Discovering old war rooms, tackling six-metre-thick concrete walls, blowing up buildings in the middle of London and racing against time … Well, at least this project wasn’t dull

Demolishing concrete walls up to 6 m thick was one of the biggest headaches facing Bouygues UK. In the end, the contractor resorted to blowing them up with explosives
Demolishing concrete walls up to 6 m thick was one of the biggest headaches facing Bouygues UK. In the end, the contractor resorted to blowing them up with explosives

The construction of a headquarters for the Home Office would have been a large and difficult project for any contractor at any time, but Bouygues had more at stake than just a £175m, 75,000 m2 building. This was one of the French giant’s first schemes in the UK PFI market, and any cost or time overruns could prejudice its chances of winning other work. As Darren Cartlidge, the project architect for Terry Farrell & Partners, says: “This is the foot in the door for Bouygues UK. They couldn’t afford to fail on this project so they had to surpass themselves.” Unfortunately for Bouygues UK, things got off to a bad start when it became involved in a war.

Or rather, with Whitehall’s planning for one. Getting rid of the 200 ft high slabs of the former headquarters of the Department of the Environment may have been relatively easy, but beneath them were two 65 m diameter bunkers, known as the north and south rotundas. These enormous bunkers had been used as war rooms, a telecoms hub and the civil service sports club. It seemed their final job was as to fortify London against the French: they certainly came as a surprise to Bouygues.

“It was only when the buildings had been demolished that we appreciated how big the rotundas were,” says Pascal Minault, Bouygues UK’s managing director. Not only were they large, but also much of the bulk was taken up by 6 m thick concrete walls. The construction programme was 34 months. “Time was our biggest challenge,” says Jean-Claude Aoustin, Bouygues UK’s project manager, putting it mildly. “We had to spend 14 months demolishing the old buildings.”

Bouygues UK had to find a way to remove the rotundas as quickly as possible. The first move was to hire specialist equipment. “We brought in a special concrete muncher from Japan,” says Minault, “but even this wasn’t effective.” The second move was more drastic: Bouygues UK decided to blow them up. This was a controversial choice as the war in Iraq had just started, and it took the contractor two months to get permission to use explosives in central London.

The explosives did the job, but it still took four months longer than planned to get rid of the rotundas. “Because it was a PFI contract it was our responsibility to find a solution,” says Minault. Bouygues UK had just 20 months to actually construct the building.

How was this lost time to be made up? The first line of attack was to modify the design. One important change centred on the rotundas, again. Because the three sisters were overweight as well as ugly, Pell Frischmann, the structural engineer on the scheme, was concerned that the new, lighter building would be affected by uplift. This could be countered by filling the void left by the rotundas with broken concrete to weigh down the building. More accurately, the plan was to build columns on the slab foundation at the base of the rotundas, fill in around them with rubble, and put the structure of the new building on those columns. A few calculations later this plan was dropped: it would take too long. Instead, the rotundas were simply filled with new concrete, eliminating the need for columns. “We gained one and a half months by doing that,” says Minault.

The remainder of the programme was then rescheduled to gain precious time. “We went back to our trade contractors to see if they could do the work in less time, and what we needed to do to make that easier for them,” Denis Motard, Bouygues UK’s production manager. The trade packages were then squashed together in time and space. In particular, services were installed straight after the concrete floors were completed. “There was some float in the programme; originally there was a gap of two floors between the concrete work and installation of the services so we reduced this to one,” says Motard. “We had to take some additional health and safety measures to accommodate this.” Work was also extended into Saturday mornings.

To keep the subcontractors onside, Bouygues UK instituted a bonus system for completing packages on time. “It was a very positive incentive; they really delivered so it was an investment that paid off,” says Minault. “Everyone was very positive on site, there were no contractual issues; we came together to discuss problems.”

However, where Bouygues UK really demonstrated Continental sophistication was in its decision to build the structural frame using hybrid construction. Little is known about this technique in the UK, but Bouygues has been using it for 30 years. As the name implies, the frame marries two kinds of concrete: insitu structural columns and precast beams and floor slabs. These elements are stitched together with insitu concrete to create a frame that is as structurally robust as an insitu frame, but is as fast to build as a precast frame (see “How the frame was constructed”).

