Stiffness of finite sand stratum under vertical vibrations

Vertical vibration tests were conducted using model footings of different size and mass, resting on the surface of a finite sand layer with different height-to-width ratios, which was underlain either by a rigid concrete base or a natural red-earth base. Using the results of resonant frequency, the equivalent stiffness was obtained using the empirical equations available in the literature. The results of the experimental study showed that the rigidity of the finite base has significant influence on the equivalent stiffness of the soil–foundation system. The equivalent stiffness obtained for model footings resting on a finite sand stratum underlain by a rigid concrete base is higher than those obtained under the influence of the natural red-earth base. The increase in dynamic force rating decreases the stiffness of the soil. At a constant height to width (H/B) ratio, for a footing with higher mass ratio when compared with a footing with lower mass ratio, the equivalent stiffness does not decrease significantly with increase in the mass ratio of the model footing, indicating the significant influence of the contact area of the footing. When a layer having significantly larger value of stiffness compared to the upper layer is present at shallow depth, it can be treated to act as a finite sand stratum.