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Researchers Work to Create ‘Seismic Cloaks’

By Kevin Wilcox

An investigation is under way to determine if properly placed bore holes can shield sensitive buildings from elastic surface waves during an earthquake.

April 15, 2014—A team of scientists and engineers in France, inspired by research into invisibility cloaks, are testing geotechnical soil modifications that show promise in one day absorbing or diverting the elastic surface waves created during earthquakes, sending then\m away from sensitive structures.
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The next step in a research effort to bend seismic waves around important structures will be to test a ring of boreholes to see if they protect the interior of the circle from waves. This computer simulation of that experiment, viewed from above, shows a pattern that might be observed in the Rayleigh waves. © S. Guenneau, Institut Fresnel

The team recently published the results of an experiment in Physical Review Letters, which is published by APS Physics. The letter, Experiments on Seismic Metamaterials: Molding Surface Waveswas authored by Stéphane Brûlé, Eur-ing, and Emmanuel Javelaud, Ph.D., Eur-ing, both of whom are engineers with the soil improvement design and construction firm Ménard, of Nozay, France, and Stefan Enoch, Ph.D., and Sebastien Guenneau, Ph.D., both of whom are researchers with the Fresnel Institute, a research laboratory in Marseille.

The project began when Brûlé read an article about the Fresnel Institute's work with bending optical waves and wondered if the concepts might be adapted to earthquake waves. The concept was to drill a series of boreholes and determine how surface earthquake waves interacted with them.

"Without Stéphane Brûlé's input, the idea of detouring surface earthquake waves using simple analogies with optics would have remained a sweet dream," Guenneau said in written responses to questions posed by Civil Engineeringonline. "Soils are highly heterogeneous and viscoelastic media so their elastic properties are far from obvious."

The team secured a grant from the European Research Council to test the hypothesis. The team first performed a series of numerical tests at the Fresnel Institute to develop the proper spacing. On the basis of a soil properties analysis by Ménard, the team selected a site in the Alpine city of Grenoble, France, which has thick, homogenous silt-clay soils.

"Ménard chose the diameter, depth, [and] spacing of the boreholes, and the frequency of the seismic source, using our results and their own knowledge of the soil's properties at the chosen location of the experiment," Guenneau said.

A team of 10 people performed the experiment over a three-day period that was chosen for its favorable weather conditions. The team drilled a series of boreholes in lines of 10, each 5 m deep and 0.32 m in diameter, with a center-to-center spacing of 1.73 m. This created a kind of seismic metamaterial, Guenneau said.

"A vibrocompaction crane with a frequency of 50 Hertz was placed close to the first row of boreholes in the soil," Guenneau said. The crane generated vibrations that were monitored and recorded by velocimeters both before and after the holes were drilled.

"The elastic signal is generated at a depth of about 2 m in the soil, and the boreholes are 5m in depth, so one could say that the structured soil behaves like a plate of clay [that] is 5 m [in] thickness. This plate vibrates atop a thick substrate (with layers of clay), but most elastic signals should be confined in the 5 m of depth of the structured soil with boreholes," Guenneau said.

The results indicated that energy at the source was actually 2.3 times higher after the boreholes were drilled. Guenneau likens it to a bright light reflected in a mirror. Conversely at the farthest boreholes from the source, energy was five times weaker.
The boreholes reflect the wave signals via multiple interferences in accordance with Bragg's law of physics. "We can achieve some shielding or concentration effect for the seismic signal, depending upon where we place the cylinders," Guenneau said. "This tells us that we can now, in principle, play at will with [the] surface (Rayleigh) elastic wave's trajectories."

The next step in this research would be to create a type of "seismic cloak" consisting of a ring of 200 such boreholes in clay soil to determine if surface waves can be diverted from the region within the ring, Guenneau said.

"This seismic cloak concept might be applied for the protection of nuclear plants, airports, stadiums or historical monuments—in fact, wherever there is enough space around a building to make a ring of boreholes," Guenneau said. "This could even be applied at larger scales to protect cities from earthquakes."


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