Nurek dam (Tadzhikistan)

The 300 m high Nurek dam across the Vaksh River is the highest rockfill dam in the world. Located in a narrow gorge on the river, it forms a reservoir with a gross capacity of 10.9 cubic km, of which 4.5 cubic km аre used for storage. The main objective of the project is generation of hydropower, for which it has installed capacity of 2700 MW.

Geology

The rocks at the dam site consist of intercalating sandstones and siltstones having a uniform monoclinal dip of 30 grades to 40 grades towards the upstream while the right bank jointing of the sandstones and silt stones is weak and developed along the bedding planes. Joints of tectonic origin extend normally to the bedding planes. All joints are partially or completely filled with sandy loam and clay.

General layout

The general layout of the dam and ancillary structures is shown in Fig. 1. The axis of the dam is straight. The hydropower plant (HPP) of open type, having 9 units of 300 MW each, is located on the right bank near the toe of the dam and is supplied by 3 headrace tunnels with separate intakes, each tunnel feeding 3 penstocks. 2 spillway tunnels on the left bank are provided to take care of any extraordinary flood.

Fig. 1. General layout of Nurek dam, HPP and ancillary structures1 - headrace tunnels; 2 - temporary headrace tunnel; 3 - penstocks; 4 - construction tunnels; 5 - superflood spillways; 6 - gully tailrace tunnel; 7 - freight tunnels; 8, 9 - road transport tunnels; 10 - freight and ventilation shaft; 11 - ventilation shaft; 12 - dam core; 13 - HPP; 14 - shoreline at normal pool level; 15 -water surface contours; 16 - drainage tunnel in penstock area.

Dam design

The dam section has a central core with slopes of 0.25:1 protected by filters on both sides. The outer pervious zones or shells are composed of coarse gravel & pebble. Oversize rockfill surcharge stones are provided on slope surfaces on both sides (Fig. 2). At the base of the dam core in the river section a concrete block 157 m long, 30 to 60 m wide with a total volume of 149000 cubic m has been provided.

Fig. 2. Cross-section of 300 m high Nurek rockfill dam with central clay core:1 – sandy loam core; 2 – shells of coarse gravel & pebble; 3 - filters (d=0-5 mm & d=0-50 mm); 4 – oversize rockfill surcharge stones on upstream slope (20-40 m thickness), downstream slope (5-10 m thickness) & downstream cofferdam; 5 – downstream cofferdam of oversize rockfill; 6 – upstream cofferdam; 7 - concrete block; 8 – consolidation grout & grout curtain; 9 – anti-seismic belts; 10 – control gallery 2x2 m; 11- control gallery 4x4 m on dam crest; 12 – contour of first-stage dam construction; 13 – temporary clay blanket of first-stage dam construction

The concrete block fills the narrowest bottom portion of the valley, in which placement and compaction would have been difficult and expensive. What is more important, it provides a plane surface for core contact, free from hollows and protuberances, so that a tight junction has been obtained. Further, it accommodates 3 interconnected grouting galleries. A special feature of the dam is 3 control and grouting galleries provided through the dam core.

The core material consists of sandy loam with a minimum of 50% material finer than 5 mm. The filter adjacent to the core has a size range from 0.1 to 10 mm with sizes smaller than 5 mm constituting not less than 60%. As there was considerable variation in the properties of the core material procured from designated borrow areas, homogenisation and moisture adjustment to the optimum requirement of 10 to 12% had to be carried out for about 8 millions cubic m of soil. This was done by placing the excavated soil in stockpiles, wetting it and later re-excavating it through vertical cuts.

As the gorge section has fairly steep slopes the possibility of tensile stresses resulting in cracking or hydraulic fracturing of the core had to be guarded against. The technique of centrifuge model testing was employed for this purpose. The tests indicated tensile zones in the upper part of the core, adjacent to the canyon walls. Cracking was found to occur at a tensile strain of 7 x 10 (power -5) or more. The cracks in the model were located at a distance of 0.2 to 0.3 m of the crest length from the abutments and were 45 to 50 mm deep, which would represent 9 to 10 m in the prototype. As remedial measures more deformable soil with a lower modulus was placed in the susceptible zone adjacent to the abutments and a larger surcharge of soil was placed on the core. Subject to these precautions, studies indicated that the compressive stress in the core would exceed the hydrostatic pressure by 20 to 30% over most of the core height, and by about 10% at its contact with the concrete block at the base.

Foundation Treatment

Besides the three grouting galleries in the concrete block, a number of galleries were provided at different elevations to enable grouting of the abutments. Consolidation grouting to a depth of 20 m was done under the contact surface of the base block. A deep grout curtain penetrating jointed rocks of relatively high permeability and connecting to tight rock has been provided both in the foundation as well as in the abutments. Multilevel drainage tunnels were also provided on the left abutment.

Compaction

Self-propelled rubber-tyred rollers on 30cm thick layers were used for compaction of the core.

Photo. Downstream view of 300 m high Nurek rockfill dam with HPP (2700 MW) & outdoor switch yard