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Iceland's national power company Landsvirkjum is building what it claims will be one of the largest concrete faced rock-fill dams in the world.Damon Schunmann investigates.

Iceland has two vital natural assets. The first, as anybody can tell you, is fish. But for an economy so heavily dependent on one industry (at 2002 figures, fishing accounted for 41% of the country's exports), the second resource, power, is becoming increasingly important.

The volcanic activity that renders much of the land infertile also means Iceland has large geothermal and hydroelectric potential. But harnessing this energy and converting it into usable power can require some heavy-duty engineering.

By anybody's standards the Karahnjukar dam in the east of the country is a huge project, as a resume of the statistics shows. About 53km of headrace tunnels at depths of 50m to 230m will feed up to 144m 3/s of glacial water from two main rivers and provide a 599m head of water to a 690MW power station. Quite a feat, but the numbers keep coming.

The main dam will contain 8.5Mm 3of fill material, standing 193m high and 800m wide to hold back 2,100Mm 3of water.

Electricity is far from scarce in Iceland and 90% of all buildings are heated by plentiful geothermal energy, so the reason for the project is more to do with exports, kind of. As Sigurdur Arnalds, public relations manager for national power company Landsvirkjum, explains: 'It is not possible to export electricity to Europe as it is too far away, so we attract industry here.'

The industry in question is energy-intensive aluminium smelting. Public relations officer Hronn Hjalmarsdottir adds: 'The reason this is so economically viable is because 100km inland where the highlands begin, the land is still only 25m above sea level. The rise there to over 500m gives the water a lot of energy.'

The $1.2bn (£651M) project, first conceived some 50 years ago, now involves yoking the Jokulsa i Fljotsdal and Jokulsa a Dal glacial rivers so they eventually feed into a mutual headrace tunnel, avoiding the need for a second large reservoir.

The 57km 2Halslon reservoir in the west is fed by the Jokulsa a Da and will provide about 75% of the water required. This then rushes 40km north east along a headrace tunnel varying from 7.2-7.6m diameter, to two pressure shafts in a valve chamber above the power station.

The other 25% of the water comes from the Ufsarlon pond to the east, which is fed by the Jokulsa i Fljotsdal river. From here, water will be carried 13km through a 6.5m diameter headrace tunnel to an intersection valve chamber about halfway along the main Halslon tunnel.

Because of its size, the scheme has been split into two parts. One is the dam and headrace tunnels, and the other is the power station area including the pressure shafts.

Separate tenders were invited for both the dam and the headrace tunnels, but Italian company Impregilo won the contracts for both jobs worth e 180M (£119M) and £182M respectively. Mott McDonald is leading the supervision team for this part of the project.

Work began in February 2003 and the enormous main Karahnjukar dam is now starting to take shape. The erosion protection on the downstream face is being built from up to 1.5m durable basalt rock laid against free draining basalt rock-fill that in turn will lean on locally quarried coarse pillow lava. The centre of the dam fill will be comprised of Moberg (a subglacial volcanic material) and pillow lava. The dam is founded over a layer of alluvial filter and transition materials covering the excavated surface.

Vibratory drum rollers provide 350kN dynamic compaction in most cases, except over areas of fine filter material of processed pillow lava or alluvial sandy gravel, where a 50kN plate vibrator is required. Lifts vary depending on the material being used but range from 2001,600mm.

The concrete face of the dam will be made from 15m wide by 120m long strips of slipformed concrete poured to create a 38infinity angle. It will be 600mm thick at the base where the pressure will be greatest, narrowing to 300mm at the dam crest.

Despite Iceland's plentiful geothermal activity, underground ice has been an issue.

Impregilo chief engineer Richard Graham says:

'During dam excavation we found permafrost and ice millions of years old which we had to blast.' Permafrost lying deeper than expected has meant that excavations have sometimes been taken down further than originally intended to get to sound rock below it.

Yet in geological contrast to this, there is a constant concern when boring the headrace tunnels.'What we dread is finding a geyser full of hot steam, ' says Graham.

The main dam is being built in a glacial valley whose riverbed has been eroded into a 40m wide by 50m deep canyon. The dam will extend 140m above the canyon shoulders, the east one being the slope of the extinct Karahnjukar volcano. A 55m wide, 45m deep and 18m thick concrete toe-wall will form the upstream dam foundation in the canyon. Above the canyon, the face of the dam will be connected to a complicated foundation plinth.

