234 TUNNELING 



resulted in the City Council suspending their contract and 

 ordering the Board of Public Works to go ahead with the 

 work. 



The first step to be taken by the engineer was to purchase 

 adequate pumping machinery and empty the tunnel. This was 

 effected Jan. 17, 1894 ; and as soon as practicable thereafter the 

 two bulkheads were removed and the tunnel cleaned, tram-car 

 tracks laid, and everything prepared for work. It was now 

 determined to go ahead on the original line of the tunnel if 

 possible, and the bulkhead here was removed and work begun. 

 Meanwhile, a safety bulkhead had been built to replace the first 

 one torn away. This was provided with a door and drain- 

 age pipes. Work was begun on the original heading, but had 

 proceeded only a little way when the water broke in, driving 

 out the workmen. This was removed three or four times, when 

 the flow suddenly increased to 3,000 gallons per minute. An 

 examination of the lake bottom above the break showed that it 

 had settled down, indicating that the new break connected with 

 the lake bottom, and making further work along the original 

 line out of the question. 



The question now arose what it was best to do. It was 

 impracticable to use a shield, as the material ahead of the break 

 required blasting, and the pressure from above was enormous. 

 On account of its expense and difficulty of application the 

 freezing process did not seem advisable, and the plenum process 

 was likewise out of the question on account of the great 

 pressure which would be required at this depth. The detour 

 to the south which had been made by the contractor had been 

 unsuccessful, and had left the ground in a treacherous condi- 

 tion. To depress the tunnel was not advisable, for it was not 

 by any means certain that the bed of gravel could be avoided 

 in that way ; and, moreover, it would be necessary to ascend 

 again further on, and thus leave a trap which would effectually 

 cut off escape to those at work on the face if water again broke 

 into the tunnel. 



