Tunnel drive under Thames complete

The London Power Tunnels job saw National Grid and contractors Costain Skanska JV having to thread the TBM below the riverbed of the Thames and above the path of the proposed Thames Tideway Tunnel.

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, Costain stated.

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, Costain said 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 EPB mode to minimise settlement as work proceeds.

The drive took some six months to complete, following its launch in February this year.