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Extreme conditions conquered in Iceland 08 Jun 2017

Patrick Reynolds for TunnelTalk

Excavation has finally finished on the Vadlaheidi tunnel, the longest road tunnel in Iceland that saw tunnellers faced with major geological difficulties almost from the outset until only the last few weeks of excavation on the delayed toll scheme.

Breakthrough for Vadlaheidi road tunnel
Breakthrough for Vadlaheidi road tunnel

Even earlier this year the progress slowed once again with challenging wet ground conditions and weak stretches, dramatically reducing the closing advance of the two drill and blast faces. However, in the final months progress sped ahead and final breakthrough on the 7.2km long single tube tunnel was achieved at the end of April 2017 by the contractor Osafl, a JV of IAV, Iceland and Marti, Switzerland.

Tunnelling on the project began in late 2013 and excavation was originally expected to finish in late 2015.

But radically different geotechnical problems at the faces being advanced from opposite ends of the T9.5 profile tunnel (66m2 section, 9.5m wide) led to major setbacks and delays.

In the west the problem was hot groundwater and major inflows, calling for large scale and long term use of grout; in the east the tunnel roof collapsed at a fault zone and the tube was flooded, stalling work for many months.

Vadlaheidargong Ltd is developing the toll road tunnel in central-north Iceland, nearest the main regional town of Akureyri, just below the Arctic Circle. The tolls will repay state funding. The developer has Geotek and Efla providing construction supervision services.

References

Extensive grouting program in Iceland 29 Mar 2016

Patrick Reynolds for TunnelTalk

Excavation is scheduled to restart next month (April) at the east end of the 7,200m-long Vadlaheidi road tunnel in Iceland – almost a year after suffering a major collapse and flood at a fault zone.

Grouting in collapse zone of Vadlaheidi road tunnel, Iceland
Grouting against hot water inflows in the west heading

A significant program of consolidation grouting and ongoing pump-out of groundwater inflows has been under way to prepare for resumption of drill+blast excavation. Support is to include forepoling, steel beams, lattice girders, mesh and shotcrete through the local weak zone, after initial excavation passes through the zone of consolidated collapse debris.

The project has continued drill+blast counter-excavation from the opposite end of the single tube alignment, but progress there has been hindered by hot geothermal inflows requiring large volumes of grouting before each excavation round. Consequently, the advance rate from the west has been restricted to approximately 20-40m per week for some months.

The original construction schedule envisaged completion of main tunnelling work by September 2015, but the complex geological and groundwater challenges experienced at both headings mean only about 70% of the tunnel has so far been excavated. The project is now at least a year behind schedule.

Contractor Osafl – a joint venture of Icelandic company IAV and Swiss contractor Marti – started blasting operations from the eastern end of the alignment in September 2014. Progress initially went well but in early 2015, when the face had passed beyond a fault zone, the tunnel roof collapsed. As a result the tunnel was flooded back up to the crest, leaving most of its length in that stretch fully submerged.

The floodwater was pumped out in October (2015), but according to Bjorn Hardarson, Chief Supervisor for the client, inflows of 100 litres/sec still need to be managed. In December (2015) an array of investigation holes were drilled overhead and into the fault zone. Then, in January (2016), rock support was installed in front of the collapse zone, and drainage holes drilled.

Consolidation grouting began in February. A total of 27 holes were drilled, to lengths of 15–20m, as part of an operation to inject 45 tonne of grout, explained Hardarson. Further consolidation work at the location of the fault debris is ongoing, with self-drilling anchors and 90mm steel pipes. It is anticipated that initial excavation will be ready to resume within a few weeks.

At the west end of the project approximately 3.5km of excavation has been completed, though over recent months the advance rate has been very slow due to hot rocks and geothermal inflows. The major ongoing grouting required to stem the inflows enables a maximum advance of only 40m per week. However, water temperatures have reduced to about 55°C from earlier peaks of 65°C, said Hardarson.

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