Interface nonlinearity of shallow foundations has been found to significantly impact the dynamic response of soilstructure-interaction (SSI) systems; however, challenges remain in both its experimental and numerical characterisation. This study presents real scale free and forced vibration experiments, as well as three-dimensional nonlinear finite element analyses, to investigate the impact of interface nonlinearity, with particular emphasis on foundation rocking, on the response of an SSI system. The system comprises a steel frame structure, EUROPROTEAS, configured with either an 18 Mg or a 9 Mg structural mass and founded on a surface footing over soft soil. The study focuses on key parameters that influence interface nonlinearity, including structural weight, interface gaps, and static normal stresses that arise at the interface due to the structural self-weight and by densification from previous testing. Further development of an interface model is proposed which allows a more realistic simulation of the static stress distribution prior to the application of dynamic excitation. Both experimental and numerical data demonstrate the significant influence of interface nonlinearity on period lengthening and damping reduction, while also highlight a scenario where it can lead to a stiffer response. Additionally, the study emphasises the significant impact that the initial stress distribution can have on the response, demonstrating that a more uniform distribution can facilitate foundation rocking. Finally, it is demonstrated that a lighter structure can exhibit stronger interface nonlinearity than a heavier one, particularly when the external excitation is applied directly to the structure.

We present the outcomes derived from controlled laboratory experiments on utilizing gravel rubber mixtures (GRM) as a geotechnical seismic isolation (GSI) stratum beneath the foundational structure of the EuroProteas prototype. The study encompassed three distinct GRM compositions characterized by varying rubber proportions relative to the total mixture weight (0%, 10%, and 30%) employed as the foundation soil. These GSI-structure systems were subjected to external harmonic forces applied atop the structure of EuroProteas across a broad spectrum of frequencies and force magnitudes. Preceding the commencement of field experiments, resonant column and cyclic triaxial compression assessments were conducted on specimens featuring identical rubber fractions and mean grain size ratios. Our laboratory tests revealed that an increase in rubber content resulted in a reduction of the shear modulus (Go) and an augmentation of the damping ratio (Do) while concurrently inducing a more linear character in the G/Go-logγ-D profiles. The findings derived from the laboratory examinations align closely with those ascertained from field investigations. Expressly, it was confirmed that a GSI layer with a thickness of 0.50 meters, composed of a GRM incorporating 30% rubber content, exhibited a discernible reduction in the overall stiffness of the GSI-structure system, accompanied by a corresponding alteration in its fundamental vibrational frequency. The enhanced damping within the system manifested through observable reductions in acceleration recordings and diminished strain development at the base of the GRM layer with 30% rubber content, encompassing a wide range of frequencies.

Time domain finite element (FE) analysis is a powerful tool for the study of Soil-Structure-Interaction (SSI) phenomena, but it requires a rigorous calibration of all aspects of the numerical model. This study presents three-dimensional (3D) FE analyses that are calibrated and validated against real scale free and forced vibration experiments on the prototype structure of EUROPROTEAS which is founded on soft alluvial sediments. The proposed calibration procedure exploits data recorded during experiments on structures with different structural stiffness, that mobilised SSI effects at different intensities. Particular focus is placed on the modelling of the soil-foundation interface, where zero thickness elastoplastic interface elements are used to allow foundation separation from the soil. A novel approach to simulate contact imperfections (gaps) between the foundation and the adjacent soil is proposed. The results demonstrate the significant impact of the interface gaps and soil nonlinearity on the response of the examined SSI systems, highlighting the importance of a rigorous model calibration.

Dynamic Soil-Structure-Interaction (DSSI) phenomena can considerably affect the structural response under dynamic loading. Time domain finite element analysis allows to study these phenomena in depth, but several computational challenges need to be addressed first to achieve rigorous modelling of both the structure and soil domain. Within this context, this study presents three-dimensional FE analyses of real-scale forced vibration tests of a steel frame structure founded on a shallow foundation. The field tests were carried out with the real-scale prototype structure of EUROPROTEAS at the Euroseistest experimental facility located in the Mygdonian Valley in Northern Greece. The structure has outer dimensions of 3 × 3 × 5 m and it is placed in an area whose geotechnical properties have been well documented by previous studies. The study focuses on the modelling of the soil-foundation interface, which is one of the major challenges of such complex SSI simulations. A novel approach to model potential contact imperfections at the interface is proposed, showing very good agreement with the field data and hence improving the reliability of numerical predictions. The analyses show that contact imperfections affect considerably the predicted motion in both the structure and the soil.

