Landslides occur both onshore and offshore. However, little attention has been given to offshore landslides (submarine landslides). Submarine landslides have significant impacts and consequences on offshore and coastal facilities. The unique characteristics of submarine landslides include large mass movements and long travel distances at very gentle slopes. This thesis is concerned with developing centrifuge scaling laws for submarine landslide flows through the study of modelling submarine landslide flows in a mini-drum centrifuge. A series of tests are conducted at different gravity fields in order to understand the scaling laws involved in the simulation of submarine landslide flows. The model slope is instrumented with miniature sensors for measurements of pore pressures at different locations beneath the landslide flow. A series of digital cameras are used to capture the landslide flow in flight. Numerical studies are also carried out in order to compare the results obtained with the data from the centrifuge tests. The Depth Averaged Material Point Method (DAMPM) is used in the numerical simulations to deal with large deformation (such as the long runout of submarine landslide flows). Parametric studies are performed to investigate the validity of the developed centrifuge scaling laws under the initial and boundary conditions given in the centrifuge tests. Both the results from the centrifuge tests and numerical simulations appear to follow the proposed centrifuge scaling laws, which differ from the conventional centrifuge scaling laws. The results provide a better understanding of the centrifuge scaling laws that need to be adopted for centrifuge experiments involving submarine landslide flows, as well as giving an insight into the flow mechanism involved in submarine landslide flows.

No description available.

Στα πλαίσια της παρούσας διατριβής έγινε θαλάσσια έρευνα στην περιοχή των Νικολέϊκων Αιγίου, στην οποία έχουν εκδηλωθεί φαινόµενα βαρυτικής µετακίνησης µαζών τόσο σε ιστορικούς χρόνους όσο και στο πρόσφατο παρελθόν. Με σκοπό τη µελέτη των φαινοµένων αυτών εκτελέστηκαν στην περιοχή ερευνητικές πορείες σε διαφορετικές χρονικές περιόδους. Κατά τις πορείες αυτές έγινε γεωφυσική διασκόπιση του πυθµένα και οπτική παρατήρηση των υποθαλάσσιων ιζηµάτων. Κύριος στόχος της έρευνας ήταν η όσο το δυνατό καλύτερη µορφολογική αποτύπωση του πυθµένα, η αναγνώριση και χαρτογράφηση των διαφόρων κατολισθητικών ενοτήτων και η διερεύνηση φαινοµένων που πιθανόν σχετίζονται γενετικά µε την κατολίσθηση (διαφυγές αερίων υδρογονανθράκων). Αποτέλεσμα της μελέτης αυτής ήταν η οριοθέτηση της υποθαλάσσιας κατολίσθησης στην παράκτια ζώνη των Νικολέϊκων, ο προσδιορισμός των κυριότερων μορφολογικών χαρακτηριστικών της, η συμπεριφορά των ιζημάτων από τη χρονική στιγμή μετακίνηση τους έως σήμερα και η προσέγγιση των γενετικών μηχανισμών της κατολίσθησης. / On june 15th, 1995, an earthquake of magnitude Ms 6,1R occured offshore of the Aigion town in the Western Gulf of Corinth. An offshore survey using a 3.5 kHz subbotom profiling system, which carried out fifteen days after the main shock has shown that the earthquake caused a small-size subaerial to submarine landslide in the Eliki fan delta deposits. Further examination of the landslide, using a side scan sonar system and R.O.V for the visual inspection of the seafloor to map the complex seafloor morphology in and adjacent to the landslide, has shown that the Eliki submarine landslide is a complex low-angle translatory slide less than 5 m deep and that the sliding took place over a single basal glide plane dipping at about 1o. The instability mechanism that triggered the landslide is liquefaction of a subsurface horizon at a depth of 5 m as suggested by the presence of sand and water injection features (sand boils) in the beach near the slide site and the conical depressions observed on the seafloor. Gas expulsions from the liquefied horizon perhaps enhanced the failure as suggested by the presence of gas charged sediments under the sliding plane and the rising of bubbles in the water column as observed by the fishermen.

No description available.

The emphasis of this dissertation is on using numerical and plasticity based methods to study two main areas of offshore geotechnics. The first part of this dissertation focuses on the undrained behavior of deeply embedded anchor plates under combined shear and torsion. Plate anchors are increasingly being used instead of typical foundation systems to anchor offshore floating platforms to sustain uplift operating forces. However extreme loading cases would create general loading conditions involving six degrees of freedom. The focus of my research was to evaluate the bearing capacity of plate anchors under two-way horizontal and torsional loading and to study the decreasing effect of torsional moment on the horizontal bearing capacity of these foundations. The study takes advantage of several approaches: Numerical simulation (two and three dimensional finite element analysis) Evaluating and modification of the available plasticity solutions Developing equations for three degree-of-freedom yield locus surfaces The same methodology is applied to evaluate the response of shallow foundations for subsea infrastructure subjected to significant eccentric horizontal loads. The second part of this study focuses on offshore geohazards. Coastal communities and the offshore industry can be impacted directly by geohazards, such as submarine slope failures, or by tsunamis generated by the failed mass movements. This study aims at evaluating the triggering mechanisms of submarine landslide under cyclic wave and earthquake loading. A simple effective stress elasto-plastic model with a minimal number of parameters accounting for monotonic and cyclic response of fine-grained material is developed. The new constitutive soil model could be used to simulate case histories and conduct parametric study to evaluate the effect of slope inclination angle, the earthquake loading with different PGA, frequency content, and duration, as well as various deposition rates to simulate different over pressure levels. This study will generate more insight on the static and cyclic behavior of submarine slopes and influencing factors on their triggering mechanisms using more comprehensive and realistic modeling tools. Several objectives are defined: Developing an appropriate constitutive formulation, Evaluating the constitutive model and material parameters for available databases.

