AARSLEFF PILING is installing nearly 400 precast concrete piles for National Grid Transco's £30M Dartford Cable Tunnel project in the shadow of the Queen Elizabeth II Bridge, which carries clockwise M25 traffic over the Thames east of London.
Piles will support concrete channels and head house buildings at the tops of two 10m diameter shafts, one at Littlebrook Power Station on the south bank of the river and the other at West Thurrock to the north.
The shafts are linked by a 2.4km long tunnel which runs under the Thames from Littlebrook and parallel to the Dartford River Crossing's road tunnels for anti-clockwise M25 traffic before turning north-east to meet the West Thurrock shaft.
The scheme will upgrade this part of the high voltage transmission system and ensure security of electricity supplies across the UK. It will carry cables replacing those installed in the M25's west highway road tunnel in the 1960s.
Aarsleff is working for main civils contractor Amec Capital Projects, which has a £16M detailed design and build contract for the Dartford Cable Tunnel to an outline design by consultant Babtie Group. Babtie is also supervising the overall project.
The firm is using its Banut 500 self-erecting fixed leader hydraulic piling rigs, with 5t drop hammers, to install about 190 continuously reinforced precast concrete piles on each side of the river.
The 250mm, 300mm and 350mm square section piles vary in length from 11m to 28m.
The longest piles are made up from two 14m long sections with integral pinned tensioned joints, which are stronger than the pile shaft itself.
Piles are driven in singles, pairs and groups of three through made ground and river gravels and up to 5m into the underlying chalk.
'The piles for the cable channels have a compressive working load up to 675kN to counteract any differential settlement, ' says Aarsleff Piling contracts engineer Colin Monk.
'The cable channel piles also cater for 30kN horizontal loadings and some have to accommodate 28kN tension as they act as restraint for the cables on isolated bases.'