Engineering characterisation of estuarine silts

Guidance is provided for geotechnical engineers designing civil engineering works in silty
soils. A detailed characterisation of two estuarine silt sites in Ireland is performed and the soil
properties are linked to their geological origin. It was found that these soils are susceptible to
densification by conventional and high quality fixed piston tube sampling and care needs to be
taken when using laboratory derived design parameters, particularly for consolidation and
shear strength properties. One dimensional consolidation and creep of these silts can be
modelled successfully by the well known Janbu formulation. Settlement predictions from
laboratory derived parameters match reasonably with measured data but tend to underestimate
primary consolidation, consistent with a sampling densification effect. Vane data should be
used with caution as measured strength values may be high and it seems more reliable
parameters can be derived from CPTU tests. Conventional techniques for determining soil
strength from triaxial tests in silt are inappropriate due to the dilational nature of the material
and more reliable and logical strength estimates can be made from a limiting strain criterion.