Sweden’s timber-piled parliament building or Sveriges Riksdag stands as the only building on the small island of Helgeandsholmen in the centre of Stockholm.
The island marks the point where Lake Malaren, around which Stockholm first developed, flows into the Baltic Sea. Regional uplift since the parliament building was constructed means the ground surface is higher, relative to sea level, than it was when building’s piles were installed.
And as a result, the top 700mm of the timber piles are now exposed to the atmosphere and are beginning to rot. Risk of settlement is considerable.
The Swedish parliament has been looking into the problem for the last 25 years and in this time has considered two main options. Initially plans were to completely underpin the building with new piles, but more recently efforts have focussed on a novel solution, which involves creating a cut-off dam downstream of the building.
This aims to raise the groundwater back to the level present when the piles were first installed; and by re-immersing the wooden piles below the groundwater level, halting their further degradation.
At the beginning of 2004 the government awarded a SEK 50 million (Euro 5.5 million) contract to a joint venture of Sweden’s Hercules and Italian contractor Trevi working as subcontractor to NCC.
The cut-off will be created largely by jet grouting, which until a few years ago was an almost untried technique in Sweden. But the Trevi/Hercules joint venture goes back more than five years and in this time has kept at least one jet grouting crew permanently busy, increasing significantly the experience and acceptance of jet grouting in Sweden, particularly around Stockholm.