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Lightweight and stiff as a board, a plastic foam material is being used to protect Utah’s natural gas pipelines from rupturing during earthquakes.
“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 is catastrophic.”
Bartlett has partnered with natural-gas company Questar to use large expanded polystyrene blocks called “geofoam” as a compressible, protective cover for natural gas pipelines buried underground.
“This low-impact technology has an advantage in urban environments, particularly if you need to realign already buried structures such as gas lines or utilities without affecting adjacent buildings or other facilities,” said Bartlett.
Geofoam has been used for decades in Europe, North America and Asia to lighten loads under roads and reduce settlement. One-hundredth the weight of soil with similar strength, geofoam blocks reduce construction time and don’t erode or deteriorate.
Bartlett previously researched the design and use of geofoam as a lightweight road embankment in the Interstate-15 reconstruction project through the Salt Lake Valley a decade ago, and more recently in the TRAX light rail line that opened last year to serve West Valley City, Utah. Geofoam currently is being used in the TRAX extension to the airport.
Questar – which provides natural gas to almost 900,000 customers in Utah, southwestern Wyoming and southeastern Idaho – is using geofoam in lightweight covers for minimizing damage to natural gas pipelines caused by severe earthquakes.
“Most pipelines are designed to withstand some ground shaking, but not several feet of sudden fault offset that may occur in a major earthquake,” said Bartlett. “When a fault breaks, it occurs in milliseconds. It is an extreme event. The problem Questar faced was how could a buried pipeline survive that offset?”
Geologists expect that when a major earthquake strikes the Wasatch fault zone in the Salt Lake Valley, a fault rupture likely will make the valley drop down relative to the mountains. As the valley drops down, a buried pipeline would start to lift up. However, most buried pipelines lie under six to eight feet of compacted soil. This weight becomes too much for a pipe to bear, causing it to rupture, Bartlett says.
Numerical simulations of earthquake fault ruptures performed by Bartlett and his students show a geofoam-protected pipeline on the valley side of the Salt Lake City segment of the Wasatch fault could withstand up to four times more vertical force than traditional soil cover.
Based on Bartlett’s experience with geofoam, Questar asked him to develop a strategy for protecting buried pipelines crossing earthquake faults in urban areas, such as 3300 South, an arterial street in the Salt Lake Valley.
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, according to Bartlett, the pipeline will remain intact and essentially undamaged.