Piling subcontractors Bachy, Stent and Skanska Cementation Foundations started work on the ticket hall in 2003, installing 550 contiguous bored piles, ranging from 900mm to 1,200mm in diameter to form the perimeter wall. This was the first of the project's many delicate operations, says Balfour Beatty Management works co-ordinator Simon Morgan.
'We had 150t piling rigs operating within metres of the Great Northern Hotel, and the eastern line of contiguous piles runs alongside the station, ' says Morgan. 'We've also done piling for Network Rail, so that they can build a concourse above the ticket hall when it's complete - it'll have an oyster shell canopy supported on columns.' To enable the machines to manoeuvre near the buildings, vaults under the Great Northern Hotel had to be filled with foamed concrete to prevent ground and structural movement.
Next, 100 large, 2.1m diameter bored piles measuring 36m in length were installed within the footprint of the box to act as a foundation for the ticket hall's massive 2.5m deep base slab.
Ground conditions were challenging at times: 'We had to pile through old foundations and tunnels - in places there was 2m of 40N to 50N concrete.' recalls Morgan. 'We were going through rebar. Some of the larger piles took 18 hours to do.' Foundation piles for the slab work through skin friction and are designed to act in tension as well as compression, says Darcy. They will complement the massive base slab which is, itself, designed to hold the structure down.
'The water table is below our structure at the moment but we have to allow for it to rise in future, ' adds Darcy.
As part of the piling works, Stent installed plunge columns - 600mm 2 I-section columns cast into 2.1m diameter piles toed at 36m below ground level and finishing 20m below where the ticket hall's base slab will eventually sit. As excavation of the ticket hall advances downward the columns will be exposed, providing support for the beams and slabs of the ticket hall's floors.
Balfour Beatty Civils is now pressing ahead with excavation of the ticket hall, using back actors and a 100t tower crane to skip the muck out.
Balfour Beatty will also be responsible for ticket hall fit out once civils work is complete.