A remote wilderness region of eastern Iceland is the scene of the Kárahnjúkar hydro power project
With a population of only 320,000, Iceland has a considerable surplus of hydroelectric and geothermal power potential. The country is not connected to Europe by cable and thus its electrical power potential is ‘stranded’. To turn this green energy into revenue for the country, power intensive industries, such as fertiliser and then aluminium production, were introduced. Iceland now produces around 800,000 tons/yr of aluminium, which is more than 2% of world production. Electrical production in the country is 16 TWh/yr, of which 75% is hydro and 25% geothermal.
Three major glacial rivers flow from the north eastern part of Vatnajökull glacier, the largest glacier in Europe which is about 9000km2 in area, covering almost one-tenth of Iceland. Plans for harnessing these rivers for electricity production were introduced as early as the 1950s. Research and preparations were gradually focused on the two easternmost rivers with about half of the total energy potential, Jökulsá á Dal and Jökulsá í Fljótsdal, flowing through separate valleys to a common estuary at the north east coast. These two rivers were finally harnessed in one combined scheme called the Kárahnjúkar hydroelectric project.
Approved by the Icelandic Parliament in 2002, construction started in 2003 with completion in 2008. The 690MW project, which has an annual generating capacity estimated at 4.6TWh/yr, was built to provide power for a new aluminium facility built by Alcoa, USA, on the east coast of Iceland. Landsvirkjun, the National Power Company, owns and operates the plant and has a 40-year power purchase agreement with Alcoa. Financing was secured by Landsvirkjun using mostly European banks.
Economic development of the project was based on a head of 600m. The low lying Fljótsdalur valley cuts the highland plateau north of Vatnajökull and creates such a high head for power production.
Jökulsá á Dal is dammed at the Kárahnjúkar hills with three dams. The largest one, Kárahnjúkar dam, crosses the southern (upper) end of the Hafrahvammar canyon and is about 700m long and 198m high. This is among the highest CFRDs in the world and is the highest of its kind in Europe. During construction the river was diverted through two diversion tunnels under the western bank of the dam. Two smaller rock fill dams with earthern core, 68m and 29m high respectively, were built on each side of the Kárahnjúkar dam.
The three dams at Kárahnjúkar create the Hálslón storage reservoir, 25km long to the toe of the glacier and 57km2 in area. The Hálslón reservoir is built for seasonal storage of 2.1Bm3 and fills up in late summer most years. Surplus water in the fall is diverted through a spillway chute at the western end of the Kárahnjúkar dam down to the edge of the canyon and from there in a 90m high waterfall, Hverfandi, into the canyon.
The eastern glacial river, Jökulsá í Fljótsdal, and two smaller streams further east on the Hraunaveita Diversion are dammed on the east side of the Snæfell mountain with minor reservoir storage at Kelduárlón (7km2) and an intake pond named Ufsarlón on the Jökulsá í Fljótsdal. The three dams are rock fill dams with earthern core, up to 37m high.
From the Hálslón reservoir, the water runs through a tunnel under the Fljótsdalsheidi moor to a juncture with another tunnel from the Ufsarlón pond. The water then runs through one combined headrace tunnel northeast to an intake above the power station at the Valthjófsstadafjall escarpment. The headrace tunnels total 53km in length and the tunnels are generally 100-200m under the surface of the plateau. The tunnels are 7.2-7.6m in diameter and designed for a flow of up to 144m3/sec.
Two vertical pressure shafts, 420m high and lined with steel, lead the water from the intake at the end of the headrace tunnel to the underground Fljótsdalur hydropower station, located about 1km inside the mountain. The total head at full storage level is 600m. The station contains 6x115MW Francis units. A tailrace tunnel and a tailrace canal take the water to the course of Jökulsá í Fljótsdal.
Preparatory construction for access started in 2002. The contract with Alcoa was signed in March 2003. The main bids from contractors were already in place and construction started a few weeks later.
The project was split into various contacts of a manageable size to suit a broader spectrum of contractors, engineering firms and suppliers, including domestic ones. The bids were mostly received in 2002 and 2003 and were in line with estimated costs and budget. The two Impregilo contracts, for the headrace tunnels and main dam, corresponded to about 55% of the project. The numerous contracts required increased overall co-ordination and effective management but this method has been very successful in recent decades at Landsvirkjun and has resulted in a stronger domestic engineering and construction industry.
Geological difficulties at the foundation of the main dam (faults) and a section of the tunnels (faults and excessive water) consumed all spare contingency of the financial budget. The fact that the geological difficulties did not result in excessive time delay was thanks to modified project execution solutions introduced by the management team and the hard efforts of the largest contractor, Impregilo of Italy, which drilled the headrace tunnel by TBM and built the main CFRD dam. Work continued all year at an altitude of more than 600m in arctic conditions, even casting concrete at the dam under protection shelters in the middle of the hard winter.
The project area was widespread and construction camps with up to 1800 people were set up at eight different locations. About one-third of the workforce was from Iceland but more than 40 nations were represented on the project.
Filling of the main storage reservoir, Hálslón, behind the high Karahnjukar dam, started on schedule in September 2006. All six turbines were fully operational in December 2007, some two months behind schedule. The reason for the minor delay was geological difficulties in the tunnelling. The eastern diversion part, providing some 25% of the total water, was completed as scheduled in the fall of 2008.
The Kárahnjúkar CFRD is the main dam with a rock fill volume of 9Mm3 and a height of 198m. Impregilo of Italy is the contractor for this dam. The rock fill is produced from a quarry inside the reservoir just upstream from the dam and was partly transported into the dam after crushing and screening by conveyor belts. The concrete face slab was cast in 15m wide bays by slip forming.
