(i HUMAN METHOD 



153 



rily of five rings of brick, but at some places in especially un- 

 stable soil eight rings of brick were employed. The arch was 

 built in concentric sections about 18 ft in length. All the 

 timl>er of the strutting above the arch and outside of the side 

 walls was left in place, and the voids were filled with rubble 

 masonry laid in cement mortar. It required about 125 mason 



Fio. 19. K<of Arcti Construction with Timber Ccntem. Baltimore Itolt Line Tunnel. 



hours to build an 18-ft. arch section. Figs. 7l and so show 

 various details of the masonry arch work. 



Owing to the very unstable character of the soil, consider- 

 able difficulty was experienced in building the masonry invert. 

 The process adopted was as follows; Two parallel 12 X 12 in. 

 tiinl>er8 were first placed transversely across the tunnel, abutting 

 against longitudinal timbers or wedges resting against the side 

 walls. Short sheet piles were then driven into the tunnel 



