TBM performance through the engineering geology of the Lee Tunnel

The Lee Tunnel is a 6·9 km long, 7·2 m dia. storm water storage tunnel, constructed in east London, at depths of up to 80 m below ground level. Ground investigation identified the three stratigraphic members of the Seaford Chalk Formation along the tunnel horizon, but with minimal variation of strength parameters between these during laboratory tests. A preconstruction geological model for the tunnel depicted a significant flint marker: the ‘Bedwell's columnar flint’, together with geological structures such as the Greenwich fault zone and Plaistow graben that indicated deterioration of ground conditions during construction. The model strongly influenced the choice of slurry tunnel-boring machine for excavation works and was also used to schedule tunnel face inspections. Monitoring of the tunnel-boring machine performance data during construction demonstrated its adaptability to the changes in ground mass quality brought about by the geological structures, as well as a pingo feature that had not been indicated on the original model.