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Parts of Copenhagen’s Metro system with City Circle Ring in red (Illustration courtesy of Metroselskabet in Copenhagen)

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The Copenhagen Metro system is being extended with a new line, the City Circle Ring. It will comprise 17 new underground stations and will be located in the heart of Copenhagen.

Wassaras water-powered drilling is used for identification and preparation of the subsoil flint layer. As flint stone is very abrasive to the hydro mills, Wassara is used for identifying and weakening this flint layer.

The Copenhagen Metro Ring

Copenhagen’s subway system have two lines with a total of 22 stations and is trafficked by fully automatic trains. The Metro is now being extended with a new line, the City Circle Ring. It will comprise 17 new underground stations and will be located in the heart of Copenhagen.

Hard flint stone and sensitive environment

Hydro mills will dig down to the bedrock, creating the place for the diaphragm walls. As a layer of flint stone is present, it needs to be identified and weakened before the milling process. Flint stone is known to be very abrasive to the hydro mills, but this can be reduced by penetrating the layer in forehand.

The surrounding presents some challenges as the City Circle Ring is being constructed in the very center of Copenhagen. Nearby buildings present a major challenge as they would face the risk of being damaged by pressurized formation. Hence, no air-powered drilling could be utilized.

The Wassara drilling

Wassaras DTH hammer is powered by water at up to 180 bar delivery pressure. When the water leaves the hammer, it loses the pressure and keeps a low flush velocity. This however is still adequate to bring any cuttings to the surface and to clean the borehole. This lowers the risk of pressurizing the formation to an absolute minimum. By this, the soft formation will not face the risk of being affected in a way that could result in severe damage to the surrounding infrastructure.

A pleased contractor:

The drilling project has been running just as well as planned. I have been working with Wassaras water-powered DTH hammers for some 18 years now. I consider them to be very useful and powerful, just make sure that you can supply the water it needs for the operating.
Maurizio Biondi

Plant Manager at the contractor Hercules Trevi


Aligning the drill rig for a new borehole


Preparing the hydro mill

Equipment used

DTH hammer Wassara hammer W80
Wassara hammer W150
Drill bit Ø 90 mm
Ø 165 mm
Pump WASP 80 D 
Drilling fluid Clean water 
Rig Soilmec SM405 and R312, Comacchio 1200 
Bore hole length 28 m in average 
Scope of drilling 110 000 meter 
Drilling formation Limestone, flint, moraine/till, sand and clay