OPEN-CUT TUNNELING METHODS 



which runs under Washington St. to its terminus at Hay- 

 market Square, where it comes to the surface by means of an 

 incline. The subway, therefore, has three portals or entrances, 

 located respectively at Boylston St., Shawmut Ave., and Hay- 

 market Square. It also has five stations and two loops, the 

 former being located at Boylston St., Park St., Scollay Square, 

 Adams Square, and Haymarket Square, and the latter at Park 

 St. and Adams Square. The total length of the subway is 

 10,810 ft. 



Material Penetrated. The material met with in construct- 

 ing the subway is alluvial in character, the lower strata being 

 generally composed of blue clay and sand, and the upper strata 

 of more loose soil, such as loam, oyster shells, gravel, and peat. 

 At many points the material was so stable that the walls of 

 the excavation would stand vertical for some time after excava- 

 tion. Surface water was encountered, but generally in small 

 quantities, except near the Boylston St. portal, where it was 

 so plentiful as to cause some trouble. 



Cross-Section. The subway being built for two tracks in 

 some places and for four tracks in other places, it was neces- 

 sary to vary the form and dimensions of the cross-section. 

 The cross-sections actually 

 adopted are of three types. 

 Fig. 109 shows the section 

 known as the wide arch type, 

 in which the lining is solid 

 masonry. The second type 

 was known as the double- 

 barrel section, :in<l is shown 

 by Fig. 110. The third type 

 of section is .shown by Fi^. 111. The lining consists of steel 

 columns carrying transverse roof girders ; the roof girders 

 U-in-r tilled between with arches, and the wall columns having 

 concrete walls between them. The wide-arch type and the 

 double-barrel (V|M- of sections were employed in some portions 



100. Wide Arch Section, Bonton Subway. 



