Non-linearity of gravelly soils under seismic compressional deformation based on KiK-net downhole array observations

In this paper, the non-linear behaviour of gravelly soils under seismic compressional deformation is investigated based on the KiK-net downhole array earthquake observations in Japan. By comparing the amplification spectra between the vertical response at the ground surface and at the bottom of downholes subjected to strong motions with those subjected to weak motions, empirical curves for constrained modulus degradation are obtained considering the different levels of vertical confining pressure. The results have shown that the non-linearity associated with compressional deformation can be as significant as that of shear deformation, for gravelly soils above water tables. The proposed curves provide satisfactory predictions for the compressional soil non-linearity investigated in previous studies. Furthermore, the proposed curves are formulated by a modified cyclic non-linear model, which can account for constrained modulus degradation under a variety of confining pressure conditions and therefore extend...