Ifsttar PhD subject

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Title : Use of cement modified soils as materials for load transfer platforms resting on rigid inclusions

Main host Laboratory - Referent Advisor   -

Director of the main host Laboratory   -

PhD Speciality Mécanique - Géotechnique

Axis of the performance contract 2 - COP2017 - More efficient and resilient infrastructure

Main location Nantes

Doctoral affiliation UNIVERSITE DE NANTES

PhD school Sciences de l'Ingénierie et des Systèmes (SIS)

Planned PhD supervisor THOREL Luc  -  Université Gustave Eiffel  -  GERS - CG

Planned financing Contrat doctoral  - Ifsttar

Abstract

During the construction of civil engineering structures or infrastructures, in situ soils, can be compressible. This causes serious problems regarding the stability and durability of the structures. To overcome these difficulties and reduce settlements, in accordance with the principles of sustainable development, the method of strengthening soils by rigid vertical inclusions has been developed. This process, which is fast in its implementation, makes it possible to keep the soil in place, which constitutes an environmental, societal and economic advantage [1].
The principle is the following; the load generated by the structure is transmitted to the rigid bedrock via a load transfer platform (LTP) resting on a matrix of rigid inclusions that pass through the compressible soil [2].
LTP concentrates the loads on the inclusions, and thus, reduces and homogenizes the surface settlements. It is usually made of a good quality filler material such as a gravel or ballast. However, this option is very expensive in the sense of sustainable development. To limit these costs, the use of in-situ soils mixed with a few percent of hydraulic binders can be an alternative [3].
Nevertheless, currently, and as is often the case with cement modified/treated soils (CMS), this approach remains empirical [4-5], and professional users highlight the need to define design rules based on material characterization tests relevant with application cases [6].
In this context, the main objectives of the proposed thesis are:
- Define the mechanical characteristics of CMS. Specifically, we will focus on defining constitutive laws of materials under static and dynamic solicitations in order to put them into numerical models.
- Define and carry out a small-scale structural test, in order to simulate the behaviour of a LTP made with CMS under dynamic loadings. This test, coupled with constitutive laws, will highlight the mechanical parameters essential for the structures design. A specific attention will be paid on the preparation method of materials that should be as close as possible of the in-situ conditions.
Moreover, in the field of CMS, it has been shown that the conventional parameters of materials characterization do not give enough data to foresee their performance [7]. This is why, in parallel of this applied mechanical approach, a fundamental mineralogical approach will be initiated. Specifically, based on basic chemical analysis techniques (spectroscopy, energy dispersive analysis), and crystallographic analysis techniques (X-ray diffraction), the aim of this approach will be to allow a fine description of soils and binders employed. The main goal will be to highlight the time-dependant parameters essential to model the interactions between the phases and the sorption or complexation reactions on the solid surfaces (titrimetric method). These studies are a necessary prelude to the microscopic description (molecular modelings) of mechanical observations on a macroscopic scale.
At the end of this work, the publication of at least two articles in the referenced international scientific literature is awaited.
We are seeking an outstanding and motivated candidate with a Master’s degree in a relevant mechanics or materials science discipline. He will have to demonstrate motivation, scientific curiosity and strong interest for mechanical assemblies. A strong command of the English language (written and spoken) is necessary.

Keywords : rigid inclusions; cement modified/treated soils; mechanical performances; mineralogical characterisation

Main host Laboratory - Referent Advisor	-
Director of the main host Laboratory	-
PhD Speciality	Mécanique - Géotechnique
Axis of the performance contract	2 - COP2017 - More efficient and resilient infrastructure
Main location	Nantes
Doctoral affiliation	UNIVERSITE DE NANTES
PhD school	Sciences de l'Ingénierie et des Systèmes (SIS)
Planned PhD supervisor	THOREL Luc - Université Gustave Eiffel - GERS - CG
Planned financing	Contrat doctoral - Ifsttar
Abstract During the construction of civil engineering structures or infrastructures, in situ soils, can be compressible. This causes serious problems regarding the stability and durability of the structures. To overcome these difficulties and reduce settlements, in accordance with the principles of sustainable development, the method of strengthening soils by rigid vertical inclusions has been developed. This process, which is fast in its implementation, makes it possible to keep the soil in place, which constitutes an environmental, societal and economic advantage [1]. The principle is the following; the load generated by the structure is transmitted to the rigid bedrock via a load transfer platform (LTP) resting on a matrix of rigid inclusions that pass through the compressible soil [2]. LTP concentrates the loads on the inclusions, and thus, reduces and homogenizes the surface settlements. It is usually made of a good quality filler material such as a gravel or ballast. However, this option is very expensive in the sense of sustainable development. To limit these costs, the use of in-situ soils mixed with a few percent of hydraulic binders can be an alternative [3]. Nevertheless, currently, and as is often the case with cement modified/treated soils (CMS), this approach remains empirical [4-5], and professional users highlight the need to define design rules based on material characterization tests relevant with application cases [6]. In this context, the main objectives of the proposed thesis are: - Define the mechanical characteristics of CMS. Specifically, we will focus on defining constitutive laws of materials under static and dynamic solicitations in order to put them into numerical models. - Define and carry out a small-scale structural test, in order to simulate the behaviour of a LTP made with CMS under dynamic loadings. This test, coupled with constitutive laws, will highlight the mechanical parameters essential for the structures design. A specific attention will be paid on the preparation method of materials that should be as close as possible of the in-situ conditions. Moreover, in the field of CMS, it has been shown that the conventional parameters of materials characterization do not give enough data to foresee their performance [7]. This is why, in parallel of this applied mechanical approach, a fundamental mineralogical approach will be initiated. Specifically, based on basic chemical analysis techniques (spectroscopy, energy dispersive analysis), and crystallographic analysis techniques (X-ray diffraction), the aim of this approach will be to allow a fine description of soils and binders employed. The main goal will be to highlight the time-dependant parameters essential to model the interactions between the phases and the sorption or complexation reactions on the solid surfaces (titrimetric method). These studies are a necessary prelude to the microscopic description (molecular modelings) of mechanical observations on a macroscopic scale. At the end of this work, the publication of at least two articles in the referenced international scientific literature is awaited. We are seeking an outstanding and motivated candidate with a Master’s degree in a relevant mechanics or materials science discipline. He will have to demonstrate motivation, scientific curiosity and strong interest for mechanical assemblies. A strong command of the English language (written and spoken) is necessary.
Keywords :	rigid inclusions; cement modified/treated soils; mechanical performances; mineralogical characterisation

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