'Robust' Rebuild Designed to Deliver Reservoir 2.0
Along the curving road leading up to Tampa Bay Water's $129-million reservoir renovation project in Lithia, Fla., signs implore crews to work safely and deliver quality work. One message, "Do it right the first time," is ironic given the project's history. The 8-year-old, 15.5-billion-gallon reservoir already was the focus of a lengthy, expensive legal battle after cracks developed. Therefore, TBW will have to settle for getting it right the second time.
With the redo, design-builder Kiewit-Gannett Fleming is leaving nothing to chance, employing what the project team calls a more "robust" set of design features. That characteristic was a design imperative, since this time the design-builder will be on the hook for maintenance for a period of five years after project completion.
Together, the reservoir's list of features may be unique, design-builders say. The new soil-cement system is thicker and stronger, as is the underlying geocomposite liner. A custom-designed seawall, patterned after a hurricane wall in Galveston, Texas, now rings the dam facility to prevent storm-induced overtopping.
Compared with the original, "This is a significantly more conservative design," says Trent Dreese, vice president and project manager for Gannett Fleming. "We [needed] a design that we knew was going to work. You're not going to see any other reservoir [like it.]"
The legal dispute between TBW and original engineer HDR centered on finding a cause of the cracking within the reservoir embankment, which measures nearly five miles around. The investigation theorized that water trapped in the embankment—which lacked a drain—created uplift pressure during drawdown, thereby causing the cracks. For the renovation, TBW's request for proposals specified a 50-year design life, with the facility able to accommodate a normal drawdown of 100 million gallons per day or an emergency rate of 160 mgd, says Ed Davis, vice president, CH2M Hill, which aided the utility with the proposal process and is providing project quality assurance.
Tear it Down, Build it Back
Work started in February 2013 and has progressed in a linear fashion along the reservoir's interior. First, to demolish the existing 16-in.-thick flat-plate soil cement, the contractor used an excavator equipped with a hydraulic hoe ram attachment, followed by a Caterpillar D10 dozer that pushed the material down the slope, says Brian Watkinson, Kiewit's project director.
Kiewit crews then followed behind with two John Deere tractors, each pulling a pair of pan scrapers, to methodically remove the 1.8 million cu yd of material included in the existing soil wedge layer—a task that took about nine months.
Once the reservoir's existing geomembrane liner became exposed, workers used another excavator to pull out the old 60-mm-thick material in sections. The contractor then sent the material to a recycler.
From there, Kiewit began building back the embankment, first by installing the roughly 4 million sq ft of 80-mm-thick geocomposite, a double-sided material manufactured by Carpi with fabric on both sides. Its installation, which started last June and concluded in April, marks only the second time this type of liner has been used, and the first time in the United States.
It's "practically bulletproof," says Dreese, who calls it "the Cadillac of the industry for lining reservoirs." The fabric on both sides helps prevent punctures from items such as aggregate, he explains.
Instead of another soil wedge, though, the more robust geocomposite allowed placing 10 in. of AASHTO No. 57 aggregate directly on top. The layer's purpose is to stabilize the embankment's water pressure.
By design, the "drainage layer has about 1,000 times more capacity than is required to keep the head in the drain equal to the head in the reservoir," Dreese explains. "So as we draw down the reservoir and fill it up, there's no uplift pressure."
While the old embankment lacked a drain, Kiewit-Gannett Fleming's approach provides a continuous outlet for the aggregate layer. A series of 2-ft-tall gabion baskets—made of stainless steel for a 50-year life—are filled with larger aggregate, forming the embankment's base. The design-builder then added a 2-ft-thick gabion cap, composed of soil cement placed in four lifts of 6 in. each, on average. Designed to 650 psi strength, the resulting soil cement is actually averaging about 900 psi, says Watkinson.