[1/7/2009 2]
Groundwater lowering complete
Groundwater at Malmö C and Triangeln is now beginning to reach normal levels again. The water table was lowered in 2005 in order to be able to carry out the Citytunnel work.
Groundwater lowering started in the late summer of 2005 in the area around Hyllie water tower and in Vintrie where Hyllie station and the Öresund bridge link have been constructed. Later, groundwater levels were also lowered at the work sites in Holma, Triangeln, Malmö C, and Lockarp, as well as in the 12 intersecting tunnels and two access shafts on Stadiongatan and Kung Oscars väg.
Over 500 drains
A network of ground drains was created in order to ensure that work could take place in dry shafts, and that the intersecting tunnels could be excavated. Most of the water removed was pumped back into the limestone via ‘reinjection drains’. By reinjecting the water, it was possible to restrict water table reduction to the immediate neighbourhood of the work. In total, around 500 drains and reinjection sites were used for groundwater lowering.
Water is pumped up from the ground via extraction wells near the work area and pumped back via infiltration wells some distance away.
The two environmental judgements for the Citytunnel project determined the extent of the area that could be affected by lowering the water table, the total amount of water that could be removed from the limestone, and the quantity that had to be reinjected. The judgements decreed that, allowing for local variations, between 70 and 90% of the groundwater had to be reinjected during the construction stage, which was complied with.
Three swimming pools an hour
When water-table reduction was at its most intensive, around 1,800 m3 per hour was being pumped out of all the work sites. This corresponds to about three normal-sized 25-m swimming pools. Throughout the period, groundwater levels and water flows were checked and documented in a rigorous control system.
During the latter half of 2008, water-table lowering was gradually reduced in most areas because the main underground concrete structures were finished and the shafts backfilled. At Malmö C, the last part of water table reduction was complete at the end of November and groundwater levels have already recovered. At Triangeln, groundwater levels are expected to be back to normal by February.
Permanent lowering
In some areas, water tables will have to be kept down even into the operating phase. These lie along the sections where the lines run through cuttings, in the region of the tunnel entrance at Holma and Hyllie station, as well as the link lines in Bunkeflo-Vintrie, and in the region around the link lines in Lockarp. On these sections, the weight of the structures is so low that there is a risk that they will be lifted by normal groundwater pressure.