The New York Rapid-transit Subway

120 DISCUSSION ON NEW YORK SUBWAY. [Minutes of The Author. Author would state that nothing in his own experience, or in his subsequent study of the experience gained in other cities, had led him to regret his decision in 1894 that for New York shallow sub- ways were to be preferred to deep tubes; but, contrary to Mr. Fitz- maurice’s opinion, the existence of rock was not a factor tending to such corroboration. This material, requiring drilling and blasting, was not attractive for open excavation with pipes being supported overhead. The determining reasons had been the greater public con- venience, lower working-expense, and consequent greater net return, of the shallow subway. Obstruction to other traffic during con- struction and annoyance to frontagers could be reduced, but at increased cost, by roofing over the excavation, as had been done in Lower Broadway, where there was no parallel street to which traffic could be diverted, or as was being done in Boston by Mr. Carson, where the streets were quite as narrow and irregular as those in London. It was only a question of money. The Harlem River crossing could have been made in open coffer-dams. The sub-contractor who had the contract for a lump sum chose the method described, as being cheaper. Less timber was needed, the cost of compressed air probably did not exceed the cost of pumping, and all risks attending an open coffer-dam against a hydrostatic head of about 55 feet, with a bottom that was not of the best, were avoided. The driving of the East River tunnels had been a very serious undertaking, the material varying from rock to quicksand, at times both being present in the same face. To blast the one without causing a run of the other called for great skill. In driving through the quicksand, the sub-contractor unfortunately so disturbed the material as to get the tube out of level. The irregularity was rectified by taking up in short sections the bottom plates on the “high” places, excavating the material, and then replacing the plates to a new gradient-line level with the low places. Special castings were provided for making up at the sides and closing at the ends. The maximum depression thus resulting was 18 inches. The tube where altered was not circular, but as it contained at the sides concrete benches carrying the ducts (Fig. 8, Plate 5) it was so strongly stiffened internally against distortion that the small varia- tion from circular section was of no importance. The stiffening effect of the benches was increased by running them up vertically to support the roof-plates. When driving a shield through a flowing material such as quicksand and under high air-pressure, there was a decided tendency to. keep the particles of sand at the sides in suspension, so that the iron ring had at the sides only hydrostatic pressure for support. Unless, the ring was restrained laterally during construe-