Piles stabilize Subway Project of Historic Dimensions

Context

The new subway line will run in a tunnel underneath the city center, the so-called Bridges districts and the town of Frederiksberg. The Cityringen will be served by the new subway lines M3 and M4; like the other subway lines, it will be operated automatically and thus be driverless.

Led by the Italian company Salini, the Copenhagen Metro Team (CMT) is carrying out construction work on the 15.5km long section that includes 17 subway stations. Advancement of the double tube tunnel is being realized using four Tunnel Boring Machines (TBMs), and tunnel excavation is carried out in Copenhagen limestone with quartenary gravel sands.

Work for a total of 4 shafts and for the 17 subway stations with average depths of 22m is being carried out from 21 jobsites. A total of 3.1 Mio t of earth has to be moved.

Construction work is made difficult by the fact that it is being carried out near historical buildings that are supported by wooden piles. In order to prevent damage to these pile foundations by exposure to oxygen, the ground water level was not to be altered by construction work. Consequently, the lowering of the ground water on the inside that was necessary for sinking the shafts, which would normally also have lowered the ground water level outside of the shafts, had to be compensated by re-infiltrating treated ground water into the aquifer. Approx. 600 extraction and reinfiltration wells were constructed for this purpose.

Solution

The subway stations and TBM starting shafts were constructed as up to 30m deep rectangular shafts with average dimensions of 65m x 20m from top to bottom using the open cut method. The roof structures of the subway stations are being built using 1.1m wide and 2m deep reinforced concrete girders spanning the shafts for their complete width. The shaft walls consist of stiff, watertight diaphragm walls and overcut drilled piles reaching down into the limestone to a depth of 46m. The shafts were built as dry excavations – the wells lowered the water level to a level below the excavation floor. Before the shaft ceilings were completed, permanent uplift anchors had to be installed in order to ensure long-term protection against uplift forces.

For this purpose, DYWIDAG supplied permanent corrosion protected DYWIDAG Gewi Piles. After drilling the boreholes, the DYWIDAG Gewi Piles were lifted in place and properly fixed in terms of position and height. Before the lowering of the ground water level was completed, the anchor heads were installed and the piles were prestressed to the predetermined service loads. Afterwards, all cavities were sealed and the anchor heads embedded in the bottom slab were encased in concrete. In total, 18,200m of 63mm Ø DYWIDAG Gewi Piles, 2,000m of 40mm Ø DYWIDAG Gewi Piles, 650m of 50mm Ø DYWIDAG Gewi Piles and 800m of 32mm Ø DYWIDAG Gewi Piles were installed in the individual shafts.