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Power steering

Geotechnics Dams

Iceland is building what it claims will be one of the largest concrete faced rockfill dams in the world.

Damon Schoenmann investigates.

Iceland has two vital natural assets. The first, as anybody can tell you, is fish, which accounted for 41% of the country's exports in 2002. But the second, power, is becoming increasingly important.

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

By anybody's standards the Kßrahnjakar dam in the east of the country is a huge project. 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.

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

Electricity is far from scarce in Iceland and 90% of all buildings are heated by plentiful geothermal energy, so the benefit is more to do with business. 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 Hr÷nn Hjßlmarsdòttir 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 £651M project, first conceived some 50 years ago, now involves yoking the J÷kulsß Ý Fljòtsdal and J÷kulsß ß Dal glacial rivers so they eventually feed into a mutual headrace tunnel, avoiding the need for a second large reservoir.

The 57km 2Hßlslòn reservoir in the west is fed by the J÷kulsß ß Dal and will provide about 75% of the water required. This then rushes 40km north east along a 7.2m to7.6m diameter headrace tunnel, to two pressure shafts in a valve chamber above the power station.

The other 25% of the water comes from the Ufsarlòn pond to the east, which is fed by the J÷kulsß Ý Fljòtsdal 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 Hßlslòn 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 £119M and £182M respectively. Mott MacDonald is leading the supervision team for this part of the project.

Work began in February 2003 and the enormous main Kßrahnjakar 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 rockfill that in turn will lean on locally quarried coarse pillow lava. The centre of the dam fill will consist 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 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 200mm to 1,600mm.

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.

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 Kßrahnjakar 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 Kßrahnjakar dam to enclose the Hßlslòn 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 1km wide.

The dams must be completed by 1 December 2006, when 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 tunnel boring machines (TBM) 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 10, 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 TBMs' 480mm cutters are able to bore their way through 60m to70m/day, and the best day so far saw 80m. When heavy support is required omega steel ribs are 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).

The drill rig is managing about 1.7m/h. Once complete, the 4.04m diameter shaft will be supported with a 30mm to 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 concrete.

The international joint venture

Fosskraft is building the 103m long, 14.5m wide and 16m high transformer hall, the 9m by 9m tailrace tunnels and the 4m by 4m cable tunnels.

It is also excavating the 115m long, 14m wide and up to 34m high power station cavern hidden 800m within a mountain. 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.

Kßrahnjakar Supervision JV, which consists of one German and four Icelandic companies, is monitoring the power station and pressure shafts, while Kßrahnjakar Engineering JV has handled the overall design.

The caverns are being excavated by drill and blast.

After the power station, water will run through tailrace tunnels for about 1km before exiting the mountain into a canal. This will carry it 2km to the J÷kulsß Ý Fljòtsdal river. Spoil is being deposited and covered near the excavation point so vegetation will grow on it.

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