SAP-sand mixtures as a geotechnical seismic isolation technology: from the dynamic characterization to a simple analytical design approach

Seismic isolation techniques are hardly implementable on existing historic structures, if their integrity is a matter of concern (Viggiani in 19th International conference on soil mechanics and geotechnical engineering, Seoul Korea, 2017; Flora in Third international symposium on geotechnical engineering for the preservation of monuments and historic sites, Naples, Italy, 2022). Recent results have shown that if a continuous barrier having a low dynamic impedance is created in the soil beneath the structure, both the absolute acceleration and relative displacement demands are significantly reduced without any direct intervention on the structure itself. Super Absorbent Polymers (SAP), mixed with the in-situ soil, are excellent candidates to the creation of these soft barriers. The first part of the paper reports a complete dynamic laboratory characterization of SAP-sand mixtures with different percentages of polymer in the soil. The results clearly show that small percentages of SAP have a minor effect on the reduction of the shear stiffness value, while when the percentage is higher than 40% a sharp drop is observed. It is also shown that SAP largely increases the damping ratio of the treated layer, being this an extremely beneficial effect for its seismically isolating application. In the second part of the paper, one and two-dimensional seismic response analyses were carried out to explore the beneficial effects of SAP-soil filtering barriers as parts of a Geotechnical Seismic Isolation (GSI) system. The paper also proposes a simplified analytical approach to design geotechnical seismic isolation using Super Absorbent Polymers.