Scour is the prevailing cause of bridge failure worldwide, leading not only to traffic disruption, but also to social and economic losses and even to casualties. Many vibration-based monitoring techniques have been proposed for identifying the scour location and extent, based on the evaluation of the changes of the bridge modal properties due to scour. This study describes the experimental and numerical research carried out to investigate the effects of scour on the dynamic properties of structures with shallow foundations. Although these are the most vulnerable ones, they have received less attention compared to structures founded on pile foundations. To fill some existing knowledge gaps, field experiments were carried out on EuroProteas, a structural prototype with shallow foundation that was subjected to increasing levels of scour. The changes of the dynamic properties of the system are evaluated by postprocessing the ambient vibration recordings and by developing various models of the soil–foundation–structural system with different descriptions of the soil–structure interaction problem. The study results shed light on the effects of scour on systems with shallow foundations and on the accuracy of alternative modelling approaches. They are presented here to inform the development and validation of vibration-based techniques and modelling strategies for bridge scour identification.

We present the results of the forced-vibration experiments performed at the large-scale prototype structure of EuroProteas founded on gravel-rubber mixture (GRM) layers acting as a means of Geotechnical Seismic Isolation (GSI). Three GRM with different rubber content per mixture weight (0%, 10%, and 30%) but the same mean grain size ratio were used as foundation soil. Each GRM-structure system was subjected to harmonic forces in a wide range of excitation frequencies and force amplitude. It was found that a 0.5 m thick GRM foundation soil layer with 30% rubber content can effectively isolate the structure. The strong effect of the rubber fraction was expressed in the detected period elongation and the dominating rocking component which leads to a more “rigid-body” response of the structure. Moreover, the developed base shear and base moment are significantly reduced regardless of the excitation frequency, while the increased damping of the system and the important energy dissipation demonstrate the effectiveness of the GRM foundation soil layer. Overall, the experimental results demonstrated that the use of GRM as a GSI system can be considered as a low-cost alternative seismic isolation technique.

We present the results of an extensive large-scale experimental campaign on the dynamic response of rubber–gravel mixtures as an innovative seismic isolation material. In the first series of experiments, the foundation soil immediately below the prototype structure of EUROPROTEAS was replaced only with gravel to serve as benchmark tests, while in the following tests two rubber–gravel mixtures with increasing rubber content per mixture weight were used. The experimental campaign included free- and forced-vibration tests. A large number of instruments of various types (accelerometers, seismometers, shape-acceleration arrays, and laser sensors) were installed on the structure, in the foundation soil and at the adjacent soil surface in order to obtain a well-instrumented 3D set of recordings to study the response of the structure and wave propagation in soil media. In this study, we seek to investigate the isolation capability of the rubber–gravel mixtures under dynamic loading. Our primary goal is to assess the effect of the rubber content of the improved foundation soil in the stiffness and the damping of the soil-structure system.

We present the results of a large‐scale experimental campaign performed on the prototype structure of EuroProteas in Thessaloniki, Greece, to assess the effectiveness of gravel‐rubber mixture (GRM) layers underneath shallow foundations as a means of geotechnical seismic isolation (GSI). We found that the GSI of structures is optimized by increasing the rubber content of the soil‐rubber mixture up to 30% per mixture weight. The effectiveness of the GSI systems has been investigated numerically and in small‐scale experiments. This article seeks to fill the gap in the lack of full‐scale experimental studies on this subject. Three soil pits were excavated and backfilled with GRM of different rubber content per weight to serve as foundation soil. A large number of instruments were installed on the structure, the foundation, the soil surface, and inside the GRM layers beneath the foundation to fully monitor the GSI‐structure systems’ response in three dimensions. The experimental investigation included ambient noise, free‐ and forced‐vibration tests. Our results showed that a GSI layer composed of a GRM with 30% rubber content effectively isolates the structure. Even 0.5 m thickness (ie, B /6 of the foundation width) of the GSI system successfully cuts off practically all emitted waves at a (horizontal or vertical) distance of B /6 from the foundation. A significant reduction in the GSI‐structure system's stiffness was apparent, leading to a rocking‐dominant response. The rise in the system's damping and the substantial energy dissipation inside the GRM layer highlight its effectiveness as a GSI system.

In this work, we design an experimental campaign to assess the attenuation performance of a medium-scale resonant wave barrier operating within the frequency range of 50-100 Hz. In particular, the dispersive properties of (i) bare soil, (ii) a configuration of "dead masses" placed over the soil surface, and (iii) a locally resonant barrier, also known as metabarrier, are compared numerically. The resonant barrier introduces a significant amplitude reduction of the surface waves in a narrow frequency range around the resonant frequency of the resonators. Multiple-frequency barriers are designed with increasing and decreasing resonant frequencies to enlarge the attenuation frequency band.

The paper presents and discusses the dynamic foundation impedance functions calculated from full-scale field tests on soil-structure interaction at the prototype facility of EuroProteas at Euroseistest in Greece. The experimental campaign included ambient noise, free- and forced-vibration tests throughout a wide frequency and amplitude range. The response of the soil-structure system was observed to be dominated by rocking. Hence, the impedances were derived under two hypotheses, i.e. considering or neglecting the foundation swaying in the dynamic equilibrium of the soil-structure system. The trend of the back-calculated impedance at high frequencies was observed to vary depending on the interpretation model and turned out to be in satisfying agreement with the available analytical solutions. Conversely, values at resonance were found almost independent of the test type and of the interpretation model. These findings imply that i) the currently widespread non-destructive tests (for instance ambient noise tests) can be effectively used to derive impedance functions; ii) the uncertainties are minimized for the resonance frequency traditionally used to calibrate the flexible-base models for soil-structure interaction analyses.

