Civil engineers at Purdue University are leading a project with the Indiana Department of Transportation (INDOT) to learn how to use a type of bridge pile, often seen in offshore applications, that could help reduce the cost of bridge building and the replacement of aging spans.

The large open-ended pipe piles are lengths of steel pipe about 100 feet long and two feet in diameter that will be used in the foundations of a bridge spanning the Wabash River near Purdue's West Lafayette campus. An advantage of the pipe piles, which will make up the bridge's center piers, is that they can be driven into the riverbed's alluvial soil more easily than the traditional closed-ended piles, which will be used for the end piers.

An advantage of open-ended pipe piles is that they can be driven into a riverbed's alluvial soil more easily than traditional closed-ended piles. Image credit: Purdue University/Erin Easterling.An advantage of open-ended pipe piles is that they can be driven into a riverbed's alluvial soil more easily than traditional closed-ended piles. Image credit: Purdue University/Erin Easterling. The researchers have 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.

Large open-ended piles have been used in offshore applications such as wind turbines and oil platforms, and INDOT is now exploring how to use them more frequently for structures on land. Because the pile is larger than conventional piles, fewer of them are needed, potentially reducing bridge construction costs.

However, little is known about how the piles behave as they are driven deep into the ground. The process, depending on the density and stresses in the soil and on the internal diameter of the pile, can cause a dense plug of soil to form in the bottom of the pile, changing how it behaves. Moreover, as the open-ended pipe is driven into the ground, the soil causes differing resistance on the outer and inner walls of the pipe.

Two types of sensors placed at one-foot increments along the length of the pile will provide data to determine precisely how much strain is generated in the internal and external pipes of the double-wall pile. Sensors will be 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. Data will be considered in the final design of the foundations for the bridge.

"These open-ended piles derive their resistance from the complex interaction of the pile with the soil plug that enters it," says Rodrigo Salgado, professor of civil engineering. "We will learn how much soil goes into the pile and how much resistance you develop as the plug becomes denser. We will also learn how the plug affects the pile response during driving, under dynamic loads and during static loading through the load test that will be performed."

The researchers will also study 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. More than 200 sensors used to instrument the piles will be linked to a data acquisition system by 24,000 feet of electrical cable.

"If the loads are not the same at different locations, that's because some load along this length was transferred to the soil, which is what you want to happen," says Civil Engineering Professor Monica Prezzi. "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."

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