Squaring the circle at King’s Cross

One of the final elements of the King’s Cross regeneration is the conversion of four huge gasholders to high end apartments, which presented unique challenges when it came to construction. James Kenny reports.

After 150 years of industrial use, the area north of King’s Cross station has been transformed over the last 15 years into a vibrant new city quarter. Among its most iconic features were the four gasholders for Pancras Gasworks which until 2001 stored much of the gas needed to heat London.

Walk through the area now and it is almost unrecognisable, as well as a number of new buildings and residences already built, one of the most striking features is the new Gasholder Park on the banks of the Regent’s Canal.

The park, built from the remnants of Gasholder No.8, has become a talking point for the new neighbourhood. And just next to this, the other three gasholders – Nos 10, 11 & 12 – are emerging from former industrial wasteland to become luxury apartments.

These three Grade II-listed megastructures are known colloquially as the “Siamese Triplets” because their decorative frames and riveted lattice girders are connected by a common spine. Contractor Carillion and developer King’s Cross Central Limited Partnership (KCCLP), a JV between Argent and Australian pension fund/AustralianSuper are delivering the development, known as “Gasholders London”. It will convert them into 145 high-end one, two, three and four-bedroom apartments housed in buildings of eight, nine and 12-storey blocks.

Architect Chris Wilkinson’s original sketch for the new gas holder homes

The project is due for completion in summer 2017, and while all parties are keeping quiet on the overall cost of the project, the end result is expected to be one of the highlights of King’s Cross’s transformation.

Such an ambitious project comes with a number of challenges, not only the restoration and reassembling of the gasholder frames, but then also dealing with the building of three different sized residential blocks and, of course, working within the confines of a circular shape. It has taken a number of different stakeholders and concepts to get the project near completion. 

When the regeneration of King’s Cross began in the early 2000s, Argent was selected as the development partner for the area’s masterplan. The site included the still standing gasholder No.8 and the already dismantled Siamese Triplets which had made way to allow construction of the Channel Tunnel Rail Link into St Pancras International.

Plans for the residential project were initially conceived in 2002 and a design competition seeking residential designs utilising the tripartite frame was launched.

Architect WilkinsonEyre won the competition, with its concept proposing three drums of accommodation at differing heights to suggest the movement of the original gasholders, which would have risen up or down depending on the pressure of the gas within.   

The architect drew inspiration from the delicate refinement of a traditional watch movement, trying to contrast the heavy industrial aesthetic with the lightness and intricacy of the interior spaces.

Above: SES’ work in progress of the LTHW plant room on the GHT project. Below right: SES’ prefabricated M&E curved corridor module.

According to architect Chris Wilkinson, although the concept was sound, nothing similar had been done like this before so they were all too aware that the practical elements of the construction could prove difficult.

“The challenge from our point of view, and I guess continuing on through the construction, was using the listed historic structures and then creating something modern that people could live in,” says Wilkinson. “It had to be decided how this was going to work and what was the best approach.”

Taking on the job was contractor Carillion, which joined the residential project in 2012.

Peter Spear, project director at Carillion, says: “When we became involved we reviewed various options for the best construction solution: how do you build inside a gasholder? Do you reassemble them; do you start construction from inside and then build around?

“Ultimately we opted to construct the concrete frame first and then re-erect the gasholder frames around the outside after.

“Work began with the basement car park, then the three concrete frames, followed by the building facade, internal fit-out and then at the beginning of this year the gasholder frames. The whole process has been over the last two years.”

Groundworks and concrete subcontractor Morrisroe was commissioned during the latter part of 2014 to undertake and complete the superstructure for the reinforced concrete of the three circular residential buildings.

One of the unusual features of the concrete work on this project is the beds for the 1.5m high holding down bolts, used to secure the gasholder plinths that support the gas tower framework. These anchor the framework of the gas towers to the ground floor concrete slabs.

There was an accelerated construction programme of 32 weeks, which was subsequently shortened as result of external delays. Morrisroe addressed the shortened time-scale by allocating additional skilled resources, peaking at 136 site workers to assist in meeting the original programme delivery dates.

According to Spear, the gasholder frames have been rebuilt and put in place over the last eight months and are due to be complete by year end.

The frames have had to be rebuilt in a certain sequence as they are not tied back to any existing or new structure surrounding the residential blocks. In total there are 123 columns, each approximately 14m high, along with connecting lattice beams and guide rails.

On top of this, each gasholder is a slightly different size, number 10 is 42m in diameter, number 11 is the widest at 43.5m in diameter, and 12 is only 31m in diameter.

“The whole gasholder re-erection process has been very steady and methodical,” Spear adds.

One way of speeding up construction and offsetting this lengthy process has been the use of offsite manufacturing. Carillion contracted SES Engineering Services to provide the mechanical & electrical, design and build services for the project.

As Spear says: “On a project like this, the devil is in the detail. Dealing with a circular structure and building around an atrium model is complicated. Floor plans are complicated and this is compounded by the fact that each of the gasholders has a different diameter, so essentially there are many different types of apartment.”

“To combat this we’ve been trying to prefabricate as much as we can, not just with SES but with all our contractors.”

