Ifsttar PhD subject

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Title : Sustainability and multi-scale characterization of the Soil Mixing material. Application tu dykes.

Main host Laboratory - Referent Advisor   -

Director of the main host Laboratory   -

PhD Speciality Géotechnique

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 so far confined to the improvement of compressible or high organic
content soils has become an attractive alternative to traditional methods for soil reinforcement, retaining walls (temporary or final), foundations and cut-off walls. But these new applications imply new requirements on the method as well as on the material.

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.

Nevertheless, crucial questions remains answered. Indeed, the research carried out at IFSTTAR demonstrate that the results achieved in laboratory do not directly translate very well to on site applications, with laboratory / in situ strength measurements ratios ranging from 1 to 3. Thus, studying the effect of the cinematic of the tool as well as the delivered mixing energy is compulsory. Laboratory and/or experimental pit results will therefore be systematically compared to on site results.

Concretely, the objectives of the thesis will be to study the influence of the material implementation method, and to propose, for in situ applications, a formulation of the material adapted to the geology encountered, as well as the tool used and the intended application.

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. Those are some of the potential problems encountered in urban, heterogeneous and anthropized soils.

This research will also rely on dikes reinforcement construction sites where IFSTTAR and CEREMA ensure the quality control. These sites will ensure the continuation of the database comparing on site and laboratory results, and will also allow the evaluation of the engineering properties of the material in a fresh or hardened state

Due to the nature of the works currently currently carried out on the dikes of the Loire, special attention will be paid to the erodibility of the Deep mixing material. This is an important issue for the validation of this technique. The first tests showed that these treated materials are indeed "erodible", with a rather high threshold stress, followed by a rather quick erosion. A parametric study on the material erodibility according to its composition, the cement content, etc. will complement the laboratory study to evaluate the aging of treated soils.

Thus, the student will be in charge of applying and developing tests, testing tools and experimental protocols to achieve the objectives described above with the help of the experience already gained in IFSTTAR and CEREMA. We will focus on the search for a test to characterize the durability of the material in a simple and reliable manner, the tests usually performed (including unconfined compression) for economic and practical reasons, being not necessarily the most suitable.

Finally, microstructural aspects and reaction mechanisms that occur during the hardening and aging of the material will be set against the associated mechanical behaviours. They will provide elements for a better understanding of the observed phenomena and to better take into account the heterogeneity of the material. 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).

Keywords : Dykes, soil reinforcement methods, Deep Soil Mixing, Hydraulic properties, mechanical properties

Main host Laboratory - Referent Advisor	-
Director of the main host Laboratory	-
PhD Speciality	Géotechnique
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 so far confined to the improvement of compressible or high organic content soils has become an attractive alternative to traditional methods for soil reinforcement, retaining walls (temporary or final), foundations and cut-off walls. But these new applications imply new requirements on the method as well as on the material. 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. Nevertheless, crucial questions remains answered. Indeed, the research carried out at IFSTTAR demonstrate that the results achieved in laboratory do not directly translate very well to on site applications, with laboratory / in situ strength measurements ratios ranging from 1 to 3. Thus, studying the effect of the cinematic of the tool as well as the delivered mixing energy is compulsory. Laboratory and/or experimental pit results will therefore be systematically compared to on site results. Concretely, the objectives of the thesis will be to study the influence of the material implementation method, and to propose, for in situ applications, a formulation of the material adapted to the geology encountered, as well as the tool used and the intended application. 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. Those are some of the potential problems encountered in urban, heterogeneous and anthropized soils. This research will also rely on dikes reinforcement construction sites where IFSTTAR and CEREMA ensure the quality control. These sites will ensure the continuation of the database comparing on site and laboratory results, and will also allow the evaluation of the engineering properties of the material in a fresh or hardened state Due to the nature of the works currently currently carried out on the dikes of the Loire, special attention will be paid to the erodibility of the Deep mixing material. This is an important issue for the validation of this technique. The first tests showed that these treated materials are indeed "erodible", with a rather high threshold stress, followed by a rather quick erosion. A parametric study on the material erodibility according to its composition, the cement content, etc. will complement the laboratory study to evaluate the aging of treated soils. Thus, the student will be in charge of applying and developing tests, testing tools and experimental protocols to achieve the objectives described above with the help of the experience already gained in IFSTTAR and CEREMA. We will focus on the search for a test to characterize the durability of the material in a simple and reliable manner, the tests usually performed (including unconfined compression) for economic and practical reasons, being not necessarily the most suitable. Finally, microstructural aspects and reaction mechanisms that occur during the hardening and aging of the material will be set against the associated mechanical behaviours. They will provide elements for a better understanding of the observed phenomena and to better take into account the heterogeneity of the material. 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).
Keywords :	Dykes, soil reinforcement methods, Deep Soil Mixing, Hydraulic properties, mechanical properties

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