Securing lift shaft walls a challenging task

In Stigbergsparken on Södermalm, work is under way to install a support structure for the lift shaft to be built for the new Sofia metro station. The support structure will act as a protective wall around the shaft to prevent the surrounding soil from collapsing and to prevent any change in the groundwater level.

News News 2022-10-20

The support structure is comprised of so-called secant piles, which are overlapping piles cast in situ to form a watertight wall.

“This task involves several challenges. On the one hand, the bedrock slopes, and on the other hand, there are large boulders in the ground that take time to bore through. However, drilling and casting the secant pile wall is a necessary part of the construction of the lift shaft for Sofia station,” says Johanna Nilsson, the geo-construction manager for Stigbergsparken.

On the left are the steel pipes that are drilled down into the ground while extracting the loosened soil. Each pipe has teeth that allow it to be drilled into the ground. These need to be replaced at regular intervals due to wear. On the right is the screw used to extract the soil.

“This requires powerful machines. At the moment, we’re working with a Liebherr 36 that drives the pipes down and then drills while also extracting the soil with a screw,” Nilsson explains.

The machine, which weighs 116 tonnes, was shipped all the way from Germany to Stigbergsparken.

To ensure that the pile wall is anchored in the bedrock, the hole for each pile is pre-drilled with a different machine equipped with a rock drilling rig. The drill bit is lowered into the casing and drilling can begin.

To ensure that the pile wall is anchored in the bedrock, the hole for each pile is pre-drilled with a different machine equipped with a rock drilling rig. Pictured in the background on the right is the Liebherr 36 machine, while in the foreground is the Bauer BG28 machine, which is equipped with rock drilling equipment.

The first thing that is done is to drill holes for every other pile. Once the pile is ready for casting, it is filled with concrete. Every other pile is reinforced. This combination ensures a watertight and robust wall. While a pile is being cast, the steel pipe is lifted up and out ready for use on the next pile. Once the concrete in the hole has hardened, a stable cast-in-situ pile has been created.