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Méthodes d'ingénierie pour l'étude du risque de liquéfaction et du tassement sous séisme

Abstract : The liquefaction of saturated soils during earthquakes is one of the most important problems facing engineers. There has hardly been a major earthquake without at least some cases of liquefaction. Settlements, tilting of buildings, lateral flows, sand boilings and slope instabilities have been some of its manifestations. The seismic design of nuclear power plants and other critical facilities systematically includes a liquefaction risk assessment.In this context, fully empirical methods are commonly used in engineering. These approaches provide design margins and limitations of use. To exploit these margins in a re-evaluation situation, we use advanced nonlinear transient calculations in which the soil behavior must be finely modeled to highlight the pore-water pressure build-up. These last calculations are expensive in terms of time and numerical skills. The objective of this research work is to reduce the conservatisms in force when using the simplified method without necessarily implementing the most sophisticated methods from the outset. To this end, we propose a method of intermediate complexity that broadens the applicability of semi-empirical models for a more detailed analysis of seismic risk.First, starting from a conventional equivalent linear calculation, a new approach for taking into account excess pore pressure is proposed under the name "X-ELM". The behavioral model employed is based on the relationship between pore pressure and plastic volumetric deformations. The new "X-ELM" approach is used to model soil response in the city of Urayasu,Japan during the Tohoku earthquake (Mw = 9.0). The model has been applied to twelve different soil profiles. The study of these cases makes possible the validation of themodel by comparing the results of the calculations with the observations in situ. The model can therefore be considered as a reliable tool for the prediction of liquefaction triggering of saturated soils.Then, a prediction tool was designed based on random process approximations, the basic mechanical properties of the soil and the characteristics of the seismic loading. In addition to its magnitude, an important feature of the input seismic signal is its duration which can lead to strong nonlinearities and an extended liquefaction state. Considering the strong phase duration, the response spectrum, the natural frequency of the model and the characteristics of soil compaction, this tool provides fast estimations of the rate of pore pressure build-up and pre-liquefaction settlement without having to perform transient calculations.Finally, a 2D dam model is studied, by examining the influence of excess pore pressure and that of the shear strains on the seismic response of the structure. An equivalent linear computation adapted to two-dimensional situations is elaborated and the predictor is used to evaluate pore pressure increase. Simulation results are compared with in situ, piezometric and accelerometric observations.In conclusion, this research work provides methods and tools of numerical computation that are efficient and accessible to engineers for the seismic evaluation of soil profiles and earth structures such as dikes or dams
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Ziad Kteich. Méthodes d'ingénierie pour l'étude du risque de liquéfaction et du tassement sous séisme. Matériaux composites et construction. Université Paris-Est, 2018. Français. ⟨NNT : 2018PESC1109⟩. ⟨tel-02132473⟩

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