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The neighborhood "Pyramiden" in central Stockholm was sinking due to old wooden piles. Wassara's technology turned out to be the most suitable and is now the only method in use.

Challenge

Old buildings in central Stockholm often have a foundation made of thick wooden piles. As the water table lowers, the wooden piles get in contact with air and begin to decay, resulting in a reduced loadbearing capability. The buildings resting on these piles consequently begin to sink.

An example is the “Pyramiden” – neighborhood, located on Södermalm, Stockholm, where the considerable drop in ground level could be observed with the naked eye. The construction company Besab was therefore commissioned by the project owner Skanska to perform a foundation reinforcement of the area.

Solution

The foundation reinforcement procedure was performed by steel piling. First, thick steel pipes were drilled down through the overburden and a bit into non-fractured rock. When the holes were completed they were filled with grout, starting from the bottom.

Besab chose to test the Wassara technology since they knew that the water DTH-hammer would improve the working environment and reduce the risk of descend of the ground level in an area connected to limestone walls.

 

Drilling conditions

The overburden is comprised of moraine, non-cohesive soils, mud and fractured rock. The ground is very rich on water which further complicates drilling as large amounts of water mixed with cuttings may end up being spread out into the basement if air-DTH is used.

 

Project size and time frame

During the initial phase of the project a total of 76 holes were drilled with a diameter of 170 mm, pile thickness of 10-12,5 mm and steel quality S460MH. In total 1 480 meters were drilled in this project. The Wassara W120 (5”) was used in combination with 114 mm drill pipes.

Since the work in the basement was complicated by the lack of space, Besab commissioned a Swedish company to develop a special rig with high maneuverability as the main requirement.

The total project has been running since 2009 and is scheduled to last through 2019. Several different technologies have been used due to high demands on the degree of ground level descent. Wassara has proven to be the most suitable method for drilling in this area since 2015 and is the only method currently in use.

Water treatment

The water used to power the hammer was municipally sourced, taken from a fire hydrant.

No water-handling system was necessary; the vast majority of the water, in fact, ran down the drill holes, and the small amounts that made it to the surface vanished through the overburden.

 

Statement from contractor:

First of all it is important to mention that it is really clean in the basement when using Wassara, also the diesel consumption of the water pump is much lower than an air compressor´s
Rani Isa

Construction manager, Besab

Equipment used

DTH hammer Wassara W120
Casing system Symmetrix (Atlas Copco
Pump Hammelmann 
Drilling fluid Clean water from fire hydrant 
Rig MD 30 SPD - built for the project 
Casing 170 mm, 10-12,5 mm wall thickness 
Drill rods 114 mm
Bore hole length ~20 m (~65 ft.) 
Scope of drilling 1 480 m (4 855 ft.) - in this phase 
Drilling formation Moraine, clay, non-cohesive soils and fractured rock
Project year 2015 - (scheduled to continue until 2019)