Soil improvement by rigid inclusions: Seismic and Dynamic Loading
This project is part of the development of the technique of reinforcement and improvement of soils by rigid inclusions (RI). This technique consists in reinforcing a compressible soil by a vertical RI mesh, all surmounted by a load transfer platform (usually made up of granular material) which allows to transfer, by arching effect and shear, the loads from the superstructure to the RI. In this way the compressible soil is less loaded and the settlements are reduced.<br />Depending on the type of superstructure, its use and location, the loads transmitted to the RI will be different. Here we are interested in two types of loading: A) dynamic loading of railway type; B) Seismic loading.<br />Topic A looks at the celerity of propagation of surface waves in a homogeneous medium with periodic vertical inclusions. The application concerns the risk of resonance between the speed of a train and the speed of propagation of surface waves in the structure of a railway, including reinforced soil. If we know the propagation rates of the waves in the soil and in the concrete made RI, we are currently in the unknown to predict the propagation rate in the composite medium, due to its geometry, the reflection of the waves propagating in a 3D medium.<br />Topic B affects a large number of geographical areas where seismic risk is high. If we know the response of a soil mass under seismic stress, what happens when the soil is reinforced by RI? When in addition a superstructure is installed on the reinforced soil? Understanding the phenomena involved, kinematic interaction and inertial interaction during seismic loading are important issues to guide the engineer’s practice with complex or simplified models, but in all cases robust.<br />This work is being done in close liaison with PN ASIRI+, which began in 2019.
Topics A and B follow the same methodology: 1) physical modelling on small scale models; 2) numerical modelling.
Topic A uses the bench MUSC (Ultrasonic Non-contact Measurement) of the University Gustave Eiffel (campus of Nantes) for the realization of models. Here wavelengths are scaled. In terms of numerical simulation, it is mainly the Planetology and Geodynamics Laboratory (Nantes campus) that provides its know-how, notably through the techniques of spectral elements and homogenization. All in conjunction with SNCF Réseau (La Plaine Saint-Denis), Terrasol (Paris) and Ménard (Orsay) who bring the vision of the work in real size and the techniques of dimensioning and realization.
Topic B is based on the Geotechnical Centrifuge of the University Gustave Eiffel (campus of Nantes) and its onboard earthquake simulator to study the effect of 1D earthquakes, while reproducing stresses similar to those of a work in real size. Here the frequencies are scaled. In terms of numerical simulation, several approaches are preferred: 3D finite elements (Centralesupélec, Gif/Yvette), macro-elements (Ecole Centrale de Nantes) and simplified models (Terrasol, Ménard and CS). The Cerema (Aix en Provence) shares its expertise in dimensioning and identifying the dynamic properties of soils (associated with EDF and UGE). The knowledge on the scale of the work in seismic zone is also provided by EDF (Aix en Provence).
The project is starting: no results at the moment.
The results of this project will contribute to the increase of knowledge, but will also have a societal impact on: 1) mobility (rail infrastructure); 2) energy (production in seismic zones).
On the economic level, the better understanding of the phenomena goes towards a better dimensioning of the works, and potentially of the savings of realization but also an opening to the construction markets in seismic zones.
The project is starting: no results at the moment.
The technique of reinforcemetn of compressible soils by vertical Rigid Inclusions (RI) is very wide-spread in France and abroad. This technique of composite foundation mixing deep and superficial elements, was developed initially for works of embankment (for infrastructures of transport), but extends in wind turbines and also in industrial buildings today (e.g. logistic platforms), of housing or offices (less than 4-5 floors), schools, hospitals, etc. This technique so fits on all the territory, urbi et orbi, impacts on the choice of the foundations of the constructions and the linear works of transport (roads and railroads), so touching in a little visible, but real way, the citizens in their living environment and for their mobility.
The issues addressed here concern the behaviour of the RI-reinforced soil mass:
i) Dynamic Loads: Modifying the celerity of surface waves in a medium with periodic inclusions
ii) under seismic stress: Inertial and kinematic effects
The methodology used is based on the experimental approach of physical modelling on reduced models combined with numerical modelling, all in conjunction with the field.
The propagation of surface waves in soil is modified by the presence of heterogeneities (vertical RI), but in what way? In order to answer this question, which concerns railway applications as a priority, reduced "geophysical" models will be carried out, based on the principle of scaling wavelengths, and implemented on the Ifsttar MUSC bench. Numerically, the spectral element method associated with the non-periodic homogenization technique will be implemented.
In the case of seismicload, the presence of Ri reinforcement necessarily changes the soil response, but in what way? To study the inertial and kinematic effects of an RI-reinforced soft soil, "geotechnical" small scale models will be tested with the earthquake simulator installed in the Ifsttar centrifuge. Here the frequencies are scaled up. A fine dynamic characterization of centrifuge soils will be carried out in parallel.The so-called macro-element numerical method as well as the so-called transfer curves will be implemented for simplified models, while non-linear 3D finite elements will be used to simulate works under seismic load (such as those studied in centrifuge), before moving to parametric studies of reference structures.
The consortium set up to try to increase knowledge on these dynamic issues brings together, around the Ifsttar, a set of partners involved to varying degrees from downstream upstream: i) SNCF-reseau, Ménard, LGP, centrale-Supelec; ii) EDF, Cerema, Terrasol, Ménard, centrale-Supélec, centrale Nantes.
Exceptional experimental equipment combined with advanced, sophisticated or simplified numerical modelling will allow to observe, understand and simulate different configurations, to bring new knowledge and make it available to the Construction Engineering.
Monsieur Luc THOREL (IFSTTAR- Département Géotechnique, environnement, risques naturels et sciences de la terre)
The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.
EDF EDF CEIDRE
SNCF Réseau / DGII / DTR VA EGV SNCF RESEAU
Cerema-Med Cerema Direction Territoriale Méditerranée
MSSMAT Laboratoire de Mécanique des Sols, Structures et Matériaux
GeM INSTITUT DE RECHERCHE EN GÉNIE CIVIL ET MÉCANIQUE
LPG LABORATOIRE DE PLANETOLOGIE ET GEODYNAMIQUE
IFSTTAR - GERS IFSTTAR- Département Géotechnique, environnement, risques naturels et sciences de la terre
Help of the ANR 718,012 euros
Beginning and duration of the scientific project: February 2020 - 48 Months