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Two Jobs Not for the Faint of Heart
"Changing site conditions" on South American dams take on a new meaning
By Stephen H. Daniels

Amid political chaos, international environmental protest and outright violence, the world's two highest roller-compacted concrete dams are taking shape in South America at a rate of up to 585 cu yd per hour.

Contractors placed the last RCC batch on the $430-million, 617-ft-high Miel Dam on Colombia's La Miel River in late July. The $568-million, 503-ft-high Ralco Dam on Chile's Biobío River is slated for completion in November.

HIGH RISK Dams on Colombia's Miel River, top, and the Biobío in Chile, bottom, both used roller-compacted concrete. In each case, the contractor opted to use conveyor systems to place RCC.

Both Ralco and Miel dwarf the highest RCC dam in the U.S., the 308-ft-high Olivenhain, the centerpiece of the San Diego County Water Authority's $774.5- million Emergency Storage Project (ENR 5/7/2001 p. 11). reaching limits. Miel is being built by a joint venture of large South American contractors-Odebrecht SA, San Salvador, Brazil; and Bogotá-based Ingetec S.A. Some wonder if it approaches the theoretical height limit for RCC dams. Ernest K. Schrader, Schrader Consulting, Walla Walla, Wash., a consultant to Miel designer Carlos Angulo, president of Bogotá-based consulting engineer Hidroestudios S.A., says this is not the case.

Schrader's opinion comes from experience. He was on the consulting team at Willow Creek. That impoundment, completed in 1983 near Hepner Creek, Ore., was the first major dam in the U.S. built with RCC. He also acted as a consultant on the first RCC dams in Peru, Australia and Brazil. "I don't think we've reached the technical limit for any kind of dam, including conventional dams." Economics are the limiting factor. "Not many dams of over 200 m are economically justifiable," says Schrader.

Odebrecht opted for a conveyor delivery system at Miel instead of delivering concrete to the site by truck. Schrader says the choice was a prudent one. "Evidence has recently shown that RCC delivery by trucks can result in damage to the RCC and decreased strength," he says. "Cores [of earlier projects] showed lower quality for the area placed by trucks. The compressive strength of cores drilled in the area delivered only by conveyor averaged 10.1 MegaPascals compared to 7.1 MPa for the area where trucks were used."

The splitting tensile strength of cores from the conveyor area was 1.84 MPa compared to 1.56 MPa where trucks were used, according to Schrader. Densities were acceptable for both areas, but more variable and slightly lower for truck delivery.

STEEP Ralco's 45Þ conveyor "sandwiches" material between two belts.

ROTEC Industries Inc., Elmhurst, Ill., the equipment supplier to both projects, was called upon to create material-handling systems that would perform under what project manager Marc Milobowski says were "near-impossible conditions." At Ralco, the slope of the abutment is roughly 45Þ, an angle originally thought to be beyond the capability of conveyors. "The first pour had everybody scared to death," says Robert Oury, ROTEC president. The answer was a cleated second belt that rode on top of the material, in effect creating an envelope that kept the concrete from draining and separating, says Milobowski.

At Miel, a long-line conveyor system to a mixing plant a half-mile from the dam site rises 100 yd on a 60Þ mountainside before entering a 325-yd-long, 3.4-yd-dia tunnel cut devoted to material movement.

To put equipment on the jobsite and deliver the RCC slurry to the face, crews employed a 2,500-tonne-meter tower crane that stretches 660 ft from hook to base. Oury describes it as "the largest in the world," with a 330-ft boom capable of a 2,500-ton pick at full extension. It could "pick a fully loaded ready-mix truck at one goal post and put it down at the other, " says Oury.

The crane, first used in Mexico, and then at Xiaolangdi Dam on China's Yellow River, was shipped to Colombia's Pacific Coast port of Buenaventura and trucked 500 miles to the site, 300 miles northwest of Bogotá.

Miel consists of an incorporated spillway at the convergence of the Miel and Moro rivers. Its diversion tunnel is accompanied by upstream and downstream cofferdams that permit construction of the main dam, which will use three 125-Mw Francis-type turbines to generate 405 Mw. Click here to view map

A 950-ton-per-hour crusher plant, located almost 3,000 ft from the main dam, processed 2.6 million cu yd of excavated material. A central automatic Bentonmac mixing plant featured four horizontal mixers and six 150-ton silos for cement, aggregate and sand. Additional cement was stored in three supplemental silos, and a warehouse stored nearly 4,000 tons of cement in bags.

