Number 26B Falkoner Allé in Copenhagen’s northern suburbs appears, at first glance, to be no different to hundreds of other five-storey apartment blocks lining the streets of the Danish capital.
But closer examination reveals a sandstone façade riddled with small target prisms; a building under the constant scrutiny of automatically controlled theodolites, and an adjacent plot of land that has hosted intense jet grouting activity for months.
For the residents of 26B are rather special. Few other people can boast of massive twin tube metro tunnels being driven just metres from their flats.
Even fewer can claim to have a contractor spending over $2M just to ensure their building feels little of the passing tunnelling machines.
This is the price tag for a complex, computer controlled jet grouting operation, and real-time settlement monitoring regime, by Bachy Soletanche engineers to guarantee that the building does not settle. And it demonstrates the measures needed to fulfil the metro client’s orders for ‘no damage to any building’.
As the metro’s final tunnelling breakthrough was being achieved into the route’s end station box just across the road from 26B, one of the two TBMs passed within 2m of the apartment block.
This 80-year-old building is founded on shallow stone strip footings lying in weak sands and gravels. As the EPB machine rose from its normal driving depth, into the same glacial till, it would doubtlessly have disturbed the structure.
That the building barely settled at all was due to a 20m long protective arch of close centred jet grouted columns through which the massive 100m long tunnelling machine ran. The arch was surrounded by a slurry wall box incorporating impressive 3m diameter grouted columns installed with the help of computerised grout measurement sensors.
"Without any protective grout shield we expected the building to settle at least 15mm and cause potentially serious structural damage," says Martin Akselsen, Comet’s chief settlement monitoring engineer. "In practice, with the jet grouting in place, it settled only 1.5mm, just half what we had predicted and caused the building no distress."
