190 Tl NNKI.INC 



platform covering erected, as shown by Fig. 112. Tliis roofing 

 work was also done at night. The subsequent work of build- 

 ing the roof arches, removing the remainder of the earth, and 

 constructing the invert, was carried on underneath the plat- 

 form covering which carried the street traffic in the meantime. 

 The successive repetition of the processes described con- 

 structed the subway. 



Where the traffic was very dense on the street above, tunnel- 

 ing was resorted to. For small portions of this work the ex- 

 cavation was done in the ordinary way, using timber strutting, 

 but much the greater portion of the tunnel work was performed 

 by means of a roof shield. In the latter case, the side walls 

 were first built in small bottom side drifts and were fitted with 

 tracks on top to carry the roof shield. The construction and 

 operation of this shield are described fully in the succeeding 

 chapter on the shield system of tunneling. 



Masonry. The masonry of the inclined approaches to the 

 subway consists simply of two parallel stone masonry retaining 

 walls. In the wide-arch and double-barrel tunnel sections, the 

 side walls are of concrete and the roof arches are of brick masonry. 

 In the other parts of the subway the masonry consists of brick 

 jack arches sprung between the roof beams and covered with 

 concrete, of concrete walls embedding the side columns, and 

 of the concrete invert and foundations for the columns. Figs. 

 109 to 112 inclusive show the general details of the masonry 

 work for each of the three sections. The inside of the lining 

 masonry is painted throughout with white paint. 



Stations. The design and construction of the stations for-- 

 the Boston Subway were made the subjects of considerable 

 thought. All the stations consist of two island platforms of 

 artificial stone having stairways leading to the street above. 

 The platforms are made 1 ft. higher than the rails. The station 

 structure itself is built of steel columns and roof beams with 

 brick roof arches and concrete side walls. Its interior is lined 

 with white enameled tiles. The intermediate columns are cased 



