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Screw piles to be used on CTRL

CHANNEL TUNNEL Rail Link (CTRL) engineers are to use more than 3,000 unorthodox screw piles on a 7km stretch of high speed rail line across east London marshland.

Screw piles - traditionally used only in Belgium - have been chosen instead of traditional contiguous flight auger (CFA) piles to support the track slab.

This will be elevated 1m or less above the Inner Thames marshes near Dagenham as part of the CTRL Section Two.

CTRL ground manager Nick O'Riordan told NCE this week that originally the 450mm slab was to be supported by a mixture of 7,000 driven and CFA piles.

However, dynamic analysis revealed that screw piles could replace the CFA because they can better withstand the large and repetitive forces produced by the high speed train over the soft marshland alluvium.

O'Riordan added that the pile change was mainly due to the move towards dynamic analysis rather than the use of conventional bridge design.

Dynamic analysis considers the train as a complex of moving loads from its suspension. This is a similar analysis to that carried out on London's Millennium Bridge to evaluate the effects of pedestrian movements.

Conventional design, using the Highway Agency's bridge design standard BD 37/01, assumes the train is a static load.

The change came because conventional calculations showed that the piled slab needed to be designed to withstand 560kN/m 2.This would have meant using more costly and noisy driven piles than the 3,500 originally envisaged and would have been hugely expensive, said O'Riordan.

The 450mm slab itself could not be thickened to reduce the pile cost because it has to be shallow to minimise visual intrusion. Instead, engineers started to design separate cross sections of the slab using worst case dynamic analyses.

'We're pretty much designing each piled slab individually, ' said O'Riordan.

As a result, it was revealed that the piled slab could instead be designed on average to carry a much smaller 300kN/m 2.O'Riordan added that together with being better able to withstand cyclic forces length for length than CFA, the displacement screw piles will also produce less spoil.

Detailed design for the piled slab is yet to be completed, but O'Riordan is expecting to use 50% screw piles of the order of 17m deep, whereas the remaining driven piles will be of the order of 10m deep.

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