I recently managed to organise myself a visit to the C315 Connaught Tunnel Crossrail Project in East London to see first-hand how the works are progressing. The works are being carried out by Taylor Woodrow – the UK Construction arm of VINCI PLC for Client Crossrail.
The project involves modifying a disused Victorian railway tunnel and associated areas to carry the new Crossrail trains on the section from Whitechapel to Abbey Wood in southeast London. Completed in 1878 this historic tunnel tunnel passes underneath two docks, the Connaught Crossing Road Bridge and the edge of London City Airport.
Your can find out more about the project including photos of the tunnel prior to construction on Crossrail’s website here.
The project includes a fair amount of surface works for the new railway either side of the tunnel but it is the tunnel section that is the most interesting. Either side of the tunnel proper are the two approach structures that are effectively large brick retaining walls. As the trackbed gets deeper these walls are propped with massive brick arch structures.
The tunnel is mostly a large diameter single arch brick structure which requires little more than some repairs and the lowering of the base to accommodate the larger Crossrail trains. The centre section however is a twin bore structure which poses more of a problem. Whilst much of the twin arch section is constructed of brick the very central section falls directly beneath the channel that links the two docks and as ships got larger it became necessary to lower this section and in 1935 it was rebuilt using steel rings.
Check out this blog of a year ago http://www.ianvisits.co.uk/blog/2012/04/11/more-photos-from-inside-the-connaught-tunnel/ for photos of the existing tunnel just before the works started.
The twin arch sections are too small to accommodate the Crossrail trains and so need to be converted to single bore structures by both lowering the base and removing the central brick stem. Either side of the central section this is being done from inside the tunnels in a sequence of painstaking stages involving significant temporary propping.
The first stage involves concreting new outer walls inside the two arch tunnels. These walls are then propped horizontally on two levels with steel beams passing through holes cored in the central stem brickwork, and vertically with more props.
The top part of the stem is then removed in strips and steel beams inserted – supported off the new concrete walls – to support the roof brickwork and ground behind before a new concrete roof is cast. The remaining brickwork stem is then removed. This stage has been completed on the east side and is almost complete on the west side.
The last stage is then to dig out the floor and wall brickwork beneath the new concrete outer walls – in narrow strips – and to concrete a new floor slab and the lower sections of the outer walls to form the complete box. It is this work that is currently under way.
The central steel lined section needs a different approach due to its poor condition. An enormous watertight cofferdam is being constructed in the dock channel above which when pumped dry will allow the steel ringed tunnel section to be safely deconstructed and then reconstructed from above. This work needs to be complete to allow the channel to be reopened for the world’s largest Defence and Security Show, in September.
Aside from the obvious technical challenges of carrying out the work described above there are other challenges like the nature of the ground around the tunnels and the not insignificant fact that there are large bodies of water not far above their heads. various techniques have had to be used – such as grouting up the ground through holes drilled in the walls – to ensure the water stays where it is meant to be.
I was ably shown around the site by Taylor Woodrow Design Manager Jonathan Wheeler and after the obligatory (and useful) Health and Safety site induction at the main offices at the Silvertown Worksite we headed underground….
All photos open to bigger versions in new windows btw. Oh – and all photos are copyright so please don’t snaffle them without asking..
Lastly a big thanks to Jonathon for showing me around and for Taylor Woodrow and Crossrail for giving me permission to blog my visit.
