Loading rate effects on the bearing capacity of shallow footings on clays

In this study the influence of loading rate on bearing capacity of shallow footings was experimentally investigated. A series of model footing tests was performed at different loading rates with a rectangular footing resting on clay samples that were either incrementally consolidated or compacted at different unit weights using static, impact, and kneading methods of compaction. Consolidated-undrained triaxial compression tests were also performed, at the loading rates used in the model footing tests, to obtain data for the undrained shear strength of the tested clay. The rate of loading was found to markedly affect the value of bearing capacity. The change in ultimate bearing capacity caused by a 10-fold change in loading rate, expressed as percentage of the ultimate bearing capacity measured at a loading rate of 0.1 mm/min, ranging between 5 and 51%. The relationship between the logarithm of loading rate and measured ultimate bearing capacity was linear in a semi-logarithmic plot. The slope of the fitted line varied between 0.08 and 0.33, depending on sample unit weight and method of compaction. The slope of ultimate bearing capacity versus logarithm of loading rate was approximately 0.33 and 0.20 for the measured and calculated bearing capacity, respectively. This should suggest that the effect of loading rate on bearing capacity of shallow footings cannot be explained only in terms of the undrained shear strength of the supporting soil