Engineered metabarrier as shield from seismic surface waves

Antonio Palermo,Sebastian Krödel,Alessandro Marzani,Chiara Daraio

doi:10.1038/srep39356

Abstract

Resonant metamaterials have been proposed to reflect or redirect elastic waves at different length scales, ranging from thermal vibrations to seismic excitation. However, for seismic excitation, where energy is mostly carried by surface waves, energy reflection and redirection might lead to harming surrounding regions. Here, we propose a seismic metabarrier able to convert seismic Rayleigh waves into shear bulk waves that propagate away from the soil surface. The metabarrier is realized by burying sub-wavelength resonant structures under the soil surface. Each resonant structure consists of a cylindrical mass suspended by elastomeric springs within a concrete case and can be tuned to the resonance frequency of interest. The design allows controlling seismic waves with wavelengths from 10-to-100 m with meter-sized resonant structures. We develop an analytical model based on effective medium theory able to capture the mode conversion mechanism. The model is used to guide the design of metabarriers for varying soil conditions and validated using finite-element simulations. We investigate the shielding performance of a metabarrier in a scaled experimental model and demonstrate that surface ground motion can be reduced up to 50% in frequency regions below 10 Hz, relevant for the protection of buildings and civil infrastructures.

Highlights

Resonant metamaterials have been proposed to reflect or redirect elastic waves at different length scales, ranging from thermal vibrations to seismic excitation
For seismic excitation, where energy is mostly carried by surface waves, energy reflection and redirection might lead to harming surrounding regions
Far from the epicenter bulk waves convey just a minor portion of the elastic energy released in a seismic event, while Rayleigh waves that travel along surface carry most of the energy[20]

Summary

Introduction

Resonant metamaterials have been proposed to reflect or redirect elastic waves at different length scales, ranging from thermal vibrations to seismic excitation. Large scale experiments showed that phononic crystals made of cylindrical holes in sedimentary soil can reflect seismic elastic energy, achieving attenuation of ground accelerations at a frequency range around 50 Hz7. It has been shown that a similar concept can be used to realize seismic lenses with large (≈100 m) gradient index able to reroute surface waves around buildings[14] Both ideas require large structures to function at the low frequencies characteristic of seismic events (1–10 Hz). A forest of man-made pillars would result in a strongly intrusive and practically infeasible solution in a highly urbanized context For this reason, we propose a feasible and effective seismic metabarrier in which soil-embedded surface resonators are used to redirect the surface waves into the bulk. We demonstrate Rayleigh waves attenuation on scaled experiments, which quantitatively agree with the attenuation predicted by the numerical simulations

Methods

Results

Conclusion