Ever since humans first began building roads, dikes and other projects, they have commonly used soils for fill. Earthen fills are low cost and abundant, yet they weigh a lot and require heavy equipment to move and compact. Furthermore, like all natural materials, they can be varied and inconsistent, which can induce differential settlement.

As an alternative, project teams are increasingly using geosynthetic materials, such as expanded polystyrene (EPS) block geofoam. The Federal Highway Administration (FHWA) describes EPS geofoam as a “lightweight, rigid foam plastic that has been used around the world as a fill for more than 30 years.” It is substantially lighter than soil, has predictable stress/strain curves and can be installed during adverse weather.

The FHWA, state DOTs and private engineering firms have specified EPS geofoam for various infrastructure projects. Applications that are suitable for EPS geofoam include roads and highways, bridge approaches, retaining walls, embankments, railways, runways and taxiways, levees, utility lines and other large public works projects. In addition to its lightweight features, EPS geofoam can also help solve some common geotechnical challenges, including soft soil remediation, lateral load reduction upon retaining structures, slope stabilization, buried utility protection and structural void fill.

Taking it Lightly

According to the EPS Industry Alliance, EPS geofoam is approximately 98 percent air, making it much lighter than other fills. It weighs only about 1 to 3 pounds per cubic foot- as much as 100 times lighter than soil (see Table 1)- yet is durable and has high, predictable compressive strengths.

Because of its low weight, EPS geofoam is easy to place by hand. Geofoam changes the traditional soil compaction phasing method because it comes out of the manufacturing facility with independently verified elastic modulus values. Additionally, since one truckload contains 120 cubic yards (equal to 12 dump trucks of earthen fill), it can help reduce construction traffic and transportation costs.

EPS geofoam has high load-bearing capacities, with compressive resistance values ranging from a minimum of 316 psf to 2,678 psf at 1 percent deformation, which is considered the conservative elastic limit stress. As long as combined dead/live loads do not exceed 1 percent strain, the material will never creep or experience plastic yield. Geofoam has been succesfully used as a sub-base material for pavement sections that bear the live loads of locomotives and jumbo jets.

Remediating Soft Soils

On sites with soft, compressible foundation soils, such as peat or soft clay, lightweight EPS geofoam can help enable the construction of roads and building foundations.

The embankment construction for Seattle’s Alaskan Way Viaduct replacement is one example of how EPS geofoam helped deal with soft soils. The ramps sit on a tide flat the city reclaimed with imported fill more than a century ago. Project engineers had to ensure the new ramps would not induce settlements on the underlying soils, which could impact the stability of adjacent elevated structures. EPS geofoam provided the necessary load support at a low weight and eliminated the need to surcharge the soil.

Another project with similar challenges was the widening of the I-80 and I-65 interchange in Gary, Ind. Located at Lake Michigan’s southern end, the site has soft soils from ancient glacial activity. The FHWA recommended a net-zero load calculation of the roadbed to prevent post-construction settlement.

Contractors installed 32 flatbed trucks of EPS19 blocks, which was equivalent to more than 400 dump truck loads of earthen fill. This reduced construction traffic in a highly congested area leading in and out of Chicago, and it helped crews maintain a tight construction schedule. In addition, the project manager reported installation labor rates of approximately 35 cubic yards per man hour.

Cutting Lateral Loads

EPS geofoam can reduce or eliminate lateral loads on retaining structures. The blocks are installed in a manner to replace the sliding soil wedge above the angle of repose. The lower lateral loads can reduce both labor and material costs of the retaining structure by requiring less over-excavation, robust forming, structural steel and concrete wall thickness, and footings. They can also reduce or eliminate geogrids or mechanical tie-backs.

Use of EPS geofoam also allows for taller walls in more narrow rights-of-way, which can reduce property acquisition time and costs, as well as minimize lane closures and other construction impacts. EPS geofoam can be constructed with a vertical face with much lower-cost fascias that act more like a “fence” then a retaining wall.

The Pacific Street Bridge over I-680 in Omaha, Neb., is one project that relied on EPS geofoam to reduce lateral loads. Typical bridge widening projects require the exisiting abutment walls to be torn down and replaced because they are not designed to withstand the increased lateral loads induced by the fill for the new lanes. By excavating the soil between the existing abutment wall and the soldier piles, the contractor was able to simply form and extend the existing wall. Approximately 2,000 cubic yards of EPS15 geofoam were installed, which allowed the bridge to be reopened to traffic in only three months.

Stabilizing Slopes

In places where landslides are potential risks under roads and railways, EPS geofoam can create stable slopes without needing to change the final slope geometry. Since it is so much lighter than soil, it reduces the weight of a slope’s driving block and thereby mitigates the risk of slides. As such, it helps decrease long-term maintenance costs by addressing the root cause of slope failure, which is gravity.

Further, because crews can manually lift and place individual EPS geofoam blocks without heavy earth moving or compaction equipment, it is easier to construct stable slopes on steep or uneven terrain-or where access is difficult.

Among the hillside road projects that utilize the material for slope repair are eight different failures on U.S. 101 in northern California; U.S. 50 near Montrose, Colo.; and Highway 12 near White Pass in Washington.

Protecting Buried Utilities

Many construction projects must account for the presence of existing utilities. For example, in Seattle, contractors used EPS geofoam to build a tightly spiraled ramp on Royal Brougham Way that crossed over large stormwater and sewer mains. One of those was a century-old, brick-lined concrete pipe with timber piles that would have been at risk for failure under heavy soil fills.

A recent study by Dr. Steven Bartlett of the University of Utah concluded that using EPS geofoam as a soil-replacing cover on top of buried pipelines not only reduces in-situ vertical/lateral stresses, but it also decreases axial strain and large bending stresses on the pipe during seismic events.

Filling Structural Voids

EPS geofoam is also used as a structural void fill in concrete forming operations. Crews can fabricate virtually any shape or slope, and the material eliminates separate concrete pours for vertical wall sections and the topping slab. These fills are used in stadium seating in movie theaters and sports arenas, stairways, podiums, loading docks, rooftop pool decks and compartment walls in water treatment plants.

As with other lightweight fills, the unit cost of EPS geofoam can be higher than traditional materials, but as a report prepared for the Transportation Research Board notes, this is “usually more than offset by savings when overall project costs are considered.”

When specifiying EPS geofoam, it is important to ensure the material is manufactured to meet the ASTM D6817 “Standard Specification for Rigid Cellular Polystyrene Geofoam.” It should be noted that at present only two manufacturers maintain a third-party certifcation program through Underwriters Laboratories (UL) to ensure physical property compliance with this standard.

It is also worth asking the manufacturer about the level of technical support and services they offer, as these vary widely. Some manufacturers can assist project teams with layout and installation shop drawings, value engineering, job start-up, field fabrication tools and training, as well as produce specific block sizes to best fill project geometry.

The next time you need fill for a project, maybe it’s time to look beyond the dirt? EPS geofoam can lower the total project cost, provide faster construction sequencing and minimize post-construction settlement.

More online

For more information on EPS geofoam, visit www.insulfoam.com, the Syracuse University Geofoam Research Center at http://geofoam.syr.edu eor the EPS Industry Alliance at http://eps industry.org/other-applications/geofoam.