Ifsttar PhD subject

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Title : Helical pile: physical and numerical modeling for marine geotechnical applications

Main host Laboratory - Referent Advisor GERS - CG  -  THOREL Luc      tél. : +33 240845808

Director of the main host Laboratory BLANC Matthieu  -

PhD Speciality Géotechnique. Géotechnique offshore.

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

Main location Nantes

Doctoral affiliation UNIVERSITE GUSTAVE EIFFEL

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

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

Planned PhD co-supervisor BLANC Matthieu  -  Université Gustave Eiffel  -  GERS - CG

Planned financing Contrat doctoral  - Université Gustave Eiffel

Abstract

BACKGROUND
With the development of renewable energies and the need to shift construction methods towards low environmental impact solutions (reduced carbon footprint and reuse/dismantling), there is now a booming market for a new type of foundation, e.g. for light structures (solar panel supports), individual houses or developments in protected or tourist/recreation areas (pontoons/bridges/barges) or for temporary structures. There are also other applications that are currently being explored, concerning anchors (operating in traction) in aquatic or marine environments e.g. floating wind turbines or solar panels on barges, or wave energy converters.

In the field of marine geotechnical engineering, the helical pile (HP) is mainly used to take compression and traction forces which consists of a metal shaft on which one or more helices are welded. For the reasons of easy to implement and removal after use, most often, these helical piles are of small dimensions (length of a few meters, diameter of the helix of about 30 cm). With the great advantages of HP, there are many examples of applications and implementations worldwide, mainly in North America and Brazil. However, HP is neither covered by the current Eurocodes nor by the specifications and standards. In addition, the stresses that the HP subjected are likely to cause a "fatigue" issue at the shaft-ground interface and in the vicinity of the helix, which can lead to long-term issues. Therefore, it is necessary to lead a comprehensive study of the HPs and to advance toward integration in the codes of design.

Finally, the 5th International Symposium on Frontiers in Offshore Geotechnics will be organized in June 2025 in Nantes, during which part of this work will be presented. This study will also enhance the communications and exchanges in the field of marine renewable energy

PROBLEMATICS
Six issues are proposed:
- what are the most appropriate methods of geotechnical investigation to build semi-empirical dimensioning guidelines (we will focus initially on the case of cohesionless soils and then move to the case of cohesive soils),

- what are the possible methods of installation for the HP, and what information can be obtained from this phase (torque and axial force measurements),

- what is the effect of an "imperfect" installation (different from a one-step drive-by-turn) on its subsequent behavior,

- how to predict the bearing capacity of these piles: we will distinguish the function in "anchorage" (traction), from the function in compression, but also the effect of cyclic loading,

- what is the influence of the geometrical configuration (one/two helices, for example) on the bearing capacity,

- which tests are proper to validate/control the long lift/behavior (short term/long term)?

The Ph.D. student will try to answer these questions and propose design guides that are adapted to the field of use of these piles and their associated stresses.

SCIENTIFIC CHALLENGES

Facing the current technological gaps, the Ph.D. student will first contribute to improving the knowledge to develop physical tests and reconnaissance protocols, as well as relevant control processes, allowing a better understanding of the behavior of the HP, and then to establish dimensioning rules for helical piles and anchors, based on existing methods.

He/She will also study the installation phase of these piles (experimental and numerical approaches with the consideration of large soil deformation) and attempt to correlate the measured parameters (force, torque, driving speed etc.) with the bearing capacity of the pile.

OUTLINE OF THIS STUDY

-Technical approach
The work will base on existing analytical and numerical developments as well as on experiments on centrifugal scale models:

Bibliographical study: a review of the scientific and technical literature, present the recent advances in the design of helical anchors, in compression, in tension under axial and non-axial loading, as well as under transverse loading, both under static and cyclic loads. The methodology developed in SOLCYP and the results of the WEAMEC REDENV-EOL project will be used. Study and summarize the numerical techniques for large deformation simulations and the impact of advanced constitutive models on numerical simulations.

Comprehensive analysis of the installation phase: measure the torque, as well as the vertical force required for driving HPs which is similar to the installation in situ; HPs of different geometries will be studied, with either an enlarged barrel at the top or multiple helices of the same or increasing diameter (for lower depths). Correlations with in situ tests will be established, in particular, in the cases of over-rotation or under-rotation (when the installation advancement is different from one-step drive-by-turn).

