Ifsttar PhD subject

French version
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Title : Behavior of rigid inclusions in seismic zones: from characterization to modeling

Main host Laboratory - Referent Advisor   -

Director of the main host Laboratory   -

PhD Speciality sismique

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

Main location Marne-la-Vallée

Doctoral affiliation UNIVERSITE DE MARNE-LA-VALLEE

PhD school SCIENCES, INGENIERIE ET ENVIRONNEMENT (SIE)

Planned PhD supervisor LENTI Luca  -  Université Gustave Eiffel  -  GERS - SRO

Planned financing Contrat doctoral  - Ifsttar

Abstract

The reinforcement of the soil by rigid elements of foundation (vertical inclusions) has developed in France since the 1990s. It makes it possible to significantly reduce the settlements of the foundation masses under the loads applied, while improving their punching stability. This reinforcing process consists in creating a composite solid mass in which the charges are distributed between the ground itself and the rigid inclusions. The national project ASIRI (Improvement of Soils by Rigid Inclusions - www.asiri.irex.asso.fr) has focused on defining, conducting and interpreting the experiments and modeling needed to understand the mechanisms by which this innovative foundation system works. Observations and modeling were thus confronted for a wide variety of works. New perspectives have emerged, they show how the combination of such different elements (soil, inclusions, granular rafts) leads to an efficient and economical composite system. At the end of this project, recommendations were published [(AFD, CFMS), 2012]. the conclusions of the project have revealed the need to study certain subjects that have not been dealt with the ASIRI NP and in particular the behavior of soil massifs reinforced by rigid inclusions under seismic loading or, more generally, under dynamic solicitations.

Rules currently exist [AFPS, 2012] to dimension the actual structures and infrastructures subject to such solicitations. However, for complex projects, relevant questions remain as evidenced by the actions undertaken in France to respond to them: the definition of a second National Project ASIRI+ and a ANR ASIRI+ SDS. The MouvGS team is involved in these projects and wishes to develop its skills in this area. A Master 2 internship on the subject was supervised by Nathalie Dufour in 2018.
The structures studied in the ASIRI National Project [IREX, 2012] were mainly embankments, pavements usually subjected to static and uniform vertical stresses. The good understanding of the behavior of the actual structures whose foundations are on rigid inclusions and that, until now were considered for static loads, leads to this technique being proposed for other purposes outside the initial scope of the ASIRI recommendations. In particular, in seismic context, the structures are subjected to complex stresses involving dynamic and non-vertical forces only.

The doctoral student will be confronted with the problem of the modeling of soil mass reinforced by rigid inclusions taking into account the different interactions that develop between the foundation proper, the raft, the inclusions, the soil as well as the superstructure.

In addition, the analysis of the behavior of reinforced soil mass leads to consider both the stresses of inertial origin (transmitted to the raft and to the ground reinforced next to the foundation) and those of kinematic origin (interaction of inclusions with the soil seismic movement in free field conditions). The work of the PhD student will lead to a realistic evaluation of the demands developed under earthquake in the inclusions, to follow an important stake of the dimensioning of the inclusions in these cases.

The rules currently available in [(AFPS, CFMS), 2012] are suitable for most small-scale projects. However, no recommendation defines how the Reinforced soil/structure interaction should be taken into account. The doctoral student will be confronted with this problem and will be able to rely on the supervisory team and on the structural engineers of Cerema and Ifsttar to be guided and to solve it.

The work will be sequenced in 3 parts. The first part will be devoted to the dynamic characterization of soils in the laboratory via different types of tests. The aim will be to define the reduction curves of the shear modulus and increase of the damping with the induced deformation for a set of samples taken on site and/or reconstituted in the laboratory. The idea is to consider/reconstruct samples according to the level of plasticity, and/or to other parameters, to define the expected variation on G and E moduli for different levels of the maximum induced strain. The soils considered will be the ones whose foundations, in the actual practice, should be reinforced by rigid inclusions. The second part of the work will be dedicated to the reproduction of stress-strain curves obtained during cyclic laboratory tests, using numerical finite element or finite differences modeling (FEM, DEM). This phase is necessary to firstly calibrate the rheological moduli that will be taken into account in the numerical modeling carried out from the small-scale experiment (in centrifuge) and, on the other hand, for the modeling of the case study concerning the site of the Prefecture of Nice. The third part of the work will concern the numerical modeling of small-scale experiments and the Nice Prefecture site by considering the soil properties and calibration obtained in the first two parts of this study as well as the seismic signals characterizing the hazard of seismic zone 4. More specifically, the rheological models that will be considered will take into account the possible generation of interstitial pressures responsible for the reduction of soil shear strength and consequently the induced soil compaction. These effects will be studied for different seismic signals (frequency content, PGA, etc.) associated with the same class of seismicity.

