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Seismic survey clears CTRL path

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INNOVATIVE 3D seismic imaging technology is being used to check for potential obstructions to Channel Tunnel Rail Link tunnelling works in east London, contractors said this week.

With no as-built drawings available of the 20 year old Ripple Road bridge, below which its CTRL Contract 250 runs, Nuttall/Wayss & Freitag/Kier JV ordered an investigation as the risk of the soft ground Lovat TBMs hitting one of the nominal 25m long piles was too high.

The steel encased raking piles, which support the bridge carrying the A123 over the London Tilbury & Southend Railway line, should be no closer than 2.5m to the twin 8.15m diameter bores which will lie directly below.

With some of the piles raking under the railway, conventional trial pits were extremely difficult, opening the way for Skanska subsidiary 3D Tomographics to try out its recently developed seismic investigation technique on the site.

'We perfected the technology on the archaeological investigation of Silbury Hill in Wiltshire, ' said 3D Tomographics operations manager Mark Kirkbride.

'The real breakthrough is a massive increase in processing power which gives us the ability to resolve small details with great accuracy.'

Four, 100mm diameter by 28m deep boreholes were located on each side of the foundations, outside railway land. Signals generated in each borehole in turn were picked up by sensitive microphones in the other seven.

Kirkbride said the data collected to date was 'excellent'.

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