Reliance on offsite manufacture has also led to improvements in onsite safety as the need for fewer trades and less waste on site means fewer accidents.

Now in the final stretch, Spear says: “In the next six months, it’s all about fit-out. This building is unique and there’s a lot of common areas and a lot of work left to do. The high-end finish brings another complication.”

Architect Chris Wilkinson adds: “Due to the radial geography, no apartment is exactly the same. Very few are in the same orientation so this is unique for the type of buyer.”

Among the amenities for residents are communal roof gardens, a residential gym and spa, screening room, lounges, private basement car parking and 24/7 concierge.

CGI concept of the gas holders (King’s Cross Central Limited Partnership)

The apartments start at £785,000 for an average 443 sq ft studio and are clad in dark steel and glass, with floor-to-ceiling windows with automatically-controlled bronze shutters.

The circular aspect of the Gasholders means the apartments are arranged around three atria, with light from skylights above. The pool of water in the central courtyard between the three Gasholder frames is designed to reflect the point where the structures meet.

One Gasholder will have a communal roof garden, while the roofs of the other two blocks form private terraces for duplex penthouses.

The exterior of the accommodation volumes are planned within each of the guide frames, expressed in a veil of metal and structural elements forming operable and static panels to control the environmental conditions inside. The dark steel cladding contrasts with elements of brass and bronze.

The final parts of the facade are now being finished. The facade is fully unitised and Spear says they worked very closely with the cladding contractor. “It’s all very bespoke, all the shutters are moveable and all very unique to the building,” he says.

Speaking about the overall development, Tom Goodall, senior projects director at Argent, says the project is not only transformative for the area but for all those involved.

“Everyone involved with this project is determined to deliver an exceptional building, which is a complete one-off: it will be something to be hugely proud of when it is complete,” he says.

“People, management, time, these are all big factors, but using offsite technology and prefabrication is what makes schemes like this possible.

“The whole process is built on mutual trust. Having SES and Carillion involved meant we could come to an agreement quickly and they could begin to inform the emerging design work to ensure the scheme was deliverable.”

Shrink wrapped and ready to go

Offsite manufactured modular risers keep the programme on track.

SES’ fully prefabricated MEPH (HIU version)

Key to getting the project complete in time has been the involvement of SES Engineering Services (SES), which is responsible for working with Carillion’s design team to develop a modularised riser system, as well as a service cupboard for each of the apartments, incorporating heat, power and plumbing systems.

These installations include a heat interface unit, MVHR units, underfloor heating manifolds, home automation, consumer unit, internet hub and space for a washing machine.

The cupboards are produced by SES’s modular manufacturing arm SES PRISM and delivered to site shrink-wrapped in batches of 15 per lorry. The frames are fitted with stiffening bars to prevent any twisting during transport, and they have wheels, so that once hoisted up the building, they can be rolled across the concrete floor slabs to the flats. Once in position, they are jacked up, the wheels are removed, and they are then lowered and fixed into position.

3D image of the GHT prefabricated core, which are Risers, Corridor Modules and MEPH Cupboards – all prefabricated off-site

SES is delivering prefabricated MEPH modules for the project and all the main service risers covering six riser shafts, each extending up the three gasholders incorporating 1.7km of finished and tested pipework, ductwork and electrical containment.

By using offsite and modularising the services, SES has been able to dramatically reduce the number of trades needed on site, as well as cutting man-hours on site to meet the construction programme.

SES operations manager, Will Newman, explains: “Building prefabrication is our first go to with any job, we have a strong relationship with Argent and Carillion and they understand this and want us to improve and be innovative.”

This led to the development of the newest version of the MEP (mechanical, electrical and plumbing) cupboard that is being used in Gasholders.

Newman explains the evolution of the M&E cupboards and where they have developed from their last project, to the Gasholders: “The MEPs have evolved; we went from 2D frames on the R4 & R5 projects to full modules in the P1 building until now. We’re now looking at full services and doing more. By using these cupboards and developing them in the factory, we’re taking about eight trades off-site. They only take about a week to make and the finished products are dust free, clean and ready to go.”

Restoring the frames

Gasholder Park – built from Gasholder No.8

In 2011 the Grade II-listed gasholder frames of No.8, 10, 11 and 12 were dismantled almost like a Meccano set and shipped piece by piece to Shepley Engineers Ltd in Yorkshire to begin cleaning and refurbishment.

The process took a total of two years and involved the paint on the gasholders being removed by shotblast so that the inspection and subsequent repair/refurbishment could commence. Each individual component was then inspected and defects and failed members identified and repair solutions applied and then all the defects registered before the painstaking fixing and repair job could begin.

In 2013 gasholder No.8 was returned to King’s Cross and rebuilt piece by piece on the banks of the canal. The restoration of the triplets continued at the same time.

Trevor Marrs, head of restoration at Shepley, says: “There are a number of processes used that we employ during the refurbishment, using a multitude of engineering skills and processes; welding, mechanical repair and enhancement solutions and brazing of cast iron right through to the application of the final paint finishes all undertaken in-house, these are highly skilled processes and in the case of the Gasholders refurbishment had to be undertaken on an industrial scale due to the large number of components and the project time-frame.  

“English heritage prefers a repair to be visible as it tells the story of its structure.”