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Crossing the Thames Marshes

THAMES PILED SLAB

One of the key elements of Section 2 is the 7km long piled slab that will carry CTRL over the Thames Marshes.This involved complex modelling and pile testing, explains Rod Allwright.

Between the Thames Tunnel and the London Tunnels, CTRL will cross the Thames Marshes. Ground conditions are typically 4m to 10m of alluvium underlain by floodplain deposits, tertiary clays and sands or chalk.

Three viaducts will be built to cross rivers, railways and the M25/Dartford River Crossing approach roads. Elsewhere, with the exception of limited areas of conventional earthworks, the CTRL will be carried on a reinforced concrete piled slab across the marshland.

The piled slab is around 7km long and typically comprises a 10m wide, 450mm deep reinforced concrete slab built in 60m lengths between movement joints. It will be supported by more than 6,000 piles, typically in rows of four to six at 5m centres. Locally, to maintain alignment with viaducts and flood compensation requirements on the floodplain, the slab will sit above ground as an elevated slab, in which case, the spans between pile rows are increased to 8m.

The proposal for the piled foundations is a combination of 600mm square driven pre-cast piles (DPC) founded in the floodplain deposits and 600mm diameter CFA piles. The latter are needed where there are noise and vibration constraints or where the floodplain deposits thin to the extent that DPC piles cannot be used. CFA pile diameters are increased to 750mm diameter when they are used beneath the elevated slab sections.

The piled slab is a relatively flexible structure and the pile/structure interaction has been carefully investigated during the design development.

Arup Advanced Technology Group carried out 3D finite element analysis to investigate the transient response of the structure at various speeds to the Eurostar, heavy freight and commuter trains that will run on the line. This provided assurance that the structure will not be subject to resonance and, combined with the effects of shrinkage, thermal effects, braking and acceleration, allowed a rational assessment of dynamic and quasi-static pile loads to be established as a basis of pile design.

Dynamic loads were shown to be less than those predicted by conventional dynamic load factors, allowing economy of design by adopting reduced live loadings.

The dynamic and repetitive cyclical nature of the pile loads requires a complex series of pile tests to be carried out to confirm design assumptions made regarding dynamic and static lateral and vertical stiffness, together with the assessment settlement under rapid cyclic loads.

It was decided that any delay to the contract while complex test assemblies were developed, results interpreted and design confirmed could be mitigated by pile testing in advance of the main contract.

Consequently, an advanced pile testing contract (APTC) was awarded to Amec Capital Projects in autumn 2000. Testing of CFA and DPC piles was carried out at two sites representative of the extremes of alluvium thickness over the Thames Marshes.

Interpretation of the APTC results allowed a relationship between predicted cyclic loading history, live load and ultimate capacity to be established, which has allowed both differential and total settlements to be calculated from loadings given by the dynamic analysis.

This showed that CFA piles working in friction required a significantly higher ultimate capacity than end-bearing DPC piles.

Upper and lower bound spring constants for the alluvium were established for pile and slab design and a method established to predict this design parameter throughout the length of the piled slab.

The main contract for this section was awarded to Eurolink joint venture in September. Since then, it has been investigating the possibility of using Atlas screw piles as an alternative to the original design. Cyclical and static pile tests were carried out successfully last year and it looks likely that the screw piles will be incorporated into the final design. Construction of the piled slab is due to start in June 2002.

Rod Allwright is RLE lead geotechnical engineer on Contract C310.

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