Who says big solutions can’t come in lightweight packages?

Earthquakes and soil movement are potentially big problems for a pipeline. According to the U.S. Geological Survey, there are 14,000 earthquakes worldwide each year that have a magnitude of 4 or greater—700 of which occur in the U.S., including Alaska.

There are several methods for mitigating the effects of seismic activity on infrastructure, one of which is EPS Geofoam, a polystyrene geotechnical product used in fill applications where a lightweight material is required to reduce stresses on underlying soils or lateral pressures. It forms a seismic buffer for buried structures and rigid retaining walls.

Six years ago, Canadian engineers Richard Bathurst, Saman Zarnani and Andrew Gaskin showed with testing and numerical modeling that Geofoam could reduce the seismic forces on rigid retaining walls. The lightweight EPS blocks that have become ubiquitous with highway embankments, green roofs, and landscape fill are growing in popularity for seismic and other buried applications. Today the spotlight is shining on Geofoam as a material with great potential for protecting pipelines.

“If an earthquake occurs, high-pressure gas lines are one of the most important items to protect,” said Steven Bartlett, associate professor of civil engineering at the University of Utah. “If they rupture and ignite, you essentially have a large blowtorch, which can be catastrophic.” Bartlett and his team have been examining Geofoam’s mitigating effects on pipeline damage due to seismic faulting since 2007.

Seismic research

Geofoam weighs roughly one-100th of the weight of soil. “During the summer of 2007, Questar Corp. requested that the University of Utah evaluate a conceptual EPS Geofoam cover system for a steel, natural gas pipeline crossing the Wasatch fault in the Salt Lake Valley,” explained Bartlett. “The fault rupture is expected to produce an earthquake with a potential magnitude of 7.5—and several feet of potential fault offset—at the pipeline crossing.”

If a major earthquake were to strike the Wasatch fault zone in the Salt Lake Valley, the fault displacement and the subsequent weight of shifting and compacted soil on buried pipelines is likely to cause them to rupture.

Many buried pipelines lie beneath six to eight feet of soil. Bartlett and his students at the University of Utah showed that a pipeline protected with a lightweight Geofoam cover could withstand the fault offset and reduce the force on the pipe by as much as four times the amount of force as a pipeline covered with conventional soil backfill.

When a 37-mile section of gas pipeline had to be replaced between Coleville and Ogden, Utah, approximately 20,000 cubic feet of EPS Geofoam was specified to reduce movement, shears, axial forces and strains imposed on the pipeline. EPS types 22 and 15 were shipped from ACH Foam Technologies’ local plant in Murray, Utah.

The goal of a Geofoam cover system on top of a buried pipeline is to reduce the lateral, longitudinal and vertical forces induced on the pipe as the surrounding ground undergoes deformation. The properties of Geofoam have distinct advantages that lead to improved pipeline performance during large ground deformation.

There are two main advantages that Geofoam has over traditional earth cover materials. First is Geofoam’s lowmass density—which reduces the vertical and horizontal stresses on buried utilities and compressive soils. This reduction in loading and deformation will likely improve the performance of a pipeline during and after a major seismic event along the fault area.

The second advantage of Geofoam is its use as a compressible inclusion for systems undergoing static, monotonic and dynamic loadings. Geofoam’s controlled compression can be used to reduce earth pressure against buried structures as well as deformation induced by structural loadings.

Bartlett’s team confirmed that the loadings that cause compression may include static and dynamic lateral earth pressure swells, frost-heave pressures, settlement of support soils, faulting, liquefaction, landslides and traffic loads.

Membrane protection

In some cases the Geofoam blocks are covered with a geomembrane. This membrane helps to reduce the vertical uplift stress by reducing the friction force between the Geofoam and the trench sidewall. In addition, placing a geo-membrane around the Geofoam block will provide added protection against a potential petroleum spill.

EPS Geofoam also has been used for a number of large transportation projects in Utah. It proved to be an incredibly time- and cost-saving material for embankments along Interstate 15 in Utah. After realizing the benefits of using Geofoam for the interstate highway, the Utah Transit Authority specified it for its TRAX light rail projects through Salt Lake City and its southern suburbs. At the same time, it was used in Utah’s Weber Canyon and 3300 South Street pipeline replacement projects.

According to Bartlett, a new approach was taken to protect the 3300 South pipeline across the south side of Salt Lake City.

“Questar Gas had to put the pipeline right down the center of the roadway. When we looked at what other countries did, they built a trapezoidal geometry above the pipe—basically just a wedge,” he explained.

Such a wedge would require many blocks of foam and would disrupt a large section of road. “This would have been a major problem in an urban area, as you might have to tear up 20 feet of lateral roadway … you’d have to shut the whole road down,” Bartlett added.

Rather than gut a major thoroughfare, Bartlett proposed a “slot trench” design in which a block of Geofoam is placed in a narrow trench between a pipeline and the pavement above. In this design, if the pipeline begins to lift up, it will displace the Geofoam block and compress it.

Although Geofoam is solid, it contains tiny air pockets that can compress without sacrificing the material’s overall integrity. As the Geofoam is compressed further, it will slide upward along the trench sidewalls and could eventually damage the pavement above.

“However, the pipeline will remain intact and essentially undamaged,” he explained. Since the 3300 South project, Questar has been installing Geofoam to protect other natural gas pipelines in the Salt Lake Valley and elsewhere.

Other uses

New research is being conducted to measure the effectiveness of Geofoam to help new buildings withstand earthquakes. The use of Geofoam backfilling against a vertical structure significantly reduces and/or completely eliminates lateral pressure on that structure, whether it is a bridge abutment, retaining wall, or foundation wall.

For example, with a foundation wall going 30 feet below grade, the compacted soil will create 3,750 pounds of vertical pressure at the wall base and 1,250 pounds of lateral pressure at the base of the foundation wall. The use of Geofoam will greatly reduce lateral and vertical pressure.

Geofoam’s light weight and compressive resistance makes it an ideal fill material for pipeline protection as well as highway embankments, landscape fill and green roofs. Also, new information is pointing to its value as a potential seismic buffer for structural and infrastructure applications.

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