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Sat November 27 2021

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Costain Skanska completes penultimate power tunnel

15 Aug 14 The Costain Skanska joint venture has completed another stage of National Grid's £14bn London Power Tunnels project.

TBM Evelyn on a previous breakthrough
TBM Evelyn on a previous breakthrough

Tunnel boring machine (TBM) Evelyn has completed a 3.4km drive under the River Thames from Wandsworth in south London to Earls Court.

The drive took around six months to complete, following its launch in February.

The 3.55m-diameter Caterpillar TBM was threaded below the riverbed of the Thames and above the path of the proposed Thames Tideway Tunnel. Going below the proposed Thames Tideway Tunnel route would have meant leaving the band of tunnelling-friendly London clay and encountering very different ground conditions, the contractors said.

Finding the correct distance below the Thames was tricky due to the presence of ‘scour hollows’ on the bed of the river and the risk of fissures in the London clay through which Evelyn was tunnelling reaching upwards to them.

The team had also to tunnel underneath the Imperial Wharf rail station without causing any settlement that could damage the track above.

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Tunnel agent Angus Mackenzie said that it was necessary to work continuously for two weeks, fixing the concrete ring sections in place immediately as tunnelling continued. Leaving any section of tunnel unsupported for any length of time between shifts could have led to voids developing and potentially causing settlement above.

The drive under the Thames also required converting Evelyn from open mode to closed earth pressure balance mode to minimise settlement.

Conclusion of the Wandsworth-Earls Court tunnel means there is now only one final section of National Grid’s London Power Tunnels project to complete. This final 5.5km stretch from Earls Court to Kensal Green will start next month, after reconfiguring the TBM back to open mode. The aim is to reach Kensal Green before the end of January 2015.

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