The quality of the bolted concrete segmental lined tunnels between Canada Water and Canary Wharf is second to none. Using two 5.13m diameter purpose- built Herrenknecht slurry machines, 2.1km of tunnel was constructed in the difficult, high pressure water bearing Thanet Sands.
Not without a struggle though. First, a change to design of the launching caisson half way between the stations at Durands Wharf meant beefing up the 30m deep by 24m long, 14.5m wide structure to allow soft tunnel eyes to be incorporated.
This was followed by difficulty sinking the massive reinforced concrete box. High water pressure in the last 10m of excavation could not be stemmed by dewatering and forced contractor McAlpine-Wayss & Freytag-Bachy to opt for compressed air to drive the water out.
Four weeks later, the caisson was slid down its soft diaphragm wall guide into position ready to launch the TBMs.
But no sooner had the machines set off than problems struck once again as the bentonite slurry separation plant could not match the required pace of the TBMs. The result was to slow progress until a second separation unitwas brought to site and installed.
The race against time to complete the tunnels ahead of the tracklaying contractor's arrival was helped by the decision to delay work at Canada Water to allow the TBMs to be lifted out. This meant the machinesdid not have to be dismantled before returning to Durands Wharf to start their journey to Canary Wharf.
With the upgraded separation plant the machines were able to beat progress targets week in week out,and despite the problematic start, finished ahead of programme.