14 August 2014

In the latest stage of the major contract to assure London’s future power supplies, tunnel boring machine (TBM) Evelyn has completed a 3.4km drive under the River Thames from Wandsworth in south London to Earls Court.

The latest instalment of the London Power Tunnels job saw National Grid and contractors Costain Skanska joint venture having to thread the TBM 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.

Finding the correct distance below the Thames was particularly 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.

To avoid this, said Tunnel Agent Angus Mackenzie, 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 – where the TBM is open to the ground through which it is boring – to ‘closed’ Earth Pressure Balance mode to minimise settlement as work proceeds.

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

The successful 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 in about four weeks, after reconfiguring the TBM back to open mode.

“We plan to arrive at Kensal Green before the end of January 2015,” said Angus.

 

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