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Title : Soil reinforcement by rigid inclusions: effect of complex surface loading

Main host Laboratory - Referent Advisor GERS - CG  -  THOREL Luc      tél. : +33 240845808 
Director of the main host Laboratory BLANC Matthieu  -  
Laboratory 2 - Referent Advisor GERS - SRO  -  BADINIER Thibault  -    -  tél. : +33 181668094
PhD Speciality Géotechnique
Axis of the performance contract 2 - COP2017 - More efficient and resilient infrastructure
Main location Nantes
Doctoral affiliation UNIVERSITE DE NANTES
PhD school SCIENCES POUR L'INGENIEUR
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

The reinforcement of soft soils by vertical Rigid Inclusions (RI) is a technique now quite widespread in the case of embankments and slabs, considered under loads distributed vertically. The surface loads are, through the Load Transfer Platform (LTP), concentrated mainly towards the RI, reducing the forces transmitted to the compressible soil, and consequently the overall settlements.
However, other loading and geometry configurations, not studied during the ASIRI National Project (2012), but covered by the ASIRI+ Project (2019-2023) are encountered in practice. This study will focus on two aspects:
1) The shalllow foundation under combined (inclined, eccentric) loading, monotonic and non-monotonic;
2) Mobile surface loading (rolling load) in the particular case of low LTP thicknesses.

The case of superficial foundation is frequently encountered in practice. The size of the foundation means that it only solicits a few RIs, and not a large number, as is the case for slabs, for which homogenization techniques are possible. Here, the transfer is local and it is necessary to better understand the operation in order to optimize the design.
Few references are available in the literature on experimental results in situ. Some tests were carried out in centrifuges (Blanc et al., 2014; Rault et al. 2018), but mainly under centered vertical load or centered inclined load but without generating any moment. In terms of numerical simulation, references are rare, probably also because simulation is necessarily 3D. The ASIRI recommendations (2012) propose to use simplified models, respectively MV1 to MV3 for a purely vertical loading and MH1 to MH3 for a lateral loading (inclined force or moment).
During this doctoral work, several approaches will be used to observe, better understand and analyze the mechanisms in order to possibly develop design methods. To do this, within the framework of the PN ASIRI+ (https://asiriplus.fr), the doctoral student may be involved, depending on the planning of this action, in the realization and interpretation of experiments in real scale (intensity of the effort respected, instrumentation of real size, phasing of the installation well taken into account, etc.), experiments on centrifuged model models (parametric study on inclination, load eccentricity, LTP thickness, foundation flexural rigidity, nature of stress – monotonous or cyclic – combination of H and V forces – “swipe tests”, whether or not there is a reinforcement geosynthetic at the base of TPC, etc.) and will have to manipulate the analytical models as well as the numerical models.
For rolling load, it is a question of identifying the validity domain of the static approaches identified in the previous part, and then proposing possible further recommendations.
A two-dimensional configuration will be studied initially, both from an experimental and theoretical point of view. The parameters studied will first be the effect of the load intensity on the load transfer for an identical LTP thickness, then several LTP thicknesses will be tested for a load similar to a service load. The nature of the LTP (with or without cohesion) will also be studied, as well as the geosynthetic reinforcement at the base of the LTP. The cyclic movement of the rolling load will consist of two variants: mono-directional loading; alternating loading.
Then, a three-dimensional configuration will be implemented with the teleoperator robot embarked in the centrifuge of the Gustave Eiffel University, on which a tool simulating an axle will be adapted. Several questions will be addressed, according to a program including the results of the 2D tests:
- Effect of the position of the rolling zone relative to the IR rows (in vertical alignement of one row or between two rows), for a monodirectional or bidirectional loading;
- Effect of a random area of rolling with movements in the horizontal plane.

Finally, a third way will be undertaken: numerical modelling of the phenomena observed in centrifuges. Depending on the configuration studied (2D or 3D, small or large deformations, etc.), the doctoral student will have to take in hand an adequate finite element code. For example, to numerically model the punching of LTP by a RI, the new methods combining the Eulerian and Lagrangian visions with possible remeshing will be compared with more traditional methods. Similarly, the use of constitutive laws that take into account post-rupture softening will be studied.
Thus, the doctoral student will have calibrated a numerical model based on data with well-controlled boundary conditions in a centrifuge that he will then be able to compare with the in-situ data obtained in the framework of PN ASIRi+. In the case of mobile load, the stationary algorithms method makes it possible to get rid of the decomposition of the movement and thus to accelerate the process of calculation by finite element (resolution of the movement with a reduced number of steps, or even a single one). For mechanisms involving large deformations, recent developments associated with finite elements can be taken advantage of. The Particles Finites Elements Methods (PFEM) adopt an intermediate posture between Eulerian and Lagrangian approaches. The models are discretized according to finite element meshes and the equations are solved according to classical methods. However, the particle medium is mobilized freely, each particle then carrying its properties, resulting in a deformed medium which is then re-meshed to continue the calculation. These methods then allow the modeling of significant deformation using a formalism of finite element associated with small deformations (Cremonesi et al., 2020 )

