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

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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.

Section snippets

The Lambeth Group Upper Shelly Beds

The Lambeth Group was deposited at the end of the Palaeocene and beginning of the Eocene epochs, approximately 54–56 million years ago (J. Skipper, pers comm., 2020), on the south western margin of a fault-controlled marine basin (in approximately the same position as the present-day North Sea), where relatively small-scale changes in sea level caused significant changes to the sediments deposited (Knox, 1996, Page and Skipper, 2000). The Upper Shelly Beds are at the top of the Lambeth Group

Thames Tideway Tunnel

The Thames Tideway Tunnel was implemented in the early 1990s under the (European Union) Urban Waste Water Treatment Directive to improve the water quality of the River Thames (Stride, 2019). It is currently under construction and will be a 25 km long, 7.2 m internal diameter storm water storage tunnel, extending west to east across London on a falling gradient of 1:890 and following the course of the River Thames through central London (Fig. 1). Construction is 30 m deep in Acton, west London,

Tideway Tunnel borehole investigation

Nearly 500 investigation boreholes were constructed for the Tideway Tunnel preconstruction geological model (Fig. 2). This was developed from the desk study conceptual model based on approximately 3000 archive borehole records held by the British Geological Survey (BGS) and Thames Water and augmented by a marine hydrographic and seismic survey (Newman and Hadlow, 2011). Most of the investigation boreholes were constructed using rotary coring techniques, with a high concentration around the

Construction geological logging

The Upper Shelly Beds were intersected by 3.2 km of the main tunnel profile and by five shafts, as well as the East and West TBM launch adits leading off the shaft at Kirtling Street; each 10 m in diameter and 37 m long (Fig. 8). In addition, a small trial pit was excavated in the shaft at Cremorne Wharf to reveal the extent of the Upper Shelly Beds beneath the final base level. This was undertaken only after thorough checks were made to ensure that the engineering design would not be

Geotechnical parameters and engineering behaviour

Despite their comparatively limited thickness, the Upper Shelly Beds have had a significant impact on construction where they have been encountered. In all of the shafts, a considerable and abrupt increase in the strength of the ground was experienced when passing from the London Clay Formation to the Top Shelly Clay Bed (the Harwich Formation, when present, was generally too thin to have any noticeable effect), typically resulting in a reduction in the rate of excavation by at least 50%.

Conclusions

Geological logging during construction of the Thames Tideway Tunnel has revealed a distinct and laterally continuous sequence of strata within the Lambeth Group Upper Shelly Beds beneath central London. This extends for over 4 km between the Albert Embankment and Cremorne Wharf sites. The sequence comprises discrete beds of typically dark grey clay, either with abundant white shells and strongly cemented shell coquinas, or with closely spaced partings and laminations of light grey silt or fine

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The author would like to convey thanks to Tideway and Jacobs for permission to publish this paper and to the FLO and BMB joint venture contractors for facilitating the excavations in which much of the field data has been collected. Special thanks go to Don Aldiss for his review of the manuscript and valuable advice and to Jackie Skipper for additional contributions. I am grateful to the two referees for their constructive comments. Thanks also go to Elena Granados, Alan Skarda and Roser Pujol

References (18)

  • R.A. Ellison

    Facies distribution in the Woolwich and Reading Beds of the London Basin, England

    Proceedings of the Geologists’ Association

    (1983)
  • J.J. Hooker

    A mammal fauna from the Paleocene-Eocene Thermal Maximum of Croydon, London, UK

    Proceedings of the Geologists’ Association

    (2020)
  • D.T. Aldiss

    The stratigraphic framework for the Palaeogene successions of the London Basin, UK. Open Report OR/14/008

    British Geological Survey, Keyworth

    (2012)
  • D. Alexandrou et al.

    Design of Primary lining (Diaphragm wall) at Kirtling Street Shaft for Thames Tideway Tunnel, Central Contract

  • BS 5930

    Code of practice for site investigations

    (1981)
  • R.A. Ellison et al.

    Geology of London. Memoir of the British Geological Survey, Sheets 256 (North London), 257 (Romford), 270 (South London) and 271 (Dartford) (England and Wales)

    (2004)
  • D.C. Entwisle et al.

    Engineering Geology of British Rocks and Soils – Lambeth Group

    (2013)
  • D.W. Hight et al.

    Engineering in the Lambeth Group

    (2004)
  • J.J. Hooker et al.

    The Woolwich Formation of Croydon, S. London, UK: a PETM fauna and flora rediscovered

    GNS Science Miscellaneous Series

    (2009)
There are more references available in the full text version of this article.

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