DYNAMIC INTERACTION BETWEEN RETAINING WALLS AND RETAINED STRUCTURES

ABSTRACT The seismic response of retaining walls that support soil layers has been examined by various researchers in the past. However, in engineering practice retaining walls are very frequently used to support, apart from soil layers, structures founded on the retained soil layers. Therefore, during a seismic event it is evident that the dynamic response of each component of this complex system (wall, soil, and structure) may affect substantially the response of the rest, and vice versa. This phenomenon, which could be adequately described as "dynamic wall-soil-structure interaction" (DWSSI), is a rather complicated issue that combines: (a) the dynamic interaction between the wall and the retained soil layers, and (b) the "standard" one-dimensional dynamic interaction of a structure with its underlying soil layers. In the present study, using numerical simulations, the influence of the wall flexibility on the free-field ground shaking behind the wall is investigated. Subsequently, a simple structure founded on the retained soil is included in the numerical models. A parametric study is being performed in order to examine at what extend the presence of the wall may affect the inertial accelerations imposed on the structure (with respect to its position and its fundamental eigen-period). In addition, it is investigated how the location and/or the characteristics of the structure may affect the dynamic earth pressures induced on the retaining wall. Numerical results provide a clear indication of the direct dynamic interaction between a retaining wall and its retained structures

ANN-Based Assessment of Soft Surface Soil Layers’ Impact on Fault Rupture Propagation and Kinematic Distress of Gas Pipelines

Large-scale lifelines in seismic-prone regions very frequently cross areas that are characterized by active tectonic faulting, as complete avoidance might be techno-economically unfeasible. The resulting Permanent Ground Displacements (PGDs) constitute a major threat to such critical infrastructure. The current study numerically investigates the crucial impact of soil deposits, which usually cover the ruptured bedrock, on the ground displacement profile and the kinematic distress of natural gas pipelines. For this purpose, a decoupled numerical methodology, based on Finite Element Method (FEM), is adopted and a detailed parametric investigation is performed for various fault and soil properties. Moreover, the advanced capabilities of Artificial Neural Networks (ANNs) are utilized, aiming to facilitate the fast and reliable assessment of soil response and pipeline strains due to seismic faulting, replacing time-consuming FEM computations. An extensive sensitivity analysis is performed to select the optimal architecture and training algorithm of the employed ANNs for both the geotechnical and structural parts of the decoupled approach, with suitable input and target values related to bedrock offset, fault and soil properties, surface PGDs, and pipeline strains. The proposed ANN-based approach can be efficiently applied by practice engineers in seismic design and route optimization of natural gas pipelines

Seismic displacements of landfills and deformation of geosynthetics due to base sliding

Summarization: Seismic design of waste landfills has been a subject of intense research over the past two decades, primarily due to the severe environmental impact of a potential failure. The majority of the related studies have been focused on the stability assessment of landfills utilizing permanent deformation methods. However, previous investigations have not fully addressed the impact of the composite liner system on the seismic performance of the geostructure, mainly expressed as potential sliding development, which is greatly affected by the geometry and the resulting initial static stress state of the landfill. Therefore, the aforementioned issues are investigated via a detailed parametric study, where the dynamic behaviour of the composite liner system is examined both analytically and numerically. The conducted coupled analyses indicated that the most significant parameters of the complex dynamic response of waste landfills can be reduced in two ratios that comprise functions of the main characteristics of the geostructure and of the excitation. Moreover, two distinct failure patterns have been identified with respect to the characteristics of the distribution of the permanent displacements along the interface and the axial deformation along the geosynthetic. The occurrence conditions of these failure patterns can be determined in terms of the two abovementioned ratios as verified by the analytical results of the critical acceleration of a simple SDOF system.Presented on: Geotextiles and Geomembrane

