Engineering Wonders of the World
Volume I

166 ENGINEERING WONDERS OF THE WORLD. BUILDING OUT THE ARCHES ON THE CANTILEVER PRINCIPLE. {From a Sketch by Howard Penton.) The arch ribs were supported by stout cables carried over temporary towers built on the piers. bed-rock, which was not reached until the shaft had been sunk 127 feet below high-water. On the St. Louis side the task was less for- midable, a depth of 47 feet being sufficient. It was here that work upon the bridge was started, and the initial undertaking was the sinking of a coffer-dam of sheet piling to , , enclose a site for the founda- Work begun. „ , tions of the abutment. The piles had to be driven through made ground, at a point where the old steamboat wharf had been widened, and luck decided that the engineers should hit on the spot beneath which the wreck of three vessels, of 300 or 400 tons, lay upon the river bed. These hulks had been sunk beside the old levee during a disastrous conflagration in 1849, and were now found right down upon the rock, their position be- ing due to “ scour ” of the current, and lying, Vagaries of the Missis - sippi. as regards two of them, the one across the other. To illustrate the action of the scour, and, at the same time, the thoroughness with which the chief engineer demonstrated his case, an extract may with advantage here be taken from Eads’s famous Report. Treating of the necessity for resting tho piers of the bridge firmly upon the rock, he says : “ It is a fact well known to those who were engaged in navigating the Mississippi twelve years ago that the cargo and engine of the steamboat America, sunk 100 miles below the mouth of the Ohio, was recovered, after being submerged twenty years, during which time an island was formed over it and a farm estab- lished upon it. Cottonwood trees that grew upon the island attained such size that they were cut into cord wood and supplied as fuel