Research Looks at Piles that Bounce Back

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Building Bridges in Florida Poses a Unique Set of Engineering Challenges

Engineering Florida’s highways is a bit different than other parts of the country given that bridges have to withstand hurricane-force winds. To do this, the pilings used to build these structures are extremely large and require enormous impact hammers to drive them in. One of the challenges in installing these pilings is that Florida soil (in most places) is made from sand, which “grabs” piles in an unusual way and in fact can bounce or push pilings back upward as much as three inches following each impact of the hammer.

“Engineers and inspectors who witness this phenomenon are amazed that piles weighing over 100,000 pounds can bounce like a basketball,” says civil engineering professor Paul Cosentino, who has been researching the phenomenon for almost 10 years. “When it occurs it can stop the installation process.” Needless to say, he continues, “The pile driving contractors question the engineers’ designs and the engineers in turn are not clear on the pile engineering performance in terms of the loads they can resist.”

With funding thorough the Florida Department of Transportation, Cosentino’s findings have led to the development of a decision tree that engineers can follow to avoid the bounce, saving the state millions of dollars in engineering trail and error.

But there is still more work to be done on this complex problem. The FDOT recently awarded Cosentino a total of $328,000 for two new projects related to building bridges in Florida’s unique sand and silt.

For the first project, Cosentino and doctoral student Brian Wisnom will study soil data from around Florida to develop correlations between FDOT pile data and various soil types. Knowing these correlations can help engineers recognize potential complications in a particular area and make adjustments before work begins.

The second grant, with help from co-PIs Matthew Jensen in Mechanical Engineering and Charles Bostater in the Department of Ocean Engineering Sciences, is to evaluate the actual pile movements during hammer impact in Florida sands using accelerometers, lasers and high-speed cameras.

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