Foundations for Dynamic Loads

Design of foundations in earthquake prone areas needs special considerations. Shallow foundations may experience a reduction in bearing capacity and increase in settlement and tilt due to seismic loading. The reduction in bearing capacity depends on the nature and type of soil and ground acceleration parameters. In the case of piles, the soil-pile behavior under earthquake loading is generally nonlinear. The nonlinearity must be accounted for by defining soil- pile stiffness in terms of strain dependent soil modulus. A comparison of observed and predicted pile behavior under dynamic loads has attracted the attention of several investigators. The lateral dynamic pile response of single piles predicted by analytical models often yields higher natural frequencies and lower resonant amplitudes compared to those determined from field tests in horizontal vibrations only. This has been found to be due to overestimated shear modulus and radiation damping of the soil. The authors made an investigation to determine a simple method to improve the theoretical predictions of piles embedded in fine soils. Based upon this investigation shear strain dependent reduction factors are proposed for determining the shear modulus and damping for pile response calculations.