In a time when new dams in environmentally conscious California are as rare as hen’s teeth, the state is about to welcome a record-setter. Twenty-seven miles north of San Diego, the tallest roller compacted concrete dam in North America is headed for completion this summer. Olivenhain Dam and Reservoir will wind up about $2 million below its $200-million budget, says Jerry Reed, project manager for the San Diego County Water Authority.

The 318-ft-tall, 2,552-ft-long concrete gravity dam is the centerpiece of SDCWA’s $827-million Emergency Storage Project. Scheduled for completion in 2012, the ESP will comprise over 90,000 acre-ft of new storage, 17 miles of pipelines, and five pump stations.

BEST FACE Dam's design offsets seismic loading by altering 1:1 slope ratio for the 318-ft-tall structure's top quarter. (Photo courtesy of Parsons-Harza)

The project will give the region a six-month backup if an earthquake or other emergency cuts imported water supplies, says Ken Steele, SDCWA’s principal civil engineer. The dam must function after a magnitude 7.2 earthquake, says Mike Rogers, resident design engineer with Parsons Harza, the project’s locally based joint venture design consultant. Olivenhain Municipal Water District is providing one-sixth of the dam’s funding and is entitled to a sixth of the reservoir’s 7.8-billion-gal capacity.

Cost and schedule drove the selection of RCC. The thick, dry mix of rock, flyash, water and cement generally costs about half of conventional concrete and requires one third the construction time, says Reed. As in road construction, RCC is placed by dump trucks and spreaders and compacted by vibratory rollers.

Tapering from 250 ft wide at the base to 20 ft at the crest, the dam crosses three valleys. Redding, Calif.-based Ladd and Associates excavated about 700,000 cu yd of material. Excavation depths ranged from 15 ft to 100 ft, with about 25 ft being typical, Rogers says.

To improve a fault-related depression at the left abutment, Kiewit Pacific Inc., general contractor for the primary structure, placed 230,000 cu yd of RCC for an 85-ft-tall shaping block below the base. And to strengthen soft, permeable rock at the right abutment, Kiewit placed 15,000 cu yd, creating a 25-ft-tall shaping block.

The location offered advantages as well as challenges. An abundant local supply of granite made importing aggregate or fines unnecessary. Kiewit’s crusher typically produced up to 1,300 tons of aggregate per hour, says Mike Pauletto, area manager for Kiewit Pacific’s Vancouver, Wash office. Concrete was conveyed to the batch plant and trucked to the dam, where it was applied in 18-in. lifts by spreaders, then compacted to 12 in. by 10-ton and 12-ton rollers.

Through extensive testing, the project team won approval from the state Division of Safety of Dams for a value-engineered, time-saving RCC mix. Kiewit proposed modifying the formula designed by Harrisburg, Penn.-based Gannett Fleming, reducing the cement content from 150 to 125 lbs per cu yd and increasing the flyash from 125 lbs to 185 lbs. The value engineered mix will achieve full strength of 4,500 psi after one year, compared with 3,000 psi for the original mix, Rogers says.

In a key contribution, the value engineered formula virtually eliminated a time-consuming intermediate step in concrete placement. To add tensile strength and prevent seepage between lift joints, bedding mix, a form of grout, is commonly added between each lift.

With its high cementatious content, the mix’s adhesive strength matches that provided by grout, Rogers says, so Kiewit could usually skip the bedding mix layer except after weekends or other long breaks in placement. The value engineered mix "resisted segregation" and "got compaction very easily," says Bruce Bennett, project manager for Washington Group International Inc.’s infrastructure unit, which serves as SDCWA’s project management consultant.

Kiewit was able to place this "contractor-friendly" mix "at a very high rate of speed," including a record 294,000 cu yd total for June 2002, Pauletto says. Production time was critical because RCC provides the best cohesion when fresh, SDCWA’s Reed points out: "The faster you go with RCC, the better the quality."

Though state officials ultimately agreed on the mix’s quality and benefits, the schedule put the team under the gun to make its case. "We had a budget. We didn’t have a lot of time to keep on doing trials to prove we didn’t need grout mix," Reed recalls.

Winning approval of RCC was also a challenge because "state officials are used to seeing it for rehab, rather than for new dams," Rogers says. DSOD expedited approval by taking the unusual step of joining in the partnering process. Pauletto calls it "a real bonus to have them on board with the partnering session every month."

RCC also "allowed us to curve the top part," providing a key seismic design feature, Rogers says. On the downstream face, the upper 83 ft step back more gradually than the 1:1 slope of the lower three quarters. The curvilinear design "eliminates the stress concentration" of seismic loads, Rogers says. Vertical joints at 100-ft intervals allow sections to move independently in an earthquake.

At each lift, the dam is tied into the 300-ft-tall, three-sided inlet-outlet tower, which Rogers calls "the money part of the job" because of its importance to the dam’s operation. With walls up to 6 ft thick at the base and 4 ft thick near the top, the tower is crowned with a control building extending 25 ft above the crest. Six gates at various locations will provide a SDCWA a choice of water quality in an emergency, Rogers says.

Because RCC dams can be subject to seepage between lifts, a drainage system is being applied to the dam’s 210,000 sq ft upstream face. A 2.5-mm-thick plastic geogrid is attached and covered with a polyvinyl chloride membrane. Every 12 ft along the upstream base, a 20-ft-long, 2-in.-dia. steel drainpipe will catch seepage and convey it to the 6-ft-wide, 8-ft-tall drainage gallery, Rogers says. With 10% to 20% of the liner still remaining to be installed, SDCWA expects to start filling Aug. 6. The process will take up to 10 months, Reed says.

The highlight of the Emergency Storage Project’s next phase will be a $199-million, 11-mile-long tunnel linking San Vincente Reservoir to SDCWA’s second aqueduct. Ranging from 50 ft to 300 ft deep, the 13-ft-dia. tunnel will contain a 10-ft-dia. pipeline.

Construction on the $199-million project is scheduled to start in late 2004. RCC will make a return appearance late in the decade for the program’s final phase, when SDCWA plans to raise 59-year-old San Vincente Dam from 234 ft to 288 ft.