Dynamic Response of Footings Resting on a Sand Layer of Finite Thickness

Dynamic response of foundations depends on several factors, namely, size and shape of the foundations, depth of embedment, soil profile and properties, frequency of loading, and mode of vibration. An attempt was made in this Technical Note to investigate the dynamic behavior of foundations resting on a sand layer underlain by a rigid layer. Model block vibration tests were carried out in a pit of size $2.0m\times 2.0m\times 1.9m$ using a concrete footing of size $0.4m\times 0.4m\times 0.1m$ and a vertically acting rotating-mass type mechanical oscillator. Using locally available river sand, a sand layer of six different thicknesses was prepared, and, for each thickness, tests were carried out for two different static weights and three different dynamic loadings. It was observed that the resonant frequency decreases with an increase in layer thickness and it nearly equals that of the half-space when the thickness of the layer is more than three times the width of the footing. It was also observed that the radiation damping of the sand layer was affected by the presence of a rigid layer at bottom. Inclusion of rigid layer causes a 9.8% reduction with respect to homogeneous sand condition even for a sand layer of thickness four times the width of the footing.