Results of a study are coming in regarding the amount of settlement in a bridge embankment experiment along SH 360 in Grand Prairie. The experiment involved using rotary kiln lightweight aggregates, also known as expanded shale aggregate (ESA) and expanded shale and clay (ES&C), in a geotechnical application to reduce the settlement of the embankment fill.
Embankment settlement, if not addressed, results in damage to vehicles and continued deterioration of the adjoining pavement, costing taxpayer dollars to repair. In much of the North Texas area where the "black gumbo" soil predominates, expansion and shrinking lead to significant settling and shifting of roadway subgrades, even with stabilization techniques.
Lightweight aggregates have been used in concrete for bridges and buildings and in asphalt mixes including seal coat throughout Texas, according to Jack Sinclair of TXI ES&C Division. Although lightweight aggregates have been used in geotechnical applications for some time, this study may open the door for more use of lightweight aggregates for geotechnical situations in Texas.
The Experiment
The State Highway 360 bridge over Green Oaks, near the Arlington/Grand Prairie adjoining city limits, was completed by Zachry Construction Corp. for the Fort Worth TxDOT District in January 2006. The contractor placed approximately 24,000 cubic yards of TXI's 3/8 x No. 10 gradation ESA (expanded shale aggregate) on the south embankment of the bridge under both northbound and southbound lanes and median, while the north embankment received about the same amount of regular fill dirt and limestone screenings.
TXI supplied their ESA from their plant in Streetman, about 75 miles south of Dallas. This is currently the only TXI plant in Texas producing expanded shale and clay, although TXI plans to stock the product at one of their Houston facilities.
University of Texas-Arlington's Engineering Department has been conducting the research under the direction of the Texas Department of Transportation's Fort Worth office. UT Arlington engineers installed vertical inclinometers in both embankments to measure the amount of settlement in the fill material on both the north and south ends of the bridge.
Results of Testing
After nine months, the north end of the embankment, containing regular fill, had settled approximately 1.5 inches while the south end of the embankment, with expanded shale, had remained settlement free, according to Richard Williammee, engineer with the Fort Worth TxDOT District. The settled area has been overlaid to smooth it out. Now, 2-1/2 years later in June 2008, those figures are about the same, he added. "All the settlement that's going to occur has occurred."
Although ESA costs almost twice as much as most other fill aggregate material, it weighs only about half as much, reducing the downward pressure on the subgrade. The weight factor is only one aspect of the equation. The high internal friction angle of ESA materials also reduces lateral shifting, according to studies.
When purchasing ESA by weight such as for TxDOT specifications, the volume makes up for the difference in price. When purchasing the ESA by volume, such as for geotechnical applications, the cost will have to be offset by long-term maintenance savings.
"We'll have to educate our design engineers to look at the long term rather than the short term," said Williammee. "The decisions [to use ESA in the future] will be based on what it will cost to haul it to our district."
UT Arlington's final report was published by the American Society of Civil Engineers (ASCE) and presented at their Annual Civil Engineering Conference last November. The white paper may be read on the ASCE website at www.asce.org.
