The Lyles School of Civil Engineering is leading a project with the Indiana Department of Transportation to learn how to use a type of bridge pile that is typically used in offshore applications. Although these piles have been used in a few states for major onshore structures, much uncertainty remains as to how best to design them.

The piles are being used in the foundations of a bridge spanning the Wabash River near the Purdue campus in West Lafayette. Called large, open-ended pipe piles, the lengths of steel pipe (about 100 feet long and two feet in diameter) will make up the bridge’s center piers.

Although data is still being collected, the results thus far have been quite encouraging, says researcher Rodrigo Salgado, the Charles Pankow Professor in Civil Engineering.

Leading the study with Salgado is Professor Monica Prezzi. The project also involves engineers at the Indiana Department of Transportation through the Joint Transportation Research Program at Purdue.

The team built a specially designed, double-wall test pile containing an inner and outer pipe, one slid into the other. Because the two segments are connected only at the top, they can deform independently, a design that makes it possible to measure precisely and separately the forces exerted by the soil on the internal and external surfaces.

“We are extremely pleased with what we have learned,” Salgado says. “When you design a bridge, you have to estimate what the live loads (such as traffic and wind) will be. It is very rare to be able to measure these loads at such a large scale ahead of time.”

Sensors are placed on both inner and outer walls of the test pile, allowing the researchers to measure how much of the load is transferred to the soil as the pile is being driven into the ground and also later, during static load tests. At the same time, the researchers are examining a second test pile that has a closed end, allowing a direct comparison. Unlike the open-ended piles, the closed-ended pile contains a pointed shoe at its base.

The more than 200 sensors used to instrument the piles will be linked to a data acquisition system by 24,000 feet of electrical cables.

“If the loads are not the same at different locations, it’s because some load along this length was transferred to the soil, which is what you want to happen,” Prezzi says. “By measuring the strains at different cross-sections, you can determine how the load is decreasing down the pile as it’s being transferred to the soil.”

The fabrication of the double-wall, open-ended pipe pile and the instrumentation of both test piles were performed at Purdue’s Robert L. and Terry L. Bowen Laboratory for Large-Scale Civil Engineering Research.