A 13-storey business hotel is ignoring the fourth floor due to client convention while cleverly re-using existing piles.
Project: Dorsett City Hotel
Client: Dorsett Hospitality International
Contract value: £15m
Contract type: JCT Design and build
Main contractor: Willmott Dixon
Demolition subcontractor: Squibb Group
Cladding subcontractor: Deepdale Solutions
Start date: June 2015
Completion date: September 2016
It seems strange that the nerve centre of a project as busy as the new Dorsett City Hotel being built in the City of London should be buried away in the quiet, somewhat spooky crypt of St Botolph’s Aldgate.
Tales persist of the church being home to the mummified severed head of Henry Grey, the 16th century Duke of Somerset and father of would-be queen Lady Jane Grey.
Given these morbid rumours, it is impossible not to feel slightly unnerved as you enter the series of vaults and the traffic-filled noise of the 21st century fades.
In its most recent past this labyrinth of cellars played host to some of London’s homeless, with more than 70 of the capital’s rough sleepers finding shelter there at any one night.
But after lying empty for 20 years following the shelter’s closure, the crypt is now home to a Willmott Dixon site team focused on delivering an altogether different kind of overnight accommodation.
Under a £15m deal, the contractor is delivering the shell and core of a new hotel on a site that is sandwiched between the church to the west and Aldgate station to the east, hemmed in by St Botolph Street to the north and Aldgate High Street to the south. It is the epitome of a tight, city centre site.
“Permissible working times, delivery times, changing traffic control measures, live underground railway adjacent. There isn’t too much we haven’t had thrown at us here”
Leon Bastajian, Willmott Dixon
Building manager Leon Bastajian knows just how much of a headache the project is. He has worked on the scheme since its earliest days in June 2015 and has had an input into every tricky task the team has been asked to overcome.
“It is a very constrained site,” he says. “We have to deal with so many restrictions. Permissible working times, delivery times, changing traffic control measures, live underground railway-adjacent. There isn’t too much we haven’t had thrown at us here.”
Project Report Willmott Dixon Dorsett City Hotel 7
Formerly the site of a small 1970s-built office development, the project team took it over once specialist demolition contractor Squibb Group had reduced the existing building down to ground-floor slab level.
Out with the old, in with the old
But instead of breaking out the existing basement slab, the team included some of the former Matrix building in the new design. It used the existing piles and the existing basement slab and boosted its bearing capacity with the introduction of a series of Pali Radice piles in clusters at points around the 1.2 m-thick reinforced concrete slab.
The Pali Radice method involves cutting through the existing slab using a bit that scabbles the edge of the core, rather than providing a clean cut. It then continues to core to the required depth before the reinforcement cage is placed and concrete poured.
No fourth floor floors City workers
Like most high-rise schemes these days, as the core at Dorsett City went up, the number of each level was recorded on the side to show progress and help workers.
But unlike those on most other schemes, the Willmott Dixon team skipped straight from level three to level five.
“We were getting lots of confused passers-by pointing out that we had missed a floor, but in fact we had deliberately missed out level four,” Mr Bastajian explains. “Dorsett City Hotel is being built for a Hong Kong-based client and the number four is considered unlucky in Chinese.
”When pronounced it sounds similar to the word for death. Due to that, many numbered product lines skip the ‘4’ and many buildings do not have a fourth ﬂoor.”
But aside from confusing passers-by, the numbering system has also lead to a little confusion on the project itself.
“We have had suppliers think we have mistakenly missed it out and so quoted for an extra floor. Obviously none of the drawings have a fourth floor, but it is still very confusing to people when they first get on site and start working on the project,” Mr Bastajian adds.
“In China and south-east Asia, tall buildings will routinely miss out any floor with ‘4’ in its title – no 14th, 24th, 34th. It takes a little getting used to, but fortunately we only have one ‘missing’ floor here.”
The scabbled edge of the concrete through the existing slab provides enough of a key against the newly poured pile to negate the need to uncover the existing reinforcement and tie in the pile cage. There are 54 of these 225 mm-diameter Pali Radice piles in total, down to depths of 18 m.
“There is no tie, only the scabbled surface of the concrete slab,” Mr Bastajian says. “If we had had to break out the slab locally we might as well have broken out the complete slab.
“The piles are cast flush with the surface of the existing slab; there is no need to break them down, so you save on material and time. You would be hard pushed to see where they have been installed.”
Biggest crane base
The crane base has also been installed on a series of piles. Thanks to the constraints of the site, it is tucked away at the position furthest away from the London Underground stop and clear of the emergency evacuation route from the station that the site team has to maintain at all times.
Project Report Willmott Dixon Dorsett City Hotel 5
It is, Mr Bastajian claims, one of the biggest bases Willmott Dixon has ever built, thanks largely to its close proximity to the London Underground lines and Aldgate Station.
“The piles for the base are deeper,” he says. “They go to around 22 m. It is one of the largest, most elaborate bases we have ever built. There is a huge factor of safety applied to the design, a third more than that of a tower crane based on a greenfield site.”
“We were going to slip both cores but because the building cantilevers out over London Underground we have had to design major temporary works to build above”
Leon Bastajian, Willmott Dixon
And the tower crane is vital to the construction methodology. With two main cores the team used slipforms to raise one core the 12 storeys (see box) from ground level, but a mixture of precast concrete and cast in-situ techniques to bring the second core up to level.
The 225 mm-thick floor slabs were cast in three separate pours using concrete pumped from the small section of St Botolph Street that the team has available to it under temporary road closure.
“We were going to slip both cores but because the building cantilevers out over London Underground we have had to design major temporary works to build above,” Mr Bastajian says.
“That design meant we were could only slip to the 10th floor. Given that we would only really be slip-forming from the second to the 10th it wasn’t really worth doing. In the end we used standard techniques to the fifth floor then after that used precast panels to bring the core up.”
Project Report Willmott Dixon Dorsett City Hotel 1
With the concrete frame well on the way to completion the team has already started the complicated process of installing the acid etched stone cladding (see box) and window panels above ground and first floor levels.
Once weatherproofed, the project will then be fully handed over to the client for specialist contractors to complete the fit-out, before opening to offer shelter and sustenance to the displaced businessmen of the world’s financial markets.
From Texas to London via Hartlepool
Parts of the unitised cladding panel systems that are being installed by specialist contractor Deepdale Solutions have travelled far by the time they are hung off the frame of the Dorsett City scheme.
The stone is quarried in Texas and then shipped over to Deepdale’s base in Hartlepool before being fixed onto the cladding panels and taken down to London.
But given the difficulties in delivery and the congested nature of the site, crane time is limited.
To get around this, the cladding panels are brought to site at the weekend in stillages. These are lifted onto loading platforms and then manoeuvred into position so that the fixing team can begin locating the panels.
“The floor-to-ceiling height is quite low here, which means it can be difficult to distribute the panels from the stillages,” Mr Bastajian says.
“The team has developed a system of trolleys to help. Once in the correct location, the team uses a monorail system that pulls the panel into position so it can slot them into place. They are fully gasketed and the only work that is done outside of the building is with a team of abseilers to mastic the joints.”