SHIELDING BUILDINGS FROM SURFACE WAVES WITH “SEISMIC METASTRUCTURES”

Phononic crystals and metamaterials constitute a broad class of artificially engineered materials able to manipulate the propagation of acoustic/elastic waves at different lengths scale. Filtering and directivity properties of these materials can be controlled and tuned by designing their fundamental building blocks, usually referred as unit cells. Phononic crystals exploit periodicity to induce total wave reflection at selected frequencies. The obtained filtering properties emerge at wavelengths comparable to the periodicity. On the other hand, phononic metamaterials exploit the coupling between propagating waves and local resonators in the fundamental unit cells. As a result, propagation of waves with frequencies around the resonance is inhibited. More recently the use of phononic crystals and metamaterials has been envisioned for large-scale applications in the field of civil engineering as a mean to shield buildings and infrastructures from natural or man-induced seismic waves. Among these, a solution based on metamaterial concepts and referred as “seismic metastructure” has been recently proposed by some of the authors. The proposed system is realized by an array of buried heavy-cylindrical steel units encased in cylindrical concrete pipes that constitute the metamaterial resonant units. The proper design and arrangement of these resonant units allows filtering longitudinal and shear waves in the typical frequency range of seismic waves (1-10Hz). In fact, analytical/numerical models as well as experimental evidences on a scaled setup proved the shielding capabilities of the proposed seismic isolation system with respect to bulk waves. Building on the initial results on bulk waves, here we design “seismic metastructure” able to mitigate Rayleigh surface waves. This is of special interest as they are considered the most harmful seismic threat. An analytical model to analyze the propagation of seismic surface waves through a soil engineered with seismic metastructures is developed. The analytical model allows predicting the frequency range within seismic Rayleigh waves are inhibited. A full 3D finite element model is used to numerically validate the prediction of the analytical model. Experimental evidences of the filtering properties of the designed metastructures are obtained on a scaled experimental setup. The excellent agreement of analytical/numerical and experimental results demonstrates the potential of this novel class of seismic isolation devices.