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Investigations begin for eastern Crossrail tunnels

GROUND INVESTIGATIONS have begun for the eastern tunnels section of Crossrail, the proposed rail route from Heathrow Airport to London's Docklands and the new international terminus at Stratford in east London.

From Paddington the route will run in two 6m diameter tunnels to Whitechapel. There it will split into two lines, one via Bow towards the Channel Tunnel Rail Link station at Stratford and the other towards the Docklands.

Extensive investigations carried out in 1992 on the central section between Paddington and Liverpool Street are helping feasibility and route development for the eastern tunnels beyond Liverpool Street.

This part of the project includes a new station at Whitechapel where the Crossrail tunnels will run beneath the Hammersmith, District and East London underground lines.

Much of the eastern tunnels section runs through the London Clay and complex Lambeth Group clays and sands. Where it passes beneath the Limehouse Link the tunnels will run deeper, in the Thanet Sand and Chalk.

Foundation and Exploration Services is carrying out the first package of the ground investigation, which has been designed by Geotechnical Consulting Group, acting as geotechnical adviser to Cross London Rail Links.

About 80 rotary and cable percussion boreholes are planned.

Insitu testing will include downhole and cross-hole geophysics;

self-boring pressuremeter and permeameter testing; pumping tests to measure the permeability of the deeper strata; and seismic cone tests.

At Bow the tunnels run through a cut-and-cover section beneath a former gasworks site where an environmental investigation will assess potential contamination.

Experience from London's Jubilee Line Extension project showed that tunnel volume losses and settlement profiles varied depending on which sub-unit of the London Clay was being tunnelled. One of the aims of this ground investigation is to identify sub-units of both the London Clay and Lambeth Group, measuring their properties insitu and in the laboratory.

The hydrogeology of the units will be studied through pore pressure monitoring devices and permeability testing. Ground investigations are being phased to continue into the second half of 2003.

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