27 Juin – Thesis defense - Ghina Darwich

10 h Amphi PABA - building B5 (Talence campus)

Modeling of the geo-mechanical behavior of a feeder and evaluation of its performance in uncertain context.

The main objective of this thesis is to better understand the mechanisms that control the geo-mechanical behavior of a feeder in order to evaluate its performance in uncertain context. It is focused on four main points:
The first point is related to the soil-pipe interaction: In the case of buried pipelines, the soil has a decisive influence on the distribution of forces, displacements and consequently stresses and deformations of the pipe. The simplified Winkler model was chosen to represent the soil-pipe interaction in our study. This model, which is appropriate for geotechnical structures, has the advantage of requiring only one parameter (the coefficient of subgrade reaction) to characterize the response of the soil and the structure under loading.
The second point concerns numerical modeling: A 1D modeling in the longitudinal direction, which considers the pipe as a beam on elastic springs, is not enough to translate the "real" behavior of this pipe. 2D plane modeling, which considers the pipe only by its cross section, is also insufficient to represent "reality". 3D modeling remains very expensive vis-à-vis the calculation time in a probabilistic context. In our study, buried reinforced concrete pipes (with steel core) carrying drinking water under high pressure, are considered. To account for the effects of internal pressure and complex surface loading, and those of the soil (in both longitudinal and transverse directions), we use a "2.5D modeling". It consists in combining the geo-mechanical response in the cross section, obtained from a 2D model, and that of a simple 1D model, so as to obtain the profile of the settlements and that of the stresses in the section all along the pipe.
The third point concerns the various uncertainties and the spatial variability of the soil: The geo-mechanical behavior of the buried pipelines is tainted with random uncertainties - related to the intrinsic variability of the materials properties and their heterogeneity - and epistemic uncertainties - resulting from a partial or total ignorance of the studied mechanism. Longitudinal soil variability is considered through VanMarcke's Random Field Theory.
Finally, the fourth point is devoted to the application of the developed approach in the context of asset management of a water network by defining and quantifying criticality indicators allowing, through reliability analysis methods, evaluating the performance of the pipe vis-à-vis previously defined limit states related to inspection, maintenance or renewal.

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