The Tarrant Regional Water District (TRWD) is now in the process of expanding its raw water conveyance system to meet the growing water demands of its 30-plus wholesale customers that serve approximately 1.6 million people in 10 North Texas counties. The entire upgrade includes 19 miles of 96-inch and 84-inch pipeline to connect TRWD's East Texas System to their West Fork system, two booster pump stations, one rated at 300 million gallons per day (MGD); a 120-million-gallon balancing reservoir; two outlet structures; and a number of other facilities.
A 78-inch-diameter pipeline has been bringing raw water back upstream from Cedar Creek and Richland-Chambers Reservoirs in the Trinity River watershed to lakes in southern Tarrant County, a distance of over 75 miles. At the northern terminus of this line is Eagle Mountain Lake north of Fort Worth.
The Eagle Mountain Connection Pipeline Contract B, a $41.7-million project awarded in 2006 to Garney Construction Inc. of Kansas City, Missouri. Garney's Pipe Group is installing:
approximately 47,000 linear feet of 84-inch mortar-lined polyurethane-coated steel pipe;400 linear feet of 78-inch pipe of the same material, supplied by Northwest Pipe Co.over 650 linear feet of 102-inch tunnels under existing roadways.
One of the unique aspects of this contract was the subaqueous, or trenched, installation of approximately 700 linear feet of 78-inch HOBAS pipe into Eagle Mountain Lake, where it fit into a concrete outfall structure with a steel discharge pipe situated well into the lake so as not to cause disturbance to shoreline activities around the marina, parks and residences.
Many lake crossings involve pressurized water or gas lines made of 8-inch-diameter to 12-inch-diameter polyethylene plastic, so they are merely weighted and submerged to the lake bed. Because a 78-inch hard pipe could be a safety hazard for boaters and swimmers, and the pipeline needed to be protected from the boaters and four-wheelers, the design called for the line to be buried.
Engineering and Fabrication
The project was designed by Freese & Nichols engineers, but the design and fabrication of the outfall structure was left to the contractor. The only requirement was that the water be expelled at a 5-degree vertical angle.
Houston-based Boyer, Inc. was contracted to design, fabricate and build the outfall structure and to install the subaqueous line. Boyer, Inc. has developed a specialty of inland marine expertise.
"We have a special marine fleet of barges. The drafts on our barges range anywhere from 6 inches to 6 feet depending on the weight they are carrying. We can go inland with our equipment to the lakes, the bayous and the shallow-water slews and operate heavy equipment from those barges. The barges are modular elements so we can connect them to make any arrangement we want."
"All our equipment is roadable," said Gordon Reavis, project manager for Boyer. "It can be disassembled and trucked to the job site."
"The hardest part of fabricating the discharge pipe was to calculate all the miters and make templates so the steel could be cut with a cutting torch, solid welded from both the inside and the outside, and come out with a result that was 5 degrees up," explained Bill O'Fiel, project manager and fabrication coordinator for Boyer, Inc.
"Everything was designed and fabricated here [Boyer's Houston facility] and trucked to the job site north of Fort Worth," said Gordon Reavis, project manager for Boyer.
"Then we had to engineer how we were going to hold it on the bottom and get the pipe to come straight into the socket," said O'Fiel.
"We actually hung it a foot above the bottom and used tremmie concrete to place a concrete slab bottom and create a seal around it," Reavis clarified. "The structure was then filled with tremmie concrete." This gave the structure sufficient weight to serve as a "dead man" to hold the cable blocks of the pulling apparatus once it was time to pull in the water line.
The aspect of the Eagle Mountain Lake project that worked in Boyer's favor was the drought. This lake like all the other in North Texas was more than 5 feet below normal by mid 2006. Even so, the water depth was about 30 feet at the farthest point from shore.
Boyer's crew downrated their 150-ton Link-Belt long-reach crane to 105 tons and mounted it on a barge that could be spudded for stability. They surrounded the dig area with 20-mil plastic silt turbidity barrier (silt fencing) on pontoons and proceeded to trench 12 feet into the lake bed.
Because silt kills fish, care must be taken to contain suspended silt during excavation in public water. These types of projects are continuously monitored for pollution control, according to O'Fiel. All dirt excavated from lake bottoms must be hauled away for disposal at approved sites.
The turbidity barrier stopped the suspended silt from migrating away from the trenching area. Volvo earthmoning trucks were driven onto a barge, pushed out to the work area for loading, then pushed back to shore where they debarked for their drive to the disposal yard.
Pulling the pipe into place along the trench proved another engineering feat. Boyer designed an apparatus with 120,000 pounds of pulling force, rigged to push the pipe to the structure. The concrete outfall structure had been built first to act as the anchoring, or dead man, point of the pulling apparatus.
"We had the Link-Belt crane mounted on the barge at the termination point where the structure was," O'Fiel described. "We actually ran cable over to the shore to activate our pushing bar on the end of the pipe. We had a special guiding system where the front end of the pipe mated with a bell spigot arrangement to make a watertight connection at the outfall structure. A rubber gasket acted as a pressure seal to make the system watertight. Each end of each 20-foot-long section of pipe had to bell-up to the next section."
Because of the friction coefficient, the maximum pull force reached was only around 80,000 pounds. Special backfill of crushed stone topped the pipeline. Less than four months after mobilizing to the site, the Boyer crew completed their contract in mid-April. "We finished on time and in budget," Reavis commented.