Manfred Wewerka looks at the uses of a geosynthetic reinforced working platform for a pile foundation over organic subgrade.

The Channel Tunnel Rail Link 310 is the connection between London and the Channel Tunnel for high-speed trains. Because of the soft organic subgrade, the track is carried in a reinforced-concrete trough constructed on a piled foundation. A working platform was required to carry pile-driving equipment with a weight of up to 80 tonnes. This platform was reinforced with polyfelt.Rock PEC high-strength geocomposites.
The route of the new line takes it in an easterly direction from Charing Cross Station, north of the Thames. Near the Queen Elizabeth II Bridge, a tunnel carries the line under the river, after which it continues towards the south-east through the county of Kent to the tunnel entrance near Folkestone. Shortly before the tunnel, Romney Marsh has to be crossed. On account of anticipated settlements, the tracks are carried in a reinforced concrete trough constructed on a piled foundation. The original design called for the construction of a 2 m high embankment on which to run the pile-driving plant. The 310 Joint Venture Partnership proposed reducing the height of the platform by half, using geosynthetic reinforcement to ensure its stability. Since suitable embankment material was very expensive and its transport by road would have had a negative impact on the environment, this had considerable merit.
Two layers of polyfelt.Rock PEC 150, one parallel and one normal to the platform axis were results of the design. polyfelt.Rock PEC is a high-strength geocomposite consisting of a mechanically bonded, continuous-filament non-woven with high-tenacity polyester reinforcing yarns.
To ensure continuity of the geotextile, the rolls of the lower layer were overlapped by 1 m, and the upper layer by 0.3 m. Both these lengths assumed a minimum distance of 5 m between the edge of the platform and the construction equipment.

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Manfred Wewerka is with Polyfelt GmbH, Linz, Austria. www.polyfelt.com