Hysteretic and dilatant behavior of cohesionless soils and their effects on nonlinear site response:: Field data observations and modeling

In this study we present evidence that nonlinearity can be directly observed in acceleration time histories such as those recorded at the Wildlife Refuge and Kushiro Port downhole arrays from the 1987 Superstition Hills, California, and the 1993 Kushiro-Oki, Japan, earthquakes, respectively. These accelerograms and others compiled in this study present a characteristic waveform composed of intermittent high-frequency peaks riding on a low-frequency carrier. In addition, soil amplification of the surface records is strongly observed compared to their downhole counterpart; this is contrary to the expected amplification reduction produced by the nonlinear soil behavior. Laboratory studies show that the physical mechanism that produces such phenomena is the dilatant nature of cohesionless soils, which introduces the partial recovery of the shear strength under cyclic loads. This recovery translates into the ability to produce large deformations followed by large and spiky shear stresses. The spikes observed in the acceleration records are directly related to these periods of dilatancy and generation of pore pressure. These results are significant in strong-motion seismology because these spikes produce large if not the largest acceleration. They are site related, not source related. Using the in situ observations from the Kushiro Port downhole array, we have modeled the 1993 Kushiro-Oki earthquake. The synthetic accelerograms show the development of intermittent behavior-high frequency peaks-as observed in the recorded acceleration time histories. Shear modulus degradation due to pore pressure produces large strains in the soil with large amplification in the low-frequency band of the ground motion. We also modeled data from the 1987 Superstition Hills earthquake recorded at the Wildlife Refuge station. The results show the importance of better soil characterization when pore pressure may develop and the effects of dilatancy in the understanding of nonlinear site response.