Keywords : rigid elements, dynamic behaviour, modeling

Main host Laboratory - Referent Advisor	-
Director of the main host Laboratory	-
PhD Speciality	sismique
Axis of the performance contract	2 - COP2017 - More efficient and resilient infrastructure
Main location	Marne-la-Vallée
Doctoral affiliation	UNIVERSITE DE MARNE-LA-VALLEE
PhD school	SCIENCES, INGENIERIE ET ENVIRONNEMENT (SIE)
Planned PhD supervisor	LENTI Luca - Université Gustave Eiffel - GERS - SRO
Planned financing	Contrat doctoral - Ifsttar
Abstract The reinforcement of the soil by rigid elements of foundation (vertical inclusions) has developed in France since the 1990s. It makes it possible to significantly reduce the settlements of the foundation masses under the loads applied, while improving their punching stability. This reinforcing process consists in creating a composite solid mass in which the charges are distributed between the ground itself and the rigid inclusions. The national project ASIRI (Improvement of Soils by Rigid Inclusions - www.asiri.irex.asso.fr) has focused on defining, conducting and interpreting the experiments and modeling needed to understand the mechanisms by which this innovative foundation system works. Observations and modeling were thus confronted for a wide variety of works. New perspectives have emerged, they show how the combination of such different elements (soil, inclusions, granular rafts) leads to an efficient and economical composite system. At the end of this project, recommendations were published [(AFD, CFMS), 2012]. the conclusions of the project have revealed the need to study certain subjects that have not been dealt with the ASIRI NP and in particular the behavior of soil massifs reinforced by rigid inclusions under seismic loading or, more generally, under dynamic solicitations. Rules currently exist [AFPS, 2012] to dimension the actual structures and infrastructures subject to such solicitations. However, for complex projects, relevant questions remain as evidenced by the actions undertaken in France to respond to them: the definition of a second National Project ASIRI+ and a ANR ASIRI+ SDS. The MouvGS team is involved in these projects and wishes to develop its skills in this area. A Master 2 internship on the subject was supervised by Nathalie Dufour in 2018. The structures studied in the ASIRI National Project [IREX, 2012] were mainly embankments, pavements usually subjected to static and uniform vertical stresses. The good understanding of the behavior of the actual structures whose foundations are on rigid inclusions and that, until now were considered for static loads, leads to this technique being proposed for other purposes outside the initial scope of the ASIRI recommendations. In particular, in seismic context, the structures are subjected to complex stresses involving dynamic and non-vertical forces only. The doctoral student will be confronted with the problem of the modeling of soil mass reinforced by rigid inclusions taking into account the different interactions that develop between the foundation proper, the raft, the inclusions, the soil as well as the superstructure. In addition, the analysis of the behavior of reinforced soil mass leads to consider both the stresses of inertial origin (transmitted to the raft and to the ground reinforced next to the foundation) and those of kinematic origin (interaction of inclusions with the soil seismic movement in free field conditions). The work of the PhD student will lead to a realistic evaluation of the demands developed under earthquake in the inclusions, to follow an important stake of the dimensioning of the inclusions in these cases. The rules currently available in [(AFPS, CFMS), 2012] are suitable for most small-scale projects. However, no recommendation defines how the Reinforced soil/structure interaction should be taken into account. The doctoral student will be confronted with this problem and will be able to rely on the supervisory team and on the structural engineers of Cerema and Ifsttar to be guided and to solve it. The work will be sequenced in 3 parts. The first part will be devoted to the dynamic characterization of soils in the laboratory via different types of tests. The aim will be to define the reduction curves of the shear modulus and increase of the damping with the induced deformation for a set of samples taken on site and/or reconstituted in the laboratory. The idea is to consider/reconstruct samples according to the level of plasticity, and/or to other parameters, to define the expected variation on G and E moduli for different levels of the maximum induced strain. The soils considered will be the ones whose foundations, in the actual practice, should be reinforced by rigid inclusions. The second part of the work will be dedicated to the reproduction of stress-strain curves obtained during cyclic laboratory tests, using numerical finite element or finite differences modeling (FEM, DEM). This phase is necessary to firstly calibrate the rheological moduli that will be taken into account in the numerical modeling carried out from the small-scale experiment (in centrifuge) and, on the other hand, for the modeling of the case study concerning the site of the Prefecture of Nice. The third part of the work will concern the numerical modeling of small-scale experiments and the Nice Prefecture site by considering the soil properties and calibration obtained in the first two parts of this study as well as the seismic signals characterizing the hazard of seismic zone 4. More specifically, the rheological models that will be considered will take into account the possible generation of interstitial pressures responsible for the reduction of soil shear strength and consequently the induced soil compaction. These effects will be studied for different seismic signals (frequency content, PGA, etc.) associated with the same class of seismicity.
Keywords :	rigid elements, dynamic behaviour, modeling

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