The project team had to build a village for 1000 students in 91 weeks on a budget that was tighter than a hippopotamus' leotard. The only chance was a risky, little-known construction method. Building found out what happened next
"We are putting up the frame faster than the external scaffolding," says Iain Baker, gesturing towards the concrete-framed building nearby. He is a project manager with contractor Laing O'Rourke and his job is to look after the teams putting up Europe's largest tunnel-form scheme – a fast-track construction method that combines concrete frame, floor slabs and internal walls all in one. He points to a block that will house 175 students; its entire superstructure was built in just 32 days.

This building is one of six that will soon be home to 1000 students. Queen Mary University of London is building a student village on its east London campus on the Mile End Road. The university needed its village in double-quick time, just 91 weeks. So the project team was compelled to look at fast-track methods of construction. At the same time, it had to be built as cheaply as possible – student accommodation programmes not being noted for their lavish budgets.

Despite the fast build programme, when Building visits the site, it seems amazingly empty. But up on the second floor, the tunnel-form gang are at work: 12 men are crammed into about 20 m2, and they're frantically stuffing steel reinforcement into the gaps between the formwork, and over the floor deck. Things are clearly moving very fast: "The tunnel-form process is quite amazing," says Ken Kinsella, the university estate department's project manager. "From one day to another it looks like a different construction site; things just seem to appear out of nowhere."

The half-finished buildings look rather like squared-off honeycombs with their sides open to the elements. Each honeycomb is a structural bay that consists of two walls running across the building joined by a structural deck to make a concrete n. Special formwork is used to build two structural bays at once, and if all goes according to plan the construction team builds two bays in 24 hours, and reuses the formwork to build another two the following day, and so on until the frame is finished.

The village's six blocks will range in height from four to eight storeys, and will be arranged as flats complete with bedrooms and kitchens. Students will have a choice of economy, standard and premium accommodation and the village will also house bars, shops and conference facilities. Even so, as Ian Taylor, partner at architect Feilden Clegg Bradley, says, "student housing is by its nature repetitive" – which makes it an ideal candidate for a factory-type construction technique.

Structural engineer Adams Kara Taylor looked at 10 different construction methods and concluded that tunnel form offered the best combination of economy and speed. AKT had no preconceptions about the method. "When we started, we studied 10 different ways of constructing this building; we stripped it back to the basics," says Hanif Kara, a partner at the firm. His colleague, associate director Jim Dunn says they looked at options such as traditional masonry, concrete and steel frame combined with a multitude of flooring options. Because it had to be fireproof and soundproof, concrete was an obvious choice. Precast concrete was ruled out because of the huge number of deliveries needed, and the negative effect on local traffic. "Having got to insitu concrete, we studied tunnel form," says Dunn.

Even though tunnel form has been around for many years it is an obscure construction method. It was introduced to the UK in the 1970s but, according to AKT, it faded out of use in the 1980s because of health and safety concerns and technical problems. Because of this, Kara says, the firm had to be careful: "We were sticking our necks out with tunnel form, as our reputation in the industry hangs on this." The firm identified some current tunnel-form projects and found the technique has been improved on in recent years. "It has been taken off the shelf and made shiny," laughs Kara. AKT produced a report on its findings to show to the rest of the project team. This was well received. "AKT did an excellent report comparing different construction methods," says Taylor. "Turner & Townsend did a cost analysis of these options and the design team felt tunnel form was the most economic approach."

As the village was procured under a design-and-build contract, the next stage was finding a contractor prepared to take on the risk of building it to a fixed price. The risk stemmed from the fact that the tender package recommended tunnel form – an unknown quantity – as the construction method. Twelve contractors applied but "several of them wouldn't touch it with a bargepole because of the risk", says Dunn. Laing O'Rourke was the only contractor prepared to embrace a tunnel-form solution.

The tight programme helped Laing O'Rourke make its decision on construction technique. "This led us to look at a quick construction method," says Gordon Latimer, project manager for Laing O'Rourke. "We looked at other options but rapidly came to the conclusion that tunnel form was the answer, and the rest of the programme would be modified to fit tunnel form." The company was also happy to make the investment in the special formwork needed.

Laing O'Rourke planned the whole programme around the construction method because it would be its speed that would drive subsequent construction operations. "It was very clear the project had to be done in a modular way," says Latimer. "For example, we could have done the bathrooms traditionally but to maximise the use of tunnel form we felt pods were best." Traditional first and second-fix was also scrapped in place of a one-visit strategy in which each trade would complete its work in one go.

