Geotechnical properties of Dublin boulder clay

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 (2005), 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 2 to 3 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 Multi Channel Surface Wave
techniques (MASW) 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% to 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 to 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.
KEYWORDS: Glacial soils, In situ testing, laboratory tests, site investigation, strength,
stiffness.