Mr. Eads Spans The Mississippi

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Because the proposed bridge would link Missouri and Illinois, enabling legislation from both states was required. A Chicago promoter named Lucius Boomer had already obtained a franchise from the Illinois legislature to build an iron truss bridge at East St. Louis. Assembling a meeting of hand-picked engineers, he got resolutions passed declaring that piers could not be founded to bedrock in the Mississippi, that spans of five hundred feet were impossible, and that the only sensible bridge to build would be a six-span truss. Boomer’s engineers ignorantly declared that bedrock foundations would not be necessary because scour—the abrasive action of the moving riverbed—did not extend “to a greater depth than thirty feet below low water.” Eads, who had spent a good part of his youth exploring the bottom of the Mississippi, knew better, and St. Louisans trusted his knowledge. “[Boomer] found himself opposed by one who possessed to an uncommon degree the confidence of his fellow citizens, and who united to the skill of an engineer great executive power and unusual resources as a financier,” observed Professor Calvin M. Woodward in his compendious History of the St. Louis Bridge , published in 1881. Without waiting for the legal tangle to unwind, Eads boldly commenced work on the west abutment. The sale of five million dollars’ worth of bonds on the New York and London financial markets depended on sufficient construction being carried out to inspire confidence in the project. Eads chose his site strictly on the basis of the bridge’s optimum location, which was where the fire of 1849 had raged. Consequently he had to plant the abutment in the midst of a fantastic jumble of steamboat debris on the river bottom. “The old sheet-iron enveloping their furnaces, worn-out gratebars, old fire-bricks, parts of smoke-stacks, stone-coal cinders and clinker, and every manner of things entering into the construction of a Mississippi steamer seemed to have found a resting place at this spot,” wrote Woodward. Two steamboats reposed, one on top of the other, the lower one shoved down through the mud to a point only two or three feet above bedrock. The wharf had been rebuilt, and the rubble dumped to support it pinned down the wrecks, making it impossible to refloat them.

The problem was how to build a cofferdam in this underwater junkyard. One of the oldest tools of bridge engineering, a cofferdam is an enclosure, sunk in a riverbed, which can be pumped out to expose the bottom and allow the construction of solid foundations. Eads devised a sort of monster chisel, with a steel blade and oak handle, to cut through the mass. The sheet piling of the cofferdam was thrust into the cleft. Despite every obstacle, including the hulls of four large barges buried in the mud, the cofferdam was completed, and the solid masonry of the west abutment began to rise from the bedrock, forty-seven feet below the city directrix (a marker on a downtown building showing the highest water in the city’s history).

With the construction of the west abutment, Boomer’s rival company gave up, and a deal was negotiated. By the expedient of adopting the rival’s name, the Illinois and St. Louis Bridge Company, Eads took over the indispensable Illinois franchise. But to win the necessary congressional approval and the support of the financial community, he had to demonstrate persuasively the superiority of his steel-arch design and the bedrock foundations. In a lucid and well-argued report he successfully did both. The foundation ‘question was more complicated than might appear, owing to the configuration of the bedrock at St. Louis. While the west abutment had reached bedrock a mere fortyseven feet below the directrix, the rock shelved away from west to east. The west pier, a third of the way across the river, would have to go much deeper than the west abutment, the east pier deeper still, and deepest of all would be the east abutment on the Illinois shore. The practical difficulties of working one hundred feet below the surface of the water had to be given serious consideration.

During the winter of 1867-68, Eads contracted a bronchial ailment and went abroad to convalesce. The leading French iron and steel firm, Petin, Gaudet & Cie., had considered bidding for the superstructure contract at St. Louis; Eads took the opportunity to show M. Petin, the head of the firm, his plans. Petin called in his chief engineer, M. Moreaux, who invited Eads to visit Vichy, where he was constructing a bridge over the Allier River. There Eads got his first look at a pneumatic caisson, one of the most noteworthy engineering developments of the nineteenth century.

French and British engineers, confronted with the problem of constructing deep foundations for bridges and harbor works, had developed the idea of sinking a huge box, full of compressed air, inside which men could work. The concept had been successfully tested at depths of eighty-five feet. To facilitate entry into the sealed box, Admiral Thomas (Lord) Cochrane had invented the air lock. The compressed air in the caisson kept the box from collapsing under the external pressure of water and mud. Ultimately, the caisson itself, filled in with rubble or masonry, became the base of the foundation. Eads’s discussions with the French engineers at Vichy convinced him that the pneumatic caisson, properly constructed, could be used at depths beyond any reached in Europe. He resolved immediately to sink the east pier foundation, the deeper of the two in-water foundations, in order to reassure the bond market that the bridge could be completed.