Suomenlinna’s undersea service tunnel

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 was carried out as a project alliance.

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 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 were 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. The location and congestion of the tunnel posed challenges to the implementation of the repair work.