PACKED IN When complete, Endesa dam will contain 2 million cu yd of material.

Each of the four mixers discharged onto a conveyor belt that fed to a high-capacity extractor conveyor. The configuration allowed unloading of several mixers simultaneously. Concrete dumped into a 59.15-cu-yd surge hopper that in turn discharged onto conveyors to the dam.

The 585-cu-yd-per-hour placing system sported a total conveyor length of about 1,000 m, a tower crane with a hook capacity of 30 tonnes at 100 m and a crawler placer.

Ralco will contain 2 million cu yd of RCC upon completion. The project has been a nightmare of immense proportions for Buena Vista, Colo.-based consultant ASI RCC Inc. and local general contractor Febrag, S.A., Santiago.

Endesa S.A., the Spanish-owned electrical developer, nearly abandoned the 570-Mw project two years ago, after a howl of international protest over displacement of 100 Pehuenche Indian families. When the project resumed early last year, back-to-back 50-year storms wiped out the cofferdam, halting work.

But the contractor says jobsite problems are tougher than social protests or storms. "The principal problem facing Ralco is difficulties in excavating the access tunnel," says Endesa project manager Beatriz Monreal H. Unforeseen geology has slowed excavation and forced a schedule change to accommodate early concrete lining of the tunnel, she points out.

Instead of rollers, Febrag has opted for immersion vibrators. To achieve compaction, the team is using grout-enriched concrete throughout the dam in place of the mix typical to earlier RCC dams. Last winter, when mean daily temperatures seldom rose above freezing, the contractor took steps to keep the job moving: heating aggregates and water, and covering exposed surfaces with thermal blankets. Despite the conditions, Febrag has been able to meet its production goal of 5,500 cu m per day.

Fighting for Land. Ralco has proceeded despite entrenched local opposition and lingering uncertainty over land ownership. In March, 100 Pehuenche Indians and their supporters blockaded a road to prevent a 225-ton transformer from reaching the construction site. They then overpowered the drivers of three trucks in the convoy and used the vehicles to block the highway. "That level of violence has not abated," says Patrick McCully, campaigns director for the Berkeley, Calif.-based environmental conservation group, International Rivers Network.

A Santiago Appeals Court ruled in May that Endesa could purchase the property of seven Pehuenche families, following condemnation proceedings. The families refused relocation offers, and their attorneys have filed an appeal with Chile's Supreme Court.

LONG RUN At Miel, 3,300 ft of conveyors moved 585 cu yd of RCC an hour.

McCully says international opposition to Ralco, the second of six dams planned for the Biobío, and violent local protests are continuing. The ecosystem, fed by glacial runoff, is considered one of the most beautiful whitewater kayaking runs in the world. "It is unreasonable to persist in our reliance upon hydropower, especially in the face of a continuing drought that has caused blackouts and economic dislocations throughout Latin America," McCully says.

"We continue to have interactions with local Indian tribes," says ASI President Jeff Allen. "This is an environmentally sensitive area." But Rotec's Oury notes that "both these dams are in incredibly difficult environments. In Colombia, drugs are a business; kidnapping is an industry. One of my guys was robbed at gunpoint twice in the same day."

The "going rate" for a prominent foreigner-engineers qualify as prominent-is $250,000, according to Oury. Most multinational firms doing business in Colombia routinely buy kidnapping and ransom insurance, although many policies forbid the holder to even confirm that coverage is carried. "Colombia is a tough place," says Chilean-born Rolo Malschafsky, ROTEC senior project manager. "If you can work in Colombia, you can work anywhere in the world."

Despite the protection of 500 government soldiers, who guarded the Miel project site, says Malschafsky, workers were forced to evacuate the site on more than one occasion. "We cannot leave the camp," he says. "If we go into town, we must be escorted by armed guards."

Despite the conditions, or perhaps because of them, says Oury, "this is the project we will be bragging about for the next five years."

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