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Nurek Dam

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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.[1] Inclusive in the dam are nine hydroelectric generating units. The first was installed in 1972 and the final in 1979. [2]

Contents

[edit] 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. [3]

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. [4]

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.

[edit] 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. [5]

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.[6]

[edit] 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. [7]

[edit] 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.[8]

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. [9]

[edit] Equipment Used

[edit] Unique Facts

  • The USA-USSR Exchanges in Earthquake Prediction rank Nurek as one most completed in terms of RIS monitoring.[10]
  • 1971 marks the year of the most substantial increase of water needed to fill the reservoir in its existence. [11]
  • 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.

[edit] References

  1. Nurek Dam. Blogspot. 2008-10-07.
  2. Nurek Dam. BBS. 2008-10-07.
  3. Nurek Dam. Eurasianet. 2008-10-07.
  4. Fill Dam. BNET. 2008-10-07.
  5. Concrete Plug. Springerlink. 2008-10-07.
  6. Concrete Plug. Springerlink. 2008-10-07.
  7. Nurek Dam. Metapress. 2008-10-07.
  8. Nurek Dam. BBS. 2008-10-07.
  9. Nurek Dam. Eurasianet. 2008-10-07.
  10. Nurek. Telusplanet. 2008-10-07.
  11. Jansen, Robert B. Advanced Dam Engineering for Design, Construction and Rehabilitation. Springer: 1988.