Geofoam embankments
Geofoam is expanded polystyrene molded into workable units, said Jeff Gebhard of Mn/DOT's Office of Materials. A rigid, low-density, cellular plastic-foam solid, it can be molded in any size and used for geotechnical applications. "Its low density is the key," he said.
Specs for the product are in ASTM d 6817; AASHTO is anticipated to announce another spec this summer with higher compressive strengths. Strength increases with higher density but so does the cost. "We try to design to the minimum compressive resistance needed for your project to keep the price down," he said.
Geofoam has a number of advantages. It is lightweight and decreases loads on the subsoil, and it can be placed very quickly: a four-person crew can place 100 yards per hour. Its versatility means it can be used in wall applications to reduce lateral loads or in landslide applications. And because it's not biodegradable, it works for long-design-life roads.
It does have some drawbacks, though. One is that it costs $35 to $65 per cubic yard (costs are lower closer to metro-area manufacturers). Limited supply can also be a concern, Gebhard said, and a staging area is needed on site for storage. Other concerns:
- Because it is damaged by UV light, geofoam must be covered or placed within a week.
- It is flammable, so the contractor and owner must keep that in mind. "It doesn't spontaneously combust, but next to a flame it can catch fire," Gebhard said.
- Petroleum will deteriorate the product, so it should be protected with a geomembrane or a concrete cap.
- It can become buoyant and rise through the pavement surface.
- Its insulation properties can cause differential icing if it is placed too close to pavement. By creating a dangerous bridge effect, geofoam will block heat from the soil that could otherwise reach the pavement and melt ice. Gebhard recommended covering geofoam with 18 to 24 inches of fill to allow heat to reach the pavement.
The first Mn/DOT job using geofoam was the TH 251 project near Albert Lea. The final bid was $65/yd3; all the geofoam was placed by hand and topped off by a geomembrane and then granular fill (a 6- to 12-inch layer to avoid damaging the geofoam or geomembrane). The road is performing extremely well with just normal transverse cracks, he said, even after 8 feet of flooding over the geofoam section during a 500-year flood, and there is no differential cracking over the geofoam.
NCHRP Report 529 has guidelines and standards: Guideline and Recommended Standard for Geofoam Applications in Highway Embankments (1.6 MB PDF). For further information, e-mail jeff.gebhard@dot.state.mn.us