A geological fault runs diagonally across the now dry riverbed and directly under the dam and toe-wall. To account for movement, the fault now incorporates a bituminous paper joint in the infill concrete which is secured to the walls of the fault using 32mm diameter, 4m deep fully grouted rock anchors.

Beneath the toe wall, site workers will build a 120m deep grout curtain from 46mm diameter holes drilled at 3m or 1.5m centres, depending on grout take.

Icelandic company Sudurverk is constructing two saddle dams to the east and west of the Karahnjukar dam to enclose the Halslon reservoir. Both will be rock and gravel structures with an earthen core. The first will be 60m high and 900m wide while the second is 25m high and over a kilometre wide.

The dams must be completed by 1 December 2006, at which point water is scheduled to begin flowing to the power station. By the time it reaches the vertical pressure shafts, it will have dropped 179m, so the final 420m above the station will give a total head of 599m.

The shafts are being bored through the mountain to the station hall, which will house the six 115MW Francis turbines that will generate around 4,600GWh/year.

Another challenge has been securing enough water for the TBMs to operate.

Impregilo project manager Gianni Porta says:

'We've had to put in 5km of water pipe from a lake that we hope doesn't freeze in the winter.'

Ironically, drilling and blasting the headrace intake tunnel has provided more than 250 litres/s of unwanted water. 'We're going from one extreme to the other, ' says Graham.

The headrace tunnel is accessed through three adits which were excavated by drill and blast to save time before the three Robbins hard rock TBMs were delivered. These machines are each powered by ten 300kw drive motors and were expected to achieve an average rate of about 26m/day through the mostly basalt ground. Where tunnel support is not needed, the TBM's 480mm cutters are able to bore their way through 60m to 70m/day, and the best day so far saw 80m.When heavy support is required omega steel ribs are being used. Less immediate strengthening is achieved using shotcrete mixed with steel fibre at a ratio of 40kg/m 3.Subcontractor Skanska Raise Boring is using a Tamrock Rhino 2008 rod string drilling rig to excavate the pressure shafts. The rig drills a 380mm pilot hole while water at 60 bar is pumped down to flush detritus from around the 330mm diameter rods. The hole is kept plumb using a German rotary vertical drilling system (RVDS) which allows for a high degree of accuracy.

The supervision team for the power station and tunnels is comprised of five companies and German consultant Lahmeyer International is overseeing the pressure shafts. Inspection engineer Bernhard Leist explains: 'Four mechanical sensors above the drill bit that are not rotating, activate hydraulic pumps that correct the drill bit continuously. There must not be more than 1% deviation over the entire length and they achieved only 14cm [deviation] over 414m, which is an excellent result.'

But things have not gone entirely smoothly.

Leist says: 'With the first shaft we had no problems.With the second one there was a problem with the hydraulics of the RVDS and we had to bring it up rod by rod to put a new one.'

The drill rig is managing about 1.7m/h and once it breaks through the reaming will commence at a slower rate of 0.8m/h. Once complete, the 4.04m diameter shaft will be supported with a 30-50mm sprayed concrete lining and rockbolts where required. German subcontractor DSD Stahlbau will then install a 50mm thick steel lining of 3.4mm internal diameter and the annulus of about 220m will be backfilled with unreinforced concrete.

The international joint venture Fosskraft is building the power station end of the job. This includes the 103m long, 14.5m wide and 16m high transformer hall, the 9m by 9m tailrace tunnels and the 4m by 4m cable tunnels.

Hidden 800m within a mountain, Fosskraft is excavating the 115m long, 14m wide and up to 34m high power station cavern. Inside, workers are installing a gantry crane to manoeuvre power station equipment during its installation.They are fixing the reinforced concrete crane beam to the wall using up to 8m long, 25mm steel rock bolts, while vertical columns will provide support.

Karahnjukar Supervision JV, which consists of one German company and four Icelandic ones, is monitoring the power station and pressure shafts, while Karahnjukar Engineering JV has handled the overall design. Supervision team deputy chief project manager Gunnar Gudmundsson says: 'We are pushing them to deliver on their design as it is Landsvirkjum's policy not to have the same company doing the design and the supervision.'

The caverns are being excavated by drill and blast. Gudmundsson adds: 'After blasting we clean the surfaces with water at 6 bar pressure.

We had to pump this water out but there has been very little groundwater inflow.'

After the power station, water will run through tailrace tunnels for about a kilometre before exiting the mountain into a canal. This will carry it 2km to the Jokulsa i Fljotsdal river.

Spoil from this area is being deposited and covered near the excavation point so that vegetation will grow on it.

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