The New York Rapid-transit Subway

18 PARSONS ON NEW YORK RAPID-TRANSIT SUBWAY. [Minutes of WATERPROOFING. As much of the subway was below tide-level, or where above tide- level was in wet ground, it was a matter of great concern that it should be dry at all times and in all places. It was therefore stated to be the « very essence of the specifications to secure a railway structure which should be entirely free from the percolation of ground-water.” In order to secure this, it was decided to build in the floor, walls, and roof, a waterproof course consisting of asphalt and asphalted felt. The latter was the ordinary roofing-felt paper, dipped in asphalt and weighing 15 lbs. per hundred square feet. The asphalt was specified to contain in the refined state not less than 95 per cent, of natural bitumen, soluble in rectified carbon bisulphide or in chloroform. Not less than two-thirds of the total bitumen was soluble in petroleum naphtha at 70 Baume, or in acetone. The asphalt, in order to be accepted, had to lose not more than 4 per cent, of its weight when exposed for 10 hours at a tempera- ture of 300° F. Such asphalt, which is very much purer than the ordinary asphalt of commerce, becomes at a low temperature hard and brittle, and in order to maintain fluidity a flux of petroleum residuum was added. At high temperatures there was found to be the reverse difficulty of too great fluidity, to prevent which powdered limestone and sand were added up to 66 per cent, of the total, in the same way as sand is mixed with cement. Such additions were not only not injurious, but permitted work to be carried on which otherwise would not have been possible. In order to put the waterproofing in place after the excavation was made, a bed of concrete, usually about 6 to 8 inches thick, was laid. After that bed had been made smooth, a bed of hot asphalt was spread, on which, while it was still hot, the felt was rolled. Then more asphalt was poured on the felt, another layer of felt was laid, and again asphalt and felt alternating, until the required number of layers of felt had been laid— usually three or four, although as many as six were used in wet ground. On the top of the waterproofing-course was laid the upper part of the subway floor. To waterproof the walls the same waterproofing-course was carried upward. Where possible, this was done by placing alternate layers of asphalt and felt against the side walls of the subway outside the beams; but in only a few cases was this possible, since as a rule the excavation was not wide enough to permit work on the back of the walls. The contractor usually built against the sheeting a wall of brick or coarse concrete, on which the waterproofing course was