|Deep Work. Spiral casings installed for Francis turbines. (Photo courtesy of Landsvirkjun)|
Iceland may be a wealthy, developed country but its remoteness and harsh highland climate demand serious pioneering skills from builders of the Kárahnjúkar hydro project. Setting up camp before the first biting winter was the Italian main contractor’s initial priority. With two-thirds of tunneling complete, the goal now is preventing further program slippage.
“I’ve worked in Inner Mongolia at minus 35° F but that was nothing. There was no wind,” says Gianni Porta, project manager with main contractor Impregilo S.p.A., Milan. For much of the year, arctic temperatures are aggravated by winds reaching 170 kilometers per hour.
A 70-km access road over frozen moors awaited the contractor, but the wilderness site was exposed and cold. “We needed to complete [camps] by mid-September,” says Porta. “This is the first project on which I have worked where every single person...has been provided with accommodation from day one.”
By the end of the first year, Impregilo had diverted the river, began building access tunnels and installed a concrete batch plant. “It was a struggle with transport,” he says. “We had 90 to 100 containers arriving per week from Rotterdam....We had to fight against weather conditions.”
Impregilo won contracts for the main dam and most of the tunnels in March 2003. Its approximately $345-million tunnel bid, in local and foreign currencies, was fairly close to its rivals, says Porta.
But the dam’s roughly $230-million price was substantially leaner, mainly due to the contractor’s use of electric rock conveyors. “Electricity was supplied free of charge,” explains Porta.
Impregilo’s work includes building the 193-meter-high, 730-m-long Kárahnjúkar dam on the glacial Jökulsá a Dal River to create the 25-km-long Hálslón reservoir. Local firms are building two smaller side dams.
Containing 8.5 million cu m of rock, the dam is world’s fourth-largest of its type, says Porta. Not having a clay core, the dam is being lined with over 92,000 sq m of impermeable concrete starting 30-m thick at the top and progressively increasing with depth. Impregilo is using a kind of slipforming to cast 15-m-wide sections of facing.
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Impregilo has completed over 90% of the dam with local basalt. It hopes to start impounding the reservoir this September, but the contractual target has been put back till next June. Seeds of the delay were sown from the very start.
To confine the dam’s upstream toe in the steep sided valley bottom, Impregilo built a 50-m-tall concrete retaining wall, one of the dam’s trickiest elements. A major fault was known to cross the wall obliquely, says Carl-Anders Andersson, a senior engineer on the supervisor’s team.
To accommodate potential movement, the wall was built in three blocks with one waterproofed joint over the fault, he says. But the geology was more difficult than expected, requiring more grouting via an additional gallery, says Andersson. That complicated construction, and was further hampered by bad weather.
Tricky ground also troubled Impregilo on the nearly 40-km-long headrace tunnel that now is over 80% built. Its basalt is interspersed with sedimentary deposits and buried bodies of pillow larva and the like. In some places, water inflows of 1,000 liters per second hindered progress. For over four months last year, two of the project’s three tunnel boring machines were down or hardly moving.
|Close Look. Engineers inspect dam�s face.|
Impregilo is building over 59 km of tunnels under a separate contract, valued at about $345 million, in two currencies. It includes 10 km of the Jökulsá tunnel linking the headrace with a small reservoir system, southeast of the main dam. Icelandic firms are building this smaller complex and the first few kilometers of the Jökulsá tunnel.
Altogether, Impregilo is boring some 49 km and drill/blasting 10.4 km. For bored sections, it has new 7.2-m-dia open-face Robbins TBMs and a 40-cm wider refurbished one. For better production and less wear, “for the first time we were using 19-in. cutters,” says Porta, who is more accustomed to 17-in. discs.
TBM3 set off from the upstream adit three in April 2004. TBM2 started from adit two, 17.2 km downstream, that July. It was followed by TBM1 moving from a downstream adit two months later. Combined rates have several times exceeded 2 km a month, peaking at nearly 2.3 km this March, with TBM3 hitting 1.19 km in August 2004. But overall progress has been patchy.
TBM3 ended its initial upstream drive last summer, 1.1 km short of its intended destination, after completing 4.8 km. Water flows were so great that Impregilo decided to let its oncoming drill/blast crew finish the drive, says Porta. Once turned, TBM3 set off downstream toward oncoming TBM2 last December. But with the drill/blast team making slow progress, a local contractor built a fourth 400-m access tunnel upstream of adit 3, and opened two new headrace faces late in May.
TBM2 also had serious difficulties. For about six months from spring of last year, it struggled through fractured rock. Then, last autumn, “a big chimney” emerged in front, says Jonathan Parry, supervisor VIJV’s project manager. Impregilo plugged the rock ahead with a concrete bulkhead and extensive forepoling and consolidation grouting. Having moved slowly through the difficult zone, the TBM has just emerged and is picking up speed, says Porta.
TBM1, meanwhile, is scheduled as the first to complete its drive of 14.7 km early this August. TBM2 will stop in September, to be relaunched from adit 2 on the Jökulsá tunnel drive next spring. That drive is due to end around Autumn 2008, with water flowing to the turbines at the end of that year. TBM3 will finish 7.4 km of downstream boring by this November.
Difficult ground conditions led to a contractual time extension and Porta hopes to end negotiations on how much extra pay his firm is due this summer. The owner granted an extension of four months. But both sides disagree over how the enhanced pay rates applicable for a built-in 60-day schedule allowance covering “extraordinary geological occurrences” should be applied.
Impregilo’s responsibilities end at two 420-m-deep penstocks that will drop water vertically onto six 115-MW Francis turbines in the 120-m-long powerhouse, excavated into the Valjófsstadafjall escarpment. By contrast to Impregilo’s struggle, “excavation went extremely well [and] finished ahead of schedule,” says Lawrence Jackson, project manager with the Fosskraft joint venture, led by Germany’s Hochtief AG, Essen.
Under a roughly $100-million civil contract, signed in September 2003, Fosskraft also is responsible for a 1.5-km-long tailrace tunnel, cable and access
tunnels that are each 1 km long, and the two penstock shafts. They are being lined by Germany’s DSD Stahlbau GmbH, Würzburg, which has lost two months in its roughly $40-million separate contract.
DSD began in April 2005 as designer and supplier of the linings from Germany, using a subcontractor for instal-lation. But DSD took charge of all work after the subcontractor went bankrupt last September, says Conzalo de La Fuente, resident engineer with the owner’s powerhouse supervising team, KSJV, led by Germany’s Lahmeyer International.
Designed to be self-supporting, the 420-m-tall penstock linings are nearly 6 cm thick at the base. “It’s one of the highest vertical shafts in construction at the moment,” says de La Fuente. DSD is lowering from the shaft tops 9-m-long sections of 3.4-m-dia linings, each weighing over 40 tonnes each. For each lift, welders spend many hours in the deep shafts joining the lining section, which are later backfilled with self-compacting concrete.
Far beneath them, VA-Tech Escher Wyss GmbH, Ravensburg, Germany, this February began installing generating equipment under its roughly $130-million contract. The first unit is scheduled to go on line this December with the rest following at monthly intervals, says de La Fuente. Commercial operations are due to start in July 2007. Should headrace tunneling be further delayed, “we will do what we can do about water,” de La Fuente adds.