A remote San Diego County canyon has just witnessed an impressive engineering feat--the construction of California's first major roller-compacted concrete (RCC) dam. At 318 feet high and 2500 feet long, it is North America's tallest RCC dam. It is designed to remain operational during and after

The dam's 24,000 acre-feet of storage is part of a plan to provide an emergency water supply in the event that drought or earthquake would interrupt San Diego's current imported water supply. The water authority will fill the dry canyon behind the Olivenhain Dam with water imported from the Metropolitan Water District. Construction is also underway to build a tunnel between Olivenhain and Lake Hodges. In the future, water may be exchanged between the two lakes to produce electricity at a generator that would be built at Lake Hodges.

According to Bruce Bennett, assistant project manager for the project's construction management firm, Washington Group International, construction materials, except cement and fly ash, have been produced onsite. Granite, blasted from an onsite quarry, was crushed and processed into three grades of rock and sand. Aggregates were conveyed to a twin-drum batch plant that produced the specialized RCC concrete at a rate of 1000 cubic yards per hour. To chill the mix, an adjacent ice plant produced 130 tons of shaved ice per day.

An earthen-tone concrete stain allows the dam to visually blend in with the surrounding local terrain.

The stepping of the dam is a cost-saving measure. Because water pressure behind the dam is greatest at the bottom and decreases as the dam gets higher, the dam can decrease in thickness as it goes tip. Vertical formed steps are less expensive to place than a curved form.

"Roller-compacted concrete dams require one-third to one-half the construction time of conventional concrete or earthfill dams and are as strong and safe," says David Akers, field engineer for California Cement Promotion Council. "This cost savings in construction makes roller-compacted concrete popular with dam owners, engineers, and safety regulators."

RCC is increasingly being used to rehabilitate earthfill and concrete dams and is used to improve the stability of existing concrete dams.

The dam required 1.3 million cubic yards of roller-compacted concrete. The stiff zero-slump mix of rock, sand, and cement, fly ash and water/ice was carried by a half mile of conveyors to the top of the dam, placed in a layer 18 inches deep using haul trucks and bulldozers, then compacted to 12 inches. Concrete was placed in the evening and morning hours to keep the mix within the specified 60[degrees] to 70[degrees]F range.

"Setting world records while maintaining quality control for the contractor and owner is the most challenging, and most satisfying, aspect of this particular job," said Bennett.