The Lambeth Group, formerly known as the Woolwich and Reading Beds, occurs in the London and Hampshire Basins, where it directly overlies the Chalk or Thanet Sand Formation, and is succeeded by the Harwich and London Clay Formations. Although generally less than 50 m thick, it's lithological variability and position beneath much of London has concerned tunnelling engineers since the early 19th century. The complex stratigraphy of the Lambeth Group was clarified during the 1990s. Recent infrastructure development in central and eastern London has provided a great deal of data on engineering characteristics.
Outcrop and subcrop of the Lambeth Group.
Geology
The sediments of the Lambeth Group were laid down in one or more embayments on the western margin of a deep-water marine basin centred on the present North Sea. The period was characterised by small but significant variations in sea level producing periodic migration of depositional environments resulting in complex lithological variation. The environments include near-shore marine, lagoonal, estuarine settings and wide floodplains. The relationship between the different depositional environments is seen in central and south-east London. Here, deposits of fine-grained sand, flint gravel beds, mottled clay, shell beds and altered beds form a complex interdigitating sequence, which is divided into three formations and several informal lithological units.
Schematic of the particularly complex Lambeth Group deposits in
central London.
The engineering challenge
Horizontal and vertical lithological variability is characteristic of the Group and a source of continuing difficulty for the construction industry. Excavations may encounter water-bearing sands; seepage from perched water tables may cause slope instability; and swelling clays may affect shallow foundations. Hard bands occur sporadically at some levels, which may slow drilling and tunnelling operations. The lithological inconsistency often makes tunnelling difficult and costly operation requiring sophisticated tunnel-boring machines.
Summary of engineering problems encountered with the Lambeth Group
- rapid changes in lithology
- clay with irregular water-bearing sand bodies
- perched water tables
- hard bands that obstruct boring, drilling, excavations and piling
- flint gravel beds, which are water bearing, and may restrict boring progress and increase wear of cutting equipment
- lignite, which may provide differential settlement
- deposits infilling dissolution features in the underlying chalk
- fissuring of clay, potentially leading to instability in cuts
- de-oxygenation of atmosphere in tunnels cut into Upnor Formation sands
Summary geological descriptions of the Lambeth Group formations
Upnor Formation
- This shallow marine deposit occurs at the base of the Lambeth Group.
- It is generally a dense to very dense, glauconitic fine- to medium-grained sand. Thin clay beds or clay stringers may be present.
- Flint gravel may occur at any depth but is generally found in courses at the base or near the top of the formation beneath central London.
- The upper part may be pedogenically altered: calcrete nodules (often described as 'limestone') occur beneath central London. Silcrete ('Hertfordshire puddingstone'), is mostly found in the north-west of the London Basin.
Example of borehole core showing the upper part of the Upnor Formation and lowest Lower Mottled Beds from a borehole in Bermondsey.
- Stiff, fissured mottled clay (Lower Mottled beds)
- Mottled glauconitic sandy clay with irregular calcareous concretion (Upnor Formation)
- Very weak limestone (Upnor Formation)
- Firm glauconitic sandy clay or clayey sand with calcareous patches (Upnor Formation)
Reading Formation
- This is mostly a floodplain deposit altered by pedogenic and biogenic processes.
- It was deposited in two leaves, the Upper and Lower Mottled beds, separated in the east by the Woolwich Formation. Elsewhere, the units are difficult to distinguish.
- It is a mottled or multicoloured, stiff or very stiff fissured clay, compact silt, and dense or very dense sand deposited in overbank (fine-grained) or channel (sand) settings.
- The Lower Mottled Beds are absent from the far eastern part of the outcrop.
- The Upper Mottled Beds are absent from south-east London and farther east.
- Pedogenic calcrete formation occurs in the upper part of the Lower Mottled beds in central London and scattered sites elsewhere.
Woolwich Formation
- This is a lagoonal and estuarine deposit found in the east of the London Basin and eastern part of the Hampshire Basin.
- It is divided into a lower part (Lower Shelly Clay and Laminated beds) in central London and to the east and in the east of the Hampshire Basin, and an upper part (Upper Shelly Clay), found in central London and sporadically to the north and east.
- The Lower Shelly Clay is a dark grey to black clay with abundant shells but may also be a shelly sand particularly in the east London Basin. Where shells predominate, thin limestone bands are formed. Lignite beds occur at the base in north Kent.
- The Laminated Beds consists of thinly interbedded fine- to medium-grained sand, silt and clay, with locally more extensive sand bodies and thin shell and lignite beds.
- The Upper Shelly Clay is mainly a grey shelly clay, and occasionally sand dominated unit and shelly limestone.
Example of borehole core showing the upper part of the Lower Mottled beds, Lower Shelly Clay and lower part of the Laminated Beds from a borehole in Bermondsey.
- Very thinkly laminated grey clay, silt and fine sand with occasional shell beds (Laminated Beds)
- Very stiff, thinly laminated grey clay and brown fine sand with occasional shell fragments( Laminated Beds)
- Very stiff, thinly laminated, grey clay with partings of brown silt, occasional thin shelly beds and scattered sand sized particles of organic matter (Laminated Beds)
- Very stiff, dark grey, organic, very shelly clay Shells locally cemented (Lower Shelly Clay)
- Very shelly dark grey fine sand. Sharp irregular base with burrows extending 0.30 m into bed below (Lower Shelly Clay)
Presentation of the geology
The Lambeth Group is being modelled in 3D to provide better information on the distribution of likely engineering problems. A typical section (below) shows the relationships between lithologies in the Lambeth Group and those in the sequences above and below. In this example, the potential for groundwater inflow can be identified where the lower, coarse-grained part of the Lambeth Group is in hydraulic continuity with the underlying Chalk and Thanet Sand Formation.
A cross-section through part of central London showing lithostratigraphy
and detailed lithologies (in borehole 'sticks'). Note the marked
changes in lithology along the section in the Lower Mottled Beds
and Upnor Formation.
Geotechnical properties
Geotechnical data from over 50 ground investigations, held in the National Geotechnical Properties Database have been analysed to assess the spatial variation in geotechnical properties across the main formations of the Lambeth Group. An example of the undrained shear strength with depth is presented below.
Undrained Strength vs. Depth plot for the Lambeth Group formations.
Acknowledgement
The BGS gratefully acknowledges the contribution made to parts of this study by Dr Jackie Skipper (Engineering Stratigrapher, Natural History Museum) based on her extensive experience of the Lambeth Group geology and associated engineering problems.
For further information contact:enquiries@bgs.ac.uk