Study of the loading (vertical and inclined) directions: For one or two helical pile geometries the effect of monotonic loading inclination from horizontal to vertical (e.g. 0°, 30°, 45°, 60°) will be investigated. These tests will be performed in a centrifuge on loose and dense sand. Correlations with the soil investigation tests will be made to guide the design of HPs.

Study of the cyclic loadings: also carry out in a centrifuge, for one (or two) geometries of helical pile. In order to find the stability domains, for certain inclinations, configurations and cyclic loadings with different amplitudes will be tested. A loading test procedure adapted to this type of foundation will be proposed. In addition, multidirectional loading will also be performed.

Geometry of the helical pile: In the case of a multiple-helixes piles, the strategy of dimensioning the helixes will be studied, so that each helix brings a contribution to resistance, considering the redesign during installation.

Numerical simulation: In the first step, establish a framework for the simulation of large deformations, at this stage a simple behavior law can be used. In the second step, the transition from the simple behavior law to the advanced one is done by calibrating a series of model parameters and validating the numerical model with the experimental data. In the third step, the validated numerical model is used for parametric analysis and simulation of other cases, then the validated numerical model will be extended to the simulation of real situations.

-AVAILABILITY of resources

The centrifuge on the Nantes campus will allow parametric tests to be carried out on scale models. The numerical simulation of the installation and loading phase by Lagrangian Eulerian Coupling (CEL) will be carried out on the platform of Abaqus with access to high-performance computational units (e.g. at the Center of high-performance computation des Pays de la Loire). Another method of simulation of large deformations, the SPH method (Smoothed Particle Hydrodynamic) can be an alternative option or a good complementary to the CEL method.

-ORGANIZATION

This Ph.D study will be carried out in Université Gustave Eiffel - Campus de Nantes (ex IFSTTAR), at the centrifuge team - lab-CG. A short stay (4 weeks) in Brazil is also planned for the Ph.D candidate within the framework of this thesis, according to the work progress and experimental plans in connection with the University of Sao Paulo.

INNOVATIVE CHARACTER
This thesis relies on a multi-approach (physical and numerical modeling) to study the behavior of HP to establish dimensioning rules that are adapted to future practical applications, for example floating wind turbine anchors, etc.

EXPECTED RESULTS & VALORIZATION
With the aim of practical application, the results obtained through advanced Methodologies should contribute to the refinement of design rules for the transfer of the outcomes to the professional area. Publications in international journals and conferences will enhance the value of this study.

REFERENCES:

Cerfontaine, B., Brown, M., Knappett, J., Sharif, Y., & Davidson, C. (2022). Overflighting screw piles in sand, a detrimental effect?. 1081-1086. Paper presented at 20th International Conference on Soil Mechanics and Geotechnical Engineering, Sydney, Australia.
Reape D, Naughton P 2018 An experimental investigation of HP subject to inclined pullout loads Proc. Civil Engng Res.Ireland Conf. CERI-ITRN2018 Dublin. 235-240
Sakr M et al. 2016 Behavior of grouted single screw piles under inclined loads in sand. El.J. Geotech.Engng, 21(2), 571-591.
Schiavon JA 2016 Behav. helical anchors subjected to cyclic loadings. Ph.D. th., Uni Sao Paulo.
Schiavon JA, Tsuha C, Neel A, Thorel L 2018 Centrifuge modelling of a helical anchor under cyclic loading conditions in sand.https://doi.org/10.1680/jphmg.17.00054
Schiavon JA, Tsuha C, Thorel L 2017. Cyclic and post-cyclic monotonic response of a single-helix anchor in sand. Geot. lett.7, 11-17, http://dx.doi.org/10.1680/jgele.16.00100
Szymkiewicz F et al. 2020 Static Pile Load Test: International Practice Review And Discussion About The European & Japanese Standards, https://doi.org/10.21660/2020..66.9432
Tsuha C et al. 2007 Physical modelling of helical pile anchors. Int. J. Phys.Mod. Geotech. 7(4) 1-12.

