Struts use in foundations works

13 November 2019

October 2019 - On the BruCity site - Near De Brouckère square in Brussels, the building which housed the iconic Parking 58 car park has disappeared from the skyline. In its place is a huge excavation, with large yellow tubes like the stripes on pyjamas. “Struts” we are told... All right, but what are struts exactly?

“A strut is a structural member which resists the deformations of the walls of a construction pit before the concrete slab is made, without interfering with the zone beyond the job site.” That’s how engineers Grégory Laurent and Vincent Parisel define it. They both work in the Engineering department at Franki Foundations. 

“Struts can be made from concrete, wood, or – in this case – steel. The use of struts is a solution offered when ground anchors are not an option, usually because of the presence of adjacent buildings or infrastructure or for administrative reasons (permits). This excavation support solution also makes it possible to do away with the issue of watertightness in the short and long term, when installing anchors under the level of the water table.” 

So why do we not see more of these in Belgium?

According to Adrien Dumont, BruCity project manager for Franki Foundations, it is primarily a question of culture. Our French, Dutch and German neighbours use this method more often. It’s also due to different regulations. In Belgium, we are still allowed to install 40 metres long anchors even in an urban environment, whereas other countries have now banned this. Besides, a particular geometry of construction site is preferred: a rectangular pit, not too wide, but symmetrical.”

Finally, this solution certainly requires a really thorough overall study beforehand, and excellent cooperation between the design offices and the various contractors. The use of struts is not something one just improvises. In fact, the planning and the phasing are of primary importance. For example at BruCity, Grégory tells us that the reinforcement of the diaphragm wall was especially adapted for installing struts. “The spacing between struts has been designed so that there is one strut per reinforcement cage, meaning two struts per diaphragm panel. This meant one could get away from a costly metal wale.”

Simply put, the whole construction site needs to adapt to the struts, rather than the other way round. One needs to look at everything in advance in order to get the best-performing solution.

Grégory Laurent
Senior Studies Engineer

What about installation? And technology?

Adrien says “In fact, installing props is like a child’s play, similar to Meccano, but on a large scale!” “Each individual unit weighs between 4 and 6 tons, and in this particular project a complete strut weighs approximately 28 tons.” So although in theory struts are easy to assemble, in practice it takes good preparation and above all rigorous execution.

As for the technology, Vincent describes the two types of struts installed at the BruCity site:
“The long cross struts are Groundforce-struts (equipped with hydraulic jacks and a monitoring system). These are 32.6 m long, 1016 mm in diameter x 20 mm, and are made from S-355 grade steel. They accommodate earth and water pressures of 300 tons, plus 120 tons resulting from the thermal effect (due to prevention from expansion when heating).
The corner struts are steel tubes, cut to size. These struts do not support directly the diaphragm wall, but prop a system made of triple metal wales which are chemically anchored in the diaphragm wall. Both the struts and the wales were reused from an earlier construction site. It’s great for synergy and environmental responsibility!”

So this solution can be recycled?

Last year a team went to La Défense in Paris, to the Eria tower job site, to see the struts in place and check if it would be possible to reuse them. For: “different sites, different situations”.

On the BruCity site, the loads are much more important as the water table is higher so the wales had to be sent back to the workshop to be reinforced before they were reused there. This operation needed a lot of preparation and imagination, both from the design and execution viewpoints.
Visiting Paris also allowed to realise what challenges would be associated with using this solution on the Brussels BruCity site.

Struts on the work site of the Tour ERIA in Paris

Why use this solution today?

Grégory and Adrien explain it to us: “We offered this option because it benefited the project overall. 

On this site, the basic solution required an execution with two levels of anchors and a concrete slab in two phases: pour the central part of the concrete slab, excavate the slope adjacent to the diaphragm wall, position reinforced and prefabricated concrete struts and complete pouring of the concrete slab.
The strutting system solution allowed for doing the earth works and pouring the slab in one single phase."

This was the right solution for the right project.

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