Geotechnical Seismic Isolation using EPS Geofoam around Piles

The feasibility of a new geotechnical seismic isolation system for pile-supported structures, such as bridge piers on monopiles, is investigated. The isolation system utilizes a soft annular zone of viscoelastic material, such as EPS geofoam, around the upper part of the pile. The presence of the EPS elongates the fundamental natural period of the structure and
also modifies its damping. For simplicity and as a proof of concept, the superstructure is considered a simple oscillator. Based on a dynamic Winkler model for pile-soil interaction, the problem is treated analytically leading to a simple design-oriented analysis method. Closedform solutions are obtained for: (a) the lateral stiffness of the structure-foundation system, (b) the corresponding fundamental period, and (c) the overall damping. Numerical analyses in the frequency domain by means of the computer code SPIAB verify the above analytical predictions. Analyses in the time domain are also carried out to examine the effect of geofoam on the dynamic response. Results in terms of base shear, maximum pile bending moment, and displacement at deck level are presented in dimensionless form, allowing for quantification of
the effect of geofoam on seismic performance. It is shown that the presence of geofoam around the pile can reduce the seismic demand, rendering the proposed method a promising, inexpensive alternative to structural protective systems.