Developers are always keen to maintain the trappings of the Victorian era. But the reality of modernising such buildings means that foundation contractors often face challenging space limitations. Gemma Goldfingle reports.
Many refurbishments of Victorian buildings have relied on Italian pali radice technology. This uses incredibly compact, yet powerful piling rigs to strengthen foundations while leaving no trace of repair on the building fabric.
This “invisible” underpinning technique has been used on a number of important heritage buildings, from the Campanile of San Marco in Venice to the Tower of London. The latest site to get the Italian job is the Corinthia Hotel on Northumberland Avenue in the heart of London.
From the outside, no one could imagine the level of activity within the confines of the building. Behind the grand doors of the 1850s structure is a congested building site with a crew of 50 hard at work. Six piling rigs are hidden around the labyrinth of corridors, begging the question, how they got there.
The new Corinthia Hotel and Residences is set on the site of the former Metropole Hotel, which was one of London’s most fashionable hotels at the turn of the 20th century. King Edward VII was reportedly a regular patron before the hotel closed its doors in 1936.
Location and glory
After years of use by the Ministry of Defence, the Corinthia Hotels Group has decided to take advantage of its prime location, a stone’s throw from the Houses of Parliament, and return the building to its former glory.
Corinthia has also bought the adjoining property, 10 Whitehall Place, to house the hotel’s luxury 2,500m² spa and a suite of exclusive residences, the largest of which spans over 1,000m².
Converting the building into a hotel involves extensive refurbishment − several of the building’s original walls will be removed to make way for the large hotel suites. Retaining the impressive Victorian façade and ornate interiors is critical to the developer, but imposes restrictions on the underpinning work needed to manage redistributed structural loads within the building. Additional column loads of up to 1,600kN need to be accommodated.
With cramped corridors just 2.7m wide, the task of installing rigs capable of drilling piles to depths of 20m or more is not easy. But geotechnical contractor Systems Geotechnique supplied a solution − it is using pali radice, a technique invented by Italian geotechnical contractor Fondedile, which the company acquired in 2005. Pali radice literally translated means “root piles”, explains Systems Geotechnique technical manager Steve Attwood.
“It is a specialist technique specifically for underpinning existing buildings in restricted spaces.”
Steve Attwood, Systems Geotechnique
“It is a specialist technique specifically for underpinning existing buildings in restricted spaces,” adds Attwood. “Rigs range from 3.5t to 8.5t, and the largest rig we would need for this job would be 3.5m high and 1.7m wide when fully mobilised, allowing piling to take place in narrow crevices around the building.”
A network of angled, crisscrossed piles is formed through the structure to achieve a bond between the existing foundation and competent subsoil strata and to provide the structure with “roots”. The raking pile network leaves the fabric untouched and needs no additional works, such as ground beams.
A lattice of 280mm pali radice piles are drilled at 20˚ angles either side of the hotel’s crosswalls.
The raked piles connect to the existing foundations, which consist of concrete footings 5m deep. Piles and wall are bonded for around 4m before the raked piles travel through to the London clay below.
A Dywidag bar reinforces the full depth of the pile. It connects together in short lengths to make the process easier in the 3.5m headroom available.
Grout, which forms the piles, is mixed on site in a 5m² central batching area outside the hotel entrance. Westminster City Council allowed the closure of one lane of traffic on a back street next to the building.
Grade 35 grout is formed by mixing one part sand to one part cement grout, which is then pumped into the working area. This system also further stabilises the original walls by pumping grout deep into any voids or dry joints that may be present.
Conditions for underpinning at 10 Whitehall Place are even more cramped. It was impossible for the rig and its operator to fit into the tiny spaces within the corridors, so Fondedile’s smallest 3.5t mini rig had to be tailored by adding a remote operation panel to overcome the space restrictions.
A total of 177 structural piles are required along the perimeter of this building’s central courtyard.
As headroom here is not a problem, a Klemm 709 can be used to form the 450mm diameter load-bearing piles. The 2m wide rig is designed for limited access sites and is 5m high when demobilised. The cast insitu bored piles are sunk to depths of 28m, and are designed for a 1,100kN working load. Temporary casings are bored through sands and gravels to reach London clay at a depth of 10m.
As with many Victorian properties a wealth of hidden hurdles lurk beneath the Metropole.
Several vaults showing signs of distress were found in ground nearby and Westminster Council plans to take action to prevent any subsidence. Fortunately, the vaults do not encroach on the hotel site.
Existing foundations did throw a spanner in the works, however. “Some pile positions were essential, like those for the new lift core. But there is no way to avoid the building’s existing foundations,” says Attwood.
“Some pile positions were essential, like those for the new lift core. But there is no way to avoid the building’s existing foundations.”
Steve Attwood, Systems Geotechnique
Piling through this material would cause big problems for most mini piling rigs, but Fondedile’s has tungsten carbide teeth that can bore through the strongest of materials, including masonry and steel. The power of these compact rigs has stimulated deviation from the traditional pali radice technique.
Their ability to core through obstructions has made the machines popular in urban sites rife with hidden foundations. Making light work of the concrete and timber foundations below the Metropole building, the rig bored 92 vertical 280mm diameter minipiles to depths of 10m.
With the rig powering through all in its wake, all 421 piles were complete two weeks ahead of schedule. These included eight additional bored piles in the centre of the courtyard, constructed to take the loads of a tower crane.
Piling is now finished, and many passers-by were left agog as an enormous 350t crane lifted plant 30m high against London’s skyline on the weekend of 26 September. All is on track for London’s newest deluxe hotel to open in October 2010.
- More geotechnical features in the October issue of Ground Engineering. For more information go to www.geplus.co.uk
A triangular courtyard will form the centrepiece of the hotel. It will have a central atrium with a garden. During construction, seven piling rigs cover what will soon be the serene garden area.
Traditional rigs would have been lifted into the courtyard using a 350t crane, but this would have meant closing a busy central London road. With a tight programme to meet the October 2010 opening date, waiting months for a road closure was not an option. Instead, main contractor Ardmore Construction decided to bring in the machinery through the main hotel entrance.
A special ramp had to be installed which is 6m lower than the hotel’s ground floor. As it covers several pile locations the 20m long, 1:4 ramp had to be removed. With nowhere to store the structure, Ardmore organised a road closure for a heavy lift at the end of the piling work when all the machinery was removed.
The road closure date was set in stone, so there was no room for delays in the tight 15 week piling programme.