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Burrowing below the Thames

By Juliet Whitcombe

Burrowing below the Thames

Crossrail's Thames Tunnel team assembled in front of TBM Sophia's cutterhead before it was lifted into place in the Plumstead portal.  

Preparation for Crossrail’s endeavour to burrow beneath the Thames continues apace as assembly of Sophia, Crossrail’s first slurry tunnel boring machine (TBM), is underway at Plumstead portal.

Sophia is Crossrail’s fifth TBM and one of only two mix shield or “slurry” machines to be used on the project.  This type of TBM is designed to cope with the widest range of ground conditions and water pressures.

At first glance her cutter head looks the same as those of our other TBMs. A closer look reveals more substantial scrapers and a line of disc cutters. These tools help to smash up the flint within the chalk she’ll be digging through beneath the Thames river bed. 

Sophia’s sister TBM, Mary, is currently undergoing assembly and factory testing.  

Unlike any other Crossrail TBMs to-date, Sophia and Mary are being assembled in the portal itself. Both TBMs will be launched from Plumstead following their assembly at the foot of the portal ramp. The challenges of completing these 115m long ‘metallic beasts’ are increased due to the proximity of Network Rail’s North Kent Line which runs along the length of the portal.

Plumstead is the most eastern point of Crossrail’s tunnelled section. Here, on a very constrained site the team are putting the finishing touches to a tunnel portal that will set two giant slurry machines on their 2.6km journey under Plumstead, the Woolwich Arsenal and the DLR, before diving beneath the River Thames to surface at North Woolwich.

At the western end of the Thames Tunnel drive lies North Woolwich portal. This will act as the TBM’s exit ramp. Diaphragm walling has begun for the portal walls to ensure that the ramp is ready for the TBMs to arrive in 2014.