Dynamic shear modulus reduction and damping under high confining pressures for gravels

To provide a more reliable material property description for seismic analyses of high rockfill dams, this study investigates the dynamic properties of gravels under high confining pressures. The dynamic shear moduli G and damping ratios D of 12 gravels derived from cyclic triaxial tests are measured for confining pressures of 227-4000 kPa. As the confining pressure increases, the shear modulus reduction curve moves upwards slightly, while the damping variation curve moves downwards. However, the tested data of both the shear moduli and the damping ratios fall within the ranges recommended by previous studies, which are derived from data tested under lower confining pressures, because the influence of the confining pressure is countered by the impact of different loading frequencies during testing. Previously proposed empirical equations were found to fit the shear modulus reduction curves well for the test gravels, but could not precisely describe the variation of damping with strain. Furthermore, one important parameter in the damping equation is difficult to determine in practice for gravels with high confining pressures. A modified equation of damping is thus proposed in which all parameters are easy to obtain from test data.