Le rift de Corinthe, en Grèce, est un jeune rift séparant la Grèce continentale du Péloponnèse. Sa partie la plus active, où la subsidence a été la plus importante durant le Quaternaire récent, a été envahie par la mer Méditerranée et forme le Golfe de Corinthe. Cette région est sujette à de nombreux aléas naturels : fréquents séismes, tsunamis et glissements de terrain côtiers. Cette thèse est dédiée à l'étude de ces processus à l'extrémité ouest du Golfe, où le l'aléa sismique, en particulier, est le plus élevée. Nous avons investigué les sédiments quaternaires accumulés sous le golfe, par l'acquisition de profils sismiques et de carottes sédimentaires. Premièrement, 22 grands glissements de terrains sous-marins ont été découverts. Leur volume varie entre 106 et 109 m3. Ces glissements eurent lieu durant 6 intervalles de temps, 4 durant l'Holocène et 2 durant le Pléistocène supérieur. Parmi les possibles facteurs ayant favorisé ou déclenché ces glissements, le rôle des apports sédimentaires semble avoir été prépondérant. Ensuite, une carte détaillée des failles en mer a été réalisée. Cette carte met en évidence, pour la première fois, des mouvements décrochants significatifs dans la partie marine du rift. Trois phases sont mises en évidence dans l'évolution tectono-sédimentaire. Durant ces phases, nos interprétations suggèrent une migration vers le nord de la déformation, conduisant à la désactivation progressive de grandes failles normales à pendage sud qui contrôlaient la subsidence durant une première phase du rifting. Par l'étude des carottes sédimentaires, nous avons ensuite recherché dans les sédiments récents des événements sédimentaires ayant été déclenchés par les grands séismes historiques. La meilleure correspondance entre l'âge des événements et l'âge des séismes est observée dans la partie la plus profonde du bassin. Finalement, quatre carottes plus longues ont été prélevées dans cette région et ont révélé une distribution spatiale et temporelle spécifique des glissements sous-marins au cours des derniers 500 à 1000 ans. Cette distribution est interprétée comme résultant principalement de variations dans la fréquence des grands séismes. Ainsi, selon ces nouvelles données, une période de quiescence sismique eut lieu entre ~1740 et ~1890 AD à l'ouest de la zone d'étude alors qu'à l'est, une quiescence sismique aurait eu lieu plus tôt, entre ~1500 et ~1700 AD. / The Corinth Rift, in Greece, is a young and active continental rift stretching between Continental Greece and the Peloponnese. The most active part of the rift, where the subsidence has been the highest during the Late Quaternary, has been covered by the sea and forms the Gulf of Corinth. This area is prone to natural hazards, including frequent large earthquakes, tsunamis and coastal landslides. The present thesis is dedicated to the study of these processes at the western tip of the Gulf, where the earthquake hazard, in particular, is considered as very high. We have investigated the Quaternary sediments below the Gulf of Corinth floor, through seismic reflection profiling and gravity coring. First, 22 large mass transport deposits were discovered. Their estimated volumes range from 106 to 109 m3. Large mass wasting events occurred in six stratigraphic intervals, four attributed to the Holocene and two attributed to the Upper Pleistocene. Among possible preconditioning factors and triggers, the likely influence of the sediment supply is highlighted. Then, an accurate map of offshore faults is presented. The map highlights for the first time significant strike-slip component in the offshore Corinth Rift, in addition to the dominant normal strain. Three phases are proposed for the Late Quaternary tectono-sedimentary evolution of the area. During these phases, the strain was suggested to migrate northward, driving the progressive deactivation of the large south-dipping faults that controlled the subsidence in an earlier phase of the rifting. Based on the sediment cores, sedimentary events triggered by large historical earthquakes in the last 3 centuries have been looked for. The best fit between the age of the identified event deposits and large historical earthquakes is observed in the deep basin. Finally, four longer cores retrieved in this area reveal specific spatial and temporal patterns of slope failures for the last 500-1000 yr. Such pattern is interpreted as resulting primarily from changes in the frequency of strong earthquakes. From these data, a period of seismic quiescence may have occurred between ~1740 and ~1890 AD in the west of the study area, while eastward, seismic quiescence would have occurred earlier, between ~1500 and ~1700 AD.

