Logging the Lambeth Group Upper Shelly Beds for the Thames Tideway Tunnel in London, UK: More than just “dark grey clay with shells”

Abstract

Despite recent advances in the procedures for logging rotary borehole core, the Lambeth Group Upper Shelly Beds are often inadequately characterised during ground investigations for underground construction. With no recognised industry guidance, there has been a lack of clarity and detail in many logging records, other than a generic description of “dark grey clay with many shells and (local) hard bands.” Improvements in describing these deposits have been hindered by the relatively limited extent of their subcrop and rarity of surface exposure, allied to a lack of precedent intersection during underground construction excavation.Geological logging during construction of the Thames Tideway Tunnel has revealed a laterally continuous sequence of strata within the Upper Shelly Beds through central London. This ranges in lithology from strongly indurated shell ‘coquinas’, mudstones and limestones to organic-rich laminated clays, silts and fine sands. Abrupt changes in lithology and fossil fauna are displayed across sharp contacts between beds, indicative of rapid fluctuations in sea level (and so potentially of climate change) during the period of deposition.Notwithstanding their comparatively limited thickness of between 1.4 and 1.6 m within the area of investigation, the Upper Shelly Beds represent significant challenges for underground construction, most notably a reduction in the rate of excavation through the stronger layers of material. Early identification of the sequence has enabled modification and improvement to the on-site construction cycle and provided valuable assurance for the existing engineering design.This paper is intended to provide guidance for recognising the sequence of strata within the Upper Shelly Beds during future ground investigations in central London, with similar benefits for early recognition of the stronger layers during engineering design.