Groundforce performs a delicate balancing act in Oslo

11 Apr
Contractors in Norway are using modular hydraulic props from Groundforce to support a major rail tunnel redevelopment in the capital, Oslo.

HAB Construction is building the Follo Line project for client Bane NOR, Norway’s national railway infrastructure company.

Currently the largest transport project in the country, the Follo Line forms part of the inter-city development heading south from Oslo. It will deliver 22km of double-track line from the capital to Ski, the largest town in the Follo municipality.

Remarkably, nearly the entire line – 20km of it – will be underground, in a twin rail tunnel. When completed, this will be the longest railway tunnel in Northern Europe.

Part of this tunnel is being built along the route of existing railway tunnels and HAB Construction is busy demolishing the old structures, enlarging the excavation and constructing the new cut-and-cover tunnels.

This is where the Groundforce propping system comes into its own.

HAB Construction is using 14 Groundforce MP150 props and 10 MP250 props to support two parallel excavations while demolition and construction work continues.

“These are quite old, existing tunnels,” explains Bane NOR’s engineering manager Daniel Eika. “We are removing existing tracks and changing the alignment of the tunnels, then merging them with the new tunnel profiles coming in from the side. Part of the transition includes separate tunnels becoming a single tunnel for both tracks.”

This section is complicated because the geometry and support conditions change every five meters while tracks are operational on both sides of the construction pit. This leads to an intricate set of construction phases and cramped working conditions. As the two tunnels merge into one the ground separating them becomes narrower and hence weaker. Lateral support is required for the loadings on the props to be transferred across from one tunnel to the other.

To avoid the collapse of this central section, the props in each tunnel have to be pressurised simultaneously and the loads balanced with great accuracy.

Two to three levels of props have been installed at approximately 5m centres. But, as Daniel Eika explains, the transition from two separate tunnels to one wide tunnel represents something of a moving target.

“Every metre there’s something different and we have to design for that,” he says.

The section of tunnel accommodating both tracks is of course wider than the single-track sections and Groundforce’s props have to span almost 40m.

For such a complex project, bespoke structural steelwork is often fabricated and installed for support. But this section of the Follo Line is located in the middle of Oslo. “It’s a very busy location and there are huge risks,” says Daniel Eika. “That’s why we have adopted this approach.”

Groundforce’s propping system is ideal for projects of this type. It comprises a range of standard components which can be assembled quickly to within 500mm of the final span.

The modular units are quick and easy to install and the hydraulic ram system is used to extend each prop to the required length and pressure. Demounting and removing the props is also quick and easy.

This is the second installation of Groundforce equipment on this project so far. The company’s contract with HAB Construction has been split into two phases: the first, which required six MP250 props, was completed in 2019.