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.
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.)
“If we can control the mellowing at different time periods, we feel we can control the heave process,” Puppala says. Currently, he believes a three-day mellowing period is ideal, but the team is still monitoring the site through its monthly collection of field data.
Puppala also is watching the long-term success of an innovative project that uses geofoam to fix the problem of bridge bump.
Since its construction in 1995, the approach slabs of the U.S. 67 bridge over State Highway 174 in Johnson County, Texas, have seen nearly 17 inches of settlement. Over time, the soil under the approach embankments compressed under the weight of its load, creating a bump between the approach and the bridge deck.
In winter 2012, the top six feet of the embankment fill material were partially replaced with expanded polystyrene (EPS22) geofoam, a lightweight fill material. Puppala and his team—led by postdoctoral student Tejo Bheemasaetti—monitored the settlement of the embankment with horizontal inclinometers. The geofoam is lightweight enough for two people to carry (it weighs between .75 and 2.85 pounds per cubic foot, compared to 110 to 120 pounds per cubic foot for soil), yet has all the stiffness of soil.
“That means there’s no pressure on the native soil underneath, so that soil won’t compress and you’ll reduce the amount of settlement,” Puppala explains. The repair also doesn’t require any big equipment, such as a heavy compactor.
In May, the two-year monitoring phase was extended for two more years. “So far, the settlement has been less than one inch,” Puppala says. “In the 15 years before the project, there was more than an inch of settlement almost every year. We’ve already cut that down.”
Snow and ice may be infrequent visitors to North Texas, but winter storms can still pack a punch. Nationwide, just over a half-million car crashes occur in wintry weather conditions every year, and traditional sanding treatments for frozen roads require manpower to both lay it down and clean it up. Salt, meanwhile, can corrode vehicles and roads.
One potential solution is geothermal energy. Civil Engineering Assistant Professor Xinbao Yu kicked off a 17-month study in April for TxDOT with Puppala to research how this sustainable resource could keep bridges free from ice and snow.
The idea is simple: Heat the bridges so falling snow melts upon contact. To accomplish this, Dr. Yu and his team are investigating how the ground’s natural temperature—which consistently measures about 60-65 degrees Fahrenheit 15 to 20 feet below the surface—could be harnessed.
“Soil is considered relatively good thermal storage,” Yu says. “And typically, when you want to store heat capacity, you use water.”
During the summer, the system could collect solar energy using water as a conductor and store it in the ground with minimal heat loss. Then in the winter, that warm water would be pumped through pipes embedded into the bridge deck, thus keeping the bridge above the freezing point and eliminating ice and snow accumulation.
“Initially, this may not provide the cheapest solution, but it’s similar in the long run to why you buy a hybrid car,” Yu says. “You may put a lot of money into buying it, but you receive your money back as you run it.”
Yu’s work is also taking him further afield. In August 2014, he began a two-year project with the California Department of Transportation (Caltrans) to evaluate the state’s bridges and determine whether or not they were designed in accordance with the federal standards set by the American Association of State Highway and Transportation Officials (AASHTO). This is the first time a UT Arlington researcher has worked with Caltrans, and the award came after a very competitive process.
According to a document published by Caltrans’ research arm, the state began designing its bridges in 2008 using Load and Resistance Factor Design methodologies instead of AASHTO’s specifications because they believed AASHTO’s methods were more conservative and “seemed wasteful.” The state now has been asked by the Federal Highway Administration to review its practices.
Yu and his team will first document California’s existing design methods, digitizing 30 years’ worth of data and then running analyses. Because testing every bridge pile is prohibitively expensive, the data only covers a fraction of the state’s actual bridge piles. As a result, Yu will use the available information (including external factors such as soil properties, traffic numbers, and load) and advanced statistical theories to extrapolate how much load the bridge piles statewide can support.
“We’ll evaluate their current design standards to see how conservative they are,” Yu says, “and we’ll optimize their design to recommend newer specifications. Their bridge designs are safe, but they want to see if they can be more economical.”
Indeed, safety and economy will be key elements for all infrastructure projects going forward, as more and more U.S. bridges and roads approach the end of their lifespans. Infrastructure is vital to economic growth, and while more funding is needed to address the problem, so too is innovation.
That’s why the work of Hossain, Puppala, and Yu is so important. The civil engineers are providing new, sustainable, and cost-effective solutions for maintaining and building our transportation networks, helping pave a brighter future for Texas roads.
Read more about The University of Texas at Arlington and other innovative ideas by reading the INQUIRY, a publication of the UTA Office of Research.
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