Between late 2002 and the spring of 2004 approximately 36km of 8.15m excavated diameter tunnels were driven under east London for the Channel Tunnel Rail Link (CTRL) high speed railway. The tunnels were driven using earth pressure balance tunnelling machines. The tunnels were constructed through a mixture of ground conditions that included various stiff clays, sands, gravels, chalk and recent estuarine deposits. The route was entirely within the London conurbation and passed under extensive existing infrastructure. Hence settlement damage as a major risk to the project.
This paper summarises observations of ground movement made during tunnelling. Throughout much of the route settlement volume loss figures averaged around 0.5%. In the best conditions prolonged runs of normal production reulted in volume losses of around 0.25%. The actual ground movement recorded is contrasted with that assumed during the planning stage. This comparison demonstrated what can be achieved with the latest tunnelling technologies and control systems in an urban area.
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