Groundforce has supplied 12 of its heavy-duty MP250 modular hydraulic props to support an enormous excavation in the Swedish city of Gothenburg.

Contractor Serneke AB is currently in the early stages of construction of the Karlastaden development, a new mixed-use district the centrepiece of which will be a 245m-tall tower – Gothenburg’s first skyscraper.

Serneke is the contracting arm of Serneke Group, which is also the project developer and construction client. The architect is US practice Skidmore Owens & Merrill.

The foundation excavation for this scheme measures 90m in length – and 90m wide. It extends to a depth of around 6m and is located on a sloping site, meaning that the levels vary across the excavation.

Gothenburg is well known for its poor ground conditions. Groundforce has worked on several projects in the city over recent years, its modular shoring system having earned an enviable reputation among contractors and developers for coping with challenging tasks.

In most instances, the Groundforce hydraulic props are used to brace one side of the excavation against the opposite side – bearing against a concrete capping beam and/or one of Groundforce’s modular steel waling beams.

Excavations wider than 30m cannot be propped without intermediate vertical support and, even with this, 90m spans are quite impractical.

In such cases the bracing is usually installed as ‘raking’ props – that is, braced at an angle with one end supporting the capping beam or waling beam and the other braced against a solid ‘thrust block’, usually a massive in-situ cast concrete structure embedded in the base of the excavation.

This is the arrangement adopted on the Karlastaden project. Here, Groundforce has supplied 250-tonne capacity MP250 props in lengths ranging from 17.5m to 20.5m, acting as raking props to support the 6m-deep excavation.

The exceptional size of the excavation (covering approximately 6,400 sq m) means it was impractical to supply enough props to support the full perimeter of the excavation all at the same time. In fact, it made more sense to construct the basement in phases, propping each section with the Groundforce equipment until the relevant section of floor slab and retaining wall was completed.  The equipment is therefore leap-frogged from one section to the next as the construction sequence progresses.

“One of the reasons our modular system was chosen is that the components are very quick and easy to install and disassemble,” says Groundforce European sales manager Sam Oldroyd.

“It would have been much slower and more complicated if bespoke fabricated steelwork had been used,” he adds.

The poor ground conditions and significant depth of the excavation required a robust support system. Groundforce proposed the use of its MP250 props equipped with 1,016mm-diameter Eurotube extensions. The standard extension tube for the MP250 is 600mm in diameter, but at spans ranging from 17m to more than 20m, these would have been too slender and subject to deflection.

Groundforce offers two types of large-diameter extensions for long-span applications – the 1,220mm-diameter Super Tube and the slightly smaller 1,016mm-diameter Eurotube.

“To get to these spans you’ve got to use larger extensions,” says Sam. “The Eurotube has been developed specifically for this type of project. The advantage is that we can get four Eurotubes on a single wagon, whereas we can only get two of the larger Super Tubes on a wagon.”

Groundforce’s main European depot is located in Hanover, Germany, and that means that most European projects involve a lot of transport. The Eurotube greatly reduces the overall transport requirement, reducing cost as well as vehicle emissions.

Serneke’s project manager Zimon Gustafsson says the Groundforce solution was a wise choice for this project.

“This is my third project in Gothenburg and we’ve used Groundforce equipment on all three. Hopefully it will save us both time and money – it’s quite obvious to me that this system is cheaper in the long run, especially as steel prices have gone up so much recently. There really weren’t many other alternatives,” he added.