National Technical University of Athens School of Rural and Surveying Engineering Laboratory of Structural Mechanics and Engineering Structures 9,Iroon Polytechniou Street Athens, 15780, Greece
Professor of Geomechanics and Engineering Structures, School of Rural and Surveying Engineering, National Technical University of Athens (NTUA)
Diploma in Rural and Surveying Engineering, NTUA (1970) Diploma in Civil Engineering, NTUA (1975) MSc in Engineering Rock Mechanics, University of London (1982) DIC -Diploma of Membership of Imperial College,(1983) Dr Eng., Applied Mechanics, NTUA (1989)
Abstract: This paper uses a computational approach to provide insight into the relationships among marine geotechnical properties that characterize the recent sedimentary cover at the head of Zakynthos Canyon in western Greece. Self-organizing maps (SOM) and generic interaction matrix (GIM) theory were used to investigate the tendency of the data to cluster and to examine the sediment property relationships. This analysis has also focused on the assessment of the dominance and interaction intensity between the related parameters following GIM theory definition. The principal results refer to the identification of clusters in the original multivariate data set. SOM-based analysis distinguished five clusters, with similar geotechnical characteristics, which led to the separation of the surficial (∼80 cm) unconsolidated sediments from the deeper normally consolidated sediments and depicted better relations between the geotechnical properties within each cluster. The combination of SOM with GIM theory also demonstrates the dominance of fine-grained sediments (especially silts) and their associated Atterberg limits. The strongest interaction intensity is observed between silt and water content, whereas the undrained shear strength of the surficial deposits appears to be least interactive. The application of computational intelligence methods in the study of marine geotechnical properties allows insight into the relationships between the various geotechnical parameters and provides a promising tool for knowledge extraction in marine geo-environments.
Abstract: Based on neotectonic mapping, analysis of active deformation through tectonic data and the Coulomb stress changes caused by major earthquakes, the active faults in Lefkada Island were evaluated. Descriptions of historic and recent earthquakes were also taken into consideration as well as the seismotectonic data concerning the last strong seismic event of 2003. According to detailed field observations, among the active and neotectonic faults that have been identified, regarding seismic hazard, the most important can be considered the Athani fault. It is running parallel to and near the west coast and shows marked similarity with the northern branch of the offshore transform fault of Kefalonia-Lefkada. These two dextral strike-slip faults exhibit same structure and state of stress as well. Moreover, the Coulomb stress changes associated with the 2003 earthquake, well correlated with the spatial aftershock pattern, are positive in the area of the Athani fault. Attention has been also focused on the geologic hazard in relation to earthquake associated phenomena, such as surface rupturing, landslides and rockfalls, liquefaction and tsunamis. The fact that surface deformation is repeated in an identical way during past and recent earthquakes provides a great advantage for the seismic protection of the island.
Abstract: A closed-form solution for the problem of the plastic zone and stress distribution around a circular tunnel in an elastic–plastic half space, derived using bipolar coordinates, is the main scope of this paper. By assuming a uniformly applied surface loading, the whole semi-infinite space is under uniform pressure: thus the gravitational effect is neglected, while the plastic zone formation around the circular tunnel is controlled by the applied internal support pressure. The plastic behaviour of the half space is described by the Mohr–Coulomb yield criterion, and the soil is assumed to be homogeneous and isotropic with earth pressure coefficient K 0 equal to unity. The critical internal pressure, where the initial yielding occurs at the tunnel wall, is derived, along with equations describing the plastic zone and plastic stresses. These equations are functions of the soil properties, which are cohesion and friction angle. This derived closed-form solution is validated through mathematical and computational analysis, and is also compared with numerical models under gravitational load, solved using the finite difference method. This innovative closed-form solution has a significant impact in practical problems by introducing a simple and effective method that allows the quick estimation of a shallow tunnel's behaviour, since it gives, in principle, the opportunity for quick and accurate calculation of the plastic zone and stress distribution around the circular tunnel. By applying different values of support pressure, the tunnel designer can easily evaluate the feasibility of different design alternatives, such as shotcrete shell and tunnel-boring machine support pressure. As a result, an efficient and innovative method of solving shallow tunnelling problems in cohesive-frictional soil is introduced.
