Seismic metasurfaces for broadband vibration mitigation in layered soil

The efficiency of seismic metasurfaces on top of layered soils for broadband attenuation of environmental ground vibration is studied using a 3D finite element–boundary element formulation. The resonators are subjected to an incident wave field generated by a point load. An array of resonators with identical resonance frequency on a homogeneous soil has been shown to create a narrow band gap resulting in large vibration reduction behind the metasurface. This narrow band is widened by inverse or classical metawedges, which are arrays of graded resonators with the capability to convert surface waves into bulk waves or evoke rainbow trapping, respectively. Considerable broadband vibration reduction is found behind the metasurface. In a layered soil, the efficiency of the uniform metasurface is significantly reduced, even though a narrow band gap is obtained. Body waves propagating from the inverse metawedge are partially trapped within the layer decreasing the vibration mitigation performance. As an alternative, a classical metawedge is employed. Although large vibration mitigation is obtained in most of the targeted frequency range, a significant drop in performance is found when the row of resonators that activates the metawedge is not excited by the body waves travelling within the layer. The observed drop in performance depends on the dynamic soil characteristics, metasurface positioning, and excitation frequency. Metawedges also evoke significant vibration reduction at a site in Lincent (Belgium) near the high speed line L2. Considerably better vibration mitigation performance is found in close proximity to the metawedge, indicating that seismic metasurfaces should be installed as close as possible to a structure. The influence of the length of the metasurface along the direction perpendicular to the line connecting source and receivers is also studied. The vibration mitigation performance is found to largely depend on the length of the metasurface in this direction. Longer metasurfaces evoke larger vibration mitigation in the shielded zone.