The dam crosses a deep canyon with vertical cliffs. It was designed to cross the canyon on the upstream side, with a massive 50m high concrete toe wall in three separate monoliths, instead of a more conventional concrete foundation plinth at the toe of the face slab. The toe wall is designed to withstand fault movements using elastic joint sealing measures and the dam as a whole to withstand heavy earthquake.
The grout curtain below the dam extends to a bedrock depth of 100m. A gallery is provided at the toe of the dam for later inspection and eventual further cement grouting.
To keep the project on schedule the dam construction, including concrete works, had to continue through the winter periods, casting concrete under cover and using heating measures. Excavation at the dam foundation started mid 2003, and the diversion tunnel gates were shut to start lake impounding in September 2006. The reservoir reached the full level of 625m asl late in the summer of 2007. Seepage under the dam at full pressure is approx 200l/sec, which is well below what was expected at this site. Settlements of the fill are also well within expectations. This is important as the dam is one of the highest of this kind in the world and the concrete membrane is sensitive to excessive settlements. Monitoring of movements show that the membrane is performing very well and movements of the joints are minimal.
Two saddle dams on each side of the Kárahnjúkar dam, one of which is 68m high, and three further dams on the eastern river are all rock fill dams with a central core of moraine. These dams with a combined volume of 6Mm3 were built by local contractors Suðurverk, Arnarfell and Istak.
The spillway at the west abutment of the Kárahnjúkar dam is an overflow spillway designed for a maximum probable flood of 2250m3/sec. This is a challenge for a height of 200m. The uppermost part is through a concrete chute down the west bank slope followed by 90m of free fall into a plunge pool in the vertical cliff canyon downstream of the dam.
This facility included several engineering challenges such as the prevention of scouring in the narrow canyon from the free falling jet. The hydraulics were tested in the hydraulic laboratory of the Swiss Institute of Technology. The plunge pool was created by digging a deep pit into the rock foundation and building a concrete weir across the canyon.
Iceland is basically made up of volcanic basalts which are very young in geological context. At the eastern part of the Kárahnjúkar project the rock is composed of layers of lavas, a few million years old, compressed during the Ice Ages. The layers dip slightly from east to west and are frequently crossed by faults and dykes. The western part, the area of the main dams and reservoir, is even younger; the youngest formations are just a few hundred thousand years old. Some of these were created under the ice during the Ice Ages, forming the moberg type of basaltic rock, a heterogeneous type frequently quite permeable.
Out of the 72km of tunnelling, some 50km were drilled by three full face boring machines, (TBM) supplied by Robbins and operated by Impregilo. TBMs had never been used in Iceland and the frequent mixed face conditions of the layered lava and the heterogeneous moberg was regarded as a challenge.
A breakthrough of the main 40km long headrace tunnel was accomplished in December 2006 after two and a half years of drilling. Drilling of the eastern diversion tunnel with one of the machines continued into springtime of 2008.
In general the TBM drillings were successful, particularly in the older eastern part, where progress of the 7.6m wide tunnel well exceeded 600m per month on average. The best day’s work for one TBM was 116m, the best week 429m and the best month 1193m.
Two main problems had to be overcome. When drilling in the far western part in the permeable moberg, a heavy inflow of water (up to 900 l/sec) was experienced. In this part the TBM drilling of one km had to be abandoned, and was excavated by conventional drilling and blasting. An extra adit entrance was also introduced.
In the centre portion a short zone contained wide faults, up to more than 10m wide each with loose material and inflow of water. This particular zone took almost six months to pass, using stabilising measures by grout injections, foam and concrete in front of the cutter head and steel rib support behind the cutter head.
The headrace tunnel was filled with water in October 2007 to enable subsequent start up of the power station turbines in November of the same year.
In general the hydraulics system is complicated as the tunnels are very long (40km and 13km) and the two headrace tunnels potentially have different pressures where they meet. This was solved by introducing an inverted siphon at the first part of the side diversion tunnel, followed by an opening to the surface, and creating free surface flowing conditions to the point of equal pressure to that of the main tunnel. A very extensive pressure transient analysis was also made of the complex hydro tunnel system, including potential surge problems.
An extensive measurement programme was executed in the headrace tunnel to accurately evaluate hydraulic roughness of the unlined TBM bored tunnel. The measured head losses after operation started were about 10% lower than had been calculated from the roughness measurement programme.
The power station
The underground caverns for the main hall, transformer hall and the access, cable and tail water tunnels were excavated during a period of only 14 months by an Icelandic, German and Danish consortium Fosskraft. The drilling by raise boring of the two 420m high vertical shafts was also carried out. The shafts were subsequently steel lined by DSD Stahlbau of Germany. The ingress of water was minimal and the geology in general very favourable.
Six turbines, 115MW each, were supplied and installed by VA Tech Esher Wyss of Germany and Austria. These are high head Francis turbines with a rated through flow of 24m3/sec each with a speed of 600rpm. The splitter blade runners are very compact with a diameter of only 1.4m. The use of Francis turbines for such a high head of 600m is relatively unusual. However economic comparison between Francis and Pelton turbines showed that the Francis units were more economical. Indeed the turbines are reported to be performing excellently
Landsvirkjun was founded in 1965, it is owned by the government and has a total installed power capacity of almost 2000MW. Landsvirkjun has four smaller hydro projects in the pipeline, all in a river system in southern Iceland, with a total installed power of 350MW. Technical preparations are well advanced and construction of the first one is planned to start late in 2009 or early 2010.
|MAIN CONSULTANTS AND CONTRACTORS|
Main consultants for design:
Main consultants for construction management:
|Karahnjukar facts and figures|
Installed power capacity (6 x 115 MW Francis turbines): 690MW
Karahnjukar dam: 198m high, 700m length, 8.4M m3 Fill materials
Tunnels: Total approx. 72km