Some geotechnical properties of the Claygate 'Beds' and Bagshot 'Beds' of south Essex

The Claygate Beds and Bagshot Beds are the uppermost formations of Eocene a ge in south Essex, the former resting on the London Clay. The Claygate Beds consist primarily of silts and clays with subordinate sands, whilst sands are more common in the Bagshot Beds. The paper describes the geotechnical p roperties of these two sedimentary units. The results form part of a larger study of the engineering geology of south Essex. The sands, particularly in the Bagshot Beds, are fine grained. All the sand s are uniformly sorted and negatively skewed. Quartz is the principal miner als in these beds and in the fine material, mica, montmorillonite, kaolinit e and chlorite figure in that order of relative abundance. The geotechnical properties of the silts and clays of both formations are g enerally similar. However, the plasticity of the Claygate Beds tends to dec rease somewhat from the lower to the upper division and the range of plasti city is greatest in the upper division. This may be due to the greater vari ation in quartz content and montmorillonite content in the latter division than in the two others. The clays in both formations tend to have normal ac tivity whilst that of the silts is both normal and active. The values of the undrained shear strength parameters, especially in the sa ndy material, are influenced by the amount of cement present, and the degre e of interlocking of grains and the degree of compaction. The undrained she ar strength of the silts and clays suggests that they range from soft to ve ry stiff.