Reliability based optimization of structures under seismic excitation

Summarization: Retaining systems are widely used worldwide for serving various purposes in struc-tures and infrastructures (embankments, bridges, ports, etc). The seismic response of various types of walls that support a single soil layer has been examined by a number of researchers in the past. Nevertheless, the dynamic interaction of the retaining walls with the structures that they are usually retained has not been investigated yet. It is evident, however, that during a seismic event the dynamic response of each component of this complex system (wall, soil, and superstructure) may affect substantially the response of the rest, and vice versa. The phe-nomenon of dynamic wall–soil–structure interaction (DWSSI) is a rather complicated issue that includes: (a) the dynamic interaction between a wall and a retained single soil layer, and (b) the "standard" dynamic soil–structure interaction (DSSI) of a structure with the underly-ing soil. In the present study, using numerical two-dimensional simulations, the influence of the wall characteristics (flexibility and smoothness) and its distance from the structure on the soil impedances (springs and dashpots) and on the distress of a cantilever wall are addressed. Emphasis is given on the variation of the soil impedance with the distance from the wall and with the exciting steady-state frequency. Subsequently, a structure founded on the retained soil is included in the numerical models, as a single-degree-of-system (SDOF). Despite the fact that there exist many open issues, the numerical results of the current study provide a clear indication of the direct dynamic interaction between a retaining wall and its retained structures. This justifies the necessity for a more elaborate consideration of these interrelated phenomena on the seismic design not only of the retaining walls but of the nearby structures as well, since the aforementioned dynamic interaction issues are not considered with ade-quate realism in the modern seismic norms.Παρουσιάστηκε στο: Thematic Conference on Computational Methods in Structural Dynamics and Earthquake Engineerin

A semi-analytical approach for simulating oblique kinematic distress of offshore pipelines due to submarine landslides

Summarization: Deep water offshore pipelines are usually directly laid on the seabed, a fact that makes them vulnerable to geohazards, such as submarine landslides and debris flows. The intersection angle between the pipeline and the moving sediments can vary taking into account the pipeline route and the unstable nature of deep seabed. The aim of the current study is to examine the distress of a seabed-laid offshore natural gas pipeline subjected to combined lateral and axial kinematic distress due to an oblique submarine landslide or a debris flow. For this purpose, a new semi-analytical model is developed combining the finite-difference method and the elastic-beam theory. Firstly, the proposed model is compared with a finite-element model for various intersection angles, as well as with previous analytical models considering exclusively lateral loading. Subsequently, various combinations of soil resistance forces and loading conditions that affect the examined problem are investigated. Realistic input data were taken from the offshore section of the high-pressure natural gas pipeline TAP (Trans Adriatic Pipeline). Finally, useful conclusions are drawn regarding the applicability and efficiency of the proposed approach to accurately represent the pipeline response under these loading conditions.Presented on: Applied Ocean Researc

A potential mitigation measure for the seismic distress of the circuit wall at the Acropolis of Athens

Περίληψη: The seismic design of new retaining structures is usually performed following modern seismic norms. Nonetheless, there are various monumental retaining structures (e.g., fortifications) with high seismic vulnerability, which must be protected against earthquakes, while there exist several limitations on the type of mitigation measures that can be applied to such cultural heritage structures. The present study investigates numerically the seismic response and distress of the Circuit Wall of the Athenian Acropolis. The Wall is a monumental masonry retaining structure surrounding the world-class archeological monuments of the hill of the Acropolis. Given the fact that the wider region of Athens is characterized by moderate seismicity, it is necessary to protect the Wall from strong ground motions. For this purpose, the geological, seismological, and topographic conditions of the Acropolis hill, as well as the geometry and the mechanical properties of the Wall, are realistically taken into account. A realistic finite-element model has been developed for a critical section of the Wall, which has been validated with available records from accelerometers being installed on the Wall. Subsequently, an efficient and suitable—according to international monument restoration guidelines—seismic mitigation measure is proposed. The results of dynamic earth-pressure distribution on the Wall are presented before and after the application of expanded polystyrene (EPS) blocks behind the Wall. Α detailed parametric study illustrates a substantial reduction of dynamic pressures on the Wall when the EPS blocks are applied, either along the entire height or only at the lower part of the Wall.Presented on: Geotechnical and Geological Engineerin

