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Specialists Awards 2012 Project Excellence: Private Sector WINNER BALFOUR BEATTY GROUND ENGINEERING AND MOTT MACDONALD

Design effi ciencies alone saved more than 11,000 cu m on excavation


“A very complex project in an extremely diffi cult location completed early, below budget and to the client’s total satisfaction”

The Terminal 2B project is part of the major capital investment programme by airport operator BAA at Heathrow Airport and will service 16 long-haul stands. Measuring 377 m long, 15 m deep, 60 m wide at the ends and 100 m wide in the middle, the terminal’s enormous basement volume is equivalent in size to 250 Olympic swimming pools. Balfour Beatty Ground

Engineering carried out piling, constructed diaphragm walls, installed ground anchors and excavated the basement. BIM 4D techniques were used to accurately plan the construction process and to assess interface risks. A culture of innovation was


embedded into the project and led to using a concreting platform for the diaphragm wall, adoption of a safe splicing technique where pile cages could be joined without having to reach inside the cages and fi xing proximity sensors on vehicles to warn of any obstructions nearby. The project also delivered

sustainability benefi ts through value engineering as well as re-use or recycling. Design effi ciencies alone saved more

60 | 5 April 2012




than 11,000 cu m on excavation and concrete volumes in the diaphragm wall and piles. Excavated clay was used as capping at Hithermoor tip and gravel was sold for reuse. Pavement quality concrete was crushed and used as sub-base material. Because BAA’s programme was

a tight 36 months, great eff ort was made to streamline and rationalise processes. For example, by taking into account consultant Mott MacDonald’s experience of the site’s ground conditions, the 45-borehole site investigation was reduced to just six borehole tests, saving more than £1 million and accelerating the design. It was also realised early on that

conventional bottom–up design would take too long, so a mixture

of top-down construction techniques for the northern and southern sections and bottom-up for the central section was adopted. Top-down construction allows

construction or excavation below ground to take place while above- ground work continues. It usually requires plunge columns to be cast into the top of piles to form part of the basement structure. Soil in the basement is then excavated through holes in the ground fl oor or lower fl oor slabs after basement walls and columns are already in place. The design used Europe’s

largest ever retained cut excavation, using a 2 km-long diaphragm wall to hold up surrounding soil. Earthworks contractor McGee still had a signifi cant challenge, excavating 575,000 cu m of spoil. This was met by removing spoil from mole holes in the top-down areas and from around the previously constructed plunge columns. The diaphragm wall consisted

of 320 panels. The depth of the diaphragm wall was reduced through value engineering, saving 5,000 cu m on earthworks and concrete.

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