OR/13/006 Introduction

Entwisle, D C, Hobbs, P R N, Northmore, K J, Skipper*, J, Raines, M R, Self, S J, Ellison, R A, and Jones, L D. 2013. Engineering geology of British rocks and soils - Lambeth Group). British Geological Survey. (OR/13/006). * Geotechnical Consulting Group (GCG)

Background to report

The Lambeth Group is a very variable sequence of near-shore marine, lagoonal, estuarine and alluvial deposits of the Palaeogene. It outcrops and underlies much of the London and Hampshire Basins. Although relatively thin, generally less than 50 m thick, its position within 50 m of the ground surface under much of London, in conjunction with its vertical and lateral lithological variation, has had a major influence on the capital’s engineered infrastructure, particularly tunnel construction. The difficulty of predicting the lithology, especially where water lain sand occurs within stiff clay, presents significant engineering problems.

To understand where and why the different deposits are found it is important to understand the depositional conditions of the Lambeth Group including the formation of ‘hard bands’, which were generally formed by soil forming processes or pedogenesis under a sub-tropical climate. In general, the distribution of the lithostratigraphical units is well known and in some places it is possible to predict the lithologies. However, in many places predicting all the lithologies that are present is often not possible. This is particularly the case in London and impacts on the design of site investigations, particularly in relation to tunnels and deep shafts, and material for earthworks. Where near-surface construction is required, and in rural areas, boreholes and geophysical techniques may provide an almost complete ground model. Tunnelling in cities makes ground characterisation much more difficult. However, identifying what is likely to occur and the increasing availability of geological and geotechnical data as well as ground investigation will improve the ground model.

The Lambeth Group is split into three formations with other units within the formations. The units are based on the original depositional environment, which impacts on their content.

The Lambeth Group causes a number of investigation and construction problems due to lithological variation, namely sand filled channel, gravel beds, lignite, hard bands, closely fissured clay, the presence of sulphide and sulphate, swelling clay, and perched water. Where it is thin and on chalk the resulting dissolution of the chalk causes collapse of the lower Lambeth Group into the voids produced. The lower part of the Lambeth Group is known under certain circumstances to contain pressurised, de-oxygenated air, which is a hazard to tunnelling operations (Newman et al., 2013).

This report on the Lambeth Group is the fourth of a series on the rocks and soils of Britain, which aims to satisfy a need of geologists and engineers for reference works describing the engineering behaviour of important geological formations. It complements, to some extent, the CIRIA report on the Lambeth Group (Hight et al., 2004).

Methodology

The properties and behaviour of Lambeth Group materials are controlled by their texture, structure, mineral composition and alteration. These factors are a reflection of their depositional environment including the climate, penecontemporaneous pedogenic alteration, diagenesis and subsequent tectonic history that also have a major influence on the engineering behaviour of the strata as a whole. Also, the near-surface zone has been influenced by more recent earth surface processes such as ‘modern’ (Holocene) weathering. The Lambeth Group study comprised several interdependent parts. An extensive literature search was carried out at the start of the study to collect and review previous work thus guiding the activities of the present study. At the same time an extensive geotechnical database was assembled from data extracted from high quality site investigation reports, which was then analysed to establish the typical range and values of the most commonly determined geotechnical parameters, and to look for lithostratigraphical and regional variation in geotechnical properties. When the scope of the database was clear, a sampling and testing programme was carried out to investigate in more detail some of the geotechnical properties and behaviour not satisfactorily covered in the database.

As lithological variation is of such importance, cross-sections based on boreholes from selected major civil engineering projects are presented to illustrate the geological units and described lithological types. Summary diagrams showing lithological and lithostratigraphical variations based on borehole descriptions were also produced for areas where there was sufficient data.