By BJ Austin, KERA News
http://stream.publicbroadcasting.net/production/mp3/kera/local-kera-868705.mp3
Dallas, TX –
A "bridge to nowhere" that's what one Council member is calling Dallas' first designer bridge over the Trinity River. KERA's BJ Austin has more on the issue and the technology that may help put the pieces together.
The problem is a deep layer of "sand" discovered in March. It was discovered when the concrete pilings for the bridge access ramps were drilled into the levees. That's a problem for the Corps of Engineers, which is in charge of flood protection. Pliable sand around the pilings could create a "soft spot" that would allow water to seep through the earthen levees - weakening them.
So the Corps ordered a halt to work within 50 feet of the levees until the extent of the "sand" problem is determined, and the levees are reinforced, if necessary.
Dallas City Council member Angela Hunt worries we could have a 120 million dollar signature bridge that no one can cross.
Hunt: I think that for some period of time, and I don't know what it will be, it will be in fact a bridge to nowhere. Again, my primary concern is now much will it cost to connect this bridge to nowhere through our levees.
Trinity River Corridor Project Director Rebecca Dugger is confident the ramps will be built. She says welding work on the steel frame of the Calatrava bridge continues, and the first section could be hoisted in place well before the ramps are completed.
Dugger: It could actually go up separately. They don't necessarily depend on one another as they're going along. So, they're kind of moving along parallel. I'm thinking later this year we should be able to start seeing the arch going up.
That's the eye-popping 40-story Calatrava arch from which a web of supporting cables stretch and descend.
The Texas Department of Transportation - building the access ramps -- expects cost increases because of delays any levee "fix" that may be ordered. How much, officials won't say. But an often-mentioned remedy is a concrete, or diaphragm wall built down to bedrock to block any water flow. Based on city of Dallas estimates, the cost of a diaphragm wall could be 9 or 10 thousand dollars a foot.
TxDot has hired University of Texas at Arlington Engineer Dr. Sahadat Hossain to use a new technology that can determine the extent of sand in the levees - starting at the Margaret Hunt Hill Bridge.
Hossain: We'll look at what is the impact of this construction on the levee or if there is any impact. Then the second part of our work will be how to remediate this so there will be no problem in the future of the levee.
Dr. Hossain and the U-T-A team will use an imaging technology new to North Texas.
R-I, or resistivity imaging, allows engineers to look at continuous segments of the inside of the levee to determine its composition. The machines are a little bigger than a car battery. Technicians place a number of steel rods about two inches into the levee. These electrodes are attached to each other and the series of R-I machines. The electrical current then creates the data that allows engineers to identify the types of soil inside the levee.
Dr. Hossain says this resistivity technology is used routinely in Europe, but not in the U.S., and he says it's more accurate and faster than boring hundreds of holes to take samples.
The speed of new and accurate information is important to Councilman Dave Neumann, chairman of the Trinity Corridor Project committee. He says, even with TxDot's delay on the ramps, he expects the project's deadline to be met.
Neumann: 2011 we'll be driving across the Margaret Hunt Hill Bridge, absolutely.