Ifsttar PhD subject

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Title : Durability of soil mixing material : multiscale characterisation and modelling. Application to geotechnical structures.

Main host Laboratory - Referent Advisor   -

Director of the main host Laboratory   -

PhD Speciality Géotechnique, matériaux

Axis of the performance contract 2 - COP2013 - Constructing, deconstructing, preserving and modifying infrastructure efficiently and sustainably

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

The Soil-Mixing consists in mixing a hydraulic binder into the soil mechanically in order to improve its mechanical properties. Because of its economical as well as its sustainable advantages, this method has become an attractive alternative to traditional methods for soil reinforcement, retaining walls (temporary or final), foundations and cut-off walls.

The first studies carried out at IFSTTAR, jointly with Soletanche-Bachy, focused on the mechanical properties of the material and its workability in laboratory, and grazed the aspects linked to the different implementation methods and its durability. The research works that followed within the research project RUFEX (reuse of existing foundations) allowed us to deepen the study of the mechanical properties of the material as well as the mixing processes and to begin to tackle the durability issues. Some key questions remain answered especially the questions relative to the evolution of the physic-chemical properties of the soil-cement mixed materials due to chemical reactions that occurred during the curing time. The latter will then have a key role on its long term durability (role of elements in the soil such as sulphates or in the water that flows in the material).

The objective of the Ph.D. work will be to qualify and to quantify the soil mixing material durability under the effects of unfavourable conditions and/or environments potentially encountered in urban, heterogeneous and anthropized soils (ageing effect of soil mixing material). This research works enters the geomaterial field with a multi-scale approach coupling experimental and modelling works. As far as modelling is concerned, up to now, the soil mixing material has been studied following soil mechanics usual procedure, an approach “concrete” type could be considered and durability models developed for concrete will be applied with some modifications, if that is possible.

To understand and to model durability, a first step is to well understand the composition and the evolution of microstructure of the soil mixing material.

This objective requires at first step a formulation of the material adapted to the geology encountered (natural soil, chemical compounds in the ground that can alter the hardening reactions, especially clays), as well as the tool used and the intended application. In order to consider real cases, the study will focus on the study of three natural soil with a composition of 50% clay (kaolinite, illite, montmorillonite) and 50% sand.
A particular attention will be given to the mixing processes on site in order to simulate them in the laboratory. Indeed, the previous research works carried out at IFSTTAR showed that that the results achieved in laboratory do not directly translate very well to on site applications.
The heterogeneity of the mixed material could be considered as responsible for the observed differences between laboratory and in situ tests. In addition, the heterogeneity must probably have a key role on the durability of the material as it modifies the microstructure and consequently the permeability and then material exchange between soil mixing material and its environment. The results could be compared to in situ samples.

Regarding the durability of the material, different curing conditions will be tested in order to quantify their impact on the long term properties of the material. More particularly, the PhD student will study the impact of temperature, hydric or loading-unloading cycles, of the presence of pollutants such as diesel, sulfates, NaCl, etc, in the confining medium, and the influence of vibrations on the hardening and aging processus.

The degradation of the mechanical (Rc, E) and hydraulic (permeability) properties will the two main parameters that will be followed at the macro-scale. On a micro-scale, the evolution of the microstructure, the pores geometry as well as the mineralogy linked to the chemical reactions during curing time of 90, 180 or 365 days will be studied. From mineralogy composition, microstructural aspects and reaction mechanisms that occur during the hardening and aging of the material will be set against the associated mechanical behaviours. The PhD student will perform micro-structural analysis of porosity (air, water or by mercury intrusion porosity measurement), mineralogical analysis by X-ray diffraction (XRD) coupled to thermal analysis, and 2D observations with the environmental scanning electron microscope (ESEM) coupled if necessary with 3D analysis (gamma bench, X ray tomography).

Then, those mechanisms should be introduced in a predictive model for the durability of the material (developed model in “concrete “approach). The objective is to simulate the evolution of the physic-chemical properties of the materials and consequently their influence on the structures reinforced by this technique.

