|High pressure releases from Seven Oaks Dam send a plume of water to its plunge pool. Unanticipated noise and vibrations led engineers to halt testing before completion of all phases. (Photo courtesy of U.S. Army Corps of Engineers)|
According to Project Design Engineer Robert Kwan, "unusual" noise and vibration prompted Corps personnel to cut short tests on a main regulating gate. Engineers were testing an outlet tunnel section repaired in 1999. At that time, debris dislodged a chunk of concrete from the chute while it was being used as a stormwater diversion route during main dam construction. In March, the gate was three-quarters open when tests were halted, Kwan says.
To repair the damage from 1999, the dam's contractor, Brazillian-based Odebrecht Construction overlaid original design elevations with 7-inch-thick lift of high-strength, silicafume-enhanced concrete with a 28-day cure strength of 8,000 psi. Paulo Suffredini, Odebrecht executive vice president in the firms U.S. office in Coral Gales, Fla., confirms that the repairs occurred.
The Seven Oaks Hydraulics Instrumentation Test Plan was implemented after an extraordinarily rainy winter filled the 145,000 acre-foot capacity reservoir to a level of 290 ft. "The inflows into Seven Oaks presented us with the first time that Seven Oaks mattered as a flood control structure," says Joe Evelyn, chief of the Hydraulics and Hydrology Branch at the Corps' Los Angeles District, in a release describing the results of the test. The $500-million rock fill Seven Oaks Flood Control Dam on Santa Ana River is a high head dam similar in design to Glen Canyon Dam. It's designed to operate under the high pressures associated with a deep reservoir," Evelyn
explains. "Fortunately, we didn't have to make our initial flood control releases during a major flood in the middle of the night. We generally expect to see some shakedown issues with a project of this magnitude.
The location of the repair happened to be the same spot on the 1,600 ft-long outlet tunnel where the full force of the water -- 2,500 cu ft per second -- from the hydraulic vertical lift gate hits the channel. Any imperfections are potential for cavitations where the water collapses onto itself, causing an implosion which takes the concrete with it, Kwan says.
Kwan says the Corps will examine data collected from embedded monitors, do a core sample of the damaged area to ensure that the surface was cleaned and roughed up correctly before the repair work commenced, test to see if the high strength concrete not bonding properly to unenhanced concrete might be at fault or if an added bonding agent is needed. It will be repaired before the next rainy season, Kwan says. Lowering the remaining pool of water to a rate of 700 cfs using a combination of a steel pipe running underneath the tunnel and the low flow regulating gate could take six to eight weeks, he adds.he Army Corps of Engineers is investigating workmanship, bonding agents and variances in the chemical properties of cement as possible causes of a 30-ft-square patch of concrete spillway base washing away, leaving only rebar during a March test of the Seven Oaks Dam flood release system.