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Foundation reinforcement in Stockholm Central station

(Photo courtesy of Jernhusen)

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The Stockholm Central station has been sinking 1 mm per year and needed reinforcement of the foundation. This was done by drilling and setting casing all the way down to the bedrock.

Niches were cut out from the side of the existing foundation walls, big enough to allow access for a small drill rig. Wassaras water-powered DTH hammer was prescribed for the project as the environment was sensitive in several ways.

Foundation reinforcement

The landlord Jernhusen hired the construction and project management company Forsenprojekt for the reinforcement project. The contractor Lemminkäinen was awarded the contract for the later part of the foundation works. This part of the project included the installation of 428 steel piles, placed every second meter in the foundation wall and down into the bedrock. The sensitive environment demanded as benign drilling as possible. No noisy machinery or dust in the air was allowed. Any vibrations must be kept at a minimum. The water-powered drilling with Wassara is the only percussion drilling technology that meets these demands.

A project with many particular demands

As parts of the building are listed as a National Heritage, strict national laws applies; the building must not be damaged. The available workspace for the operation was quite cramped, due to bulky foundation walls. To further complicate the works, multiple parallel construction projects were running at the same time.

As the public area was in full operation, some 200 000 commuters were passing very close to the drill site every day. This pushed the environmental aspects with low noise even further. It also made the moving of drill rigs and other machinery limited to off-peak hours.

Drilling inside the foundation walls

Niches were cut out from the side of the foundation walls, big enough to allow access for a small drill rig. The drilling and casing advancing was done all the way down to the bedrock, an average of 16 meters. The formation was full of oak logs that had to be drilled through. After the casing was set, arrangements were made in the walls to enable the support of the new pillars. This was repeated every second meter of the wall.

Creating new space

The Central station’s lower hall accommodates several stores and small shops. In order to create more space, the number of the rather bulky foundation walls needed to be minimized in this area. Some 3 600 m2 of new area has been created in the lower hall. This was achieved  by excavation of new areas and lowering of the floor level, as well as replacing bulky walls with supporting steel pillars.

Pleased contractor:

The drilling went as planned, he says. When drilling indoors, we arranged for the return water to be collected and pumped away. The oak logs didn’t cause too much of a problem. Our biggest challenge was the logistics; working so close to the 200 000 commuters that walked by every day as it demanded special arrangements for our machinery and hoses.
Oskar Nillsson

Site manager, Lemminkäinen

The new pillars (in the bottom) are now supporting the foundation. The niche is ready to be closed and filled with concrete. (Photo courtesy of Lemminkäinen)

Setting up for the next casing, indoors under the Central Station. (Photo courtesy of Lemminkäinen)

Equipment used

DTH hammer

Wassara W80
Wassara W100
Wassara W120
Wassara W150

Casing system Ruukki RD90, RD 115, RD140, RD170 & RD220
Pump Lemminkäinen HP500/180 
Drilling fluid Clean water 
Rig Geomachine Kelpo S
Drill rods 76, 89, 102 and 140 mm 
Bore hole length 10-25 meters 
Scope of drilling 5 000 meters 
Drilling formation Sand, gravel and boulders, together with oak logs