OPEN-CUT TUNNELING METHODS 183 



"Trenches about 12 ft wide shall be excavated across the 

 street to as great a distance and depth as is necessary for the 

 construction of the subway. The top of this excavation shall 

 be bridged during the night by strong beams and timbering,, 

 whose upper surface is flush with the surface of the street. 

 These beams shall be used to support the railway tracks us well 

 as the ordinary traffic. In each trench a small portion or slice 

 of the subway shall be constructed. Each slice of the subway 

 thus built is to be properly joined in due time to the contiguous 

 slices. The contractor shall at all times have as many slice- 

 trenches in process of excavation, in process of being filled with 

 masonry, and in process of being back-filled with earth above 

 the completed masonry, as is necessary for the even and steady 

 progress of the work towards completion at the time named in 

 the contract" 



In regard to the success of this method Mr. Carson, in his 

 fourth annual report on the Boston Subway work, says : 



" The method was such that the street railway tracks were 

 not disturbed at all, and the whole surface of the street, if de- 

 sired, was left in daytime wholly free for the normal traffic." 



Tunnels on the Surface. It occasionally happens when 

 filling-in is to take place in the future, or where landslides 

 are liable to bury the tracks, that a -railway tunnel has to be 

 built on the surface of the ground. In such cases the construc- 

 tion of the tunnel consists simply in building the lining of the 

 section on the ground surface with just enough excavation to 

 secure the proper grade and foundation. Generally the lining 

 is finished on the outside with a waterproof coating, and is 

 s uii'-timcs banked ;uid partly covered with earth to protect the 

 masonry from falling stones and similar shocks from other 

 A recent example of tunnel construction of this char- 

 u t r \\MN .l.-s.-rilird in "Engineering News" of Sept 8, 1808. 

 In constructing the Golden Circle Railroad, in the Cripple Creek 

 mining district of Colorado, the line had to be carried across a 

 valley used as a dumping-ground for the refuse of the surround- 



