Late finish on tough Norway road project 29 Aug 2019

Need for localised heavy support and extensive pre-grouting to reduce water inflows slowed work on the Lyshorn underground section of the E39 Svegatjørn-Rådal road link to Bergen, delaying completion by about a year. At 9.3km long, Lyshorn is the longest of the twin-tube elements of the project. Drill+blast work began in April 2016, with Atlas Copco rigs opening profiles of 70m², and was scheduled to have finished in 2018.

Fig 1. A tough route through to Bergen

Construction of the Svegatjørn-Rådal section of the E39 West Coast Highway development near Bergen is being built in two lots (Fig 1):

• The North Lot, comprising Rå and Sørås sections, plus a complicated underground junction, by Implenia Norway
• The South Lot, comprising the Lyshorn and the shorter Skogafjell alignments by Veidekke.

Veidekke began main excavation of the 1.4km Skogafjell section in February 2016 and finished just over a year later, despite some high groundwater ingress. Lyshorn, immediately north of Skogafjell, was expected to present greater challenges with more complicated geology over a much longer length. The package also included the 1.7km single-tube Hamre water tunnel, which is almost completed.

Last blast finishes Lyshorn

Consultancies on the project are Norconsult (design work and construction phase assistance to NPRA) and Sweco (tunnel support supervision for NPRA). Multiconsult is designer for the fitting system, which is outside the scope of the Veidekke contract.

Expected geology and excavation strategy

The rock types expected along the Lyshorn alignment included granitic to granodioritic orthogneisses, banded gneiss, anorthosites, metagabbro, amphibolite and greenschist/greenstone. There are also zones of gneiss, quartzite, mica schist, schistose amphibolite plus some marble. Overburden depth varied between 35m and 300m, with less than 12m at the portals.

Based on Q-System classification, rock mass quality was expected to be mostly fair (Q-values 4-10) to poor (1-4). Site investigations during planning mapped numerous fault and weak zones with groundwater water expected in the weak zones, said Bremnes.

Drill+blast advances key road link

Excavation advanced with the Atlas Copco rigs on six principal faces, two from the south and four from an intermediate adit. Short tunnels were blasted from the north portal. Works also involved locally widening the main tubes every 500m for safety stopping zones. Further excavations included pre-grouting and blasting of 36 cross passages, each about 10m-12m long with T5 profiles, joining the main tunnels.

By spring 2017, Lyshorn excavation had advanced reasonably well, completing almost 40%, or about 7km of the 18.6km (2 x 9.3km) total. “The major rock types were seen but not always where they were predicted. The same went for some of the weakness zones, but within acceptable limits,” Kari Bremnes, Project Manager for Norwegian roads authority, Statens Vegvesen, NPRA, on the South Lot, told TunnelTalk. “Some of the expected minor weak zones were not always found, and we encountered a few minor ones that were not mapped. The major ones were mostly there. The general difficulty was the acute angle between the strike of the weak zones and the tunnel axis.”

“Rock mass quality was actually significantly better than expected, with less rock support required,” continued Bremnes. About two-thirds had Q-value of good to very good (>10). Most of the rest was classed as fair to poor, about 31% in total, compared to the 78% anticipated. The better rock conditions led to more spot bolting before shotcreting with less systematic bolting after shotcreting. Heavy rock support was needed, as expected, at the acute angle strike of the tunnel axis with the weak zones, including thick shotcrete lining, steel arch ribs, and long rock anchors.

Despite delays, workers celebrate completion

Bremnes added that the Q-System is not used to determine the type and amount of rock support but NPRA guidelines are employed, as Norwegian rock tunnels “usually occupy a relatively narrow band horizontally across the Q-diagram.”

The project met the overall post-construction groundwater limits, although it was “especially wet” where the alignment passed below a surface reservoir and inflow limits were tighter, down to 5l/min per 100m in each main tunnel. Probe drilling was used ahead of the face with pre-excavation grouting used significantly, Bremnes said. Where used, the face would have 35-45 holes drilled to 24m depth for grouting ahead by more than three blast rounds, each typically 5.3m. Typically, about 12hours of grouting was then expected, using about 12t of cement, although both time and materials needs “varied much”.

In some cases, after only one blast round to advance the face and using control holes to check for water or where water was flowing out of the face, extra pre-grouting was needed and much sooner. Several excavation and rock support complications also had to be overcome at some cross passages.

Lyshorn moves from excavation to completion phase

Finishing the job

Waterproof membranes are now being installed on the walls and roof, while the drainage system and road are being built. Lining of the lower walls will be completed with concrete shell panels, while the roof will be finished with mesh and shotcrete. The final sizes of the two-lane main tunnels will be 53.6m² and 9.5m at road level.

The initial expectation was for main excavations on Lyshorn to finish in the first half of 2018 with full completion of the tunnel in August 2020. With excavation finishing about a year late, the project is now due to open for traffic in mid-2022, said Bremnes.