Professors devise new method for slope stabilization

News November 21, 2000
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Plastic pins are at the heart of a system for repairing and maintaining ramps, embankments and other slopes along U

Plastic pins are at the heart of a system for repairing and maintaining ramps, embankments and other slopes along U.S. highways. The technique, developed by two civil engineers at the University of Missouri at Columbia, could cut the cost of shoring up these slopes by 25% or more.

"Slopes are used to build up highways, exit ramps and overpasses," said Assistant Professor Erik Loehr, whose specialty is slope stability. "But no matter how well they’re built, slope failure is common. Due to the high cost of current techniques, work crews often just push the failed soil back up the slope, only to have it fail again. Our goal is to create an affordable, permanent solution."

The system conceived by Loehr and Associate Professor John Bowders supports the slope by driving recycled plastic pins (RPPs) into the ground to keep the soil from sliding down the bank. The 4-in. x 4-in. pins are made of a variety of recycled plastics, saw dust and other byproducts. They range in length from 8 ft to 18 ft and are driven into the ground with a hydraulic hammer drill. The pins are arranged in a grid with a spacing of 3 ft and alternating rows offset.

The professors contend that their scheme is less invasive and faster than traditional methods as well as being less expensive. They say the average cost of using RPPs is $4 per sq ft, compared with $20 per sq ft for a method where holes are drilled and filled with grout and steel bars.

Loehr and Bowders have successfully completed phase one of a three-phase program to test their RPP stabilization technique. They tested RPPs on two slope failures along I-70 near Emma, Mo., and both have met all expectations. Phase two is now under way. In phase two, the system will be tested in a variety of soil types, slopes, depths and pin sizes and arrangements at a number of sites around Missouri.

Final analysis and design recommendations will be completed in phase three.

Loehr and Bowders hope to have the method in wide use by 2004. "For years, highway maintenance crews have been rebuilding slopes over and over again because it was too expensive to stabilize them with traditional methods," Loehr said. "This technique finally will allow them to afford what they’ve wanted to do all along."

Funding for the RPP slope stabilization system came from a grant from the Missouri Department of Transportation. Tamko Composite Products in Lamar, Mo., provided the RPPs.

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