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A team triumph

T5 Earthworks

As the main terminal buildings begin to take shape, the massive earthworks at T5 are starting to scale down. Alan Sparks pays tribute to the team responsible.

By the time the first 30M passengers have swarmed through the new terminal buildings in 2008, the sheer scale of the earthworks involved in the T5 project will no longer be obvious. No one on site right now, however, has any doubts about the triumphs achieved by the earthworks and enabling (E&E) team.

Consider the sheer volumes involved. By completion, the total muckshift will amount to 8.6Mm 3-with 6.9Mm 3of cut. For a site that is no more than 260ha in size, this is phenomenal.

'Because of the quantities involved, intensive design and planning was needed to prevent us tripping over ourselves, ' says Laing O'Rourke E&E production leader Alan Burford.

'Hitting key milestones was essential for everyone on the project otherwise the whole site would have effectively clogged up.'

T5 has 70 key milestones that have to be passed on or before time if the monster project is to meet its target completion date. Of those passed so far, 15 have been down to the E&E team - and all have been hit on time.

More than 70% of the muckshift is complete, and the team is on the starting blocks to begin work on the site of the existing twin rivers following their successful diversion into new channels.

All muck was to be reused on the project with only contaminated top layers removed. Huge stockpiles, including earth for the new M25 spur road embankments, were built up in between the Western Perimeter Road and the M25. Every truck load had to be taken over the perimeter road on the temporary Jansen bridge. At 3was excavated each week.

Design of the earthworks was essential ahead of work to predict the eventual site level and optimise truck movements around the congested site. This was the domain of TPS Consult earthworks design leader Roger Yenn. He says: 'One of our main challenges is to make sure we have the right type of material in the right place.

Having excessive differential settlement in the pavement areas would be a disaster.'

The geology of the area means that a range of materials must be managed. The upper 2m of made ground is contaminated by the old sludge disposal works on the site.

Below this is around 5m of waterbearing Terrace Gravels, polluted with ammonia in parts. Further down is London Clay, which is also being churned out by the eight tunnels being driven as part of the scheme.

Although the main terminal building's roof will draw maximum attention at a height of 40m above ground level, excavations as deep as 22m had to be dug into the clay.

'Modelling clay slopes this high in temporary conditions is particularly challenging. There have been many failures in the past with deep excavations in London Clay so we went to great lengths to manage the risks and achieve a lean design that was safe, ' says Yenn.

Coping with the wide variety of spoil and the range of uses to which it would be put meant exhaustive analysis of potential heave and settlement.

Obviously, a major factor is the water content. 'When the wet weather struck, the site threatened to become a complete bog. But with over 500,000m 3of lime modification so far, we have been able to continue undeterred, ' says BAA head of civils Rob Stewart.

The flip side of this, which has caused more concern, is the problem of dust. 'In the summer we had to water down some areas to control airborne particulates, ' says Burford.

'We monitor this closely to ensure we meet strict air quality controls under the planning agreements.'

Undertaking the soils modification is main earthworks contractor CA Blackwell. Five teams have tackled the muck to make it suitable as fill. Blackwell is also responsible for pavement foundation works, which involves cement stabilisation of the Terrace Gravels using GPS-based machine guided and controlled systems.

Also under the E&E team's banner is drainage and dewatering. Controlling the water on site is a key constraint. 'In addition to dealing with the surface water and ground water, with over 5,200 site workers, there is a lot of sewage to dispose of - 250,000 litres each day, all of which must be removed by tanker.'

The solution to disposal of surface water is the early completion of the Storm Water Outfall Tunnel (SWOT) which is on programme and ready to receive run-off, easing the pressure on existing drainage infrastructure.

'This important milestone delivery was essential to accommodate run-off from the terminal building roof as assembly begins, ' says Burford.

A major challenge in keeping the programme on schedule was the work of the archaeology team which worked in tandem with excavation. 'This is unpredictable work and obviously we had no idea what was to be unearthed from one minute to the next. Thankfully, we escaped unscathed from any serious impacts on programme, ' adds Burford.

On top of technical constraints, operations and logistics have proved a constant headache due to the headaches associated with working on a single access site.

Although the sludge works has been demolished, there is still an operational Thames Water facility on site. This will remain in place until a new plant is built in 2005.

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