Design of subsea links on Faroe Islands 25 Jul 2019

Rock pillar creates roundabout for undersea route

The Eysturoy subsea road link on the Faroe Islands is particular in design to include an underground roundabout and bifucation to portals on different islands. The 11.2km long bifurcating route connects the Faroese capital, Torshavn, with both sides of the Skala fjord to the north. In accessing both sides of the fjord, the project faced the unusual challenge of creating an underground cavern for the roundabout. A rock column was left in situ to support the cavern, with vehicles circling around the pillar.

The construction contract was awarded to NCC of Norway in late 2016 together with a second subsea route to the south of Torshavn and running from Gamlaraett on Streymoy to Tradardulur on Sandoy (Fig 1). The client, P/F Eystur-og Sandoyartunlar (ESTunlar), is a special purpose company set up by the Faroese Ministry of Industry to own, build, and operate both the Eysturoy and Sandoy toll roads. Geotek ehf (Ltd) is the engineering geology consultant for the client. The Eysturoy link advanced ahead of 10.5km Sandoy route and achieved final breakthrough in June despite some challenging conditions.

Fig 1. Route of Eysturoy subsea link

Fig 2. Location of the Eysturoy and Sandoy subsea links

Geology along the Eysturoy route, with its deepest point at 189m below the seabed, is primarily basalt. The southern end is at Hvitnes and the portals at the y-shaped northern end are at Strendur and Rokin (Fig 2). The single-tube, two-lane, bi-directional tunnel has the standard Norwegian T10.5 profile of 82m² in cross-section and 10.5m wide at road level.

“Construction on the Eysturoy project began in early 2017,” said NCC Project Manager, Alf Helge Tollefsen. “Excavation was undertaken from opposite ends using two Atlas Copco XE3 jumbos with wire mesh and shotcrete support and rock bolting as required.” Weekly advance reached a peak of almost 190m in a single week in April 2018.

“One of the main challenges,” said Tollefsen, “was that much more pre-excavation grouting was needed than expected. This included one small part of particularly bad rock quality.” Approximately 7,500 tonne of grout was used on Eysturoy, “considerably more than was typical for two earlier road tunnels on the Islands.”

Construction of the roundabout with the profiled rock pillar in the cavern was a further, and unusual, challenge. “The rock quality was quite good but the roundabout was systematically grouted,” said Tollefsen. Standard excavation was performed “but with focus on smaller sections, with immediate bolting and shotcreting. These elements together were quite complex.”

Artist impression of the roundabout cavern and its pillar support

Portal design for the subsea connections

Shotcrete on roundabout rock pillar

Drill+blast excavation of the 82m² cross section

While working on the remaining tasks at Eysturoy, the NCC team is also managing construction of the Sandoy project with the lowest point at 157m below the sea bed. Work started on the straight 10.5km undersea alignment at the start of 2019. Following completion of the main excavations at Eysturoy in June, the two NCC Atlas Copco jumbos were quickly moved across the islands to being blasting at Sandoy.

The Faroese Parliament approved the projects in 2014, aiming to improve transport infrastructure to the immediate north and south of the capital. Local company, Jardfeingi, completed geological investigations, Norconsult performed the tunnel design, and Sintef acted as technical advisor.

Subsea breakthrough celebrations

NCC previously built the 4.9km long Vaga and 6.2km long Nordoya subsea road links on the islands, completing them in 2002 and 2006, respectively. Other sea connections on the archipelago of 18 islands rely on bridges and ferries. Road tunnel construction on the islands began in the 1960s and there are plans for about 30km of additional subsea road connections.