DRIVEN
To preserve and update the state’s aging roadways, UTA civil engineers are turning to novel solutions, from slopes shored up with plastic pins to bridges with built-in heating systems.
by melinda mahaffey icden
As the cliché goes, everything is bigger in Texas—and that includes the infrastructure. The Lone Star State has over 50,000 bridges and 300,000 miles of roads, more than any state in the nation. But it all comes at a price: The Texas Department of Transportation (TxDOT) estimates that replacing the state’s aging travel networks will cost $500-750 billion. Effective—and cost-effective—solutions are in high demand, and a trio of UT Arlington engineers is answering the call. The professors are offering innovative ways to help maintain the roads and bridges we have today and build better ones for tomorrow.
Pinning Your Slopes
North Texas, it turns out, is built on very shaky soil. The ground beneath our feet contains highly expansive clay, which functions like a sponge. In wet periods, the clay absorbs water molecules and increases in volume; in summer, the moisture disappears and the clay shrinks. Over time, this eats away at soil strength, causing extensive damage to the infrastructure on top of it—like our roads.
“Eventually, the failure load becomes greater than the resistance load, and the slope supporting your road fails,” civil engineering Professor Sahadat Hossain says.
To combat the problem, in March 2011 Dr. Hossain and his team installed commercially available recycled plastic pins into the sinking slope on two 50-foot test sections of U.S. 287 in Midlothian. They used a method previously developed by the University of Missouri but under different soil conditions. An untouched control section was left in between.
The installation took two days, with 192 pins in the first section near a bridge and 225 pins in the second. A year later, they installed a third section with 238 pins and established a second control section. The length of the pins varied between 8 and 10 feet.
During the subsequent monitoring period, the three reinforced sections moved only 1 to 2 inches, while the first control section settled about 15 inches and the second almost 9 inches. Additionally, about two years after the first pins were installed, the slope on the other side of the highway failed in two spots.
“All along the road, in every other place, the slope is failing,” Hossain says. “That shows us that the pins we installed are really working. There’s no doubt about it.”
Hossain uses resistivity imaging—which he likens to an “X-ray for the soil”—to get a continuous profile of the soil’s properties and high-moisture areas to establish possible failure locations before installing the pins. He prefers this geophysical method over traditional boring, which only provides information about the particular spot where an engineer inserts the probes.
Anand Puppala and his team test soil mixtures to reduce pavement cracks.
Using the information gleaned from the imaging, Hossain worked with Ashfaq Adnan, assistant professor of mechanical and aerospace engineering, to develop an easy-to-use model to predict the geotechnical properties of clay soils, enabling TxDOT to design pin placement once they have an area’s slope measurement and soil properties. (The researchers found that the pins are effective if the failure depth of the slope is 6 to 8 feet.)
The Midlothian project went so well that once the two-year monitoring period ended, Hossain received another $1 million TxDOT contract. He’s using a portion of it to repair a slope on State Highway 183 in the agency’s Fort Worth District and one in the Dallas District at Interstate 35 and Mockingbird Lane. The project runs until August 2016.
The team’s success also is generating national interest. The traditional solution for sinking slopes—a concrete retaining wall—can cost a half-million dollars and take months to build. Recycled plastic pins, on the other hand, take about one month from site investigation to installation and cost only $100,000. Plus, it’s an environmentally friendly solution that diverts plastic from landfills but benefits from the plastic’s long lifespan.
As Hossain notes, “This year, we had much more rain, and the Midlothian slope is still holding. If the pins can take that kind of moisture, they can take anything.”
Banishing Bumps in the Road
Calcium-based additives such as lime and cement have long been used to strengthen and improve the resistance of soil subgrades during road construction. But when that soil contains high levels of sulfates—as parts of North Texas’ gypsum-rich soil does—and the mixture is exposed to water, a chemical reaction occurs. The result? A mineral called ettringite, which can swell to more than 100 percent of its original volume. Under a road, this phenomenon, known as sulfate-induced heave, leads to bumps and pavement cracks. Removing the calcium is not feasible, as it is essential to the stabilization process. Instead, Anand Puppala, Distinguished Teaching Professor and associate dean for research in the College of Engineering, is investigating how to minimize the heave mechanisms in high-sulfate soils.
In previous laboratory work involving soil testing, Dr. Puppala found that when a lime and soil mixture is compacted immediately after mixing, the soil swells. But pre-compaction mellowing—allowing the mixture to sit undisturbed for a period of time before compacting it—gives the ettringites time and space to grow, reducing later heaving.
So for his two-year field implementation project, Puppala and his team are experimenting with three test strips on a 2.1-mile section of U.S. 82 near Bells, Texas. They’re using a lime and fly ash mixture with extended mellowing on one strip, lime with extended mellowing on another, and lime with a three-day mellowing period as a control section. (Fly ash, a coal power plant by-product, is a calcium-based stabilizer that has been shown in studies to decrease the swell and shrinkage of soils.)
