Ifsttar PhD subject

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Title : Vulnerability of dykes to climate change: a multi-scale approach to studying the long-term behaviour of soil-cement mixtures and the effect of untreated soil inclusions during the mixing process

Main host Laboratory - Referent Advisor GERS - SRO  -  LE KOUBY Alain      tél. : +33 181668272

Director of the main host Laboratory REIFFSTECK Philippe  -

PhD Speciality Géotechnique, durabilité des ouvrages

Axis of the performance contract 3 - COP2017 - Planning and protecting regions

Main location Marne-la-Vallée

Doctoral affiliation UNIVERSITE PARIS-EST

PhD school SCIENCES, INGENIERIE ET ENVIRONNEMENT (SIE)

Planned PhD supervisor GUEDON Jeanne-Sylvine  -    -

Planned financing Contrat doctoral  - Ifsttar

Abstract

To remedy the risk of failure by internal erosion of the earthen levees of the Loire, cut-off walls have been built since 2013 using the Deep Soil Mixing (DSM) method, in particular with the Trenchmix® technique. This technique mechanically mixes the soil in place with cement and water. However, due to the varying properties of the soil to be treated and the mixing process, soil inclusions not mixed with cement appear in the soil-cement mixture. Given that this method is relatively recent, feedback on the hydromechanical performance and durability of these DSM materials is still very limited. The work carried out during this thesis therefore aims to explore their hydromechanical behaviour, in the presence of soil inclusions, in face of various hydraulic, mechanical and hydric solicitations. First, the description of DSM materials' microstructure, a key characteristic governing the material's performance and durability, allowed us to develop an approach for successfully predicting the water permeability of soil-cement matrix, which is crucial in the context of waterproofing work on earth levees. In addition, this thesis developed innovative 2D and 3D methods involving image analysis to realistically characterize the heterogeneities of DSM materials that are inevitably present on site, thus revealing their impact on the engineering properties of DSM materials coming from different sites in France, in particular the impact on water permeability, which had not been explored in the literature. Numerical simulations, using real inclusion mesostructures (generated using methods developed during this thesis) or artificial ones, have enabled 2D and 3D models to be set up to accurately assess the hydromechanical properties of DSM materials, while taking into account their specific features, such as the presence of unmixed soil inclusions and the interfacial transition zone (ITZ) that surrounds them. Indeed, microstructural investigations carried out on DSM samples from the site partially revealed the existence of an ITZ several hundred micrometers thick surrounding the soil inclusions, a less cemented zone with poorer mechanical and microstructural properties than the soil-cement matrix. Finally, this thesis explored the degradation of the properties of DSM materials under the environmental conditions to which they are exposed in the soil surrounding the DSM structure, by introducing a new durability test based on percolation cells.

Keywords : Loire levees, Deep Soil Mixing, Cut-off walls, Soil-cement mixtures, Water permeability, Microstructure, Accessible porosity, Soil inclusions, Image analysis, PIM; ITZ, Durability, Percolation, Mechanical properties, Numerical simulation,

Main host Laboratory - Referent Advisor	GERS - SRO - LE KOUBY Alain tél. : +33 181668272
Director of the main host Laboratory	REIFFSTECK Philippe -
PhD Speciality	Géotechnique, durabilité des ouvrages
Axis of the performance contract	3 - COP2017 - Planning and protecting regions
Main location	Marne-la-Vallée
Doctoral affiliation	UNIVERSITE PARIS-EST
PhD school	SCIENCES, INGENIERIE ET ENVIRONNEMENT (SIE)
Planned PhD supervisor	GUEDON Jeanne-Sylvine - -
Planned financing	Contrat doctoral - Ifsttar
Abstract To remedy the risk of failure by internal erosion of the earthen levees of the Loire, cut-off walls have been built since 2013 using the Deep Soil Mixing (DSM) method, in particular with the Trenchmix® technique. This technique mechanically mixes the soil in place with cement and water. However, due to the varying properties of the soil to be treated and the mixing process, soil inclusions not mixed with cement appear in the soil-cement mixture. Given that this method is relatively recent, feedback on the hydromechanical performance and durability of these DSM materials is still very limited. The work carried out during this thesis therefore aims to explore their hydromechanical behaviour, in the presence of soil inclusions, in face of various hydraulic, mechanical and hydric solicitations. First, the description of DSM materials' microstructure, a key characteristic governing the material's performance and durability, allowed us to develop an approach for successfully predicting the water permeability of soil-cement matrix, which is crucial in the context of waterproofing work on earth levees. In addition, this thesis developed innovative 2D and 3D methods involving image analysis to realistically characterize the heterogeneities of DSM materials that are inevitably present on site, thus revealing their impact on the engineering properties of DSM materials coming from different sites in France, in particular the impact on water permeability, which had not been explored in the literature. Numerical simulations, using real inclusion mesostructures (generated using methods developed during this thesis) or artificial ones, have enabled 2D and 3D models to be set up to accurately assess the hydromechanical properties of DSM materials, while taking into account their specific features, such as the presence of unmixed soil inclusions and the interfacial transition zone (ITZ) that surrounds them. Indeed, microstructural investigations carried out on DSM samples from the site partially revealed the existence of an ITZ several hundred micrometers thick surrounding the soil inclusions, a less cemented zone with poorer mechanical and microstructural properties than the soil-cement matrix. Finally, this thesis explored the degradation of the properties of DSM materials under the environmental conditions to which they are exposed in the soil surrounding the DSM structure, by introducing a new durability test based on percolation cells.
Keywords :	Loire levees, Deep Soil Mixing, Cut-off walls, Soil-cement mixtures, Water permeability, Microstructure, Accessible porosity, Soil inclusions, Image analysis, PIM; ITZ, Durability, Percolation, Mechanical properties, Numerical simulation,

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