and the dry holes in the last. Thus each advance of 7 ft. of the whole tunnel section was made by means of forty holes fired in five rounds which consumed 277 lbs. of dynamite with an average additional quantity of 76 lbs., making a total of 353 lbs. With the exception of the center cut, where 60% dynamite was used, all the other holes were discharged with 40% dynamite.
Cross Section.