Mitigating high-speed train vibrations with EPS blocks for various soil conditions

Summarization: Ground-borne vibrations due to high-speed trains passage strongly depend, apart from the speed of the train, on the geometry of the railways as well as the properties of the underlying soil layer(s). The main aim of this study is to investigate the effectiveness of expanded polystyrene (EPS) blocks in mitigating soil vibrations induced on railway embankments for different subsoil and railway embankment material conditions. The EPS blocks are placed in suitable locations, either as embankment's side fill material, or trench filling material, or combination of the above. An efficient three-dimensional numerical model has been developed -in conjunction with a user-developed subroutine for applying the moving loads-to accurately calculate the dynamic response of the coupled embankment-soil model. Four typical soil types - categorized as rock, dense sand with gravels, stiff and soft clay - are investigated. In addition, the mechanical properties of the embankment material have been altered to assess to what extend they can affect the HST vibrations.Presented on: Soil Dynamics and Earthquake Engineerin

Mitigation of high-speed trains vibrations by expanded polystyrene blocks in railway embankments

Project supported by Greece and the European Union (European Social Fund) through the Operational Programme “Human Resources Development, Education, and Lifelong Learning 2014–2020” in the Context of the Project “Strengthening Human Resources Research Potential via Doctorate Research–2nd Cycle” (No. MIS 5000432)Έργο με συγχρηματοδότηση από την Ελλάδα και την Ευρωπαϊκή Ένωση (Ευρωπαϊκό Κοινωνικό Ταμείο) στα πλαίσια του Επιχειρησιακού Προγράμματος «Ανάπτυξη Ανθρώπινου Δυναμικού, Εκπαίδευση και Δια βίου Μάθηση 2014-2020», Πράξη «Ενίσχυση του ανθρώπινου ερευνητικού δυναμικού μέσω της υλοποίησης διδακτορικής έρευνας - 2ος κύκλος» (MIS 5000432)Summarization: The vibrations induced by the passage of high-speed trains (HSTs) are considered a crucial issue in the field of environmental and geotechnical engineering. Several wave barriers have been investigated to reduce the detrimental effects of HST-induced vibrations. This study is focused on the potential implementation of an innovative mitigation technique to alleviate the developed vibrations. In particular, the use of expanded polystyrene (EPS) blocks as partial fill material of embankment slopes was examined. The efficiency of the proposed mitigation technique was numerically investigated. More specifically, a 3D soil-track model was developed to study the cross-section of a railway track, embankment, and the underlying soil layers. The passage of the HST, Thalys, was simulated using a moving load method, and the soil response was calculated at several distances from the track. Several parameters influenced the effectiveness of the examined mitigation measure. Therefore, to ensure an optimal design, a robust procedure is necessary which considers the impact of these factors. Hence, the implementation of EPS blocks on several embankments with different geometry, in terms of height and slope angle, was investigated.Presented on: Journal of Zhejiang University Science

Efficient mitigation of high-speed trains induced vibrations of railway embankments using expanded polystyrene blocks

Summarization: The main aim of this work is to present an efficient mitigation measure of ground vibrations induced by high-speed trains (HST). It is very important to propose such mitigation measures against soil vibrations due to various negative impacts to the population, structures, as well as railway infrastructure. This study examines the application of expanded polystyrene (EPS) blocks as an efficient mitigation measure against the ground vibrations induced by HST’s passage. EPS is a high-performance geosynthetic fill material, which is widely used due to its low weight and great compressibility. In the present numerical study, an three-dimensional (3D) model was used, utilizing the finite element software ABAQUS in conjunction with a user-developed subroutine in order to accurately simulate the complex dynamic phenomenon of soil response during the passage of HST. For this purpose, field data of a typical soil embankment from Paris– Brussels Thalys line were used to validate the adopted numerical approach. Subsequently, the use of different types of EPS schemes was investigated and compared in order to obtain an optimal geometrical configuration of EPS blocks that significantly reduces train-induced vibrations.Presented on: Transportation Geotechnic