After a literature review on the different approaches and on the object itself of the isolated surface load on RI-reinforced soft soil, the work will have several components:
- Critical analysis of existing analytical methods, their limitations and their range of validity. These methods will be compared with existing experimental results.
- Detailed analysis of full-scale experiments carried out under the NP (according to implementation schedule).
- Experimental parametric study in centrifuge.
- Numerical modeling of a studied case and parametric study around it.

This work will mainly be carried out in Nantes, but may require travelling, depending on the projects in progress.


Bibliography
Almeida M.S.S., Fagundes D.D.F., Thorel L., Blanc M. 2020 Geosynthetic-Reinforced piled embankments: numerical, analytical and centrifuge modelling. Geos Int doi.org/10.1680/jgein.19.00011 27(3), 301–314.
ASIRi (2012). Recommandations pour la conception, le calcul, l’execution et le controle des ouvrages sur sols ameliores par inclusions rigides verticales, Presses Ponts. IREX, Paris.
Blanc M., Rault G., Thorel L., Almeida M. 2013. Centrifuge investigation of load transfer mechanisms in a granular mattress above a rigid inclusions network. Geotext Geom.36,92-105./dx.doi.org/10.1016/j.geotexmem.2012.12.001.
Blanc M., Thorel L, Girout R. Almeida M. 2014 Geosynthetic reinforcement of a granular load transfer platform above rigid inclusions: comparison between centrifuge testing and analytical modelling. Geos int,21, 37-52. DOI: 10.1680/gein.13.00033
Blanc M., Thorel L., Macé D., Neel A., Rault G. 2014 Raft above rigid inclusions - Centrifuge investigation of complex loading. 8th ICPMG Int. Conf. Phys Mod Geot, Perth 14-17jan 591-596.
Cremonesi et al. 2020 SotA PFEM Arch. Comp. Meth. Eng 27(5), 1709-1735
Fagundes D.D.F. Girout R., Almeida M.S.S, Blanc M., Thorel L., 2015 Behaviour of piled embankments without reinforcement Geotechnical Engineering Vol.168, GE6, 514-525. doi.org/10.1680/geng.14.00155
Fagundes D.D.F., Almeida M., Thorel L., Blanc M. 2017 Load transfer mechanism and deformation of reinforced piled embankments. Geotext Geom. doi.org/10.1016/j.geotexmem.2016.11.002, vol. 45(2), 1-10.
Girout R., Blanc M., Dias D., Thorel L. 2014. Numerical investigation of a load transfer platform above a piled network reinforced with geosynthetic – Validation on centrifuge tests. Geotext. Geom.. 42, 525-539. DOI 10.1016/j.geotexmem.2014.07.012.
Girout R., Blanc M., Thorel L., Da Fagundes D., Almeida M.S.S. 2016 Arching and deformation in a piled embankment: centrifuge tests compared to analytical calculations. J. Geot Geoenv Eng DOI: 10.1061/(ASCE)GT.1943-5606.0001557. 142(12)
Girout R., Blanc M., Thorel L.,Dias D. 2018 Geosynthetic reinforcement on piled-supported embankment Geos. Int. 25, 1 37–49. doi.org/10.1680/jgein.17.00032.
Kearsley E.P. et al. 2014. Centrifuge modelling of Ultra Thin Continuously Reinforced Concrete Pavements. Proc. Phys Mod Geot Gaudin & White (Eds) Taylor & Francis
Lukiantchuki JA et al. 2018. Centrifuge modelling of traffic simulation on a construction waste layer.Int J Phys Mod Geot 18(6): 290–300, doi.org/10.1680/jphmg.17.00012
Rakitin B., Xu M. 2014. Centrifuge modeling of large-diameter underground pipes subjected to heavy traffic loads Can. Geot J. 51: 353–368 dx.doi.org/10.1139/cgj-2013-0253.
Rault G., Blanc M., Macé D., Thorel L., 2012. Semelle sur inclusions rigides soumise à des sollicitations inclinées : expérimentations en centrifugeuse. Rapport ASIRI. 169p.
Simon B., Briançon L., Thorel L., 2020. Amélioration des sols par inclusions rigides : le rôle des géosynthétiques dans la plateforme de transfert de charge. RFG 162, 1.doi.org/10.1051/geotech/2020003

Keywords : Rigid Inclusions, Compressible Soil Reinforcement, Localized Surface Loading, Physical and Numerical Modeling, Geotechnical Centrifuge
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