Arizona DOT using new technique to combat freeway subsidence on Loop 101


The Arizona Department of Transportation (ADOT) is using a new technology approach to combat the problems of freeway subsidence without digging up the road surface, saving time and avoiding major traffic disruptions.

The agency is using the technique along Loop 101 (Pima Freeway) in the Scottsdale area, where ADOT is injecting a foam that hardens as it expands to stabilize the soil and lift the sunken section of road surface. ADOT crews have removed a minor roadway dip without the disruption of digging up and replacing part of the freeway.

The foam injections are being done as part of the ongoing ADOT project to widen and improve Loop 101 between Shea Boulevard and Loop 202 (Red Mountain Freeway). Repairing the depression in the freeway’s northbound lanes near McDonald Drive is one of the project team’s last work items before completing the addition of a new layer of smooth rubberized asphalt in that area. In addition to the current soil-stabilization work, the Loop 101 widening project is adding new outside lanes and other improvements along an 11-mile (17.7km) stretch of the freeway, with the entire project scheduled for completion by this fall.

Previously ADOT and its contractors have used the expansion foam at other freeway locations where the ground and road surface have settled. It has also been used a number of times to lift concrete slabs at either end of freeway bridges. Before the foam injection is scheduled, ADOT’s geotechnical staff have the job of examining the area where the ground beneath the freeway has settled, to determine if the use of the foam is the best course of action. More advanced soil testing is then done using equipment to map the ground where settling has occurred, which helps give crews the information they need to inject the foam and stabilize the soil.

“This is certainly less disruptive than digging up the freeway to do reconstruction work,” said Steve Boschen, director of ADOT’s infrastructure delivery and operations division. “It is a high-tech fix for long-term ground settlement beneath a freeway. Expansion foam isn’t the answer in all cases, but we’ve had some great success in offsetting the effects of ground settlement.”

Dan Haskins, ADOT’s resident engineer on the Loop 101 widening project, commented, “It’s already made a noticeable difference with the dip that drivers may have noticed as they traveled northbound on Loop 101, near the Arizona Canal, crossing north of McDonald Drive. We’re able to work during overnight hours with lane closures. The contractor drills very small holes in the freeway pavement and crews are then able to run narrow hoses to a depth underground where open space in the soil can be filled with the expansion foam. In this case we’re going as far as 30 feet (10m) beneath the freeway.”

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Tom has edited Traffic Technology International (TTi) magazine and its Traffic Technology Today website since May 2014. During his time at the title, he has interviewed some of the top transportation chiefs at public agencies around the world as well as CEOs of leading multinationals and ground-breaking start-ups. Tom's earlier career saw him working on some the UK's leading consumer magazine titles. He has a law degree from the London School of Economics (LSE).