Planning the Cityringen

02 March 2009

In planning the Cityringen Metro, priority was given to completing the project rapidly and in an environment-friendly, financially responsible manner with as little inconvenience to the city as possible.

Before the actual construction work on the Metro could begin, it was necessary to relocate many utility supply lines.

A year before construction work started, Copenhagen City Museum undertook a series of archaeological surveys. During the excavations, traces of Copenhagen's medieval fortifications were found. As a result, Copenhagen City Museum will follow works on the Metro during the rest of the construction period, so that archaeologists can intervene if anything of archaeological significance is found.

Groundwater table
Performing construction work 20 m underground for this project poses several problems. Avoiding harmful disturbance to groundwater levels is paramount, because a lowered water table could cause the surrounding buildings to settle. In some parts of Copenhagen, building foundations were made according to old piling methods using wooden piles that are under water.

A lowering of the water table would expose these foundations to air, and they could disintegrate in a few years as the result of fungal attack. As a result, the construction method used for the tunnel stations prevents harmful groundwater lowering. To monitor groundwater levels, a number of bore holes are being drilled along the entire Metro. The groundwater level in these holes will be continuously checked.

Built from the top down
Tunnel stations will be built from the top down. The basic procedure will start with the establishment of a watertight outer wall surrounding the station box. Next, the actual construction pit will be excavated within the reinforcements. This ensures that construction work occurs in a stable, dry pit that is impervious to water penetration.

The outer walls of the station will be formed from concrete secant piles that are so tightly placed they will form a cohesive watertight wall. The secant piling method was chosen because it is more environmentally acceptable through reduced noise and vibrations over methods such as sheet piling.
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