Even if the terminal was being built in the Gobi desert, BAA would still want to use as much prefabrication possible, for reasons that you will no doubt be familiar with. For example, it means components can be assembled off-site in clean and efficient factories, which will give improved quality and reliability and reduce the amount of materials wasted or purloined. And, if sufficient quantities of the same component can be made in the production runs, BAA will benefit from economies of scale. And with most workers based in a factory, it is easier to ensure they adopt safe working practices.
In addition to improving safety, prefabrication reduces pressure on southern England's labour market. With factories located away from the enormous site, it means that the workforce can be drawn from a wider pool of workers and it means that fewer people are needed on site to fit these preassembled components.
And because T5 is so congested, the use of prefabricated components will mean fewer workers need to be squeezed into an area, which should mean faster build times and increased productivity. In fact, BAA estimates that prefabrication has led to an increase in productivity of between 10% and 15% of that of the average building site.
So how is it done? Here we look at three different examples of where prefabrication is being pioneered at T5.
Case study 01 Modular services
Prabat Garga, Amec Services' project director, is under no illusions about the scale of the task his company faces. It is undertaking the detailed design, procurement, installation and commissioning of all the main terminal's M&E services.
"It's one of the largest M&E installations ever undertaken by a UK contractor," he says.
Despite the gargantuan nature of the project, Amec is undertaking it with less than half of the site labour that would conventionally be required – and yet six months has been cut from the project's build programme.
Impressive statistics indeed, but how have they been achieved? Garga says: "The conventional approach is to erect 80% or more of the M&E services on site, which is a laborious and time-consuming activity." Whereas at T5, Amec is making and assembling 60% of the services off site.
For the terminal building, Amec developed a modular services system. Each module consists of a lightweight rectangular frame into which the pipes, ducts and electrical cable trays are installed. More than 5000 modules, based on 11 standard types, will be supplied to the main terminal building alone. The module's dimensions vary, but all were designed to be transportable by road.
The concept design for the services was developed by consulting engineer DSSR. This basic plan is to take the main services, such as electric power and hot and chilled water, from the site's adjacent energy centre and direct them to key riser points within the building's basement. These 11 service cores have plant rooms at their root, below the IT rooms, stair towers and pipework risers that distribute the services vertically through the building. The horizontal pipework distribution runs and the more complex vertical cores will be supplied to site as a series of modules. As with the roof and the substructure of the main terminal, the services will be installed from south to north.
One of prefabrication's benefits is that it is compatible with state-of-the-art design systems. Before the modules are assembled, their digital forms are inserted into the T5's single-project model (see pages 64-66), which allows Amec to eliminate several nightmare scenarios in advance. In addition to detecting any clashes, the single-project model allows what Garga describes as "construction studies" to take place. This is where the service modules are put together into runs and risers to work out the most efficient way of assembling them. The single-project model also allows BAA's facilities management team to "walk through" the building and check for problems with maintaining and servicing plant. The 3D CAD models are transferred directly to the fabricators to be used in their computer-aided manufacturing processes.
Case study 02 Prefabricating the roof
T5's roof is huge: 400 × 170 m to be precise (see pages 22-26). But even this is assembled from prefabricated elements. The steel beams, for example, may be 156 m long, but they are made up of pieces up to 30 m long, transported to site by truck and then simply bolted together. "We designed the steel structure to maximise off-site prefabrication," says Steve McKechnie, an associate at Arup, the roof's structural engineer.
Even more impressive than the steelwork is the way that 68,000 m2 of roof is being built from prefabricated roof cassettes. When it first started on the project in June 2000, roofing contractor Hathaway Roofing was looking at a conventional solution.
"We were taking away concepts on a weekly basis and pricing them," says Ian Coverdale, Hathaway's T5 project director. That all changed when the architect asked Coverdale if it would be possible to fabricate the roof "in lumps".
Coverdale developed the lump concept on the train on the way back to Hathaway's Bishops Auckland factory. He established that the ideal size of roof cassette for transport purposes would be 3 × 6 m, as 10 of them would fit on a lorry at once.
This was the first time such a system has been used, and it had large implication for the safety of operatives. If preassembled cassettes were used to cover the roof, it would reduce the number of hours roofers needed to work on site. And if the cassettes could be fitted before the roof was raised into position, it would cut the number of hours they worked at height – and it would take the weather out of the equation.
As with the services modules, Hathaway built 12 prototype cassettes using different materials and tested their acoustic performance.
Once a production model had been chosen, it was decided to assemble it in a specially constructed annex to Hathaway's factory, the cost of which was partly borne by BAA. The factory will produce more than 3000 cassettes for the main terminal and 950 cassettes for its T5B satellite.
Case study 03 Prefabricating the concrete reinforcement
Despite the size of the T5 site – it is roughly the size of Hyde Park – there is nowhere to store materials on it. "There is only limited lay-down space – often, we can only store enough for a day's use, or less sometimes," says Stuart Barr, demand fulfilment manager at Laing O'Rourke.
For the teams responsible for the construction of the mass of concrete substructure beneath the new terminal, one way of overcoming the site's storage problems, and at the same time increasing the speed of construction, was to put together the steel reinforcement off-site and deliver it on demand.
It is being preassembled in an industrial shed at BAA's Colnbrook logistics centre near Heathrow (below). The prefabricated cages are then craned into place straight from the delivery vehicle – which reduces on-site work and eliminates on-site storage. According to Barr, prefabrication has other advantages. "Factory production improves the quality of the assembly, the factory is a safer working environment and production is quicker under factory conditions," he says.
Stepping into the plant, the visitor is greeted by frantic activity and noise. Groups of men tend clattering machines, which cut and bend the steel rods before disgorging them noisily onto racks. Nearby other gangs are hard at work assembling the bent rods and bars into steel reinforcing cages and rollmats by wiring the rods together to form a mesh. Rectangles of this mesh will be placed on a lorry and delivered to site later in the day.
On site, the reinforcement cage is lifted from the lorry directly into place in the substructure. It is then attached to the reinforcement already installed. Later that day, the shuttering will be added, and in the evening the concrete will be poured.
Colin Potts, T5 substructures production leader for Laing O'Rourke, is as evangelical as Barr about the advantage of using prefabricated reinforcement. "Rather than taking a week to build a column base we can do it in six hours on site," he preaches. "We've moved from 20% preassembled reinforcement to 80%."
Pre-planning is essential for the prefabrication operation to be successful, but this has to work in close liaison with the site: the "pull" from site has to control the production of reinforcement and tells the plant what is needed on a particular day. The plant then tells the steelwork suppliers what steel it needs to take delivery of, and by when, to meet the site's needs.