I9I0.] DAVIES— TUNNEL CONSTRUCTION. 185 



road companies, traffic was suspended on the railroad track im- 

 mediately above the caissons. 



The conditions were very much worse, however, in the case of 

 the construction of the approaches to the Church Street Terminal 

 Station under Cortlandt and Fulton Streets, where the arrangement 

 of switches desired was such that the tunnels could not be built by 

 shields and the caisson method was used. In this case the business 

 had to be continued on the surface of these streets at all times, from 

 beginning to end of the construction work, and further, all sewer 

 pipes, steam pipes, electric conduits, etc., had to be in use and opera- 

 tion throughout the entire period of construction. The difficulty, 

 due to adjacent buildings not having foundations which extended 

 below the cellars, made necessary other methods than those adopted 

 in Jersey City. The cofferdam enclosing the entire area of the 

 buildings had already been sunk, by a series of caissons, to bed 

 rock around the entire area, and these approaches had to be con- 

 structed contiguous with the exterior of the cofferdam taking in 

 practically the entire width of Cortlandt Street and Fulton Street, 

 building line to building line. 



In this case an absolutely new departure in caisson construction 

 was adopted, which involved sinking boxes, forming short sections 

 of tunnel, with solid sides, the same being built up as the caissons 

 descended. These boxes had to be sunk entirely from below the 

 level of the street, and their sides were built up five feet at a time 

 as they sank, special types of air locks being necessary to enable 

 the work to be carried on in this manner. These caissons, too, con- 

 trary to the usual custom, were sunk without roofs, that is to say, 

 the permanent floor of the tunnel was designed as the roof of the 

 working chamber, and the sides of the box built up of reinforced 

 concrete, but the ends with steel plates reinforced with pin-connected 

 truss girders. These boxes were sunk by loading with pig iron 

 in order to obtain the necessary weight, and when they had reached 

 their permanent position timber struts were put in to take the ex- 

 ternal pressures on the side walls, pending the construction of the 

 roof. When the various caissons had been sunk, one against the 

 other, until the whole strip of tunnel had been completed, the 



