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Battling geothermal challenges in Iceland Nov 2014

Patrick Reynolds, TunnelTalk
Iceland’s complex geology is known to present groundwater challenges but two road tunnel projects that are currently under way in the country are having opposite experiences. While the Vadlaheidi Project is wrestling with a stalled drive due to major geothermal flows and hot rocks, the Nordfjordur Project, now just over half finished, has met far drier conditions than expected in sedimentary stretches, though progress is reported as slow through the layers of weak tuff. Special correspondent Patrick Reynolds reports on progress for TunnelTalk.

Hot water inflows and rocks are severely restricting progress on the west drive of the 7.5km long Vadlaheidi tunnel in Iceland, with groundwater temperatures recorded at a challenging 46ºC and higher. Inflows of up to 350 litres/second have been encountered, according to the client, the Icelandic road authority Vegagerdin; rock temperatures have been recorded at up to 60ºC.

Geothermal hot water flow at the Vadlaheidi tunnel face
Geothermal hot water flow at the Vadlaheidi tunnel face

Early in the year, drill+blast works had reached almost 2.6km when the difficult conditions surfaced. These have become steadily worse, with the drive advancing just 825m between February and August (2014). Major efforts have been made to grout the fissures but the effort has so far met with only limited success. As a result weekly advance rates have varied considerably week by week, with many days often dedicated to grouting alone, according to Vegagerdin.

A number of changes in both work and logistical patterns, and in some of the equipment used, have been introduced to try and help deal with the difficult heat and wet conditions. Shift durations were reduced from 12 to eight hours to enable respite from the heat and humidity, which has led to steam forming at and near near the tunnel face, reduced visibility in the first 1km of tunnel, and affected mucking out with vehicles forced to travel in convoys.

Extra ventilation and heaters were installed to reduce the steam clouds, with some success. But even this effort created problems of its own, with even more new equipment needed to help counter the effects of hot water flowing along the tunnel floor and heating the air being blown into the tunnel. The new equipment helped to improve the cooling capacity of the ventilation system, and therefore working temperatures, but at 26-28ºC these are still too high for prolonged work, said Vegagerdin.

After battling to reach 3.4km of advance from the western side, the client decided in August to change its strategy. Focus has now shifted to the east drive while plans are developed to resolve the geological and geothermal problems in the west. While it is Vegagerdin’s intention to proceed with the stalled excavation, options for how to resume tunnelling from that side are still under discussion. Later this month (November 2014) further grouting will be carried out and more ventilation installed, said Vegagerdin.

The Vadlaheidi tunnel is being excavated by local contractor IAV and Marti of Switzerland. Work began in February 2013, with tunnelling beginning in the third quarter of 2013. The 9.5m wide, single tube tunnel is being blasted through a mountain near Akureyri in the mid-north of the country. Geology along the alignment is basalt with sedimentary beds, and mixed face conditions were anticipated for a large part of the tunnelling.

For the client to open the tunnel by the end of 2016, the schedule was for major tunnelling work to be completed by September 2015. However, progress on the unit price contract has suffered from the hot water inflows and hot rocks experienced by the contractor JV on the west drive.

Presently, the contractor is pushing ahead with the east drive. Tunnelling from that end of the alignment started in September, and by the beginning of this month the drive had advanced 381m.

In total, almost 3.1km – or nearly 43% – of the Vadlaheidi tunnel has been excavated. The project includes 320m of portal structures as well as 4km of access roads.

Nordfjordur Project progress

Quite different geological conditions have been met on the other road tunnel project currently under way at present in Iceland – the 7.9km long, single tube Nordfjordur scheme, which recently broke the 50% excavation milestone. However, it too has had own excavation challenges.

Nordfjordur is located in east Iceland, on the coast. The tunnel is 8m wide at the base (T8 profile), and its total length includes 366m of portal structures. The project also includes 7km of access roads.

Tuff layer at face on Nordfjordur road tunnel project
Tuff layer at face on Nordfjordur road tunnel project

Geology in the location, and along the alignment, is mostly basalt rock layers with variable scoria thickness at the boundary layers, and some sedimentary – colourful orange tuff – layers.

In terms of groundwater, little was expected at the south side and some inflows were anticipated in the north side – but there has been little ingress anywhere, said Vegagerdin. The client noted that the possibility still exists for groundwater inflows, but as the tunnel progresses – now more than 4km, or just over 55% excavated – that likelihood is declining, it added.

On Nordfjordur, the challenge for the contractor – a JV of Czech firm Metrostav and local company Sudurverk – is to work through a number of extensive, and weak, tuff layers on the south side of the scheme, near Eskifjordur. As basalt gives way to tuff layers of varying thickness, the advance rate of the drive drops significantly, decreasing from an average of about 60m/week in good rock down to about 15m/week, said the client.

In the dry tuff the contractor inserts pre-support ahead of the face, and the length of each drill+blast round is only about 2m. The main excavation support is lattice girders at 2m centres, and shotcrete.

Currently, the expectation is major tunnelling on Nordfjordur could be completed just under a year from now, for an anticipated opening date in the third quarter of 2017.

Nordfjordur is Metrostav’s second road tunnel project in Iceland. In 2010 it completed construction of the country’s longest road tunnel system, located in the mid-north coastal region near Akureyri. That project comprised two tunnels – Olafsfjordur and Siglufjordur which are single tubes with a combined bored length of more than 10km. Tunnelling on that scheme also experienced tough groundwater challenges.

           

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