Tough subgrades and cold winters make thermal cracking a common – and expensive – problem throughout the Midwest. And while initial bearing values after chemical treatment are often acceptable, freeze-thaw cycling and moisture infiltration eventually make chemically treated subgrades unstable and lead to thermal cracking in pavements.

"Our strategies for soil stabilization were based on chemical treatments with lime, cement and fly ash," says Randy Roths, a former senior lab technicians with the City of Wichita's Materials Division and now quality assurance manager for Cornejo & Sons, Inc. "We would take soil borings at every new residential site and test the soils to determine which type of chemical stabilization to use. But the hodge-podge of soils in our area always made it hard to know if one approach would work for an entire site."

"Three-inch wide thermal cracks are not unusual," notes Owen Hunn, Tensar International north central region manager. "Cracks allow water to infiltrate and supersaturate the subgrade. And that accelerates the breakdown of flexible pavements."

WICHITA PIONEERS MOVE TO GEOGRIDS
In 1992 Wichita began using Tensar® Biaxial (BX) Geogrids to improve subgrade conditions. "Our specifications called for covering the geogrids with 5 in. of crushed concrete followed by 5 in. of asphalt," Roths says. "We have abundant supplies of crushed concrete for base applications, and it is much less expensive than importing limestone."

"The concrete's alkalinity isn't a problem since the polypropylene used to make the Tensar BX Geogrids easily tolerates a wide range of pH values," adds Hunn.

PERFORMANCE WINS OVER HAYSVILLE
Roths says city personnel noticed a big difference in road performance. "We were able to move our maintenance schedule from five-year intervals to 12 or more years. There weren't nearly as many cracks. And when they did appear, it was much later."

News of a better option for subgrade improvement soon reached the City of Haysville, located just south of Wichita. "With fly ash and lime, our roads routinely developed 150- to 200-ft longitudinal cracks," says Public Works Director Randy Dorner. "Now, it's not much of an issue."

Dorner adds that geogrids are the department's material for choice for sandy soils and fatty clays. "They bridge soils very well and can be laid down no matter the weather. A little rain on a fly ash job can mean a week's delay."

MORE CITIES FOLLOWING WICHITA
Other nearby cities, including El Dorado, Maize and Derby, have started using Tensar BX Geogrids as well. In total, they have installed more than 10 million sq yds of geogrids so far.

"We used to use 8 in. of fly ash or lime subgrade modification with 9 in. of asphalt," says Scott Rickard, assistant city engineer for the City of El Dorado. "With Tensar Geogrids, we were able to adopt a new section with 6 in. of recycled concrete and 6 in. of asphalt. It's a lot more economical."

"Projects also move along more quickly since Tensar Geogrids aren't sensitive to temperature and moisture conditions," notes Rickard. For city residents, that means fewer disruptions during construction and maintenance projects.

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