Enginuity 2011

Transportation infrastructure

Repairing roads and monitoring mega-loads

Mien Jao and Mark BourlandA safe and well-maintained roadway and bridge system is critical to any state. That’s why the Texas Department of Transportation has several research projects underway that pertain to safety and maintenance. Civil engineers Mien Jao, professor, and Mark Bourland, assistant professor, are working on two of these projects: one on super-heavy load criteria for bridges and another on pavement-repair strategies.

“In general, these are parts of a larger effort of the Texas Department of Transportation to preserve and protect transportation infrastructure,” Bourland said. “The Federal Highway Administration and other states are also involved with similar programs.”

The bridge study was launched because of the increased frequency of super-heavy loads being transported over Texas roadways and bridges. The Texas Department of Transportation classifies super-heavy loads as those greater than 254,300 pounds gross vehicle weight or greater than 200,000 pounds gross vehicle weight if they are less than 95 feet long. Loads less than these values are not considered to be super-heavy, and bridge analysis is not performed. These criteria are often much less conservative than the criteria in other states.

“The current super-heavy load criteria were established in the 1970s,” Jao said. “Accelerated deterioration of some bridges has been observed. The deterioration may be caused by many factors including the repeated heavy loads that were not well considered in the analysis approach used to establish the current super-heavy load criteria. Therefore, it is essential to verify and evaluate the current super-heavy load criteria to better protect and preserve bridges in Texas.”

More than 1,000 super-heavy loads now travel Texas roads each year, and the number and average weight of the loads are increasing. Many of these loads are repeatedly routed across the same bridges. The result: The bridges are wearing out faster than originally expected.

Why have the loads trucks carry increased? One reason is a result of engineering. Trucks and trailers are now capable of carrying heavier loads than they carried decades ago. Another reason is the need for huge pieces of equipment to be delivered to places including energy plants and refineries. New-generation power plants fueled by natural gas are one of the contributing factors to an increase in the number of super-loads on U.S. highways, Bourland said. These destinations are often in remote areas that can only be reached by truck. The loads can weigh more than 1 million pounds.

“In Texas, the roadways are currently getting a lot of use by trucks involved with wind-energy plants,” Bourland said, adding that the Texas Department of Transportation Motor Carrier Division is proud that it has never denied a hauler's request to move a load. “They always find a way,” he said.

The entourage is also huge when it comes to moving these monstrous pieces of equipment. “It's not just the load,” Bourland explained. “There's a large contingent of trucks and personnel that move with the loads.”

Last year, a super-load left Houston Jan. 5 traveling state highways and farm-to-market roads and arrived in Riesel Feb. 10. The part the trailer carried for a generator at a power plant weighed 886,249 pounds. The combined gross weight, including the pull tractor, trailer with payload and two push tractors, was 1.7 million pounds. About 15 semi-trailer trucks traveled with the load carrying support equipment.

soil sampleAlthough the study isn’t finished, the team has made some conclusions. “Load intensity, which may be thought of as the weight of the load divided by its length, is a better indicator of load severity related to bridges than is gross vehicle weight,” Bourland said.

The team’s second project deals with pavement repairs on rural roads. Many farm-to-market roads in Texas are experiencing longitudinal cracking and severe edge failure. These roadways have received numerous repairs, and the current maintenance approaches are not working as the distresses reappear in a short time. Often these roadways are narrow, have little or no base, have poor drainage and are constructed over soils that experience relatively large deformations under loads when they are saturated with water. Funds are not available for extensive widening or full reconstruction of these roadways, and an innovative maintenance approach is required. “It is very important to come up with an innovative and cost-effective maintenance and construction approach to solve the problems,” Jao said.

The project is in the final-report phase. The team found that in Southeast Texas, 22-feet-wide farm-to-market roads benefit from an added 2-foot to 3-foot shoulder. This prevents longitudinal cracking that leads to other pavement-maintenance costs. Adding shoulders is expensive, Bourland explained, but the cost can be reduced by substituting recycled asphalt pavement for new asphalt to build the shoulders.