Rate effects of rapid loading in clay soils.

The study of the relationship between the shear strength of the clay and the rate
at which it is loaded is relevant to the application of a new rapid load pile testing
technique called Statnamic. There are problems associated with interpreting the test
results in clay soils due to the non linear variation in shear resistance with rate of
shearing.
An investigation has been conducted for two clay soils which were used in an
associated research project. These were a reconstituted kaolin clay (KSS) used for
model pile tests and undisturbed glacial clay taken from a full scale prototype pile
testing site near Grimsby. Monotonic and multistage strain controlled triaxial tests were
carried out on both clays using a, pneumatic computer, controlled rapid load triaxial
system at rates from 0.001 mm/s to 200 mm/s. The shear strength increased and the
excess pore pressure decreased as the rate of shearing increased. A power law was
proposed relating dynamic and static shear strength. The damping coefficients and
hence the rate effects, defined as a function of strain, were similar for both clays
Based on the triaxial test results and a back analysis of Statnamic and "static"
constant rate of penetration data from the associated model and full scale pile tests in
both clays, a non-linear model has been proposed relating the static resistance of a pile
to the measured Statnamic load taking into account the rate effects and the inertia of the
pile. The non-linear model was used to develop a new multistage interpretation method
for the analysis of Statnamic tests in clays.