Keywords : Helical pile, soil-structure interaction, physical modelling, numerical modelling

Main host Laboratory - Referent Advisor	GERS - CG - THOREL Luc tél. : +33 240845808
Director of the main host Laboratory	BLANC Matthieu -
PhD Speciality	Géotechnique. Géotechnique offshore.
Axis of the performance contract	2 - COP2017 - More efficient and resilient infrastructure
Main location	Nantes
Doctoral affiliation	UNIVERSITE GUSTAVE EIFFEL
PhD school	Sciences de l'Ingénierie et des Systèmes (SIS)
Planned PhD supervisor	THOREL Luc - Université Gustave Eiffel - GERS - CG
Planned PhD co-supervisor	BLANC Matthieu - Université Gustave Eiffel - GERS - CG
Planned financing	Contrat doctoral - Université Gustave Eiffel
Abstract BACKGROUND With the development of renewable energies and the need to shift construction methods towards low environmental impact solutions (reduced carbon footprint and reuse/dismantling), there is now a booming market for a new type of foundation, e.g. for light structures (solar panel supports), individual houses or developments in protected or tourist/recreation areas (pontoons/bridges/barges) or for temporary structures. There are also other applications that are currently being explored, concerning anchors (operating in traction) in aquatic or marine environments e.g. floating wind turbines or solar panels on barges, or wave energy converters. In the field of marine geotechnical engineering, the helical pile (HP) is mainly used to take compression and traction forces which consists of a metal shaft on which one or more helices are welded. For the reasons of easy to implement and removal after use, most often, these helical piles are of small dimensions (length of a few meters, diameter of the helix of about 30 cm). With the great advantages of HP, there are many examples of applications and implementations worldwide, mainly in North America and Brazil. However, HP is neither covered by the current Eurocodes nor by the specifications and standards. In addition, the stresses that the HP subjected are likely to cause a "fatigue" issue at the shaft-ground interface and in the vicinity of the helix, which can lead to long-term issues. Therefore, it is necessary to lead a comprehensive study of the HPs and to advance toward integration in the codes of design. Finally, the 5th International Symposium on Frontiers in Offshore Geotechnics will be organized in June 2025 in Nantes, during which part of this work will be presented. This study will also enhance the communications and exchanges in the field of marine renewable energy PROBLEMATICS Six issues are proposed: - what are the most appropriate methods of geotechnical investigation to build semi-empirical dimensioning guidelines (we will focus initially on the case of cohesionless soils and then move to the case of cohesive soils), - what are the possible methods of installation for the HP, and what information can be obtained from this phase (torque and axial force measurements), - what is the effect of an "imperfect" installation (different from a one-step drive-by-turn) on its subsequent behavior, - how to predict the bearing capacity of these piles: we will distinguish the function in "anchorage" (traction), from the function in compression, but also the effect of cyclic loading, - what is the influence of the geometrical configuration (one/two helices, for example) on the bearing capacity, - which tests are proper to validate/control the long lift/behavior (short term/long term)? The Ph.D. student will try to answer these questions and propose design guides that are adapted to the field of use of these piles and their associated stresses. SCIENTIFIC CHALLENGES Facing the current technological gaps, the Ph.D. student will first contribute to improving the knowledge to develop physical tests and reconnaissance protocols, as well as relevant control processes, allowing a better understanding of the behavior of the HP, and then to establish dimensioning rules for helical piles and anchors, based on existing methods. He/She will also study the installation phase of these piles (experimental and numerical approaches with the consideration of large soil deformation) and attempt to correlate the measured parameters (force, torque, driving speed etc.) with the bearing capacity of the pile. OUTLINE OF THIS STUDY -Technical approach The work will base on existing analytical and numerical developments as well as on experiments on centrifugal scale models: Bibliographical study: a review of the scientific and technical literature, present the recent advances in the design of helical anchors, in compression, in tension under axial and non-axial loading, as well as under transverse loading, both under static and cyclic loads. The methodology developed in SOLCYP and the results of the WEAMEC REDENV-EOL project will be used. Study and summarize the numerical techniques for large deformation simulations and the impact of advanced constitutive models on numerical