202 TUNNELING 



penetrate. This fact is generally determined by making dia- 

 mond-drill borings along the line, and the object of ascertaining 

 it is to determine the method of excavation to be adopted. If 

 the material is permeable and the tunnel must pass at a small 

 depth below the river bed, a method will have to be adopted 

 which provides for handling the difficulty of inflowing water. 

 If, on the other hand, the tunnel passes through impermeable 

 material at a great depth, there will be no unusual trouble 

 from water, and almost any of the ordinary methods of tun- 

 neling such materials may be employed. Shallow submarine 

 tunnels through permeable material are usually driven by the 

 shield method or by the compressed air method, or by a method 

 which is a combination of the first and second. 



It is not an uncommon experience for a submarine tunnel 

 to start out in firm soil and unexpectedly to find that this 

 material becomes soft and treacherous as the work proceeds, or 

 that it is intersected by strata of soft material. The method of 

 dealing with this condition will vary with the circumstances, but 

 generally if any considerable amount of soft material has to be 

 penetrated, or if the inflow of water is very large, the firm- 

 ground system of work is changed to one of the methods 

 employed for excavating soft-ground submarine tunnels. The 

 Milwaukee water supply tunnel and the East River gas tunnel, 

 described elsewhere, are notable examples of submarine tunnels 

 began in firm material which unexpectedly developed a treacher- 

 ous character after the work had proceeded some distance. 

 Occasionally the task of building a submarine tunnel in the 

 river bed arises. In such cases the tunnel is usually built by 

 means of cofferdams in shallow water, and by means of caissons 

 in deep water. 



Submarine tunnels under rivers are usually built with a de- 

 scending grade from each end which terminates in a level middle 

 position, the longitudinal profile of the tunnel corresponding to 

 the transverse profile of the river bottom. Where, however, 

 such tunnels pass under the water with one end submerged, and 



