Sodra Bancohuset, the old national bank building in Stockholm, is literally something of a Swedish national treasure – it’s the building depicted on the Swedish 500 Kroner banknote (about Euro 80).
Situated in Gamla Stan, Stockholm’s Old Town, this historically important seventeenth century building is suffering from long-term settlement and has been sinking at up to five millimetres a year. Largely this is due to regional groundwater lowering, again of around five millimetres a year. As a result, the old timber piles that have supported the building for over three hundred years have become exposed to oxygen and are starting to rot.
This is not the whole story, says Mats Johansson, of consultant Ebab. When the Sodra Bancohuset – a collection of three buildings built between 1676-1712 – was constructed, he explains “they didn’t have the same foundation standards or drilling methods as today.”
It’s built on a couple of metres of old fill material and although the pioneers of Swedish foundation engineering knew they could transfer the loads through the fill using wooden piles, according to Johansson, “many of the wooden piles don’t actually extend through the fill – they got stuck on obstructions and that is another reason why the building is sinking.”
The solution is to underpin the building using micropiles taken deep into the underlying glacial deposits. Swedish foundation contractor Peab Grundlaggning started the project in September and will be onsite installing some 420 piles until May 2004.
What makes the work all the more challenging is that it is carried out from inside the building, from an existing basement where headroom does not exceed 2.7m