Keywords : Soil mixing, Sustainability, laboratory tests, in situ tests, multi-scale charactrisation, modelling

Main host Laboratory - Referent Advisor	-
Director of the main host Laboratory	-
PhD Speciality	Géotechnique, matériaux
Axis of the performance contract	2 - COP2013 - Constructing, deconstructing, preserving and modifying infrastructure efficiently and sustainably
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 The Soil-Mixing consists in mixing a hydraulic binder into the soil mechanically in order to improve its mechanical properties. Because of its economical as well as its sustainable advantages, this method has become an attractive alternative to traditional methods for soil reinforcement, retaining walls (temporary or final), foundations and cut-off walls. The first studies carried out at IFSTTAR, jointly with Soletanche-Bachy, focused on the mechanical properties of the material and its workability in laboratory, and grazed the aspects linked to the different implementation methods and its durability. The research works that followed within the research project RUFEX (reuse of existing foundations) allowed us to deepen the study of the mechanical properties of the material as well as the mixing processes and to begin to tackle the durability issues. Some key questions remain answered especially the questions relative to the evolution of the physic-chemical properties of the soil-cement mixed materials due to chemical reactions that occurred during the curing time. The latter will then have a key role on its long term durability (role of elements in the soil such as sulphates or in the water that flows in the material). The objective of the Ph.D. work will be to qualify and to quantify the soil mixing material durability under the effects of unfavourable conditions and/or environments potentially encountered in urban, heterogeneous and anthropized soils (ageing effect of soil mixing material). This research works enters the geomaterial field with a multi-scale approach coupling experimental and modelling works. As far as modelling is concerned, up to now, the soil mixing material has been studied following soil mechanics usual procedure, an approach “concrete” type could be considered and durability models developed for concrete will be applied with some modifications, if that is possible. To understand and to model durability, a first step is to well understand the composition and the evolution of microstructure of the soil mixing material. This objective requires at first step a formulation of the material adapted to the geology encountered (natural soil, chemical compounds in the ground that can alter the hardening reactions, especially clays), as well as the tool used and the intended application. In order to consider real cases, the study will focus on the study of three natural soil with a composition of 50% clay (kaolinite, illite, montmorillonite) and 50% sand. A particular attention will be given to the mixing processes on site in order to simulate them in the laboratory. Indeed, the previous research works carried out at IFSTTAR showed that that the results achieved in laboratory do not directly translate very well to on site applications. The heterogeneity of the mixed material could be considered as responsible for the observed differences between laboratory and in situ tests. In addition, the heterogeneity must probably have a key role on the durability of the material as it modifies the microstructure and consequently the permeability and then material exchange between soil mixing material and its environment. The results could be compared to in situ samples. Regarding the durability of the material, different curing conditions will be tested in order to quantify their impact on the long term properties of the material. More particularly, the PhD student will study the impact of temperature, hydric or loading-unloading cycles, of the presence of pollutants such as diesel, sulfates, NaCl, etc, in the confining medium, and the influence of vibrations on the hardening and aging processus. The degradation of the mechanical (Rc, E) and hydraulic (permeability) properties will the two main parameters that will be followed at the macro-scale. On a micro-scale, the evolution of the microstructure, the pores geometry as well as the mineralogy linked to the chemical reactions during curing time of 90, 180 or 365 days will be studied. From mineralogy composition, microstructural aspects and reaction mechanisms that occur during the hardening and aging of the material will be set against the associated mechanical behaviours. The PhD student will perform micro-structural analysis of porosity (air, water or by mercury intrusion porosity measurement), mineralogical analysis by X-ray diffraction (XRD) coupled to thermal analysis, and 2D observations with the environmental scanning electron microscope (ESEM) coupled if necessary with 3D analysis (gamma bench, X ray tomography). Then, those mechanisms should be introduced in a predictive model for the durability of the material (developed model in “concrete “approach). The objective is to simulate the evolution of the physic-chemical properties of the materials and consequently their influence on the structures reinforced by this technique.
Keywords :	Soil mixing, Sustainability, laboratory tests, in situ tests, multi-scale charactrisation, modelling

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