Resonant metasurfaces for surface shear horizontal wave attenuation in unconsolidated granular media

Abstract

Arrays of mechanical resonators, referred to as metasurfaces, have proven capable of controlling in-plane polarized surface waves across multiple length scales, i.e., from the micrometer scale, for the design of devices for sensing applications, up to the metric scale, for the attenuation of mechanical and seismic vibrations. In the context of seismic waves and groundborne vibration mitigation, researchers have recently proposed metasurfaces composed of meter-size resonators arranged around the foundation of critical infrastructures as innovative seismic isolation systems. While diverse studies on metasurfaces for Rayleigh wave attenuation are present in literature, investigations over the interaction between metasurfaces and surface shear waves are scarcely reported thus far. A recent numerical study demonstrated that it is possible to extend the concept of metasurfaces to anti-plane surface waves existing in semi-infinite layered media, the so-called Love waves. Furthermore, by varying the mechanical parameters of the resonators, the target frequency to control the wave propagation can be controlled. Initiating from the aforementioned works, we experimentally investigate the potential of resonant metasurfaces for the mitigation of surface shear waves propagating in an inhomogeneous material. An unconsolidated granular medium is considered, featuring a 1D power-law velocity profile, mimicking waves in a scaled sedimentary basin. In particular, we first design and fabricate a small-scale resonant metasurface consisting of an array of mechanical horizontal oscillators buried under the surface of the unconsolidated granular substrate. Then, we study the interaction between the metasurface and the horizontal shear waves originating at the surface of the granular medium. Our findings demonstrate wave attenuation around the horizontal frequency of the resonators, revealing the potential of realizing seismic resonant metamaterials for shear surface waves.