In a remote San Diego county canyon, an impressive engineering feat is under way – the construction of California's first major roller-compacted concrete (RCC) dam. When complete at 318 feet high and 2500 feet long, it will be North America's tallest RCC dam. It is designed to withstand a 7.2 (magnitude) earthquake and remain operational during and after the quake.

The dam's 24,000 acre-feet of storage is part of the San Diego County Water Authority's plan to provide an emergency water supply for San Diego in the event that drought or earthquake interrupts the city's current imported water supply.

The water authority will fill the dry canyon behind the Olivenhain Dam with imported water from the Metropolitan Water District. Construction is also underway to build a tunnel between Olivenhain Dam and nearby Lake Hodges. In the near future, water may be exchanged between the two lakes to produce electricity at a generator that would be built at Lake Hodges.

On a recent tour of the construction site, an international audience of construction professionals witnessed the benefits that have made RCC the material of choice for building new gravity dams and rehabilitating existing dams worldwide.

"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," explained David Akers, field engineer for California Cement Promotion Council and one of the tour hosts. "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. It is proven as the best material for providing overtopping and spillways for earth dams and is used to improve the stability of existing of existing concrete dams.

Bruce Bennett, Assistant Project Manager for the project's construction management firm, Washington Group International, pointed out the features of the Olivenhain Dam's remarkably self-contained construction site. With exceptions of cement and fly-ash, construction materials for the project are produced on-site.

Granite is blasted from an on-site quarry, crushed and processed into three grades of rock and sand. Aggregates are conveyed to a twin-drum batch plant that produces the specialized RCC concrete at a rate of 1000 yards per hour. To chill the mix, and adjacent ice plant produces 130 tons of shaved ice per day. All this is powered by a row of trailer-sized generators.

An impressive on-site lab provides quality control for the aggregate, batch plants and field samples of RCC. Fifty samples of raw materials and RCC move through the lab every day. In addition to standard compression tests of the RCC, accelerated curing tests are also performed. Concrete samples are heated, simulating long-term curing conditions, to provide early information on the strength of the RCC structure.

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

The dam will require 1.3 million cubic yards of roller-compacted concrete. The stiff zero-slump mix of rock, sand, cement, fly ash and water/ice is 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 must be placed during the evening and morning hours to keep the mix within a specified 60 F – 75 F degree temperature range.

Round the clock construction began October 2001. Plans are to begin filling the dam July 1, 2003.