It's the area of eight football pitches and St Paul's Cathedral could fit inside the hole.
Emma Crates visits the main terminal excavation for T5 and finds that, once again, size is everything at Heathrow
THE APPROACH to Heathrow's Terminal 5 compound gives the impression of a frontier site at the edge of the earth.
With airport security tighter than a tango dancer's trousers, the only way in is through a nine-lane toll booth. Five lanes feed in, but only four lead back to the outside world.
At this stage of the project, roughly 150 vehicles are passing through the booth every hour and the cargo changes as the day progresses.
At the beginning of the day it's buses full of people.
The number of workers on site is hitting the 2,500-a-day mark. Then come dump trucks full of excavated material and low loaders carrying crane sections.
Reinforcing steel bars and other materials arrive on a regular basis from BAA's two logistics centres - Colnbrook and Heathrow South. Later there are vans delivering consumables such as drill bits and PPE equipment on a four-hourly cycle.
The traffic enters a world dominated by giant stockpiles of earth and one of the largest excavations ever undertaken in London clay.
The scene is one of concentrated activity, carried out in a landscape dominated by crane masts and piling rigs.
At peak times more than 80 articulated dump trucks, are grinding away, shifting earth on one-way hauls of up to 2 km. The project keeps 18 prime earthmovers and more than 50 excavators and dozers, besides the 30 tower cranes and 20 crawler cranes.
The proliferation of plant is understandable. T5 has 18 major projects running at full tilt and more than 200 work faces on site.
Work is going at full stretch on the satellite building, on tunnelling projects such as the Piccadilly and Heathrow Express Extensions, and on the twin rivers diversion around the perimeter of the airport. But the most impressive feature is the 478 m x 156 m x 20 m excavation, the area of eight football pitches and deep enough to fit St Paul's Cathedral entirely inside it, for the main terminal building.
It is impossible to talk about virtually any aspect of T5 without marvelling at its size. At present an estimated 500 tonnes of rebar and 2,500 kg of concrete are going into the project each day. There are four concrete batching plants on site.
'We've had to break the programme down into manageable sizes. We can't let the scale confuse our vision, ' says Steve Cork, director with Laing O'Rourke, the main civils contractor on the project.
Mr Cork prefers to focus his mind on the 70 'milestones' that the project team has to meet by 2007.
One looming milestone is October 27, where the parts of the main terminal - which is also referred to as T5 A - are handed over so that work can begin on the superstructure. Getting the roof on as quickly as possible is a key element of BAA's programme and erection of the roof will be well under way before the substructure work is completed.
The excavation was originally planned as an open excavation but, as with everything on T5, speed is of the essence. In order that work can start early on the superstructure contiguous pile walls have already been installed on the south and west sides.
'This has given us programme gain. Open cast excavation is slower and takes up space. But with a contiguous pile wall we can use the space behind it, ' says Howard Tucker, design leader for the T5 main terminal substructure team.
The 850 bored cast in situ piles range from 750 mm to 2.1 m in diameter. Because the piles are taking hefty loads - of around 3,000 tonnes - extensive pile testing was carried out.
Mr Tucker describes the ground conditions as very good - London clay with terraced gravel above.
'Our biggest worry was ground heave, ' he says. 'We are expecting around 300 mm and are using compressible void formers under the slab to protect the substructure from swelling forces.'
The basement design has also been rationalised for speedy construction. Whereas the design previously had steps and deeper sections for facilities such as baggage handling systems, it now has a flat bottom.
'We worked with the architects and core collaboration team to fit those operations in elsewhere, ' explains Mr Tucker.
The substructure is supported by a heavy-duty 16,000-tonne steel frame made up of 600 m x 600 m welded H sections. While the steel columns of the frame are attached to the piles, they are effectively divorced from the reinforced concrete slab during the construction phase.
'This allows concrete and steel works to progress independently of one another, ' says Mr Tucker.
But he admits that working with such a large building grid brings its challenges.
'We're dealing with 18 m spans here, rather than the more conventional 8 m or 9 m spans. It just means that tolerances are magnified.'
The team has also found that the scale has also brought the benefits for repetitive construction.
At the lowest basement level, which covers 250 sq m, precast concrete units are being laid.
For the upper basement levels, where the footprint is less uniform, in situ concrete is poured. But even here the scale has enabled the team to develop a cunning plan for laying reinforced steel. In the early stages of the project the rebar was laid manually, But T5 has a voracious need for rebar - 190,000 tonnes in the project as a whole - and so far only 15,000 tonnes have been laid.
The project team has speeded up the process by developing 'roll mats', whereby the rebar is rolled out like a carpet.
'It's a continuous improvement process, ' says Mr Verling. 'When we first introduced the roll mats they were rolled out on site by hand. But one of the site workers suggested that we roll up air lines - a little like firemen's hoses, with the roll mats. Once in place, the lines were filled with air and the mats roll out automatically.'
The method has transformed the speed of production.
At the start of the project one man was laying roughly a tonne of rebar a day. Now that the rollmats have been introduced, three men can handle 5.5 tonnes of rebar in 15 minutes.
