Nurek
Dam stands 984 feet (300 m) tall and is considered one of the largest rockfill
dams in the world. It is situated near the Rogun Dam, which, at 1,099 feet (335
m), is another one of the world's largest dams. These structures are located on
the Vakhsh River in Tajikistan in central Asia—a region characterized by high
seismicity.
The
Nurek Dam was constructed between 1961 and 1980 by the Soviet Union, when
Tajikistan was still considered a part of the republic of the Soviet Union.
Inclusive in the dam are nine hydroelectric generating units. The first was
installed in 1972 and the final in 1979.
Construction
History
The
dam was constructed in the design of a pyramid along the Vakhsh River canyon,
reaching more than 2,296 feet (700 m) over the top and consisting of 70.6
million cubic yards (54 million m3) of earth and cement.
Because
of the abundance of material on-site, the Nurek Dam was constructed using
materials found locally, making the construction considerably less expensive
than the alternative types of dams built using other materials.
The
construction phase involved preparing the materials to be used in the dam’s
construction, compacting them, and preparing layers onto which they would be
poured and settled.
Although
the majority of the dam is a rock- and earthfill dam constructed with locally
found materials, an impermeable barrier was built out of cement to separate the
central core from the 984-foot (300-m) dam.
The
construction project involved filling the mound with 71 million cubic yards (54
million m3) of material.
Construction
of Concrete Plug
Because
of the location of the dam, a concrete plug was constructed for the purpose of
helping the structure handle high pressure and seismic activity combined with
the strength of corrosive waters. With a pressure head of 918 feet (280 m) and
seismic activity of nine, a concrete plug was a necessity to keep the dam
stabilized.
To
combat these factors, the construction of a clay loam core for the dam was
considered, along with a foundation built entirely out of rock. Three concrete
grouting galleries were set up and arranged in a symmetric order under the dam
core.
Excavators
penetrated rock and galleries were constructed, enabling the planning stage for
the grouting procedure to begin. The dimensions of the central gallery were
measured at 15 x 18 feet (4.5 x 5.5 m), and another gallery extending up and
downstream was measured at 11.5 x 18 feet (3.5 x 5.5 m).
The
concrete plug was constructed at the river section of the dam core and was used
to fill the gap left by excavated rock. The concrete plug, measuring 515 feet
(157 m) long and 98 to 196 feet (30 to 60 m) wide, has a volume of 194,884
cubic yards (149,000 m3). At this size, the plug is able to tightly constrict
the junction that used to exist between the dam’s core and the rock.
The
core was constructed to have smooth curves, intended to prevent cracks and
perching that could risk the stability of the core materials used in
construction.
In
addition to this, the concrete plug raised the dam base to the level of the
rock base located under the core beneath the canyon. This is beneficial in that
it reduced the amount of rock blasting from excavation processes that take
place in the pit.
The
construction of the grouting galleries within the plug made the foundation of
the dam. Along with three interconnected galleries are several chambers. The
rocks between these chambers were grouted as deep as 65 feet (20 m) and run the
entire length of the plug until reaching the rock.
A
stainless steel strip was used in lieu of bitumen to seal the joints of the
concrete structure.
Because
the canyon was quite narrow, a concrete-placement system was carried out using
cranes mounted on Type SE-3 excavators and Type MAZ-503 dump trucks that poured
the concrete directly into blocks that would make up the canyon’s bridging.
Compaction of the Earthfill in the Nurek Dam
The
Nurek Dam is the largest earthfill dam in the world, filled with rock to a
volume of 73 million cubic yards (56 million m3). The compaction of the
earthfill was completed in seven zones.
The
dam was constructed in a layer-by-layer method by assigning dry materials
(aggregate) into zones. Although the method was not conventional, it was
necessary in preparation for filling the reservoir. Once prepared, soil was
loaded by excavators with bucket capacities of six to 7.3 cubic yards (4.6 to
5.6 m3) into 27-ton capacity dump trucks.
