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Suomenlinna’s undersea service tunnel, renovation

The tunnel’s critical importance to the functioning of Suomenlinna; its narrowness and its location between the island and the continent pose particularly significant challenges for the renovation work. For this reason, the project will be realized as a project alliance.

Photo: Jouni Maidell

The Suomenlinna maritime fortress is a UNESCO World Heritage that attracts more than 900,000 visitors each year. Two out of three visitors come from abroad. The fortress is also the home of 800 inhabitants, the workplace of 400–500 people and a district of Helsinki, the Finnish capital. The renovation will ensure the continuous supply of municipal engineering services in Suomenlinna, guarantee the safety of the tunnel itself during use and maintenance, and lower the tunnel’s cost of use while extending its life cycle.

The project developer is the Governing Body of Suomenlinna, while Sito Rakennuttajat Oy will be responsible for the construction works. The developer’s project manager and expert is Jouni Maidell of Sito Rakennuttajat, with Tapani Lyytinen as his partner.

The Suomenlinna service tunnel was built in the early 1980s, serving as access for municipal engineering from downtown Helsinki to the Suomenlinna islands. The tunnel runs below sea level from the Länsi-Mustasaari island of Suomenlinna, passes under the island of Särkkä, and emerges in Kaivopuisto in continental Helsinki. The total length of the tunnel is 1,300 meters and the area of its cross-section is some 15 m2.

The development phase of the alliance will be completed in the summer and fall of 2016, and actual construction work in the tunnel by the end of 2017.

The tunnel was initially built to house pipes for municipal engineering only. A connection for vehicles was added later, enabling emergency vehicles to pass through if necessary. In line with construction practices at the time, the tunnel is fitted with just a few technical reinforcements that meet tunnel construction requirements. Such reinforcements do not meet modern safety requirements. Furthermore, large quantities of water seep into the tunnel, causing significant corrosion in the tunnel structures and high radon levels.

The key goals of the service tunnel renovation works are to lengthen the tunnel’s life cycle, improve working conditions by reducing radon and strengthening the tunnel structures, and to reduce the tunnel’s cost of use by blocking leakages and modernizing the tunnel’s building services.