Large-scale underground structures excavated during the construction of the Thames Tideway Tunnel have facilitated observations of the in situ geology and revealed sedimentary characteristics that were not fully evident in disturbed samples from ground investigation boreholes or laboratory tests. These have led to significant improvements in the understanding of groundmass behaviour of several strata within the London Basin geological sequence that are not easily studied by intrusive ground investigations, especially those involving sand channels and the ‘Pebble Bed’ within the Lambeth Group deposits. Further validation of the ground model and geology projected over the scheme during the preconstruction design has been gained from logging the geology within deep shafts, open-face connection tunnels and the main 8.8 m-outer-diameter tunnel from behind the tunnel-boring machine cutterhead. It is recommended that an archive of field logs, as obtained during construction, be established to augment archives of borehole logs.

Thematic collection: This article is part of the Geology of London and its implications for ground engineering collection available at: https://www.lyellcollection.org/cc/london-basin

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