Using periodicity to mitigate ground vibration

Introduction of trenches, barriers and wave impeding blocks on the transmission path between a source and receiver can be used for mitigation of ground vibration. However, to be effective a barrier must have a depth of about one wavelength of the waves to be mitigated. Hence, while great reductions of energy transmission can be obtained within the medium and high frequency ranges, barriers are a poor solution at low frequencies.

As an alternative, a periodic change in the properties of the soil can be introduced—to the geometry and/or to the material. Periodic structures are known to act as filters for wave propagation, effectively reducing the transmission of energy in certain frequency bands known as stop bands or band gaps, thus only allowing propagation in the so-called pass bands.

In this paper, a stratified ground with two soil layers is considered and two types of periodicity is analysed: A soil with periodic stiffening (ground improvement) and a ground with periodic changes in the surface elevation obtained by artificial landscaping. By means of a two-dimensional finite-element model, the stiffness and mass matrices are determined for a single cell of the ground with horizonal periodicity. Floquet analysis is then performed in order to quantify the number of propagating wave modes as well as modes with low degrees of attenuation. As a conclusion of the analysis, effective mitigation in the low frequency range can be established. The position of stop bands can be manipulated by engineering the soil with periodic changes to the geometry and material.