| Summary: | The behaviour of sands during loading has been studied in great detail. However, little
work has been devoted to understanding the response of sands in unloading. Drained
triaxial tests indicate that, contrary to the expected elastic behaviour, sand often exhibit
contractive behaviour when unloaded. Undrained cyclic simple shear tests show that the
increase in pore water pressure generated during the unloading cycle often exceeds that
generated during loading. The tendency to contract upon unloading is important in
engineering practice as an increase in pore water pressure during earthquake loading
could result in liquefaction.
This research contributes to filling the gap in our understanding of soil behaviour in
unloading and subsequent reloading. The approach followed includes both theoretical
investigation and numerical implementation of experimental observations of stress
dilatancy in unload-reload loops. The theoretical investigation is done at the micromechanical
level. The numerical approach is developed from observations from drained
triaxial compression tests. The numerical implementation of yield in unloading uses
NorSand — a hardening plasticity model based on the critical state theory, and extends
upon previous understanding. The proposed model is calibrated to Erksak sand and then
used to predict the load-unload-reload behaviour of Fraser River sand. The trends
predicted from the theoretical and numerical approaches match the experimental
observations closely. Shear strength is not highly affected by unload-reload loops.
Conversely, volumetric changes as a result of unloading-reloading are dramatic.
Volumetric strains in unloading depend on the last value of stress ratio (q/p’) in the
previous loading. It appears that major changes in particles arrangement occur once peak
stress ratio is exceeded. The developed unload-reload model requires three additional
input parameters, which were correlated to the monotonic parameters, to represent
hardening in unloading and reloading and the effect of induced fabric changes on stress
dilatancy. The calibrated model gave accurate predictions for the results of triaxial tests
with load-unload-reload cycles on Fraser River sand.
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