Laing O'Rourke's Baker says the system is simple to master. "Everyone on this job was new to tunnel form," he says. "You can learn it in a week; in two weeks they know the cycle, and what they are doing. You don't need particularly skilled people but people who are prepared to graft."

Once the superstructure is finished, the building is made weatherproof. A lightweight Metsec steel frame with insulated panels is used to close off the outside of the building. Internal partitions separate each structural bay into four bedrooms. Because all the units are the same size the steelwork, studwork and plasterboard can be supplied cut to size, saving time, money and waste. Wiring is run through the partitions to minimise the need to include services voids in the concrete frame; modular trunking is used for electric sockets.

And tunnel form has another advantage. "The beauty of the tunnel form system is that with the steel shuttering you get a very high quality finish to the concrete as struck, so there is minimal preparation before decorating," says Latimer.

But the method is not as easy as it looks. According to Latimer, the tunnel-form process needs very careful management. The team builds two structural bays every 24 hours. "It is a very labour-intensive process, so it is absolutely vital that the process is uninterrupted so you meet the 24-hour turnaround," Latimer explains. Crane time is critical to move formwork, reinforcement and concrete, and three tower cranes are needed to keep pace. "It demands so much crane time it can restrict other activities if not carefully planned," says Latimer.

Two sets of formwork, made by French company Outinord, are being used on the project. One is used to build five blocks with a 6.3 m wide and 10.3 m deep structural bay. The sixth block, building E, has bays only 5.1 m wide with three bedrooms in each one instead of four. This building is adjacent to the railway line and the architect wanted it to perform some marketing work. "This building is next to the eastern counties main railway line, so we felt it should be a dramatic icon that people would associate with Queen Mary," says Feilden Clegg Bradley's Taylor.

Taylor was pleasantly surprised to find tunnel form was more flexible than he first thought, and could be used to make building E stand out. "As long as the main part of the building is repetitive there is time to do the detail," he says. And the detail in this case makes the building look as if it has sprouted a series of flippers. Every other bedroom is cantilevered over the one below, with the projection tapering to meet the main building. The cantilevered floorplates were made by bolting in extra sections on to the formwork.

The team has clearly got tunnel form down to a fine art – as proven by the fact that the project is ahead of schedule. Latimer is full of praise for tunnel form: "I think it is a superb system, and is the way the industry will move forward; for us as a company, as the benefits far outweigh any concerns or disadvantages it might have."

He believes it will be useful for a whole variety of projects. "I can see it being widely used across the industry for residential to offices to student accommodation." This isn't wishful thinking either; Laing O'Rourke is set to build Royal Holloway College, a 600-bedroom student block using the same method later this year.

What a difference a day makes: The 24-hour tunnel-form cycle

One set of formwork can create two structural bays every 24 hours: 8:00 The first stage is to take down the formwork that has been shuttering the concrete poured the day before. Four L-shaped sections make up a complete set of formwork, and these have to be unbolted from each other. Once dismantled, the formers are craned into their new position. They have wheels to make final positioning easier and each of the four folding L-shaped sections has special arms that lock it into position with a simple push. Any voids for services in the walls and floor are created using polystyrene void formers. 13:00 With the formwork in place, the steel fixers start positioning reinforcement inside the walls, and on top of the formwork to reinforce the structural deck. Most of the steel reinforcement is prefabricated in sections to the right size for the walls, which saves time. 15:00 The crane lifts skips full of concrete, which is then poured into the formwork. The concrete contains admixtures to make it flow more freely and cure quickly. The concrete is vibrated to compact it, as self-compacting concrete is uneconomical. 18:00 Fabric curtains close off the end of each structural bay and heaters are left on overnight. 18:00-8:00 The heaters help cure the concrete so it is strong enough for the formwork to be dismantled in the morning. The work is sequenced so there is time to construct “kickers”, the bottom 100 mm of the wall, in advance. These locate the formwork in the correct position.

Credits

Client Queen Mary University of London Architect Feilden Clegg Bradley Landscape architect Kinnear Landscape Architects Structural engineer Adams Kara Taylor Services engineer Max Fordham M&E contractor Aqua Mechanical Service Formwork supplier Ischebeck Titan Main contractor Laing O'Rourke