Section 2 Of the Channel Tunnel Rail Link (CTRL) high-speed railway includes 36l, of 8.15m running tunels driven through soft ground conditions beneath east London. The tunnels pass beneath buildings, bridges, suface railways, and underground station and under six existing operational metro tunnels. The latterwere considered a significant hazard due to their close proximity.
Systematic assessment was undertaken for each tunnel crossing with the capacity of each metro tunnel calculated in terms of an allowable bending curvature. The cast iron tunnels were found to the most susceptible to the possible movements. In cases where the calculated capacity was deemed insufficient mitigation works were undertaken in advance of tunnelling. Structurally this was limited to the release of selected circle joint bolts in the existing linings.
Ground movement during tunnelling was limited through close tunnel boring machine control and the effects observed through intensive monitoring of the metros. System of monitoring trigger levels and associated contingency plans were prepared to manage any adverse events.
The actual movements observed were genrally lower than initial expectations. Back-analysis demonstrated that the assessments of the effects had proved to be reasonably accurate. All of the metro crossings were conducted safely without distruption to the normal operation of the existing railways.
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