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
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
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
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.
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
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.
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.
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,"
"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
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."