simulations. Comprehensive analysis of the installation phase: measure the torque, as well as the vertical force required for driving HPs which is similar to the installation in situ; HPs of different geometries will be studied, with either an enlarged barrel at the top or multiple helices of the same or increasing diameter (for lower depths). Correlations with in situ tests will be established, in particular, in the cases of over-rotation or under-rotation (when the installation advancement is different from one-step drive-by-turn). Study of the loading (vertical and inclined) directions: For one or two helical pile geometries the effect of monotonic loading inclination from horizontal to vertical (e.g. 0°, 30°, 45°, 60°) will be investigated. These tests will be performed in a centrifuge on loose and dense sand. Correlations with the soil investigation tests will be made to guide the design of HPs. Study of the cyclic loadings: also carry out in a centrifuge, for one (or two) geometries of helical pile. In order to find the stability domains, for certain inclinations, configurations and cyclic loadings with different amplitudes will be tested. A loading test procedure adapted to this type of foundation will be proposed. In addition, multidirectional loading will also be performed. Geometry of the helical pile: In the case of a multiple-helixes piles, the strategy of dimensioning the helixes will be studied, so that each helix brings a contribution to resistance, considering the redesign during installation. Numerical simulation: In the first step, establish a framework for the simulation of large deformations, at this stage a simple behavior law can be used. In the second step, the transition from the simple behavior law to the advanced one is done by calibrating a series of model parameters and validating the numerical model with the experimental data. In the third step, the validated numerical model is used for parametric analysis and simulation of other cases, then the validated numerical model will be extended to the simulation of real situations. -AVAILABILITY of resources The centrifuge on the Nantes campus will allow parametric tests to be carried out on scale models. The numerical simulation of the installation and loading phase by Lagrangian Eulerian Coupling (CEL) will be carried out on the platform of Abaqus with access to high-performance computational units (e.g. at the Center of high-performance computation des Pays de la Loire). Another method of simulation of large deformations, the SPH method (Smoothed Particle Hydrodynamic) can be an alternative option or a good complementary to the CEL method. -ORGANIZATION This Ph.D study will be carried out in Université Gustave Eiffel - Campus de Nantes (ex IFSTTAR), at the centrifuge team - lab-CG. A short stay (4 weeks) in Brazil is also planned for the Ph.D candidate within the framework of this thesis, according to the work progress and experimental plans in connection with the University of Sao Paulo. INNOVATIVE CHARACTER This thesis relies on a multi-approach (physical and numerical modeling) to study the behavior of HP to establish dimensioning rules that are adapted to future practical applications, for example floating wind turbine anchors, etc. EXPECTED RESULTS & VALORIZATION With the aim of practical application, the results obtained through advanced Methodologies should contribute to the refinement of design rules for the transfer of the outcomes to the professional area. Publications in international journals and conferences will enhance the value of this study. REFERENCES: Cerfontaine, B., Brown, M., Knappett, J., Sharif, Y., & Davidson, C. (2022). Overflighting screw piles in sand, a detrimental effect?. 1081-1086. Paper presented at 20th International Conference on Soil Mechanics and Geotechnical Engineering, Sydney, Australia. Reape D, Naughton P 2018 An experimental investigation of HP subject to inclined pullout loads Proc. Civil Engng Res.Ireland Conf. CERI-ITRN2018 Dublin. 235-240 Sakr M et al. 2016 Behavior of grouted single screw piles under inclined loads in sand. El.J. Geotech.Engng, 21(2), 571-591. Schiavon JA 2016 Behav. helical anchors subjected to cyclic loadings. Ph.D. th., Uni Sao Paulo. Schiavon JA, Tsuha C, Neel A, Thorel L 2018 Centrifuge modelling of a helical anchor under cyclic loading conditions in sand.https://doi.org/10.1680/jphmg.17.00054 Schiavon JA, Tsuha C, Thorel L 2017. Cyclic and post-cyclic monotonic response of a single-helix anchor in sand. Geot. lett.7, 11-17, http://dx.doi.org/10.1680/jgele.16.00100 Szymkiewicz F et al. 2020 Static Pile Load Test: International Practice Review And Discussion About The European & Japanese Standards, https://doi.org/10.21660/2020..66.9432 Tsuha C et al. 2007 Physical modelling of helical pile anchors. Int. J. Phys.Mod. Geotech. 7(4) 1-12.
Keywords :	Helical pile, soil-structure interaction, physical modelling, numerical modelling

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