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Prime movers: The spotlight on piling

A tricky deep-basement in Knightsbridge is providing a useful showcase for Cementation Skanska’s Crossrail credentials. Paul Thompson reports.

For the UK’s geotechnical contracting elite, Crossrail contracts already look set to be the most sought after of this coming decade. With only a handful of contractors offering the necessary technical and operational capabilities to take on the coveted station boxes, Cementation Skanska is currently engaged in what is in effect a full dress rehearsal.

Across the road from the ultra-luxurious One Hyde Park residential development in London’s Knightsbridge, Cementation Skanska is working on Prime Development’s equally impressive super deluxe hotel and residential development. The work involves constructing a six-level deep basement in an extremely confined inner urban site immediately alongside substantial existing buildings, roads and services.

Innovative technology

In addition to showing off its deep basement construction capabilities, Cementation has adapted its innovative energypile technology − a ground source heat pump-based renewable energy source − for use in the basement’s perimeter diaphragm wall. This is certainly a first for the UK and provides a massive tick not only for Prime Development but also for Cementation when it comes to evaluating Crossrail tenders; particularly since politicians have identified public sector procurement as a means of driving the transition to a low-carbon economy.

But right now Cementation is more concerned with the redevelopment of the site. Designed by Antonio Citterio & Partners, in conjunction with Squire & Partners, the 10-storey structure will include penthouse apartment on the upper levels, while the six subterranean levels will provide a grand ballroom, spa and leisure facilities, business centre and private screening room.

Cementation moved on to site in September last year and is on schedule to complete its £4M foundation work this March. The key challenge of the job, says Cementation’s project manager Neil Abbott, is using the large plant and equipment on such a small site in a very busy location. The construction sequence and on-site management is obviously critical to maintaining a safe and productive site.

Cementation Skanska

Cementation Skanska get to work

The basic strategy is to complete the 30m-deep perimeter diaphragm wall and then install the load bearing piles.
Given the scale of the foundation operations, managing space and delivery logistics is a key capability − particularly locating the bentonite cleaning plant, site offices and welfare facilities, to enable the excavation rig, crane, reinforcement, concrete and muck-away lorries to manoeuvre around the site. Careful scheduling is enabling Cementation to operate with just one major reorganisation of the site’s layout.

At the start of the contract Cementation covered the entire site in a reinforced concrete working platform. This has proved to be a sound decision for keeping the site clean, an important consideration given the proximity and sensitivity of its neighbours.

A delicate operation

Cementation is constructing the 155m-long, 800mm-wide diaphragm wall in panel lengths of 3.2m to 7m. The 35 low cut-off bearing piles and 15 tension piles are 1.5m diameter and are also bored under bentonite − to depths of up to 60m. All bearing piles are plunged with steel columns, with the cut-off level some 23m below ground surface.

The ground source heat pump system requires incorporating closed loops of pvc piping within the wall and piles. The challenge is to it ensure the pipes are not damaged either in the construction or the subsequent excavation and build phase.

The method for installing loops in the wall is essentially the same technique trialled at Bankside − where last autumn, Cementation installed the UK’s deepest ever energy piles, a process that Abbott describes as “simple but effective”. That’s if you know how.

Super luxury

Prime Development derives its name from the fact that it develops only in the “world’s super-prime locations” − which, of course, include London’s prestigious Knightsbridge.

The original Knightsbridge Palace Hotel was built in 1911 and in its day became a by-word for elegance.

It was renamed the Normandie in 1937, but closed in 1977 and the upper floors were converted to apartments for holiday lets.

In redeveloping the site Prime Development intends to create one of the world’s leading super luxury hotels, with club suites and penthouse apartments which all promise a “21st Century

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