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Groundforce has recently supplied 10 large hydraulic props incorporating their wireless load monitoring system, and including six of their largest - the 500-tonne capacity MP500 Super Strut - to support a basement excavation for 3 Merchant Square, part of a multi-million mixed-use development just off the Edgware Road at Paddington in West London.


The 9m deep basement excavation is located just a few metres away from Paddington Basin on the Grand Union Canal and is lined with a combination of steel sheet piles and a secant piled wall.


Poor ground conditions and the close proximity of the canal mean that significant and variable lateral forces are imposed on the piled retaining wall. To ensure stability, groundworks contractor PJ Carey cast a heavy capping beam over the piles and specified Groundforce struts for structural support.


Four knee-braces comprising 150-tonne working load capacity MP150 struts are used to support one end of the excavation; at the other, support is unnecessary as an existing concrete retaining wall provides stability.


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 supplied 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, technical sales manager with Groundforce.


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.


The Groundforce equipment was chosen primarily to maximise the amount for working space inside the excavation so PJ Carey could cast the basement slab and begin construction of the central core without undue interference due to 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,” said Mr 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 massive Tyne Tunnel project for French contractor Bouygues. Essentially a bespoke product designed for a specific purpose, it has now been updated, refined and launched as part of Groundforce's Major Projects range.


“The MP500 is the flagship of our fleet,” says Mr 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 obtain 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 Groundforce struts save time thanks to the speed with which they are installed. Even the massive MP500 struts took less than half a day to fix in place.


During the excavation phase, loads on the support structure will be 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, contract manager - Merchant Square, with PJ Carey


The installation was completed in June, each strut being installed in sequence, and they will remain in place until late autumn 2012.


Merchant Square, a development by European Land and Property, is due for completion in summer 2014.

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