No fewer than 17 of Groundforce’s modular hydraulic props – some spanning almost 50m with no intermediate support – have featured in a complex structural support contract for a new luxury housing development in Hammersmith, London and the first project completed by Groundforce to EC3 Eurocde.
Developer St George West London is transforming the old Kings Mall car park in Glenthorne Road to provide 418 luxury one, two and three-bed apartments in a multi-storey complex called Sovereign Court.
The roughly triangular site will incorporate a two-storey basement excavated by groundworks contractor Joseph Gallagher which has hired in the Groundforce equipment to support the secant-piled retaining wall during the excavation process.
The hydraulic props were installed at ground floor level to support the reinforced concrete capping beam. Due to the shape of the excavation, most of the 17 hydraulic props were not perpendicular to the capping beam and special steel brackets were cast into the beam to accommodate the angles.
The props are all high capacity units, mostly MP250s with a load capacity of 250 tonnes. Unusually – and due to the very long spans involved, most of the props were combined with Groundforce’s 1.2m diameter ‘super-tubes’. These large-diameter tubes (twice the standard diameter) provide the necessary section modulus to prevent any risk of deflection over the long spans.
“We had strict performance requirements that had to be satisfied”, explains Ajay Nagah, Major Projects Design Manger, , with Groundforce Shorco.
“First of all, there were stringent deflection criteria due to the presence of the existing LUL [London Underground] line against the southern boundary of the site. Secondly, we had to provide a clear space beneath the props for Gallagher to excavate the basement”
“The large size and irregular shape of the excavation added to the complexity,” adds Ajay.
To minimise deflection, Groundforce not only prescribed its largest props – including two MP500 Hybrid props, with a load capacity of 500 tonnes – but also positioned them relatively close together, at 7.5m centres.
The Hybrid props combine both the hydraulic rams, used to quickly extend the prop and apply pre- load, as well as mechanical screw-jacks that could be locked off to take the full compressive design load and ensure a solid immovable support. The Hybrid props were fixed to the capping beam using bespoke steel corbels, designed by Groundforce and cast into the beam.
“A corbel enables the shear forces acting at prop ends to be transferred efficiently into the RC capping beam, with a steel solution also proving optimum when dealing with the removal of props,” explains Ajay.
To provide assurance that there was no movement, Groundforce also supplied and fitted its proprietary load monitoring system which provides a constant read-out.
The four longest props spanned 47.5 m clear across the centre of the site from north to south. With the props in place, Gallagher were then free to continue the excavation down to formation level.
The basement slab was then cast, followed by an intermediate slab to provide the first storey basement. Then came the ground floor slab, level with the capping beam.
“The construction sequence dictated that the groundfloor slab had to be cast prior to the props being removed,” explains Ajay. “The concrete pour was therefore done in sections, with the props removed a few at a time.”
In some local cases, the use of load monitoring combined with on-site deflection monitoring had enabled Gallaghers to remove props prior to the groundfloor being cast by virtue of an observational approach – which proved highly practical for the site team, whilst also saving significant time off the programme of works.
For Groundforce, this project marked a significant milestone, as Ajay explains: “This entire structure was designed to the latest EC3 Eurocode and is the first project we’ve completed using the Eurocode”.
Brought in to harmonise structural steel design throughout the EU, Eurocode 3 is set to replace the long-standing British Standard BS5950.
“We have decided to fully embrace the new Eurocode, and we’ll now use it to design all our major projects,” says Ajay.