Eads Bridge

Pneumatic Piling

The pneumatic process for installing bridge foundations is mentioned several times in documents related to Eads Bridge. In his 1864 report, St. Louis city engineer Truman J. Homer offered a cost estimate for a hypothetical bridge founded on pneumatically-placed piling. The bridge promoted by Eads' competitor Lucius B. Boomer featured pneumatic foundations. Eads himself, in his 1868 Report of the Engineer-in-Chief, discusses pneumatically-placed foundations before concluding that they are not appropriate for his bridge. [1]

Speaking at the convention of civil engineers that convened in St. Louis in August 1867, C.L McAlpine described the pneumatic piling used at the recently-completed (1865) Harlem River Bridge, in New York. The piles were hollow cast iron cylinders, about 6 feet in diameter. Their bottom end was open and provided with a beveled cutting edge to help it dig into the riverbed. The top was capped and fitted with an airlock. After sinking a pile to the riverbed, compressed air was applied to displace the water from inside it. After the pile had been emptied of water, the compressed air was quickly released, causing the water to flood back. The inrush scoured material from below the cutting edge, undermining the pile and causing it to drop, sometimes as much as 9 or 10 feet in a single stroke. After the pile stopped settling, compressed air was applied again, forcing the water out so that workers could enter through the airlock to excavate the mud and sand that had been carried in by the pressure release. This cycle was repeated as needed until the pile reached the required depth. After emplacement, the piles were filled with concrete.[3]

Arrays of cylinders could be installed as piling, with their caps located below the level of the riverbed to create a platform on which to construct a masonry pier, or cylinders could extend all the way up to a bridge's deck to directly support the superstructure.

According to James Eads, pneumatic piling had several drawbacks which rendered it problematic for deep foundations in fast-moving water. Because they were relatively slender, they were subject to buckling. Below the riverbed this was mitigated by the support of the surrounding sand and mud, but this support would be lost if the riverbed eroded. Above the riverbed, the slender piers could be reinforced by a system of bracing connecting adjacent cylinders, but this bracing was subject to corrosion and was vulnerable to damage by unequal settlement of adjacent piles.[2]

Kansas and Missouri Bridge, across Missouri River near Leavenworth, Kansas, completed 1872.

From the journal Scientific American, March 15, 1873

The Kansas and Missouri Bridge, above, is typical of bridges using pneumatically-placed foundations. The piers were iron cylinders, installed using the pneumatic process and carried up to deck level. Masonry cutwaters were constructed around the piers to protect them from floating ice and other debris.