Seismic Metamaterials: Rayleigh Wave Control in an Elastic Half-Space

Mohammed, Shoaib Ayjaz and Somala, S N (2019) Seismic Metamaterials: Rayleigh Wave Control in an Elastic Half-Space. Masters thesis, Indian Institute of Technology Hyderabad.

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Abstract

Seismic surface waves and Rayleigh waves, in particular, are usually responsible for the failure of structures during an earthquake as they carry the highest energy and have maximum amplitudes. In order to mitigate the hazard associated with earthquakes, seismologists and engineers have been exploring novel ideas that can lead to a paradigm shift. One such idea which has recently been explored is that of seismic metamaterials. The idea of metamaterials, originally conceptualized in electromagnetics for applications on the nanoscale has been extended and applied to seismic waves on the geophysical scale. Metamaterials have been found to interact with waves in an exceptional manner not necessarily seen in nature. In this thesis, two different classes of seismic metamaterials have been comprehensively reviewed. To understand the physics of these metamaterials we perform numerical simulations in both frequency-domain and time-domain. Periodic inclusions arranged in the soil can lead to permitted and prohibited frequency bands for seismic wave propagation. Both 1D and 2D periodic media using concrete as the inclusion material have been studied. The results show that bandgaps can be obtained when the size of the inclusion is comparable to the wavelength. By anchoring the ends of the piles into a stiff layer, complete stop bands for seismic waves can be obtained. Another way to achieve this is by exploiting the localized displacements of resonant mass units to dampen propagating seismic waves. A locally resonant structure has been designed to obtain a very wide bandgap for Rayleigh waves in the low-frequency range. It has been found that the stiffness of the soil plays a vital role in deciding the design and performance of the barrier. An unprecedented way to improve the sensitivity of Terrestrial GW Detectors in the low-frequency regime (< 20 Hz) has been proposed using a metabarrier of locally resonant devices. Another barrier consisting of clamped piles is suggested for a site where the bedrock is at a shallow depth. This can potentially address the problem of seismic gravity gradient noise which currently limits the detector sensitivity in the 9-15 Hz band. The spectral- element method has been used to carry out numerical studies in time-domain for evaluating realistic design of seismic barriers. The metamaterials approach also has a good scope in the shielding of infrastructure such as Nuclear Power Plants which pose a high environmental risk and also for those structures located in regions where uncertainty in the hazard estimation is high. From this study, we conclude that seismic metamaterials indeed offer an appealing alternative to mitigate the destructive surface Rayleigh waves.

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