SEM petrography of samples of the London Clay of Southern England

Abstract

This report summarises work undertaken in support of the Ground Movements: Shrink/Swell Project undertaken as part of the BGS Physical hazards Programme. It provides petrographical descriptions acquired using scanning electron microscopy (SEM) of a suite of samples (n = 24, from 19 sites) collected from the London Clay of the London and Hampshire Basins in southern England. The analysis indicates that the samples from both basins display considerable variations in grain size both on a millimetric scale (mm-scale laminae of clay-rich, or silt-rich or fine-sand material), and also on a regional scale, with a broad trend to increasing grain size and decreasing clay-content from NE to SW. The clay content of a given mudstone is likely to be an important factor in governing the shrinking and swelling capacity of the London Clay at a given site – clearly if clay is volumetrically of minor significance within the sediment, then it will have proportionally less impact on sediment volume as it shrinks/swells, irrespective of it’s mineralogy. As might be anticipated, porosities, and hence permeabilities, will be higher where grain sizes are coarser, this will affect the susceptibility of the lithology at a given site to wetting/drying. It is recommended that a more quantitative assessment of sediment grain size is carried out. X-ray microchemical analyses qualitatively confirm the results of XRD analysis (Kemp and Wagner 2006), indicating that the clays are dominated by illite and smectite species, which might be expected to undergo significant shrinking and swelling in response to wetting and drying