Norway trims ship tunnel cost 21 Nov 2019

The cost of the long-planned ship tunnel project in Norway has been significantly reduced thanks to extra site investigation work and other design revisions.

Artist impression of ship at tunnel portal

While the cost reduction for the 1.7km long, 36m wide x 50m high project is below levels required by the Government transport plan, the revision came too late for the project to be included in the recent national budget and will have to wait for the next round of the national budget approvals.

Terje Andreassen, Project Manager of the Norway coastal agency Kystverket, said, “that means a minimum one year delay,” putting back potential contract award and start of major excavations to 2022, if the project is approved in the next budget round in late 2020.

Further geological investigations and review of plans for the ship tunnel by Kystverket followed a second round of external quality assurance studies, by Atkins and Oslo Economics, following five years of Phase 1 investigations. The findings of the Phase 2 study were given to the Ministry of Transport and Communications in May 2018. The project was then estimated to be about a third more than the NKr2.7 billion (or about US$320 million in 2016 prices) allowed in the national transport plan.

Based on the Phase 2 findings, the Ministry ordered a rethink of project costs at the end of 2018, calling for changes to bring the estimate back into line with the national transport plan. The Ministry also wanted the work to produce a better cost-benefit ratio by looking at greater social benefits as well as re-examining the concept and technical solutions, estimates, uncertainty, and support needs of the project.

Fig 1. Outline of the 36m wide x 50m high cross-section

Kystverket undertook the extra investigations and value analysis over the first half of 2019. The outcome was a revised project cost estimate of just less than NKr2.7 billion (US$295 million in 2019 prices), beating the allowable level in the earlier 2016 prices, but the gap is even greater when the level is price-adjusted to today at almost NKr3 billion (US$328 million in 2019 prices).

Consultants advising on the overall design are Norconsult, Dr Techn Olav Olsen, and Snøhetta. There was no early contractor involvement to help develop the plans said Andreassen.

Longitudinal core sampling

A key strategy in the extra work to cut project costs was to get more and better geological data. Andreassen told TunnelTalk that consultancy Concreto AS suggested using longitudinal steer-drilling for core sampling along much of the tunnel length, working from each portal area. It is an uncommon technique for civil engineering projects but common for drilling in the oil and gas industry.

Kystverket hired contractor Diamant Wire Teknikk to perform the longitudinal core drilling, and drill steering was done by Devico AS, which completed a similar task for the proposed 15.5km long Romsdal subsea road tunnel in Norway. Planned to be built in the same region, the Romsdal tunnel is also awaiting funding approvals as part of a larger, multi-project fixed link fjord crossing.

Fig 2. Concept plan of excavation stages

Andreassen told TunnelTalk that expert help on the extra geological studies in the gneiss and slate involved assistance from Multiconsult and Sintef.

The original plan, he said, was to drill the whole length of the tunnel for the longitudinal core sampling but limited time led to only 60% of the alignment being probed. The core samples are 50mm in diameter and 1m in length, taken from along a path steered to run under the centre of the tunnel crown. Following the core sampling, groundwater inflow tests were performed along the core holes, at the actual depth of the tunnel.

The previous site investigation mainly performed limited inclined core drilling close to the portal areas and those did not reach down to the tunnel level, said Andreassen. However, a single vertical core hole did go as deep and was used to retrieve rock samples for laboratory testing and also, on site, insitu stress testing was done, in 2016, he added. In the portal areas, site investigation also included refraction seismic investigations in the sea and above where the portals are to be excavated.

“We have determined where the most variation of rock quality is expected along the alignment," said Andreassen. Findings “did not change the understanding of the geology but we got more precise information” about the amount of different rock types, and “information on cracking and the orientation of gaps”.

Impression of ship approaching tunnel portal

Overall, the rock along the tunnel alignment is better than previously expected, and there is a more favourable rock stress direction, although some more local leakage of water, he added. The more precise data on rock type, characteristics and stress distribution, and location of weak zones, have helped to reduce uncertainty and risk for future tunnelling works which call for large, multiple bench excavations and some use of pre-excavation grouting in weaker zones.

Andreassen acknowledged the improved understanding of the geology would allow for better, faster tunnelling progress but said client planning did not address construction time but focused on the cost/m³ of drill+blast excavation. The tunnel will be larger and longer than a typical underground cavern for hydropower machine hall. Tunnelling will be similar with a crown heading and benches, except at a larger scale with five benches to be excavated in stages (Fig 2).

Benefits of extra site investigation

Following the extra site investigation, and better pinpointing the weak areas, the planned length of the tunnel remains at 1.7km, as does the cross-section at 36m wide x 50m high, Andreassen said. Further cost savings were achieved at the portals with shorter quay approaches found to be possible.

Other key factors in helping to reduce projected costs include:

• simplifying the entrance structures to the tunnel, and introducing new computer-assisted sailing;
• modifying the construction of the tunnel portal at Molde fjord;
• minor changes to the ship guide structure along the tunnel; and,
• removal of a walkway in the roof of the tunnel.

Project planning for the construction phase also includes provision of a service tunnel adit to support tunnelling logistics.

Simulation of piloting ship through tunnel

Procurement

If funding is approved in the next national budget round, procurement of the project would get underway about one or two months later with prequalification and a competitive dialogue process in bids. Main construction should then commence the following year.

Kystverket plans to award the works as a single design-build contract with the client retaining all geological risk, Andreassen confirmed to TunnelTalk.

Excavation is expected to take three years, including the final step of blasting and dredging the temporary coffer dams at the portals to allow excavation to more than 13m below mean sea level. The overall project is planned to take four years to complete.

While there are tunnels for small boats and barges in different parts of the world, Andreassen said "nothing exists for passage of large ocean-going vessels such as being planned by Kystverket".