Reusable Cofferdam Keeps Project Green
The small town of Gruene (pronounced 'green') is barely a speck on the Texas map near New Braunfels, between Austin and San Antonio, but it is brimming full of fun. It is home to the historic Gruene Dance Hall, built in 1878 and steeped in German and Czech polka heritage. In more recent years, numerous country-western musicians have honed their skills within its humble wooden walls. Gruene has also become a haven for artists, crafters — and shoppers. Guadalupe River rafters and tubers know Gruene as a take-out point just short of the low-clearance Gruene Bridge. The bottom of the bridge was usually just a couple of feet above the water level of the river, trapping debris that required frequent cleaning and posing a danger to river enthusiasts.
It is the latter group that will be most pleased with this construction project. TxDOT has undertaken the rebuilding of Gruene's Guadalupe River Bridge, raising its clearance to be about four feet above normal water level. Austin Bridge and Road was awarded the project in the summer of 2007, but actually began work in October after the rafting season ended. Completion was scheduled for May 2008 so that work would not interfere with the beginning of the 2008 rafting season over the Memorial Day holiday.
The New Bridge
The new concrete bridge, 172 feet long by 44 feet wide, is two lanes wide like Gruene Road, and because this area is a popular tourist spot, it has 6-foot-wide sidewalks outside the vehicle lanes.
The bridge is supported by three 5-foot-tall by 50-foot-long solid concrete piers at the interior bents. To create the substructure supporting the new bridge, WW Foundation Drilling drilled the 20 pier shafts 30 inches in diameter to depths of 25 feet and 35 feet.
"Approximately 90 percent of the pier is below the natural river bottom," described Dan Holycross, project manager for Austin Bridge and Road. "Three caps, 5 to 6 feet tall by 50 feet long, sit on top of the concrete piers."
A Green Choice
TxDOT and the city of Gruene wanted the demolition and rebuilding to be conducted in a safe and environmentally conscious manner. While still in the engineering phase, TxDOT contacted Portadam to handle the diversion of river water with their cofferdam system.
"It was the green aspect that drove this project to use Portadam," said Jim Kirby of Portadam. "They didn't want that debris to fall into the river during demolition of the old bridge. They were able to remove the debris and silt instead of sending it downstream."
"The entire Portadam system is reusable," added Dan Deavers. "Everything that goes in, comes out."
"There was also concern about river water reaching the top of the dam," Kirby stated. "That's why they called us."
Portadam's technology makes use of a steel frame structure that supports an impermeable fabric liner, which can form a wall capable of holding back up to 10 feet of standing or running water.
The main beam of the frame slants away from the water side at a 45-degree angle. Bob Gotta, president of Portadam explained, "The pressure of the water at a 45-degree angle pushes down on the legs. The difference in the force between the wet and dry side creates a seal."
As a result, working areas that were previously hazardous or hidden under water are dry and easily accessible for work and repair. Portadam allows water to be temporarily diverted and is environmentally preferred over sandbags or cumbersome cofferdams.
The Portadam cofferdam system may be placed under existing spans and also works well on an uneven bed terrain. Here in Gruene, the Guadalupe bed is gravel punctuated with large root balls from trees that line the banks and shallows, creating a shady canopy. Because the Portadam frame does not penetrate the bed of the stream, it was the perfect answer to preserving the trees adjacent to the bridge.
"Each Portadam is site specific in design. Primarily for this reason, Portadam continues to be solely a rental product," explained Deavers.
For the Gruene dam, Portadam actually designed and installed two cofferdams. For the first phase, a 160-linear-foot Portadam system was installed in shallower water around the southern half of the bridge, where demolition began. It, as well as the dam for the second phase, was L-shaped due to the trees in the river shallows close to the bridge. The city would not allow the trees to be cut down.
The steel frames of the Portadam system extend 23 feet upstream and outward in front of the triangular frame. The Portadam crew bolted pairs of the triangular steel frames together on shore. An excavator positioned on the bridge reached out to position these frames into the water in pairs. Divers then clamped the pairs together with link bars, which can be spread out for adjustment. The plastic liner was then attached at the tops of the framework and was allowed to sink to the river bed, then secured with sand bags.
The water was then pumped out from behind the cofferdam. In Gruene, a 6-inch diesel Godwin pump removed the original water. Because the cofferdam was L-shaped, or two-sided, the area did not stay as dry as it would under other circumstances. Two smaller Godwin pumps removed the seepage and rainwater as the project progressed.
The bridge was built in two phases, according to Holycross. "Once approximately two-thirds of the existing bridge was demolished, abutment No. 1, interior bents No. 2 & No. 3 were constructed. In addition to the construction of these structures, the finish work (sand blasting) on the structures and the stone rip rap in front of abutment No. 1 had to be completed prior to the Portadam being removed and relocated. Once the Portadam was relocated, the remaining existing bridge was demolished and interior bent No. 4 and abutment No. 5 was constructed. Once again the finish work and the stone rip rap in front of abutment No. 5 had to be completed prior to removing the Portadam."
"After they demolished half the bridge, we flipped the dam over to the other side," said Kirby. "We used 150 linear feet [of Portadam] in the second phase, which was the deeper water." It took about two days to install each dam, and just one day to dismantle it, according to Kirby.
"The objective was that any work that needed to be done that fell within the limits of the river needed to be completed prior to removing the Portadam," Holycross continued. "We were required to remove all of the existing bridge debris from the river and this would not have been possible with the river flowing normally."