A Chapter on Foundations. 287 



At tbe building of Skilligalee Light House, a great deal of dif- 

 ficulty was encountered. The shoal upon which the light is situ- 

 ated is in the northern end of Lake Michigan some miles from the 

 east shore and is composed of gravel and boulders. As it was 

 impossible to drive piles and the mass so irregular in consistence 

 other means had to be employed in order to get a safe foundation. 



The area was first enclosed by a secure cribdam, inside of this 

 an iron cylinder sufficiently large to enclose the foundation of the 

 tower was placed, open at top and bottom. It was supposed at 

 first that the water could be kept down by means of pumps and 

 the excavation be made it the open cylinder. It soon appeared 

 that this was impossible; sheet piling was out of the question, 

 and no matter how tight the cribdam might be made, the leaks 

 through the boulders and gravel from below would still remain, 



A diaphragm was constructed in the cylinder and the excava- 

 tion continued under the plenum process until the cylinder 

 reached about 13 feet below the lake water level. 



Stone and olher materials were passed through an air lock to 

 the workmen below, and the lower part of the cylinder was built 

 up with solid masonry until there was sufficient weight to exclude 

 the water ; the diaphragm was then cut away and the work con- 

 tinued from above. The unavoidable manner of operating 

 brought the work into what we know at present "caisson," an ex- 

 ceedingly expensive and tedious plan, and, as in the case above 

 mentioned, should only be a last resort when no other plan will 

 promise success. 



The latest wonders in construction are now before us. One 

 the great steel arch bridge so successfully completed across the 

 Mississippi at St. Louis, and the other now in process of construc- 

 tion across the East river at New York. In the case of the St. 

 Louis bridge, the certainty of reaching bed rock was a great in- 

 ducement to the engineer to adopt the caisson and place his work 

 thereon, although a stratum of good earth was found under the 

 river drift, which continued the same to the bottom. Had it been 

 impossible to reach rock, a coffer-dam enclosure would have 

 enabled the excavation to be made and bearing piles to be driven 

 and would, in my estimation, have made a foundation as safe as 

 the present one. 



