The cracks in Tampa Bay Water's $146-million reservoir were caused by the way it was built, the engineers now believe.
The finding comes after two years of study and is expected to lead to lawsuits.
"Analysis of soil cement cracking at the C.W. Bill Young Regional Reservoir indicates that it may be due to reservoir design, construction, construction oversight or a combination of these," the utility's manager and attorney wrote in a letter to the board Thursday.
After checking a dozen different theories, engineers concluded "filling the reservoir up and draining it – it's the action of lowering the water level" is what is causing the cracks, Don Polmann, the utility's director of science and engineering, said Friday.
Water is getting under the top soil-cement layer of the reservoir wall and instead of draining back into the reservoir, it's producing water pressure under the layer, causing the cracking, Polmann said.
"There's enough evidence to suggest it's not working the way it's supposed to," he said.
Although the utility has not figured out yet how to fix the cracks, "it is necessary to initiate litigation against potentially responsible parties," the memo to the board says. Lawsuits will have to be filed soon, too, "due to impending deadlines imposed by statutes of limitations," the memo noted.
The utility's board will vote on the recommendation Oct. 20.
The likely targets of the utility's legal action: HDR Engineering, which designed the massive reservoir; Barnard Construction Co., which built it; and Construction Dynamics Group, which oversaw the construction.
In addition to recommending a lawsuit, the memo from utility manager Gerald Seeber and attorney Richard Lotspeich recommends the board fire HDR from its job of filing monitoring reports on the reservoir.
The reservoir — the largest in Florida, covering about 1,100 acres — is supposed to hold 15-billion gallons of water for use by customers in Pinellas, Pasco and Hillsborough counties. But for now it contains only 6.5-billion gallons. Tampa Bay Water has kept it half-full as engineers searched for the cause of the cracking.
The regional utility opened the reservoir, named for the longtime congressman from Pinellas County, in June 2005, as a place to store water skimmed from the Alafia River, Hillsborough River and Tampa Bypass Canal.
Water demand during the 2007 drought drained the reservoir to 2.4-billion gallons. Since then, though, the utility has been able to build the water level back up.
The reservoir's ability to save the region during such dire times made it stand out as a success for the wholesale utility, which for years was plagued with embarrassing problems with its dysfunctional 25-million-gallon-a-day desalination plant and, to a lesser extent, its 66-million-gallon-a-day surface-water treatment plant.
The reservoir's walls consist of an earthen embankment as wide as a football field at its base, averaging about 50 feet high, with an impermeable membrane buried in the embankment to prevent leaks.
The top layer of the embankment is a mixture of soil and cement to prevent erosion.
The cracks in that layer were discovered in December 2006. An independent inspector hired by the state Department of Environmental Protection reported that the cracks were up to 400 feet long and up to 15 1/2 inches deep.
Although the utility hired crews to fill in the cracks with a special type of grout, many of the cracks opened again in the spring.
The cracks have occurred in the soil-cement on the inside surface of the reservoir wall. Below that is a layer of soil 10 to 25 feet deep between the soil-cement layer and the impermeable membrane, then more soil in the embankment.
Because the cracks are comparatively shallow, Tampa Bay Water officials say that the residents living around the reservoir in rural Hillsborough County shouldn't worry about a breach opening up and sending a wall of water into their homes.
When the reservoir first began operation three years ago, it was honored as Environmental Project of the Year by the Florida chapter of the American Public Works Association, which hailed the work by Nebraska-based HDR on siting, design, permitting and construction oversight.