But all eyes are on that October 27 deadline, when parts of the excavation are handed over for the construction of the roof.
Already on site you can see the first basement being concreted over and ready for crane rails being installed to aid construction at deeper levels. And in readiness for the roof construction, the project team will be putting the slab over to apron level in key areas.
'We'll be erecting off the apron level slab before the substructure is completed. The slab will have to be able to take the weight of mobile cranes and cherry pickers, ' says Mr Cork.
Remarkably, the giant stockpiles of earth will start to disappear back into the project as it progresses. Much of the earth will be used as backfill to raise the ground around the main terminal around 8 m, to bring it up to apron level.
And in the meantime, the project team is putting its shoulder to the relentless schedule.
As early as the beginning of next year, five or six new airport stands, currently part of the construction site, will be handed over to BAA to help the airport operator relieve its straining schedules. The sections of baggage and services tunnels that run underneath them have already been completed.
Mr Cork is keeping a cool head.
'Our biggest challenge is delivering what we said we would, at the time we said we would. We have 70 milestones to meet by 2007 and the focus is on hitting those.'
WITH an average 700,000 man hours worked a month, BAA is particularly keen to keep its accident frequency rate to a minimum. 'We're doing everything we can to achieve a one-in-a-million (one accident in a million man hours) safety regime, says Steve Cork, Director of Terminal 5 Project for Laing. 'We've got safety committees, monthly forums, and our supervisors audit each other.'
One initiative, introduced since the project started, has been with pile cap removal.
At the start of the project workers were removing pile caps using drills but, to avoid white finger vibration, and for general safety purposes, they have switched to automated grabs. The grabs remove the concrete in one lump, it is then crushed and reused as subbase elsewhere on the project.
But on a site the scale of T5, the senior team admits that it is virtually impossible to monitor safety at all areas of the site at the same time. For this reason BAA has selected specific areas to demonstrate safety excellence. So far, Railbox West, the Satellite Building and the Southern Ancilliary Area have been targeted.
'We've got a very stringent safety regime.
We take our new starts on tours of the area and tell them that this is the standard we expect. We'll be setting up more centres of excellence soon.'
The value of design
THE FIRST thing you notice about senior T5 project members is that they are reluctant to introduce themselves by the company they work for.
'This job is about delivery. It's not about the suppliers, ' says Laing O'Rourke director Aran Verling.
BAA's development of its integrated team, where client and supply chain work seamlessly together, in theory at least, has been well documented on earlier projects.
Many of these, such as the Stansted extension, were used as dress rehearsals for T5. But it is here at Heathrow that the BAA buzz phrase 'cross-functional team' has come into its own.
At design definition stage, BAA held process mapping sessions with the key players, including civil and structural engineers, architectural planners, service engineers, baggage engineers, to build an understanding of each team's needs.
All parties are now working from the same design model - the so called 'single model environment' using computerised 3-D design model that contains real time data.
'The transparency of the overall design is apparent to all, enabling a value engineered approach to be adopted, ' says Mr Tucker.
The results have been impressive. The T5 team claim that working out design details is normally a six-week process. On this project it has been refined down to five days.
'It all happens when you've got four or five people sitting down at the same desk.
Recently the reinforced concrete detailer sat down with the steel fixer and solved a problem [that could have taken days to sort out], ' says Mr Verling.
The software also links the design process to the project flow, ensuring that materials arrive on a 'just in time' basis. For example, Laing O'Rourke delivers the reinforcement for the reinforced concrete works on a just in time basis from its logistics centre at Colnbrook.
The project team also uses virtual reality software to help it plan sequencing and logistics. The single model is built up, to show the virtual completed T5 project, then broken back down again into the original elements.
Teams represented by members of all parties huddle together in virtual reality centres, where they use virtual reality to finetune the build sequence.
'We're showing pictorially how it all fits together, a bit like a Haynes car manual, ' says Mr Tucker.
Mr Verling adds: 'The site workers are really craving this information. They keep coming up to us and asking for models of later parts of the project.
Steel fixers, for example, pointed out that the tie bars were going to clash with the reinforcement on one part of the site, so tweaked the detailing.
The design process is also linked with the production of materials, which, BAA claims has paid dividends for its just in time delivery programme. Currently the site only has room for materials for one day only.
'We wouldn't want to do this all the time, but if we were up against it, we could change the details in the morning and the steel would turn up in the afternoon. We can do it in a day if we have to, ' says Mr Cork.
As a result BAA claims that the cost and time predictability on the project are 94 per cent accurate, compared with a 70 per cent figure given for public projects by the National Audit Office.
Terminal 5 project details T5 statistics:
6 million cu m of bulk excavation
I million cu m of concrete
190,000 tonnes of rebar
50,000 sq m of curtain walling
100,000 sq m of roofing
Main terminal building (Concourse A)
2 million sq ft
8 primary floors
6-platform rail station
11 service cores
£850 million build cost
Satellite building (Concourse B)
0.75 m sq ft
7 primary floors
17 stands (13 jetty)
6 service cores
£330 build cost