OPEN-CUT TUNNELING METHODS 197 



cement, two parts sand, and four parts broken stones. The 

 double-track subway will be built in the same way, except that 

 only one column is placed between the tracks for the support 

 of the roof. 



All the concrete masonry of the roof, foundations, and side 

 walls, must contain a layer of waterproofing, so as to keep 

 perfectly dry the underground road, and prevent the perco- 

 lation of water. This waterproofing must be made up as 

 follows On the lowest stratum of concrete, whose surface 

 is made as smooth as possible, a layer of hot asphalt is spread. 

 On this asphalt are immediately laid sheets or rolls of felt; 

 another layer of hot asphalt is then spread over the felt, and 



Fio. 114. Park Avenue Deep Tunnel Construction, New York Rapid Transit Railway. 



then another layer of felt laid, and so on, until no less than 

 two, and no more than six, layers of felt are laid, with the frit 

 between layers of asphalt. On top of the upper surface of 

 asphalt the remainder of the concrete is put in place so as to 

 reach the required thickness of the concrete wall. 



Tunnels. At three points the Standard Subway will be 

 replaeed liy tunnel lilies. The location of the three tunnels 

 will le between 33d and 42d St. on Park Ave. ; undei 

 Central Park, northeast of 104th St., and under the Har- 

 lem River. The Park Ave. const ruction (Fig. 114) will 

 consist of two parallel double-track tunnels, located on each 

 side of the street, and about. 10 ft. below the present tunnel. 

 The soil being composed of good rock, the tunnels will be 



