226 TUNNELING 



instances ; but the caisson method, although proposed for sev- 

 eral projected works, has never actually been employed. The 

 Van Buren Street tunnel, built to carry a double-track street 

 railway under the Chicago River, was completed in 1894 by 

 the cofferdam method. The special features of the tunnel * 

 are : (1) the unusually large dimensions of the cross-section of 

 30ft. X 15ft. 9 ins.; (2) its construction inside of coffer- 

 dams of great length and wdith ; (3) the construction under 

 some very high buildings calling for great care and very strong 

 temporary and permanent supports. 



The special feature of the work for our present purpose 

 was the construction of the tunnel across the river. To accom- 

 plish this a cofferdam was built out from the west shore of the 

 river to its middle, and the tunnel constructed within* it like 

 the building of any other structure within a cofferdam. Trans- 

 verse and longitudinal sections of this cofferdam are shown by 

 Fig. 119. As will be seen, it was a simple double-wall coffer- 

 dam, with a clear width between the walls of 58 ft., and braced 

 transversely as shown. Inside of this a single-wall cofferdam 

 of piles was constructed, with a clear width just sufficient to 

 allow the construction of the masonry within it. When the 

 tunnel end reached the channel end of the cofferdam, a crib-wall 

 was built over the end of the completed tunnel, as shown by 

 the drawings. This crib wall was intended to form the end 

 wall of another cofferdam, which was built out from the east 

 shore, and within which the remaining half of the tunnel was 

 built as the first half had been. The drawings show the char- 

 acter of the tunnel masonry and of the centering upon which 

 it was built. 



In this connection it will be interesting to mention briefly 

 the most pretentious proposition for tunnel construction by 

 means of caissons. Some years ago, Prof. Winkler proposed 

 to construct a tunnel under the River Danube to connect the 

 various portions of the Vienna, Austria, underground railway, 



* " Eng. News," April 12, 1892. 



