Engineered metabarrier as shield from seismic surface waves

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.

Hybridization of Guided Surface Acoustic Modes in Unconsolidated Granular Media by a Resonant Metasurface

“…1(e)]. We observe that the dispersion of this confined mode is akin to the lower branch of Rayleigh waves hybridized by surface resonances [10,14]. For Rayleigh waves, the lower branch is accompanied by a repelling upper branch, which leads to an avoided crossing behavior and a related surface-to-shear wave-conversion phenomenon above the resonance frequency.…”

“…Such dynamic stress can be seen as a boundary condition at surface depth h res . We remark that similar resonant surface conditions have been successfully used to describe the hybridization of Rayleigh waves with resonances at a different length scale [10,14]. Additionally, at the depth h res , the unconsolidated medium is subjected to a pressure p h res ¼ ρgh res , which acts as an adhesive force between the grains, ensuring nonzero elastic properties for the granular medium.…”

“…Similar setups have been used to analyze the propagation of GSAMs [18,21] and to probe the jamming transition of granular media [16]. We embed subwavelength resonators [14] (height 24 mm, diameter 20 mm) below the surface of the granular medium. The resonators consist of a rigid cylindrical shell and a soft spring, both 3D printed in acrylonitrile butadiene styrene, and a steel cylindrical mass [inset, Fig.…”

“…Within this frequency range, Rayleigh waves are deflected away from the surface, in the form of shear vertically polarized waves. The Rayleigh-to-shear wave conversion has been predicted and observed at microscales [10] and macroscales [11], and its use has been proposed for applications in sensing [12] and seismic waves deflection [13,14].…”

“…In this work, we aim at understanding (i) how GSAMs hybridize with local resonances, and (ii) if a resonant metasurface can induce energy delocalization, like in homogeneous media [10,13,14].…”