186 TUNNELING 



Another serious diffioulty of tunneling under city streets 

 arises from the danger of disturbing the foundations of the 

 adjacent buildings. This danger exists only where the depth 

 of the tunnel excavation extends below the depth of the build- 

 ing foundations, and where the material penetrated is soft 

 ground. Where the tunnel penetrates rock there is no danger 

 of disturbing the building foundations. To prevent trouble of 

 this character requires simply that the excavation of the 

 tunnel be so conducted that there is no inflow of the surround- 

 ing material, which may, by causing a settlement of the neigh- 

 boring material, allow the foundations resting on it to sink. 



The Baltimore Belt tunnel, described in a preceding chap- 

 ter, is an example of the method of work adopted in construct- 

 ing a tunnel under city streets through very soft ground. 

 This may be classed as a deep tunnel. Another method of 

 deep tunneling under city streets is the shield method, ex- 

 amples of which are given in a succeeding chapter. Two 

 notable examples of cut-and-cover methods of tunneling are 

 the Boston Subway and the New York Rapid Transit Ry., a 

 description of which follows. 



Boston Subway The Boston Subway may be defined as the 



underground terminal system of the surface street railway 

 system of the city, and as such it comprises various branches, 

 loops-, and stations. The subway begins at the Public Garden 

 on Boylston St., near Charles St., and passes with double 

 tracks under Boylston St. to its intersection with Tremont St., 

 where it meets the other double-track branch, passing under 

 Tremont St. and beginning at its intersection with Shawmut 

 Ave. From their intersection at Tremont and Boylston streets 

 the two double-track branches proceed under Tremont St. with 

 four tracks to Scollay Square. At Scollay Square the subway 

 divides again into two double-track branches, one passing 

 under Hanover St., and the other under Washington St. At 

 the intersection of Hanover and Washington streets the two 

 double-track branches combine again into a four-track line, 



