Buried Rubber Layers Could Shield Buildings From Earthquakes, But Only Up to a Point

An earthquake does not deliver its energy in a single, predictable punch. Some ground motions shake buildings slowly with long, rolling waves rich in low-frequency content, while others jolt structures with rapid, high-frequency tremors. Which of these characters an earthquake displays can make a dramatic difference to how a building responds, and now a team of civil engineers in Türkiye has quantified that difference for one of the most promising sustainability-minded technologies in earthquake engineering: the geotechnical seismic isolation layer, a cushion of soil mixed with recycled rubber placed beneath a building’s foundation.

Researchers Kasif Furkan Ozturk, Tufan Cakir and Onur Araz of Gümüşhane University examined how the frequency content of ground motions and the thickness of such an isolation layer jointly govern the dynamic response of a five-story building. Their parametric study, published in the Bulletin of Earthquake Engineering, combined analytical modeling with a carefully classified set of earthquake records, comparing the structure resting on natural soil against the same structure founded on a geotechnical seismic isolation, or GSI, layer of six different thicknesses. The results carry practical weight for designers considering this technology.

The appeal of GSI systems lies in their simplicity of concept. Rather than installing mechanical base isolators, such as lead rubber bearings or friction pendulum devices, between a structure and its foundation, engineers instead surround or underlie the foundation with a engineered soil mixture, often soil blended with granulated scrap tire rubber. This softer, more damping-rich medium partially absorbs and dissipates seismic energy before it can climb into the building, filtering the shaking at the ground level itself. Because the approach recycles petroleum-derived waste materials such as discarded tires, it also addresses a genuine gap in waste recovery, turning an environmental liability into structural protection.

To test how well this filtering survives the messy variety of real earthquakes, the researchers grouped their ground motions according to frequency content and ran dynamic analyses across the full set. The findings on this front were consistent and, in some respects, sobering. For structures founded both on natural soil and on GSI layers, earthquakes with low frequency content produced structural responses greater than the average responses calculated across all the investigated records. In other words, slow, long-period ground motions tended to be the harshest critics of the building, regardless of what was buried beneath its foundations, and designers who evaluate structures against a generic suite of motions without regard to frequency content risk underestimating the demands that the most damaging classes of events can impose.

This frequency dependence echoes a growing body of research on soil-structure interaction. Previous work by members of the same group has shown that the frequency character of ground motion shapes the seismic response of retaining walls, box culverts, and buildings of varying aspect ratio when interaction with the surrounding soil is included. The new study extends that lesson into the GSI domain, confirming that an isolation layer does not erase the influence of how the shaking oscillates through time; the layer reshapes and attenuates the transmitted motion, but the character of the input still leaves its fingerprint on the building above.

The thickness of the isolation layer emerged as the other central variable, and here the study delivers its most actionable result. Increasing GSI thickness significantly attenuates structural responses, as expected, since a thicker compliant layer forces more of the seismic energy to deform and dissipate within the mixture rather than propagate upward. But the relationship is not linear, and the benefits diminish decisively with depth. Beyond a thickness of three meters, the researchers found that adding more material does not lead to a relatively significant further reduction in the dynamic responses of the investigated structural system.

That finding translates directly into a design recommendation. For the structural system, the natural soil conditions, the rubber-soil mixtures, and the earthquakes examined in the study, the authors suggest that selecting a GSI thickness in the range of two to three meters, inclusive, may be regarded as a practical approach for mitigating dynamic responses. Thicknesses beyond that range provide only progressively limited improvements, meaning the extra excavation, material, and cost buy shrinking returns. The proposal offers engineers a defensible starting point for a technology that has historically lacked the codified design procedures available for conventional base isolation.

The broader context makes the study timely. Geotechnical seismic isolation has advanced rapidly over the past decade, propelled by laboratory characterizations of sand-rubber and gravel-rubber mixtures, centrifuge tests demonstrating their protective performance, and shaking table experiments on medium-rise buildings founded on EPS bead-sand mixtures. Researchers have explored related variants, including soil mixed with recycled tire rubber, bitumen, and high-damping polyurethane, and have applied the concept to rural residences, highway embankments, and cut-and-cover utility tunnels. Analytical design models and fragility frameworks have followed, gradually building the engineering infrastructure needed to move GSI from promising research to routine practice.

What distinguishes the Turkish team’s contribution is its systematic treatment of two factors that interact: the spectral character of the earthquake and the geometry of the isolation layer. By coupling a frequency-classified ground motion set with a six-point thickness parametric study, the researchers generated a response surface that reveals where protection is most effective and where it saturates. Their analyses also confirm that the GSI system’s benefit is best understood as partial energy dissipation before the waves reach the foundation, a framing that positions the technology as a complement to, rather than a wholesale replacement for, structural isolation systems in high-consequence applications.

The authors note that their results should support the development of vibration control strategies aimed at improving the seismic resilience of structures, and that the data are expected to be useful for a better understanding of GSI applications in general. With no external funding and no competing interests declared, the study stands as a focused analytical contribution from a group with a sustained record in soil-structure interaction research. As cities in seismically active regions search for affordable, sustainable ways to protect ordinary buildings, the message from Gümüşhane is refreshingly concrete: bury a modest, two-to-three-meter cushion of rubber-enhanced soil beneath the foundation, recycle waste tires in the process, and expect the greatest gains in shielding structures from the low-frequency shaking that matters most, while resisting the temptation to assume that more isolation layer always means more protection.

Subject of Research: Seismic performance of buildings resting on geotechnical seismic isolation layers of varying thickness under earthquakes with different frequency content

Article Title: Effects of earthquake frequency content and isolation thickness on seismic behavior of a structure system resting on geotechnical seismic isolation layer

Article References: Ozturk, K. F., Cakir, T., & Araz, O. (2026). Effects of earthquake frequency content and isolation thickness on seismic behavior of a structure system resting on geotechnical seismic isolation layer. Bulletin of Earthquake Engineering. https://doi.org/10.1007/s10518-026-02658-8

Image Credits: AI Generated

DOI: 10.1007/s10518-026-02658-8

Keywords: geotechnical seismic isolation, earthquake engineering, rubber-soil mixtures, soil-structure interaction, frequency content, isolation thickness, seismic isolation, waste tire recycling, structural dynamics, vibration control, Bulletin of Earthquake Engineering, Gümüşhane University