The Thames Tideway Tunnel is 25 km long and extends west – east through central London, beneath the River Thames for most of its route. A detailed preconstruction ground model has been assembled, using data from borehole and river-borne seismic reflection survey investigations. The two data sets have together delineated several significant geological structures along the route.

The investigations have led to an improved understanding of the morphology of some structures, such as the Greenwich Fault, London Bridge Fault Zone, Millwall Anticline and Greenwich-Plaistow Syncline, which were only generally indicated during preliminary desk studies. Other structures, such as the Putney-Hammersmith Fault Zone, Chelsea Embankment Fault Zone and Lambeth Anticline, are entirely new discoveries.

Most of the structures described here have characteristics compatible with strike-slip displacement and, although this has been previously widely suspected, this paper presents new evidence towards this. When intersected by the tunnel during its construction phase, they have imposed significant changes in geological strata, leading to changes in the performance of tunnelling plant or creating adverse ground conditions. Their early identification by the ground model has assisted engineering design and planning, for the benefit of construction cost efficiency and, importantly, Health and Safety of underground personnel.

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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