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Reinforcing with water

When the Pyramiden neighborhood in central Stockholm, Sweden, began sinking due to old, wooden pile foundations, Wassara’s water-powered drilling technology was chosen as the most suitable solution for the reinforcement work


Old buildings in central Stockholm are often supported by foundations made of thick wooden piles. As
the water table drops, the wooden piles get in contact with air and often begin to decay, resulting in a reduced load-bearing capability of the foundations. The buildings resting on these piles consequently begin to sink.

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

The foundation reinforcement procedure is performed using steel piling, installed with a standard
casing advancing system (Atlas Copco’s Symmetrix). First, thick steel pipes are drilled down through the overburden and a bit into non-fractured rock. When the holes are completed, they are filled with grout, starting from the bottom.

Besab chose to test Wassara’s technology for the project, since the company knew that the water down-the-hole (DTH) hammer would reduce the risk of the ground level dropping in an area connected to limestone walls and improve the working environment.

Igor Kordas, project manager at LKAB Wassara, explains:
“First and foremost, pollution is minimized compared to air, as water does not have the same up-hole velocity and as a fluid it is non-compressible, while the air as a gas expands when leaving the hammer. So when drilling with air DTH, you get a fast stream of air going up to the surface, also containing material that was produced during drilling. Needless to say, it is not a clean environment and it takes a long time to clean up the area (especially relevant when drilling in basements and similar spaces), but, furthermore, you also get oil mist in the air as air DTH requires oil-based lubrication.”

The overburden in the area consists of moraine, non-cohesive soils, mud and fractured rock. The ground is very rich in 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. “First of all, it is important to mention that it is really clean in the basement when using Wassara,” says Rani Isa, construction manager at Besab. “Also the diesel consumption of the water pump is much lower than an air compressor’s.”

During the initial phase of the project a total of 76 holes were drilled with a diameter of 170mm,
pile thickness of 10-12.5mm and S460MH steel quality. In total 1,480m have been drilled in this
project. The Wassara W120 (5in) has been used in combination with 114mm drill pipes. Since the work in the basement is complicated by lack of space, Besab commissioned a Swedish company to develop a special small rig with high manoeuvrability as the main requirement, as well as a telescopic mast.

The project has been running since 2009 and is scheduled to last through 2019. Several different technologies have been used due to the strict requirements related to the degree of ground level lowering. The company has been using Wassara’s drilling technology in this area since 2015, and it is currently the only method in use. The water used to power the hammer is being municipally sourced, taken from a fire hydrant. No water-handling system is thus necessary. The vast majority of the water, in fact, runs down the drill holes, and the small amounts that make it to the surface have vanished through the overburden.

Courtesy Geo Drilling International
Wassara and Besab representatives observing the
work being done

“The company has been using Wassara’s drilling technology in this area since 2015, and it is currently
the only method in use”

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