Sparsely populated Iceland sits on the northern run of the Mid-Atlantic Ridge where the Eurasian and North American tectonic plates are moving apart to be filled with extrusive igneous rocks. However, not all of the country is experiencing rifting and volcanism; the active zone runs diagonally through the land, cutting from the southwest to the northeast. This spine of the island is flanked by gently dipping basalts.

The Icelandic bedrock is dominated by subaerial basalts (100MPa-300MPa) but they are commonly faulted. The dominant features of the bedrock are the relatively thin layers of the basalt succession of porphyritic, tholeitic and olivine rocks, and the gentle dip of the strata.

The top and bottom of each lava layer are composed of fragments, resulting in the formation of porous breccia – scoria – which is usually well consolidated in Tertiary bedrock. The upper zone of a layer often has sedimentary deposits. Below, in the body of the solidified lava layer the rock usually has irregular columnar joints.

Most faults are sub-vertical and long these zones there is usually breccia of sheared and crushed rock that is well-cemented with the bedrock. There are also frequently basaltic dyke intrusions with horizontal jointing patterns.

Groundwater typically runs along the lava contacts and cooling joints but the prime conduits are the faults and dykes through the layers, including sedimentary interbeds acting as impervious or semi-impervious barriers.

Groundwater Challenge
Many road tunnel excavations in Iceland have met fairly reasonable conditions but there have been occasions of difficult groundwater inflows, such at a basaltic dyke during excavation near the junction for Breidadalsheidi-Botnsheidi plateau scheme in the northwest of the country, on the Muli tunnel works in the north and most recently on the Hedinsfjordur Tunnel in the same vicinity.

Constructed in the mid-1990s, the Breidadalsheidi-Botnsheidi scheme comprises three branch tunnels – Breididalur, Botnsdalur and Tungudalur. Earlier tunnelling experience indicated there might be considerable but scattered inflows (up to 50 l/s per km). On the 2.9km long Botnsdalur drive the inflows increased significantly when excavation was 450m from the T-junction of the three tunnels – pressures began at 6-12 bar and eventually reached 19 bar; open joints were up to 10cm wide and connected, which resulted in little success with cement grouting – used for the first time in Iceland. Competent rock, relief boreholes and resumed excavation eventually helped take the drive forward.

However, on the 4.1km, long Breididalur drive, some 800m from the T-junction – and while anticipating major faults but despite using long probeholes – the right side of the tunnel unexpectedly collapsed on the uphill drive. The cold inflows from the crown were initially 2,500-3,000 litres per second but reduced to almost half the rate in a few weeks. The drive was halted and work focused elsewhere on the project. Eventually, an extensive combination of bolting and shotcrete support, drainage holes and channels, and grouting – including the use of chemical grout with polyurethane (PU), enabled the section to be passed. But the flow, while reduced significantly, has never stopped. Instead, it was piped to become a replacement spring-fed water supply for the town of Isafjordur, and is also now used for small hydropower generation.

A key lesson drawn from the project is that post-grouting with cement mix is very difficult, if not impossible, in high transmissivity basalts.

Groundwater at edinsfjordur Tunnel
Trollaskagi mountain range in the north of Iceland is formed by a complex of basic to intermediary rock types with sedimentary interbeds. The volcanic complex is of a lower Tertiary age, when short periodical lava eruptions alternated with short periods of volcano clastics sedimentation.

In the late 1980s, road tunnelling works in the area also experienced some groundwater problems, on part of the 3.4km long Muli project, near the small town of Olasfjordur. While the problems were somewhat similar to those later met on the Breidadalsheidi-Botnsheidi scheme the challenge of the inflow rates and pressures were not so great.

But the challenge of high inflows and pressures was met again in the region, on the recently-built tunnel at the other side of the Olafsfjordur valley – the almost 7km long tube, called Olafsfjordur (‘Olafs’) Tunnel, that is part of the Hedinsfjordur Tunnel scheme.

Olafs and its sister tunnel, Siglufjordur (‘Siglu’), pass through mainly olivine basalt except for a relatively short section of porphyritic rock close to one end of Siglu. Along their alignments, the rocks are sub-horizontally bedded, disturbed by north-south trending tectonic faulting. As per the general context of bedrock in Iceland, along their alignments the rock mass holds many faults, dykes and joints. These made for often difficult – and sometimes exceedingly tough – tunnelling work in some sections.

The overburden height varies between 5-800m. Some rock bursting was anticipated along stretches of high overburden. Spalling occurred where the overburden was more than 500m, though began in Siglu with a depth of around 400m. The layered basalt lava succession in the tunnels with thin scoria and sedimentary interbeds resulted in mixed face conditions for a large proportion of the excavation.

While olivine basalt with sub-horizontal bedding is dominant in Siglu (with changing porosity and transfer to fine-grained facia) the tunnel has faults of 1-5m thickness. Groundwater conditions were easier and the temperature not as cold on Olafs but the challenge arose on the downhill drive and some cement grouting was done and occasionally some PU pre-grouting.

Geology along Olafs is mostly fine-to-medium grained basalts and partially volcano clastics, and there were some interbedded, nearly horizontal fine-grained tuff layers. The main joint sets are either perpendicular or parallel to the axis, but dip steeply. In many places the rock is heavily broken due to tectonics. The tunnel encountered many dykes but more brittle and broken than Siglu.

While the possibility existed for thermal inflows, and there was monitoring to assess any effects on geothermal water sources near some portals, in the event it was inflows of cold water that were encountered starting about 1km into the uphill drive from Olafsfjordur and continuing for a further 3.5km. In all, almost two-thirds of Olafs had groundwater problems.

Oshlid Tunnel
Just to show the variation in tunneling experience in the complex geology of Iceland, relatively little water problems were met in construction of the 5.4km long mined section of Oshlid Tunnel (near Breidadalsheidi-Botnsheidi scheme).

The Oshlid tunnelling works did, however, experience some rock stability difficulties in sections where there were thick sedimentary layers between basalt layers.

Future challenges – Vadlaheidi Tunnel
To be constructed through the mountain east of Akureyri, the 7.8-8km long Vadlaheidi road tunnel is to be a single tube toll route.

If funding is agreed before the end of the year, Vegagerdin anticipates a prequalification call to contractors in 2011.

Geology along the tunnel alignment is basalt with interbedded sedimentary layers, and expected to be reasonably similar to that of Hedinsfjordur. Medium quality rock, in general, is expected with groundwater problems expected in some locations. The overburden is up to 450m.

Further core drilling is underway – 600m in total – in vertical and inclined bores – which follows earlier site investigation (2005) that performed 915m plus 200m of percussion drilling. Most of the core drilling is near the portals, others are further up in the mountain.

Cold, high-pressure water was frequently encountered during tunnelling