The California Dept. of Transportation is nearing completion of a $5-million experimental project using a plate-pile system to stabilize almost 2 miles of embankment at an interchange near Colusa, Calif. Caltrans officials say the method may shave $3 million off the cost of a conventional method.

Caltrans is using a proprietary system in which steel plates are affixed to poles, or piles, and inserted into stable strata underlying loose soil, transmitting the slide forces downward to the stiffer material.

The technique has been used on several municipal projects but is a first for Caltrans, says Richard Short, founder and president of Slope Reinforcement Technology (SRT), Danville, Calif.

Areas of expansive clay fill embankments on steep slopes along Interstate 5 where it meets Route 20, causing creep, says Caltrans transportation engineer Mark Hagy. This creeping leads to tension cracks in the road, because water seeps in and erodes the pavement.

Bill McCormick, engineer with Kleinfelder, San Diego, studied stabilization options, including excavation and rebuilding to slope gradient, rock slope protection, lime treatment and building retaining walls. The least expensive option, at $3.4 million—less than half the price of some other alternatives—was installing the plate piles.

The plate piles are ¼ –in.-thick, 1-ft- wide steel plates welded onto 2 ½ -in. or 3-in.-thick steel angle poles, which are then pounded into the slope to create lateral resistance. The test project uses 2,810 6-ft-long and 6,325 10-ft-long plate piles, which crews place at 4-ft to 10-ft intervals in a staggered pattern. The method has the benefit of not requiring additional rights-of-way or intrusion into wetlands.

Stabilization work began in April. Petaluma, Calif.-based North Bay Construction is using customized hydraulic hammers to install as many as 400 plate piles a day. At that rate, the project may be complete in August.

“Plate piles were not only the lowest cost solution,” says Hagy. “In some cases they were the only viable solution because of space and time limitations.” Getting approval to do slope flattening in environmentally sensitive areas would have set the project back considerably.

Caltrans estimates the installation cost at $200 per ft of embankment width versus $460 per ft for flattening. If the test project performs as expected, the plate-pile system could be added to Caltrans’ prequalified products list, which requires a new product to be environmentally friendly and fulfill an ongoing need.