The Imperial County, CA government operates Imperial County Airport, an airfield with two runways that support general aviation and commercial aircraft. In 2009, the county needed to rehabilitate a taxiway to maintain its capability of serving this traffic.
The county wanted an affordable option for creating a new taxiway with a superior projected design life. With regard to site conditions, the subgrade soils consisted of lacustrine deposits, which varied from lean to fat clay (CL to CH) and silt (ML). Shallow groundwater also created pockets of extremely soft soils.
An alternative solution the county considered involved using a 19-inch thick aggregate base with and without lime stabilization. Both of these options were rejected because of concerns over cost, construction scheduling, and lifecycle performance.
The Tensar Solution
The county decided to use the Spectra Roadway Improvement System, a mechanically stabilized layer incorporating TriAx Geogrid as a lower cost, longer lasting alternative to installing a very thick base layer or remediating the subgrade with lime.
“Our client had used geogrid on another airport project,” says Geotechnics, Inc. project engineer Matt Fagan.
“The client believed that the use of geogrid could improve the lifespan of the new taxiway without adding substantial costs to the pavement section. Granite Construction completed the installation by removing the existing pavement section and 15-inches of soil.
“Next, they leveled and rolled the subgrade and covered it longitudinally with TriAx Geogrid. To complete the installation, they installed 15-inches of aggregate base and 4-inches of asphalt concrete.
“In areas with significant groundwater, the company excavated the saturated material, installed a lower layer of TriAx Geogrid, and covered it with recycled asphalt [stockpiled when the original taxiway was removed] to return the subgrade to the required profile depth.”
“The TriAx Geogrid’s rib has a higher profile and a unique hexagonal shape that gives 360-degree load distribution,” says Tensar International Corporation regional manager Lars Nelson.
“These properties allowed the taxiway designer to reduce the aggregate layer by four inches while still providing superior base confinement and strength,” he adds.
“The use of TriAx Geogrid helped us establish a firm and unyielding subgrade throughout the site, which translated to improved relative compaction in the aggregate base and asphalt concrete,” says Fagan.
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The full cost for the projects totals nearly $35 million.