Unstable soil drives up highway prices

03/11/2001
By Tony Hartzel / The Dallas Morning News

Left untreated, North Texas' notorious shifting clay soil can wreak havoc on multimillion-dollar, multiton structures such as the President George Bush Turnpike. So the North Texas Tollway Authority is taking extensive - and expensive - steps to make sure the road and the dirt under it don't move.

On most road projects, crews treat soil with powdered lime to prevent it from expanding and contracting as the moisture content changes. In Irving and Carrollton, sites of the agency's latest construction projects, crews have encountered a problem: gypsum veins in the soil. "If we used traditional methods, it would cause more problems," said Mark Bouma, director of engineering for the tollway authority. "The cure would be worse than the ailment." Gypsum, a component of sheetrock, would react with the lime in cold, moist conditions and cause the soil to shift. A smooth parkway would turn into four-wheel-drive terrain almost overnight, a phenomenon known as "sulfate-induced heave." Instead of using lime, the authority is using a liquid chemical to treat the black, sticky soil. Crews treat three feet of soil in 12-inch layers. Then the dirt is packed and used as a base for the road. Once treated, the soil becomes solid base material. That process costs about $500,000 per mile more than building on more stable ground, Mr. Bouma said.

Much of North Texas has clay soil, said Syed Haneefuddin, a geotechnical engineer for the Law Gibb Group in Fort Worth, a company that helped build the runways at Dallas/Fort Worth International Airport. Although some areas have five to 10 feet of clay soil on top of more stable Austin chalk, Irving, Las Colinas and Carrollton have clay soil that reach depths of 25 to 30 feet. In some cases, the clay runs as deep as 100 feet, he said. "In Las Colinas, we've seen buildings with 10 inches of movement," Mr. Haneefuddin said. "That's a bad area."

In most situations, keeping the soil's moisture content steady with regular watering can prevent much of the movement that the road builders are trying to prevent. Although homeowners can water the soil around a house foundation, it's not practical to water around a freeway.

Construction workers have discovered the temperamental nature of the western Dallas County soil. Work on a new bridge at Gateway Drive, south of State Highway 114, had to be halted when the concrete structures shifted an inch after heavy bridge beams were set, Mr. Bouma said. Shifting soil and an inadequate foundation were to blame. To fix the problem, crews had to remove the beams, drill a deeper foundation and rebuild the structure at an extra cost of up to $100,000, plus a 60-day delay. "These are rigid structures," Mr. Bouma said. "They're not designed to move at all."