As many as 500 geotechnical borings to a depth of between 30 and 50 metres along the entire alignment combined with comprehensive pumping tests and seismic measurements – microscopic vibrations created by sound waves – help to create the most precise image of the city’s underground ever.

The mapping needs to be finished by 2009, when the projects are sent out for tender. Surprises at that point will be far from welcome.

Different soil types"The construction will be done in different soil types – fill, sand, clay and limestone. Each one of them has special characteristics that we need to take into account," says project manager Jens Gravgaard of Metroselskabet.

"We can use the numerous studies we’ve made to calculate the optimal alignment and then come up with an overall strategy for how we deal with soil and groundwater long before the construction crews begin tunnelling or building stations or shafts. We’ve chosen to carry out very detailed mapping during this phase of the project, because being prepared has clear environmental, technical and financial advantages," he says.

Wooden piles One of the biggest challenges will be how to deal with groundwater. Tunnels, stations and shafts are all being constructed below the groundwater table, and the construction will, on the one hand, have to ensure that stations remain dry, while at the same time being careful not to drain off the water that protects the foundations of
Copenhagen’s historic buildings.

Many of the buildings in the medieval part of the city were built on wooden piles that will decay if they are exposed to air for too long.  Preventing that from happening requires keeping the groundwater table at its current level.

By Eva Isager
Published: 02.07.2009