Bernard Cambou

Change of scale in granular materials

This paper analyzes relationships allowing global variables to be deduced from local ones in granular materials. This change of scale is now clear for static variables but rather confusing for kinematic variables. This paper compares the various established formulations for kinematic variables and also proposes original formulations. By running numerical simulations all the formulations in question are compared and the basic premises to define the change of scale for kinematic variables in granular media put forward.

Micromechanical modelling of anisotropic non-linear elasticity of granular medium

In this paper a macroscopic elastic model is derived from a microscopic Hertz-Mindlin elastic contact law using an homogenisation technique. The results obtained in the case of an initially isotropic granular medium submitted to an isotropic stress state are first presented. The influence of the static and kinematic internal variables defined in the homogenisation approach is then discussed. Extensions to the cases of an isotropic granular medium submitted to an anisotropic stress state and of an anisotropic medium loaded isotropically are analysed and discussed. As often as possible, comparisons with experiments or numerical simulations are considered.

SOIL-STRUCTURE INTERACTION IN SHIELD TUNNELLING IN SOFT SOIL

Abstract The development and extension of large cities creates a need for multiple shallow tunnels in the soft ground of building areas. Prediction of the ground settlement caused by the tunnel excavation is a major engineering challenge. A numerical simulation using a finite element method was implemented in the aim of developing a procedure to predict the movement induced by shield tunnelling in soft soil. This study describes a two-dimensional modelling and compares two procedures. The first procedure is done in a simple way (called “deconfinement modelling”) simulating the excavation using a stress decrease vector exerted on the excavation boundary (inside the tunnel) described by a stress release scalar parameter λ (named the “deconfinement factor”). The second procedure is composed of a complete stage of modelling (called “phase modelling”) taking into account different phases which simulate the different kinds of interactions between the tunnel and the soil (deconfinement, lining installation, pore pressure applied on the lining, and weight of the lining). Using a shallow lined tunnel with homogeneous soil conditions, the two procedures are analysed and compared. Then, the second modelling procedure is applied to the case of the metro of Lyon where field data have been obtained. Observations of the results and comparison with the experimental data demonstrate that the proposed modelling is adequate for the analyses of settlement induced by tunnelling in soft soil.

From a constitutive modelling of metallic rings to the design of rockfall restraining nets

This paper considers recent contributions concerning metallic structures designed to stop rockfalls. These structures are composed of metallic nets generally made with anti-submarine rings. Both the singular technology and the spatial array of the rings confer outstanding mechanical properties to the net sheets, which are used today in order to control rockfall trajectories. A spatial description of the net is proposed, and a constitutive modelling in finite strains is presented. The proposed modelling is based on both the experimental and analytical approaches. But the main feature of this modelling is that the local behaviour of the net can be described in the usual framework of elasto-plasticity. Mechanical analysis of each material body has been carried out, but the complexity of the resulting system imposed the building of a numerical scheme. Finally, this research has led to the development of original computational software, for both industrial and academic purposes. In order to give a clear indication of the capacity of both the modelling and the computational software, a typical example of a simulation is proposed and discussed. Copyright

Modélisation hiérarchisée du comportement des sols

ABSTRACT In this paper, a tool for better controlling the use of constitutive models in numerical methods in geotechnical engineering is proposed. For this purpose a constitutive model based on CJS model is developed. This model is hierarchically composed of several levels of constitutive modelling, respectively from a simple level to a more complex level. The transition from one level to another is easy in operating only over some constants of the model. The strategy of identification and physical meanings for all of the constants are clearly defined. In order to validate the different levels of the model, the results of homogeneous drained and undrained tests, in the same way as boundary problems concerning pressuremeter and tunnel are presented and discussed.

Etude du comportement mécanique des ouvrages souples de protection contre les éboulements rocheux

ABSTRACT This paper deals with recent research concerning rockfall restraining nets. After a presentation of the general context of the study, the scientific approach is described in details including: the assumptions about physical phenomena involved, the experiments used to appreciate the local behaviour of the structure, and the numerical method employed. In order to give a clear indication of the capacity of the software, several examples are discussed.

Forecast of creep settlements of heavy structures using pressuremeter tests

Abstract This paper presents a new method proposed for the prediction of long term settlement of very heavy structures based on a numerical interpretation of long term pressuremeter creep tests. The constitutive model used for the modelling of the soil creep is described. The constants of the constitutive model are determined by fitting a simulated curve, given by a simple Finite Element Method, to the results of a long term pressuremeter creep test. The calculation of long term settlement under nuclear power plants, using the soil parameters defined by this method, is presented and compared with the in situ measured values.

Relevant local variables for the change of scale in granular materials

This paper deals with different ways to performe a change of scale in granular materials. Two different local levels are considered and analysed: the local level defined at the contacts between particles and the local level defined at the local array level. The difficulties to perform a change of scale, considering these two local levels are analysed and discussed. The different theoretical approaches presented in this paper are illustrated by a numerical simulation using a DEM code.

Tassements et pouvoir endommageant d'un mouvement sismique

ABSTRACT The design of an index measuring the damaging power of a seismic record is generally undertaken within the frame of Structural Dynamics. Herein, we propose to undertake it within the frame of Soil Dynamics. The global settlement undergone by a soil profile is regarded as an indicator of the damage caused by a seismic motion. The ability of some classical damage index to predict the settlement is analysed. A predictor of expectable settlement is proposed. This predictor, which account for essential features of the soil profile, appears therefore as a new improved damage index.

Granular Materials: Mesoscale Structures and Modeling

Abstract: A straightforward and reliable modeling of granular materials currently remains an open issue. The difficulty lies in the very discrete nature of these materials composed of grains in contact. In this chapter, we explore a new upscaling approach including a mesoscale.