Figure 44. — Opening of the St. Clair Tunnel, 1891. {Photo courtesy of Detroit Library, Burton Historical Collection!, 



shafts were started near both riverbanks, but before 

 reaching proper depth the almost fluid clay and silt 

 flowed up faster than it could be excavated and this 

 plan was abandoned. After this second inauspicious 

 start, long open approach cuts were made and the 

 work finally began. The portals were established in 

 the cuts, several thousand feet back from each bank 

 and there the tunneling itself began. The portions 

 under the shore were driven without air. When the 

 banks were reached, brick bulkheads containing air 

 locks were built across the opening and the section 

 beneath the river, about 3,710 feet long, driven under 

 air pressure of 10 to 28 pounds above atmosphere, 

 for most ol the- way. the cla\ was firm and there was 

 little air leakage. It was found that horses could not 



survive in the compressed air, and so mules were 

 used under the river. 



In the firm clay, excavation was carried on several 

 feet in front of the shield, as shown in the model (fig. 

 42). About twelve miners worked at the face. How- 

 ever, in certain strata the clay encountered was so 

 fluid that the shield could be simply driven forward by 

 the rams, causing the muck to flow in at the door 

 openings without excavation. After each advance, 

 the rams were retracted and a ring of iron lining 

 segments built up, as in the Tower Subway. Here, 

 for the first time, an "erector arm" was used for 

 placing the segments, which weighed about half a ton. 

 In all respects, the work advanced with wonderful 

 facility and lack of operational difficulty. Consider- 



238 



BULLETIN 240: CONTRIBUTIONS FROM THE MUSEUM OF HISTORY AND TECHNOLOGY 



