How to build the ‘world’s first’ ship tunnel

Norway is pressing ahead with building the world’s first ship tunnel through a rocky peninsula, with a feasibility study due on 12 May 2017 – but how do you make a permanently wet tunnel safe?

The Stad Ship Tunnel will help freight ships and cruise liners of up to 16,000 tonnes bypass a dangerous region of sea that can have up to 106 stormy days per year. It will cut into the narrowest point of the Stadlandet peninsula, measuring in at 37m high, 26.5m wide and cover a cross-sectional area of 1625m2. It is expected to cost 2.7 billion kroner (£250m) and the Norwegian Coastal Administration (NCA), the governmental body behind the project, hopes to start construction in 2019 – and open the tunnel in 2023.

So how will it be done?

To dig through the 8 million tonnes of rock, Terje Andreassen, the project manager, said the engineers will use the tried and tested approach of conventional blasting using underground drilling rigs and pallet rigs. This will have to be done on several levels right down to 12m below sea level, he explained. One of the biggest challenges in carrying it out will be finding the best way to transport the rock from the blast regions due to very limited and small roads surrounding the peninsula, as well as building infrastructure inside the tunnel.

Mark Leggett, part of the British Tunnelling Society, said that the Norwegians are “very good at constructing tunnels” and using drill and blast techniques through granite is largely self-supporting. Caverns of similar width and height have been built in Scandinavia for various purposes, so the project is achievable, Legget stressed.

However, he added that the biggest impediment will most likely be weather and keeping the sea out of the tunnel during construction, especially during a storm. “At the portals, the rock is likely to be weathered and may require greater support to be installed in quite aggressive conditions,” he added. Another challenge will be transporting the huge amounts of rock being excavated, and taking the digging equipment to and from the site. It will most likely be done on barges, says Andreassen.

Alan Bloodworth, an expert in tunnelling at Warwick University, said that excavating the tunnel below sea level is not as big a problem as it might appear to be. The engineers will design and construct a temporary wall, most likely by piling into the sea bed or carefully sinking precast concrete elements to create a “cofferdam” in the water across each end of the tunnel. “Only once the whole tunnel is completed, lined and with all services such as lighting installed will it be flooded,” explained Bloodworth. This will be done in a controlled, gradual manner by allowing water slowly through a valve in the cofferdam.

Once the tunnel is flooded, however, it will be open to “an aggressive and salt laden environment” which could cause corrosion, said Legget. This will mean that the engineers will have to protect and design fixtures, fittings and reinforcements that will be resistant to these conditions.  

Once the tunnel is ready, a ship will be able to pass through in just 12 minutes, and up to five ships will be able to enter it in a queue, according to the NCA team’s computer simulations. Ships will be able to remotely alert the digital traffic management system and be told where and when to enter the tunnel and which place they will be in the line.