184 TUNNELING 



ing mines. To protect the line from this refuse, the engineer 

 constructed a tunnel lining consisting of successive steel ribs, 

 filled between with masonry. 



Concluding Remarks. From the fact that the open-cut 

 method of tunneling consists first in excavating a cut, and sec- 

 ond in covering this cut to form an underground passageway, 

 it has been named the " cut-and-cover " method of tunneling. 

 The cut-and-cover method of tunneling is almost never employed 

 elsewhere than in cities, or where the surface of the ground has 

 to be restored for the accommodation of traffic and business. 

 When it is not necessary to restore the original surface, as is 

 usually the case with tunnels built in the ordinary course of 

 railway work, it would obviously be absurd to do so except in 

 extraordinary cases. In a general way, therefore, it may be said 

 that the cut-and-cover method of construction is confined to the 

 building of tunnels under city streets ; and the discussion of 

 this kind of tunnels follows logically the general description of 

 the open-cut method of tunneling which has been given. 



TUNNELS UNDER CITY STREETS. 



The three most common purposes of tunnels under city 

 streets are : to provide for the removal of railway tracks from 

 the street surface, and separate the street railway traffic from 

 the vehicular and pedestrian traffic ; to provide for rapid 

 transit railways from the business section to the outlying 

 residence districts of the city ; and to provide conduits for sew- 

 age or subways for water and gas mains, sewers, wires, etc. 

 Within recent years the greatest works of tunneling under city 

 streets have been designed and carried out to furnish improved 

 transit facilities. 



Conditions of Work. The construction of tunnels under city 

 streets may be divided into two classes, which may be briefly 

 defined as shallow tunnels and deep tunnels. Shallow tunnels, 

 or those constructed at a small depth beneath the surface, are 



