An unassuming apartment block in a Geneva suburb is actually a significant landmark for a UK construction equipment company, as Margo Cole reports.
Two years ago excavation support and propping specialist Groundforce Shorco managing director David Williams told NCE about the firm’s ambitious plans to expand into mainland Europe (NCE 15 September 2011). Despite the recession in many European countries, the strategy has proved successful, particularly in Germany, where the hire firm now has three depots.
Groundforce’s plan for further expansion involves building on the success in Germany and winning orders in neighbouring countries. Again, this plan appears to be working, as the company has just picked up its first job in Switzerland, supplying huge props to support a basement excavation in a suburb of Geneva.
Like many of its competitors, Groundforce hires out traditional trench shoring systems, like big steel trench boxes, rolling strut boxes and slide rails. But in recent years it has also developed a reputation for supplying props for big excavations, with the capability to span anything from 0.5m to 47m.
The Geneva basement is fairly typical of this type of work - an 80m long, 26m wide and 7m deep rectangular excavation that will house a two level car park for the seven storey apartment block being built on top. But in Switzerland - as in much of mainland Europe - the use of proprietary props for an excavation like this is not typical. Here, the norm would be to use structural steelwork to form a support structure, as contractor Implenia project leader Jean-Pierre Binétruy explains: “Normally it is done in structural steel, which is much denser than we have here, because you [would] have main struts and cross struts and vertical supports.
“All over Europe there are these big basements. We’ve just got to find the right projects it’ll work on ”
David Williams, Groundforce Shorco
“We’ve been doing that for 25 years, and we own the steel,” he adds.
Implenia is Switzerland’s largest construction company, employing 6,500 people and turning over around €2.5bn (£2.1bn) a year. In addition to building bridges and tunnels, the company has a foundations division that specialises in piling, diaphragm walls, shaft excavation and propping.
At Chêne-Bourg, a suburb to the east of Geneva, the firm was brought in to install the diaphragm wall for the basement, and to support it while another contractor, HTP, carried out the excavation. A third firm, Belloni is responsible for building the apartment structure above and the internal structures within the basement.
The site had previously been parkland, with a single level underground car park built within a 3m retaining wall. Implenia has replaced this with a 12m deep concrete diaphragm wall, installed in 7m wide panels through the underlying clay using a 12t grab. Although Binétruy describes the clay as “not very hard, and easy to excavate”, the company did have to use bentonite at times to provide support while the wall panels were installed.
In total, the company installed 2,300m2 of 600mm thick diaphragm wall. It has been designed to carry all the loads from the seven storey building, and is heavily reinforced with 450mm wide reinforcement cages.
The decision to prop the excavation using Groundforce’s struts was made on the basis that it would provide far more space for the rest of the construction activities than a structural steel frame would have done, as well as making the job a bit quicker. “For the building contractor there is sometimes not enough space to lower elements like formwork, because you have cross struts,” explains Binétruy, who also says a steel frame would require piled foundations for the vertical supports, as well as gaps to be left in the base slab and intermediate floor slabs to accommodate these vertical elements.
Eight rows of struts
At the Chêne-Bourg site the basement was supported by eight rows of tubular struts on two levels, all spanning 26m across the width of the excavation. The higher level struts measure 510mm in diameter (600mm at the flanges where the tubes are joined together), and were capable of carrying loads of 125t. The lower struts have a load capacity of 250t and were 600mm in diameter (800mm at the flanges). In total there was around 150t of steel inside the basement excavation.
They were installed sequentially as the excavation progressed from one end of the site. Once the groundwork contractor had removed the first section of earth, the top struts were installed. A Komatsu PC240LC long reach excavator was then used to dig beneath them, creating space for the lower props to be installed. “There was not enough space underneath the first level for an excavator and trucks, so we used a long arm and dug from the top,” explains Binétruy.
This process was repeated, with the excavator working backwards from the far end of the job until the entire basement had been excavated and all the struts were in place, with the site’s tower crane being used to lift in the props, the largest of which weighs 6.8t.
Incorporated within two of the struts - the upper and lower props that were nearest to the base of the tower crane perched on the edge of the excavation - were load monitors, which constantly recorded the load going through the prop. These were set to give a warning if the load exceeded a pre-set limit. According to Binétruy the design load for the lower struts was 70t, although the actual load recorded on the day NCE visited the site was around 90t. However, this was not causing any alarms, as the props were designed to cope with the forces from the ground, and thermal loading resulting from the struts expanding and contracting as the temperature changes.
“For the building contractor there is sometimes not enough space to lower elements like formwork, because you have cross struts”
Jean-Pierre Binétruy, Implenia
“In Geneva we usually count for around 30% more load linked to heat,” says Binétruy.
Inclinometers within the diaphragm walls recorded a variation of around 25mm in the vertical plane, while the struts themselves deflected by a maximum of 80mm - well within the design tolerance of 120mm.
With the excavation complete and all the struts in place, HTP was able to start pouring the base slab and intermediate floor slab for the underground car park, as well as some concrete walls for other structures within the basement. As soon as the slabs are complete, the props are no longer needed and can be removed - something Implenia did last week, again using the tower crane to lift them out.
Groundforce sees the project as a very successful introduction into the Swiss market, not least because Implenia is thinking of using the struts again on other basement projects. “We have moved into a completely new market with the biggest [construction] company in Switzerland, who is already talking to us about the next two projects, so that’s a significant and successful start,” says Peter Richardson, an independent piling and foundations consultant who is helping Groundforce with its expansion in mainland Europe.
According to Binétruy, Implenia has built around 90% of the basements in Geneva and the surrounding region, so the success of this first project is an important step for Groundforce. “We can use them [the struts] for pretty much anything apart from projects where you don’t have any access to remove them from underneath, and you would have to cut them out,” he says, adding that he can easily see Implenia using the system for around 50% of its basement jobs.
That is music to the ears of Williams, who wants to double the firm’s size within the next five years by expansion in Europe and believes basement propping is one way to do that. Props currently account for around 25% of the firm’s stock. “All over Europe there are these big basements,” he says. “We’ve just got to find the right projects it’ll work on.”