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Numerical study of the dynamic response and mitigation of seismic distress of the Circuit Wall at the Acropolis of Athens

Katsirakis Emmanouil

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Summary

The Acropolis of Athens is one of the most prestigious monumental complexes in the word. On the hill of Acropolis great word heritage monuments are located, such as Parthenon, Propylaia, the temple of Athena Nike and the Erechteion. One important part of Acropolis is the perimetric circuit wall, which functions as a retaining structure for the side fills of the hill. The ancient masonry structure is characterized by complexity, due to the damage by natural hazards and man-made damages and various interventions. Based on historical data as well as recent instrumental recordings and studies, it appears that a critical factor for its structural integrity is strong seismic excitations.In the context of this master's thesis, a detailed numerical study of the dynamic response of the highest part of the circuit wall has been performed. In addition, an effective and appropriate mitigation scheme for reducing dynamic pressures has been investigated. Initially, a series of preliminary numerical dynamic analyses have been performed to investigate various aspects of soil response and retaining walls dynamic response, focusing on the special characteristics of the examined case. These analyses included the dynamic response of a simple horizontal soil layer and retaining walls of different dimensions and stiffness with either a horizontal or an inclined bedrock, aiming to gradually build an accurate computational model of the complex Acropolis circuit wall.For the numerical investigation of the actual problem, a realistic finite element model has been developed, based on quite accurate representation of the existing wall conditions at the specific location of the highest part of the south wall. The model included a simplified representation of the geometry and properties of the wall-bedrock-backfill material system that enabled a quite accurate calculation of the magnitude and the distribution of the developed dynamic pressures on the wall. To ensure the accuracy of the numerical results available acceleration time-histories from two recent earthquakes measured at the bottom and at the top of the examined part of the wall have been used.Subsequently, a detailed investigation has been carried out focusing on the reduction of the seismic pressures imposed on the wall for real seismic records and simpler Ricker pulses. The mitigation of dynamic pressures has been achieved via the application of expanded polystyrene (EPS) geofoam behind the wall. A parametric study regarding the thickness and the mechanical properties of the EPS has been conducted. In addition, the impact of EPS height has been studied, i.e., whether it is placed at the upper part or at the lower part of the wall. For this study, have been used.This thesis consists of eight chapters. Initially, Chapter 1 presents a detailed description of the different types of retaining walls as well as the different methods for calculating the imposed static and dynamic pressures. In addition, computational approached based on finite element method are described, while a brief overview of seismic norms and guidelines for retaining walls is also provided. Then, in Chapter 2, the mechanical properties, the categories of EPS geofoam and selection criteria are described. Various successful applications of EPS in geotechnical projects and retaining walls are given. In Chapter 3, the hill of the Athenian Acropolis and historical data for its monuments are presented. Moreover, special reference is made to the Circuit Wall and the main interventions that have been carried out in several periods.Chapter 4 highlights the geological and tectonic characteristics of the Acropolis hill. The most important faults of the wider region are described together with the most important seismic events that have affected the monuments of the hill. The modern network of accelerometers on the Acropolis hill is briefly presented, while previous studies on the static and dynamic response of the circuit wall are reported. In the sequence, Chapter 5 provides all the preliminary dynamic analyses of various types of retaining walls. Chapter 6 presents the study of the dynamic response of the south circuit wall at its current state (i.e., without any mitigation measures). Chapter 7 includes the results of the extensive parametric study of the impact of EPS geofoam on the reduction of dynamic pressures on the wall. Finally, Chapter 8 summarizes the current study, as it provides the conclusions that have been derived, as well as suggestions for further extending the research on this very interesting and important topic.

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