Importance of strong motion in the design of earth reinforcement

In the design of earth reinforcement structures in a seismic area it is important to understand the nature of the strong motion arriving at the site. It is known that the propagation of the stress waves through the soil strata overlying the bed rock will alter the characteristics of the bed rock motion. The recording of strong motion traces at different depths in the reclaimed area of Port Island during the Kobe earthquake provides us an opportunity to study the changes in the bed rock motion as it travels to the soil surface. This paper concerns itself with the analysis of the data recorded at this site and concentrates on the implications of strong motion modulation in the design of earth reinforcement structures. It was observed that there is significant attenuation of the peak ground acceleration in the upper strata of soil at this site. The N-S component and the E-W component during this earthquake are 180° out of phase as the strong motion reaches the soil surface. Lissajous figures of these two components were constructed at all the four depths. These figures suggest that the strong motion polarizes in the N-E and S-W direction as it approaches the soil surface. These findings suggest that the orientation of the soil reinforcement within the soil structure will determine the efficacy of the soil reinforcement during an earthquake event. Also the arrival times of stress waves at the soil surface were compared between the U-D components induced by the P waves and the horizontal components induced by the shear waves. As expected the P waves arrive earlier than the shear waves at the soil surface. Frequency analyses were carried out for strong motion recorded in both N-S and E-W directions. These analyses suggest a strong attenuation of high frequency components while selective discrete frequencies are amplified. This suggests that there is a need to understand the behavior of a reinforced earth structure at discrete frequencies. Also it has strong implications on the type of input motion that may be used while modeling the dynamic behavior of a reinforced earth structure in a geotechnical centrifuge.