Expansion: it’s a win for communities when roads reach out and connect people to business, resources and formerly rural areas. But another form of expansion—soil that expands when water is added and shrinks when it dries out—can thwart this progress by making it hard to economically build and maintain long-lasting roads.
“Expansive soil” is in every state in the country, but Texas, Colorado, California, Virginia, North Dakota, Oklahoma and Montana have higher concentrations according to Civil & Structural Engineer Magazine. Combined with high usage of roadways and the expanding/shrinking cycle (clay particles in soil can grow to 15 times its original size), strains are created in the asphalt. This can lead to cracks and heaving that lets in water seepage. This cycle then repeats for years and greatly shortens a road’s life span.
Expansive soil under roads can cause damage that may not be noticed for years but will ultimately lead to costly repairs. Transportation engineers use various methods to address these potentially problematic subgrades in road building. Conventional techniques include removing and replacing the expansive subgrade soil, mixing in a chemical additive to temporarily dry out and harden the soil, or disking (drying and recompacting the clay). And, more and more engineers are incorporating geosynthetics into designs, that experts say, can cost effectively address subgrade issues.
Research proven, road tested
Dr. Jorge G. Zornberg Ph. D, a University of Texas geotechnical engineering professor, has looked extensively at the use of geosynthetics like Tensar’s TriAx Geogrid (a weblike product utilizing triangle/hexagon shapes to best absorb forces from traffic weight) to manage expansive soil under roadways. His lectures and research note the “beneficial effect of geosynthetic stabilization of pavements,” minimizing cracks and offering “promising sustainable opportunities” that could save road-building communities millions of dollars in repair and maintenance.
Tensar’s TriAx Geogrid product is a geosynthetic designed to solve multiple problems in the engineering process. These include mechanical stabilization of the aggregate base (creating a stiffer base for the pavement), separation (keeping subgrade soil from seeping into the other base layers), and filtration (allowing water to move through but not soil/base particles).
Importantly, the geogrid can be used to preserve the pavement surface by protecting the underlying sensitive subgrade soils. The product should be placed, “at the interface between the base and sub-base layers or within the base course layer of the flexible pavement,” writes Zornberg. “This leads to lower stresses over the subgrade than in unreinforced flexible pavements.”
In his lecture titled, “A New Application: A Stabilization of Flexible Pavements Over Expansive Clay Using Geosynthetics,” Zornberg notes the use of geosynthetic “was found to effectively minimize the detrimental effects of expansive soil subgrades on flexible pavements . . . significantly improving the performance of roadways.”
Such sustainable options are greatly needed in areas where heaving, shifting and cracking roads are common due to expansive soil.
Practical application: Building success
In one middle-Texas small town where expansive soil is an issue, investing in TriAx Geogrid for smooth roads is already having huge payoffs in terms of attracting new businesses and residents.
Located between the cities of San Antonio and Fort Worth, the town’s leaders took an innovative approach to growth by investing in new roadways/transportation, utilities and water plans. The community’s rapid growth (the population has jumped from 5,000 to over 40,000 since the 2000 census) is a testament to the power of durable, economical roadways.
“You really get a better road for less money,” says Tensar’s Senior Regional Sales Manager Derek Wiatrek. “I want it to last as long as possible for the least amount of upfront cost,” something which an investment with TriAx Geogrid ensures throughout the life cycle of a road, he notes. “They [city officials] chose TriAx based on how much they could save on aggregate (sand, gravel, crushed stone).”
Wiatrek points out that more than 10 other communities in the area between San Antonio and Fort Worth have taken a similar approach in terms of building new roads to accommodate and facilitate rapid growth. By using TriAx Geogrid, they are looking ahead to the geologic challenge that expanding soil brings to the region, while ensuring low-maintenance, smooth roads for many years to come.
Wiatrek points to studies done that show reduced cracking on TriAx Geogrid-stabilized roadways, leading to less costly repairs years later. An unstabilized pavement typically fails from the bottom, as the subgrade moves, and cracks propagate up through the pavement surface. The only way to fix a failure like this is complete removal and replacement of the entire pavement section, which is costly. A TriAx-stabilized pavement protects the subgrade and stiffens the aggregate base, encouraging top-down failure of the pavement. This is a much more cost-effective repair.
Primed for growth, the community went from a town without a stoplight, or even a grocery store, to one that is building business and retail sites while welcoming new families and continuing to flourish. By investing in strong roads through technology, leaders there are looking forward.
This article was first published on the Tensar blog, https://info.tensarcorp.com/overcoming-obstacles-tensars-triax-geogrid. Used with permission.