Two structural configurations of the EuroProteas prototype structure, defining two test structures with different structural stiffness, were subjected to dynamic excitation to study the influence of soil-foundation-structure interaction effects on the recorded response. The first test structure was braced in all directions making a stiff structure frame based on soft ground. In contrast, we removed the bracing in the direction of loading in the second structure to significantly reduce its structural stiffness and the relative structure-to-soil stiffness ratio. Ambient noise measurements, free-vibration tests over a wide range of pull-out forces and forced-vibration experiments over a wide range of frequencies were included in the experimental series performed on both structures. The strong effects of the soil-foundation-structure interaction in the response of the stiff structure were expressed in the detected period elongation and the dominating rocking component which increased the radiation damping. The identified rocking stiffness was found to be frequency-dependent, in contrast to the lateral stiffness. On the contrary, the most significant proportion of the introduced energy was dissipated in the structural members of the second test structure, and the measured translation and rotation of the foundation were almost negligible.

A series of experiments to study soil-foundation-structure interaction (SFSI) is performed in the full-scale prototype structure of EuroProteas constructed in the experimental facility of Euroseistest in Northern Greece. Its outer dimensions are 3x3x5m and its structural mass and stiffness are reconfigurable, allowing for four different configurations and covering a wide range of resonant frequencies. A large number of instruments of various types have been installed both on the structure and in the surrounding soil in order to obtain a well-instrumented 3D set of recordings to study SFSI and wave propagation in soil media. In this paper, we evaluate the effects of SFSI phenomena in the full-scale experiments such as period elongation. Additionally, we examine the extent of participating foundation soil mass in the structure response, based on the waves that are created in the foundation soil because of the vibration of the structure.

A sequence of experiments to study soil-structure interaction and wave propagation in soil media due to structural oscillation is performed in the full-scale structure of EuroProteas. EuroProteas is a simple test structure constructed in the experimental facility of Euroseistest in Northern Greece particularly designed to promote soil-structure interaction phenomena. Six experimental campaigns were performed including ambient noise measurements, three freeand three forced-vibration sets of tests. Response was recorded by a dense 3D array of more than 80 instruments (accelerometers, seismometers, shape acceleration arrays) monitoring structural, foundation and the surrounding soil response. The data collected includes acceleration and velocity records. A brief description of the EuroProteas facility and field testing is presented. Experimental data recorded was organized in a database. This data can be used for applications such as the evaluation of soil-structure interaction effects, the back-calculation of experimental impedance functions and the validation of theoretical impedance functions, the determination of system damping etc. Examples of the data usage is demonstrated. The database is organized and processed in a Jupyter Notebook, an open-source web application which combines code blocks with markdown cells allowing the user to interact with data.

A full-scale symmetric structure was constructed in the Euroseistest experimental facility in the North of Thessaloniki, Greece, to promote studies of wave propagation and soil-structure interaction in real field conditions. The 3-by-3-by-5 m model structure, called EuroProteas, comprises of a reinforced concrete slab resting on the soil surface, and four steel columns (with or without bracing) supporting one (or two) reinforced concrete slabs acting as the superstructure mass. The reconfigurable stiffness and mass of the structure, in conjunction with the well-known soil profile in Euroseistest, allow for the investigation of soil-structure interaction for different structure-to-soil stiffness ratios. More than 80 accelerometers and seismometers on the structure and on the soil surface, as well as down to a depth of 12 m, were used to record the response of the soil-structure system during a set of field tests under ambient noise, free- and forced-vibration loading. Following a detailed description of the EuroProteas testing facility and the experimental campaign, field evidence on critical SSI issues is reported based on recorded data. Particular emphasis is placed on the period elongation and damping of the SSI system due to the foundation soil compliance by employing well-known data interpretation methods. Softening of the soil-structure system, reflected in the elongation of the SSI period with respect to its fixed-base counterpart, and attenuation of soil response away from the oscillating structure with increasing amplitude of loading, are discussed.

Analytical solutions for the evaluation of system damping are validated through freeand forced-vibration experiments performed in the large-scale prototype structure of EuroProteas located in the experimental facility of Euroseistest in Northern Greece. EuroProteas is a simple stiff model structure founded on soft soil and is especially designed to mobilize strong soil-structure interaction (SSI) effects. Its outer dimensions are 3x3x5m and its structural mass and stiffness are reconfigurable, allowing four different model configurations and covering a wide range of resonant frequencies. We describe analytical methods proposed in literature and guidelines for calculating system damping. We apply these methods to the experimental data obtained from a wide range of freeand forcedvibration tests in order to validate them and calculate the damping ratio of the system. The results indicate strong influences from both hysteresis in foundation-soil interaction and radiation damping. For large forces the presence of soil nonlinearities is identified.