“The frame would have taken twice as long to build if we had constructed it from insitu concrete,” says Aoustin. “The advantage is the day after pouring the concrete you strip the formwork with just a bit of shoring below the beams to support them, and you can immediately start placing the precast slab. You then put a concrete topping on top of that to cover the beams and slab, and you can immediately start installing the services below the concrete. Otherwise we would have to have waited two weeks before the concrete was strong enough to support itself.” Bouygues UK constructed a 4200 m2 floor in each of the three office buildings every 11 days.

To preserve its gains, Bouygues UK ensured that the complex facade was installed without any potentially time-sapping problems. “It’s quite a complex piece of work as the architect didn’t want a plain facade,” says Minault. The skin includes windows set into the concrete frame, stone cladding to cover the frame and a multitude of different types and sizes of glass, including glazing with letters fritted onto the surface. Minault says Bouygues had its own in-house facade engineer managing the facade contractors to ensure all the critical points of design, fabrication and installation including interfaces with the frame and other trades. “It went very well,” he says.

Finally, Bouygues adopted a sensible risk management strategy.

It treated the residential buildings behind the offices as an entirely separate project – indeed, this part of the scheme isn’t due for completion until May. Different construction methods, such as an insitu concrete frame, were used and different specialists were employed to spread the risk. Although the Home Office building and the residential scheme each had a separate project team, it was co-ordinated by the senior managers. Bouygues also employed more than one specialist to fit out the Home Office building so if one hit a problem the other could pick up the pieces.

The result of all this was a triumph for Bouygues UK: it finished the building bang on time. “We are very happy and proud of the end result,” says Minault. “It’s true the difficulties have increased the cost, but overall we are more or less where we were planning to be.” Cartlidge, the architect, is full of praise for the French contractor. “We found them very good to work with; one of the best contractors we have worked with for some time. It was a very collaborative experience and an excellent working relationship which I thoroughly enjoyed,” he says. “Being French, their organisation is different. It’s difficult to put your finger on, but it’s really down to attitude, and team spirit. A lot of people talk about partnership. On this project it happened.” Cartlidge does have one concern, however. “For Bouygues UK, this was too important a project for it to fail. Whether they operate in this way on other projects remains to be seen.” If it does perform as well, its domestic competitors better be taking notes …

How the hybrid frame was constructed

The Home Office has two floors underground and six overground, all built using a hybrid concrete frame based on a 7.5 m square grid. The hybrid technique combines insitu columns with precast beams and floor slabs. The combining is done with a series of special shuttering towers that have a platform at the top to support the precast beams while the concrete for the columns is being cast.

The first stage is to place the rebar for the column. After that has been done, the towers can be wheeled into position – the exact spot is located by a small area of concrete protruding from the floor called a kicker. Then the 6.9 m long precast beams are then craned into position and are supported by the shuttering tower. The beams have a T-shaped section – the crossbar is there to support the precast slabs once the column has been constructed. The beams have rebars protruding from their ends, so once the column is poured they are physically tied into the structure. Concrete is also poured into the shuttering to form the “stitch” – the gap between the ends of the beams and the top of the column, where the rebars from the column and precast beams meet.

The following day, props are placed under the beams and the shuttering tower removed. Lorries containing 6.9 m long, 1.2 m wide hollowcore floor slabs arrive on site and are unloaded by crane directly onto the precast beams. Once all of these are in place steel mesh is placed over them and a 50 mm concrete screed is poured over the whole surface. Reinforcement protruding from the tops of the beams engages with the steel mesh to provide a structurally integrated floor slab. Special beams are used at the perimeter of the building; these have one side of the T missing and are designed to accommodate windows or special spandrel panels that are cast in with the columns and beams ready to accept waist height windows. Finally, the high quality of the concrete in the columns allows them to be covered with a skim finishing coat rather than plasterboard.

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