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Super struts

27 Oct 12 PJ Carey is using huge hydraulic props to support a London basement excavation

In the Paddington district of west London, formerly derelict canal land is being transformed into a plush new neighbourhood with smart apartment buildings, offices and shops.

Merchant Square, a development by European Land & Property, comprises six large buildings within walking distance of the West End. Flagship is Number 1 Merchant Square, which at 42 storeys will be the tallest building in Westminster.

Currently under construction is Number 3 Merchant Square, a 21-storey residential building just off the Edgware Road, which will house 201 upscale apartments.

When it came to excavating the basement, poor ground conditions presented a certain technical challenge to groundworks contractor PJ Carey. The 9m-deep basement excavation, just a few metres away from Paddington Basin on the Grand Union Canal, is lined with a combination of steel sheet piles and a secant piled wall.

There are significant and variable lateral forces imposed on the piled retaining wall and so, to ensure stability, Carey cast a heavy capping beam over the piles and specified huge struts for structural support.

Groundforce, part of the Vp equipment hire group, supplied 10 hydraulic props incorporating its wireless load monitoring system. Six of the 10 are the 500t capacity MP500 Super Strut, the largest props in its inventory.

Four knee-braces comprising MP150 struts – with 150t safe working load capacity - are used to support one end of the excavation; at the other end an existing concrete retaining wall provides stability, making additional support unnecessary.

Most of the lateral support is required across the width of the huge excavation, which is more than 100m long and 49m wide for most of this length. Support here is provided by the five MP500 Super Struts located at the point of highest calculated load.

“The loadings are not consistent or evenly distributed so at this point we have had to use our largest strut to ensure stability,” says Mark Whitmore, Groundforce’s technical sales manager.

All 10 of the struts are braced against the capping beam, which has specially designed steel corbels cast into it to accept the connectors at the ends of the MP150 and MP500 struts.

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The Groundforce equipment was chosen primarily to maximise the amount of working space available inside the excavation so that PJ Carey could cast the basement slab and begin construction of the central core without too much restriction from supporting steelwork.

“The MP500 struts employ our 1220mm diameter Super Tube extensions which give them the strength and rigidity to span the 49m width without intermediate support,” says Whitmore. “Using our biggest struts also allowed us to minimise the number of supports required.”

The MP500 was first developed in 2008 for use on the Tyne Tunnel project for French contractor Bouygues. Essentially a bespoke product designed for a specific purpose, it has now been updated, refined and added to Groundforce’s Major Projects range.

“The MP500 is the flagship of our fleet,” says Whitmore. “We are seeing a growing demand for these large struts as a cost-effective solution for all types of major excavation.”

The hydraulically extendible strut uses modular tube sections to get the required length and has a hydraulic adjustment of +/- 0.5m. The strut is assembled in-situ, fixed to the corbels and extended and pressurised between supports before being mechanically locked off with twin load-bearing screws.

Besides providing a clear working space within the excavation, the struts save time by being quick to install. Even the MP500 struts took less than half a day to fix in place.

During the excavation phase, loads on the support structure have been constantly monitored in real time using Groundforce’s own remote load monitoring system. This uses load pins in the strut connectors to measure loads and send readings via secure SMS or wireless internet links to authorised recipients.

“Each 47m long MP500 prop was installed in less than half a day, which is far quicker than a traditional fabricated solution. The system does not require any mid-span support and has the added benefit of incorporating a load monitoring system,” said Bradley Barham, PJ Carey’s contract manager at Merchant Square.

The installation was completed in June, each strut being installed in sequence. The struts are scheduled to remain in place until the end of October 2012.


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