Abstract: This paper studies the long-term (>30 years) settlement behavior of the Mornos dam on the basis of finite element analysis and vertical displacement data. It compares actually measured deformations resulting from a continuous geodetic monitoring record of the dam behavior with a numerical back analysis. Our aim is to explain the actual deformation history on the basis of the mechanical behavior of the dam. The deformation monitoring record consists of precise leveling data of a large number of control stations established along the crest and the inspection gallery of the dam, as well as settlements derived by magnetic extensometers placed inside the dam body. Overall, the available data cover the phases of construction, first filling of the reservoir and most of the operational time of the dam. The numerical modeling assumes 2D plane-strain conditions to obtain the displacement and the stress–strain characteristics of the abutments and the dam at eleven equally-spaced cross sections. Comparative evaluation of the results of the geodetic monitoring analysis against the findings from the finite element back analysis simulating characteristic stages in the lifetime of the structure, shows a very good agreement (on average 0.03 m) between the measured and computed displacements, which testifies to the correctness of the geotechnical parameters and loads used in the analysis.
Abstract: The current paper illustrates the application of computational intelligence tools in slope performance prediction both in static and dynamic conditions. We present the results obtained by using the back-propagation algorithm, the theory of Bayesian neural networks and the Kohonen self-organizing maps, one of the most realistic models of the biological brain functions. We estimate slope stability controlling variables by combining computational intelligence tools with generic interaction matrix theory. Our emphasis is given to the prediction and estimation of the following: slope stability, coefficient of critical acceleration, earthquake induced displacements, unsaturated soil classification, classification according to the status of stability and failure mechanism for dry and wet slopes. Finally, we present an integrated methodology for assessing landslide hazard coupling computational intelligence tools and geographical information systems.
Abstract: In this paper a decision support system is presented, which integrates the technology of geographical information systems in evaluating slope stability. A dynamic tool has been developed in ARC∕INFO 7.0.1 Macro Language, the “Landslide Hazard Assessment Tool,†which produces safety factor contours giving, as an output, a landslide hazard zonation map. The factors contributing to the manifestation of the phenomenon have been studied as well as the geomorphologic and quantitative information referring to the geotechnical parameters. To represent and interactively examine the results, a series of thematic maps have been created, using this overlay concept of geographical information systems. The study area is located in Vouraikos Valley, in northern Peloponeese, Greece. This particular area has been selected in order to test the model in an area strongly affected by landslides, belonging to the neotectonic regime of the Gulf of Corinth. The final conclusion is that proper implementation of engineering geological information can decrease the geohazard vulnerability of infrastructure. Such a model could be most helpful in the stage of decision making in the construction industry.
Abstract: An experimental study was undertaken concerning the propagation of a slant crack under compression. By using the equations of the deformed shape of a crack and by introducing a correction model in order to prevent the crack lips from incompatible displacements, an estimate of the stress distribution along the crack borders was achieved. It was found that an opening-mode K 1 stress intensity factor (SIF) must be introduced at the crack tips in the overall compressive stress field, in order to give the required space for the lip-slip phenomenon, due to shearing, to take place. This local dilatation in the vicinity of the crack tip, together with the lip-slip phenomenon, due to which the initial crack tip is displaced into a new position along the deformed crack borders, causes the out-of-plane propagation of the crack, either towards the largest compressive stress in the case of biaxial stress field, or towards the applied compression in a uniaxial compressive field. A series of experiments on PMMA (perspex) rectangular specimens with pre-existed cracks and slits has been executed and the type of the stress field in the front of the propagated branches is examined by using the method of caustics. It was found that the crack propagation under compression is an interactive process of two conjugate branches, which is strongly influenced by the boundary conditions of the pre-existed discontinuity.
Abstract: The behavior of the two main types of flows encountered in materials such as rocks, i.e., cracks and slits, under compressive and shear stresses has been examined. It was shown that the existence of a gap between the edges of a slit is allowing the inward movement, causing the development of a negativeK I -SIF at its cornered extremities. On the other hand, for a slant ideal crack additional compressive stresses should develop because of the noncongruent deformation of its lips thus opposing the incompatible interpenetration of the crack flanks and causing an openingK I -SIF. Finally, for an ideal crack normal to the global stresses in the plate the cracked body behaves as a continuum without any singularity.
In order to verify the validity of the above arguments the method of caustics has been used. After developing the appropriate equations of the caustics a series of experiments were executed giving results which support the already developed theory.
