Redevelopment of the Regent Palace Hotel in London’s West End has been one of the most imaginative and demanding of minipiling projects. Paul Wheeler reports.
Crown Estates’ Quadrant 3 redevelopment of the Regent Palace Hotel, a prominent landmark on the corner of London’s Piccadilly Circus and Regent Street, looks like a hugely complicated demolition job.
The hotel, built in 1914, is a grade II-listed building, and as part of the redevelopment, Crown Estates and developer Stanhope are preserving many of the original features.
Half of the historic facades are being renovated and retained, and four glamorous 1930s art deco entertainment venues are being completely restored and transplanted.
Timber veneer, marble, brass, mirrors, ceilings, even wallpaper from the Atlantic Bar & Grill, Titanic Restaurant, Dick’s Bar and Chez Cup, have been carefully stripped from the building and labelled so the rooms can be put back together as they were.
The scheme will replace the 1,000-bedroom hotel with a mixed-use development comprising seven floors of offices, a ground floor incorporating shops and nine residential units.
With most of the work on site (except the heavily braced retained facades) now approaching street level, you might expect to see the piling rigs moving in.
But they are, in fact, just about to move out.
For, over the past year, if you had ventured deep into the hotel’s labyrinthine subterranean bowels, you might have been surprised to stumble upon foundation contractor Cementation Skanska hard at work, with demolition contractor Keltbray working simultaneously above and occasionally alongside.
Cementation Skanska was invited to tender for what appeared to be a fairly conventional, large diameter bored pile-based foundation scheme in August 2008, explains its ground engineering project manager Kevin Hague.
A value engineering exercise, instigated by construction manager Sir Robert McAlpine, identified that a large number of the bored piles could be replaced with rafts, supplemented with minipiles, king posts and other bracing structures.
Although simple in concept, it was nevertheless a bold new design, particularly because the formation level of the new development is 4.5m below the basement level.
The basic approach is to prop the original basement structure, freeing up the centre of the site for efficient slab construction.
The biggest saving in this alternative design is time - 14 weeks were cut from the original programme.
This is largely because the new solution enabled foundation work and demolition to take place concurrently.
While Cementation Skanska is installing fewer piles than first envisaged, the work has proved to be extraordinarily complex and demanding.
Primarily, it’s a matter of space.The original hotel was built with three basement levels, one of which contained a rudimentary air conditioning system.
This lowest basement level is a conduit of passages which took warmed and cooled air from boiler and refrigeration rooms within the basement, along up-risers and into the public spaces of the hotel.
While fascinating from a building services point of view, this resulted in a very “piling rig-unfriendly” layout of tight corridors, conduits and undercrofts.
As a result, most of the work has been carried out from the slightly more accessible middle basement level and occasionally, when access permitted, the lower basement.
Cementation Skanska’s work includes installing more than 360 bearing piles together with sections of king post retaining structures and some ancillary temporary piling work to support cranes and gantries.
Work includes installing more than 360 bearing piles together with sections of king post retaining structures
All piles are cased auger, and are being installed using electric mini-piling rigs.
Typically, the casing is required to extend through the void of the lower basement level.
Often, it has been necessary to pre-core through the floors of the two lower basement levels as well as break out the ceiling above to increase head space, enabling the rig mast to be manoeuvred into position.
In many cases, temporary supports have been needed to strengthen the middle basement level as the work progresses - as these slabs became increasingly riddled with holes, in what Hague describes as the “Swiss cheese” effect.
In addition to the extremely challenging space and access limitations, the other big challenge at the site has been the scheduling of work, particularly the coincident demolition and piling activities.
To manage this, Sir Robert McAlpine set up a “dashboard” system, in which all parties active on site attend daily planning meetings where they run through their programmes of work for the day.
Different activities are denoted on a schematic schedule and floor plan displayed prominently as a giant wall chart just inside the site entrance.
This provides a clear indication of where everybody is working - and an excellent check to ensure that no two activities are taking place in the same place at the same time.
This daily scheduling is linked to a weekly coordination planning meeting to agree the planning and sequencing of work among the different trades.
And with up to six activities working on site at any one time - such as demolition, piling, slab pre-coring, asbestos removal, steel fixing, scaffolding and so on - this has proved to be a simple and reliable method and has been, reports Hague, “executed very smoothly”.
Cementation Skanska first moved onto site in October 2008, and as a result of the phasing of the demolition work has programmed its activities across three site visits.
The team returned for the third time in early September 2009 to construct the final load bearing piles.
Upon completion, the piling contractor will have installed 368 300mm diameter bearing piles using two low-headroom Technodrill TD308 minipiling rigs.
The cased auger piles extend deep into the underlying London clay, typically to 25m, and are designed to carry loads of 560kN in compression.
Throughout the piling, working headroom within the basement has been no more than 2.5m and operating widths just 1.4m.
This restricted environment has proved to be a major logistical challenge for the crews and equipment, especially during the installation of the 6m-long reinforcing steel cages.
These were typically installed in three 2m sections spliced together at the top of the pile casing, with the site crews using hand-operated mini-cranes to safely lower the cages to the desired levels.
Communication between the piling operatives and grouting squads proved to be another interesting challenge, which was overcome using a two-way radio system that was able to penetrate the three basement levels.
Some work was executed outside the facade, including seven additional 31m deep bearing piles installed from the pavement, but through existing lightwells, using a Klemm 709 rig.
A further eight 600mm diameter king post piles, extending to 14m depth, were also installed from the pavement to provide a retaining function for a new service tunnel.
Temporary works installed within the confined basement included 87 450mm diameter retaining wall king post piles extending to 7.5m.
These were constructed using two Hutte 202 rigs. An additional four 300mm diameter bearing piles formed part of the facade retention system.
Other key challenges involved getting materials including casing, grout and steel reinforcement to the job.
For example, the high shear grout mixer was located just outside the retained façade and mixed grout was pumped 150m to the pile locations.
Getting spoil away diligently and safely was also tricky.
The challenges and space restrictions extended beyond the site itself, which is located on the densely developed edge of Soho with its very narrow streets.
These are crammed with services, whose positions are not all reliably recorded.
At ground level, the streets are filled with people all day and much of the night, which again makes deliveries to and from the site difficult.
In terms of overcoming these very job-specific limitations, Hague puts the successful delivery of the mini-piling works down to the in-depth co-ordination and planning of the project both in advance of the site works and during their execution.
Equally he cites the “active management in tandem with Sir Robert McAlpine’s project team” as the key to providing secure foundations
for this “imaginative but demanding” project.
He says this enabled the “systematic completion at all levels” of the safety, logistics and detailed planning elements.