Geotechnical properties of Dublin Boulder Clay

Abstract

Geotechnical characteristics of Dublin Boulder Clay (DBC), based on detailed site investigation and site experience from some recent large projects in Dublin, are presented. This paper attempts to synthesise available information in parallel with recent work by Skipper et al., who provide an updated understanding of the geology of the DBC. Having assessed the effects of sampling disturbance, the paper characterises the various formations and sub-units of the DBC. The interpreted material behaviour is related to observed engineering performance. It was found from the behaviour of earth retained structures that intact, clayey, DBC formations are two to three times stiffer than assessed from high-quality laboratory tests on block samples. DBC is shown to be significantly stiffer than other well-characterised tills. Relatively inexpensive multichannel surface wave techniques can give very reliable estimates of in situ small-strain stiffness. High undrained triaxial compression strengths were measured, and it appears that simple UU tests on high-quality specimens give good results. Significant strength anisotropy was suggested by undrained triaxial extension strengths that were only 30–50% of the triaxial compression strengths. Field horizontal permeability values of the intact clayey till units have a representative mean of about 10−9 m/s, and when compared with laboratory values suggest that the material may exhibit some degree of anisotropy of permeability. Overall, the measured engineering parameters for the DBC are favourable for many construction projects. Further work is required in order to understand the in situ horizontal stress profile and the stiffness anisotropy of the till.