236 



TUNNELING 



Upon examining the lake bottom a cavity over 60 ft. deep and 

 10 ft. in diameter was found directly over the end of the tunnel, 

 which had been caused by the gravel breaking into the tunnel. 

 Having now reached an elevation where it was possible to use 

 compressed air, it was determined to put in double air-locks 

 and use the plenum process. The locks were built, and some 





Longitudinal Section' Showing Method of Y v 

 Construction in Rock Covered with Quicksand. 



Sketch "a". 



Section A-B-C-D. 

 Sketch "c". 



Wi (Jay Packing 



Longitudinal Section of Tunnel . 



Sketch. "!>'. 



Cross Section Showing Manner of 

 Constructing Lining around Boulder. 



Sketch U" 



FlG. 121. Sketch Showing Methods of Lining, Milwaukee Water- Works Tunnel. 



670 cu. yds. of clay were dumped into the hole in the lake 

 bottom. On Aug. 4 the air-locks were tried with 26 Ibs. air 

 pressure ; but, upon a temporary release of the pressure, the 

 water passed around the locks and back of the tunnel lining 

 for some distance, and even forced through the lining, carrying 

 considerable clay and fine sand with it. Upon sounding the 