La chaine de Taïwan représente l’une des zones les plus actives au monde. Depuis le début du XXe siècle, plus d’une vingtaine de séismes _Mw 7 ont affecté l’île. Cependant la probabilité d’occurrence de séismes plus importants (_Mw 8) est toujours matière à débat. Dans un tel contexte, il apparait donc important d’obtenir des enregistrements plus anciens pour évaluer l’occurrence de tels évènements. La paléosismologie marine, basée sur l’enregistrement des dépôts issus de la sédimentation gravitaire, apparait comme une alternative sérieuse afin d’illuminer l’histoire de la sismicité sur des périodes anciennes. Ce travail de thèse apporte de nouvelles contraintes sur l’histoire sismique au large, à l’est de Taïwan, au cours des derniers 3000 ans.La première partie de ce travail consiste à caractériser les systèmes sédimentaires récents. Cette étape fut essentielle pour la compréhension des processus, faciès sédimentaires et les facteurs de contrôle de la sédimentation qui régissent l’évolution de la pente sous-marine. Ces résultats ont montré que l’Est de Taïwan est caractérisé par une grande variabilité de processus et que les courants de turbidité dominent l’enregistrement sédimentaire.Les courants de turbidité sont générés par deux facteurs distincts : l’activité tectonique/sismique et l’activité climatique. La deuxième partie de ce travail est consacrée à l’approche paléosismique en utilisant les dépôts de turbidités comme marqueurs des paléoséismes. Pour ce faire, nous avons dans un premier temps testé et validé l’approche à l’échelle du siècle dernier. Ensuite une fois calibré, nous avons pu étendre les séries temporelles à l’échelle de l’Holocène. Nous avons daté les trois plus récents évènements turbiditiques autour de 2001 ± 3 AD, 1950 ± 5 AD et 1928 ± 10 AD. En utilisant des relations empiriques intégrant magnitude, distance et valeur du "peak ground acceleration", nous avons pu calibrer la source sismique et ainsi corréler ces trois turbidites à trois séismes instrumentaux : le séisme de Chengkong (12/10/2003) (Mw 6.8), le séisme de Taitung (11/24/1951) (Mw 7.1) et le séisme de Lutao (9/4/1935) (Mw 7.0). Au-delà du siècle dernier, les datations et modèles d’âges nous permettent d’établir une partie de la chronologie des évènements extrêmes sur une période de 3000 ans et d’estimer un temps de retour pour des évènements de l’ordre _Mw 7. Les résultats ont toutefois montré que ces temps de retour sont nettement supérieurs à ceux connus sur le siècle dernier, ce qui suggère que tous les séismes ne sont pas enregistrés par les dépôts sédimentaires marins. Ce travail de thèse a donc permis de montrer que les dépôts issus de la sédimentation évènementielle peuvent être considérés comme marqueurs de la sismicité et que cette approche peut constituer un outil complémentaire pour les études portant sur le risque sismique. / Taiwan is a young mountain belt, known as one of the most active area in the world. Since the beginning of the 20th century more than twenty _ Mw 7 earthquakes have struck the island. However, the occurrence of larger events (_Mw 8) is still a matter of debate. In this framework it is of key importance to obtain longer record in order to evaluate the occurrence of large past earthquakes. The sub-aqueous paleoseismology, based on the record of the sedimentary gravity deposits, appears as a serious alternative to approach this thematic and is a rapidly advancing field that has the potential to illuminate the long-term history of seismicity.The first part of this work was to investigate the present sedimentary systems off east Taiwan, essential to understand the morphosedimentary features, sedimentary facies and processes governing the evolution of the submarine slope, and the controlling factors of the recent sedimentation. Our results showed that the offshore slope east Taiwan is affected by a variety of sedimentary systems and processes, and that turbidity currents appear as the main erosional processes covering nearly 60% of the sedimentary record. Turbidity currents are generated by distinct controlling factors such as tectonic and climatic activity that enabled us to define twoend-members relative to turbidity currents initiation: Turbidity currents preconditioned by tectonic activity and triggered by earthquakes shaking and likely deposited into intra-slope basin and turbidity currents driven by climatic activity such as extreme floods or typhoons, generated in basin directly connected with onland rivers.The second part consisted to apply a paleoseismic approach, based on turbidites record, at two time-scales. First, we tested and validated the method by correlating turbidites deposits with instrumental earthquakes. Then, once calibrated we extended the time-series back in time. We dated the three most recent turbidites layers circa 2001 ± 3 AD, 1950 ± 5 AD and 1928 ± 10 AD. Using empirical relationship that link peakground acceleration, distance and magnitude to calibrate the seismic sources, we correlate these three turbidites with instrumental earthquakes: the Chengkong Earthquake 12/10/2003 (Mw 6.8), the 11/24/1951 Taitung Earthquake (Mw 7.1) and the 9/4/1935 Lutao Earthquake (Mw 7.0) respectively. Applying criteria to discriminating the different triggering mechanisms for turbidity current generation, we propose that earthquakes are the main triggering mechanisms. Dating and age modeling provided a part of the chronology of extreme events since the last 3000 years and allowed us to estimate return time for earthquakes Mw _ 6.8.This work represents a good starting-point for future investigations in order to better assess Holocene time series of extreme events.

No description available.