The
borrow pit, where a lot of preliminary preparation was carried out, consisted
of an alluvial fan of dry wash – mostly loam and loam rubble measuring 3.1
inches (80 mm) in size. Once the material was gathered, it was dumped into
segregation cones. Upon mixing the materials proportional to the cones, any
large fractions (usually larger than 7.9 inches or 200 mm) were removed and the
sections were moistened.
The
near-contact zone was also moistened. This zone comprised soil on the core that
was arranged in stockpiles, with fractions exceeding 2.8 inches (70 mm) to be
moistened and carried out. Of the prepared soil, only aggregate 60 to 80
percent smaller than 0.2 inches (5 mm) was permitted -- this excluded 10
percent of clay.
On
the downstream side of the dam (between the core and the shoulders) is a
two-layer filter, which makes up zones 2 and 3. Zone 4 is the upstream side of
the dam and has both a two-layer filter on the same level as the reservoir, and
a one-layer filter located lower down.
The
first layer of the filter on the downstream side, also known as zone 2,
consists of artificial sand and materials that were filtered by screening and
crushing. Crushed into a fine aggregate, the sand was measured into fractions
between 0.002 and 0.4 inches (0.05 to 10 mm).
Zone
3, or the second layer of filter on the downstream side, consists of rubble
measuring between 0.002 and 1.6 inches (0.05 to 40 mm). Similar to the sand
used in Zone 2, the rubble was also screened and crushed at the filter plant.
Zone
4, which is made up of one layer of filter on the downstream side, consists of
a mixture of gravel and sand, also prepared at the filter plant and screened so
that the fractions were no larger than 1.6 inches (40 mm).
Zone
5, the shoulders, also consist of a mixture of gravel and pebbles. The mixture
is coarse and fractions are larger than 5.9 inches (150 mm).
Zones
6 and 7, which are a surcharge of slopes, comprise broken rocks anywhere from
16 to 28 inches (400 to 700 mm) located on the dam’s sides. These two zones use
a variety of materials, but mostly limestone and sandstone.
Once
the zones were allotted and prepared for filling, the materials would be loaded
by excavators into dump trucks and hauled to the dam. The soil would fill in
the zones and be leveled by heavy machinery.
Special
vibrating rollers, PVK-70EAs, were used in combination with interchangeable
roller drums to level out and compact the filled materials. The frequency of
the vibration provided by the rollers acted at a high density and reduced the
amount of time needed for compaction, a benefit when constructing the Nurek or
any other type of earth or rock dam.
Because
of the speedy compaction of the dam, the geotechnical monitoring of the soil
placement could be carried out—a practice required due to the unstable region
in which the dam was constructed.
Building
the Reservoir
The
reservoir, known simply as Nurek, is the largest of its kind in Tajikistan at
43 miles (70 km) in length, with a surface area of 61 miles (98 km). The
purpose of the reservoir was to feed the hydroelectric plant near the dam with
fuel, as well as act as a source of irrigation for the local agricultural
region. When irrigation is needed, water travels through the 8.9-mile (14-km)
Dangara irrigation tunnel.
One
of the drawbacks concerning the reservoir is that it may be a contributing
factor the area’s high seismic activity.
The
reservoir makes up 6.5 cubic miles (27.1 km3) of water and extends over a
surface area of 60 square miles (98 km2) that reached up 43 miles (70 km)
towards the direction of the valley. It is the largest reservoir in Tajikistan.
Unique
Facts
The
USA-USSR Exchanges in Earthquake Prediction rank Nurek as one most completed in
terms of RIS monitoring.
1971
marks the year of the most substantial increase of water needed to fill the
reservoir in its existence.
The
Nurek Dam was the tallest dam as of 2005. It was anticipated that the Rogun Dam
would exceed the Nurek upon the construction completion date. The Rogun Dam
would stand at 1,099 feet (335 m).
Each
of the nine hydroelectric turbines were built with the capacity to generate 300
megawatts, but have since been reconstructed to generate as much as 3,000
megawatts. It generates so much energy, in fact, that it accommodates 99
percent of Tajikistan’s electricity needs.
The
Varkhsh River is home to five completed dams and four that are currently under
construction.
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