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Bouygues' Triangle mega-office in Cambridge takes shape

Bouygues UK is replacing 11 offices with one purpose-built site for the university’s exam board operator thanks to some huge programme savings.

Project: The Triangle, Cambridge
Client: Cambridge Assessment
Contract value: £126m
Contract type: NEC Option A – Design and build
Region: East of England
Main contractor: Bouygues
Piling subcontractor: Fayat Piling
M&E subcontractor: Allied M&E Services
Start date: September 2015
Completion date: December 2017

If the barometer for the relative strength of the construction industry in any town or city is the number of cranes on its skyline, then Cambridge is in very rude health indeed.

One site that is garnering plenty of attention is ‘the Triangle’, a new office development set to provide accommodation to one of Cambridge’s most important employers.

Cambridge Assessment is responsible for the operation and management of the university’s exam boards and carries out leading academic and operational research in education.

It currently employs thousands of staff in 11 different offices across the city but, in a rationalisation bid, is moving them to a single purpose-built office to the south of the city centre.

“It makes more sense,” says Cambridge Assessment group construction and property director Rob Smart. “For us to be able to house all of our business units under one roof will be a real advantage.”

Rapid delivery

Key to that will be the successful delivery of the new headquarters building by main contractor Bouygues UK. It has taken the £126m design-and-build contract and has until the end of 2017 to complete the scheme, a deadline that Bouygues UK project director Neil Pixsley is confident about hitting.

“We are where we want to be in terms of the programme. It is a challenging project to complete within the two years, but I think one of the reasons that we won the contract was because we could hit that target,” he says.

Open Doors Week 2017

Bouygues UK is opening up the Triangle site as part of Open Doors week 27 March to 1 April.

To find out more about the initiative, and arrange a site visit, go to: opendoors.construction

The team tinkered with the design to make the build more suited to some of the innovative construction methods being used. As a result, the firm’s proposals were able to reduce the programme by a massive 20 weeks compared with its rival bidders.

That requirement for a speedy, efficient project delivery has been evident from the very first stages of the scheme.

“It is a challenging project to complete within the two years… I think one of the reasons that we won the contract was that we could hit that target”

Neil Pixsley, Bouygues UK

“Contractor interviews were staged in April 2015 and the team started on site at the end of September. It was a very streamlined process,” Mr Smart says.

The building features two four-storey blocks to the north and south of the triangular site (see below), which are linked by a three-storey glazed atrium. Three fingers jut out around podium-level courtyards on the northern block while the block at the southern end of the site, hemmed in by road, rail and the route of the Cambridge Guided Busway, curls around another central courtyard.

Bouygues The Triangle 2

Bouygues The Triangle 2

In the corner of the southern building a 39 m-high observation tower will act as a landmark, welcoming visitors from the south into Cambridge.

Concrete redesign

With the well-advanced design already worked through to RIBA Stage 3, there were only a few areas that the project team could really influence. But with Bouygues’ experience working with concrete on the continent, these were areas that have proven hugely advantageous.

Initially the thinking was that there would be an SFS façade, but reality dawned that it could only be a concrete-framed building, according to Mr Pixsley. “By proposing an alternative design for the façade using a formwork system developed by our own engineers, we could offer considerable time, quality and health and safety improvements.”

Acting almost as a horizontal jump-form, the façade formwork is a prefabricated steel-faced system, which is assembled on site. It features a working platform as part of the assembly and is fixed into the upstand of the storey below. The concreting team can then work safely from the platform as they cast the next sections of wall and upstand before the formwork is struck, re-oiled and moved to the next bay length.

And with the formwork casting the upstand, steel fixing and concrete pouring for the post-tensioned slabs could be carried out safely from within the structure without the need for any additional edge protection.

“It is a very quick, very clean system. It is also very safe,” Mr Pixsley explains. “It offers built-in edge protection, which has enabled us to post-tension the floor slabs from within the structure, reducing the risk of working at height and making the whole process simple and safe.”

With a client requirement for clear soffits and a 9 m grid across the upper floors, post-tensioning was the most logical method the team could use to provide them. The tendons for the 270 mm-thick slabs were tensioned to 25 per cent on the first day after pouring with 100 per cent tensioning carried out by the third day. The plain reinforced-concrete slabs at podium level were much thicker at 400 mm.

The structures façade is a mixture of curtain walling, mainly on the ground floor and throughout the link atrium, and rendered elevations, precast stone panels, windows, cladding and brickwork finishes.

Heavy building, solid foundations

All that concrete means a heavy building and solid foundations to match. Initially there had been some hope that the project would be able to use the foundations and slab that was in position thanks to the site’s previous guise as the print works for the Cambridge University Press. Unfortunately, that turned out not to be the case and so more than 350 CFA piles were installed by subcontractor Fayat Piling.

Bouygues The Triangle 9

Bouygues The Triangle 9

Piling company Fayat installed more than 350 CFA piles to depths of 30m across the site

These varied in diameter with the bulk being 400 mm and 500 mm, installed to depths of 30 m through the made ground and soft clay and on into the underlying boulder clay beneath.

The Bouygues UK team was again able to improve efficiency, reduce safety concerns and cut time from the programme thanks to Fayat’s ingenuity.

It used its own method of cutting piles, which negated the need for traditional noisy and inefficient pile trimming methods.

“By proposing an alternative design for the façade… we could offer considerable time, quality and health and safety improvements”

Neil Pixsley, Bouygues UK

A circular plywood section is introduced at the correct height on the reinforcement cage with plastic sleeves placed over the stub ends of the steel bars. The cage is then placed in the pile as usual. Once the concrete has cured, the ends can be excavated and the top of the pile broken off in one chunk thanks to the plywood disc, which acts as a crack inducer.

“It is very simple and has proven to be very accurate. It is also much quieter than other pile-breaking methods. With the area being partly residential that was an important factor here,” Mr Pixsley explains.

Bouygues The Triangle 11

Bouygues The Triangle 11

Prefabricated roof-top plant systems are lowered into place

Windows are being fitted across the site and the team has made a start on each of the cladding systems to the façade. The project’s mechanical and electrical fit-out has already begun with work to install the raised access flooring and chiller beams continuing (see box, below). The scheme has now reached the stage when the internal work will really start to accelerate away.

That end-of-the-year completion date looks safe enough.

Locals feature on large M&E focus

With such a strong focus on the mechanical and electrical delivery on the Triangle project, the site team looked at how they might be able to tailor the work in a move that could help reduce the overall exposure and free up work for Bouygues UK’s local tier two contractors.

By directly procuring the main plant and equipment itself the Bouygues team could break down the rest of the M&E work into smaller packages, ensuring that those tier two contractors could contribute to the project.

There has been an emphasis on prefabrication with bespoke service modules developed by Allied M&E Services. They feature pre-insulated plastic pipework and lightweight prefabricated ducts and are supported using lightweight punched steel frames.

Throughout the project there are more than 4.5 km of chilled beams going in. These are integrated chilled beams with 7,000 light fittings pre-installed and are linked to sensors which shut down the beam when ducts in the window mullions are opened to allow fresh air into the building.

“Key to our brief is that the office should be light and airy,” Mr Smart explains. “That’s why we wanted the clear, flat ceiling soffit and the chilled beams. We did want openable windows but needed to compromise on that. The larger mullions on the windows offer our staff the best of both worlds.”

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