Semi-empirical relationships to assess the seismic performance of slopes from an updated version of the Italian seismic database

Funder: Dipartimento della Protezione Civile, Presidenza del Consiglio dei Ministri; doi: http://dx.doi.org/10.13039/100012783; Grant(s): ReLUIS research project - Working Pachage 16: Geotechnical Engineering - Task Group 2: Slope stabilityAbstractSeismic performance of slopes can be assessed through displacement-based procedures where earthquake-induced displacements are usually computed following Newmark-type calculations. These can be adopted to perform a parametric integration of earthquake records to evaluate permanent displacements for different slope characteristics and seismic input properties. Several semi-empirical relationships can be obtained for different purposes: obtaining site-specific displacement hazard curves following a fully-probabilistic approach, to assess the seismic risk associated with the slope; providing semi-empirical models within a deterministic framework, where the seismic-induced permanent displacement is compared with threshold values related to different levels of seismic performance; calibrating the seismic coefficient to be used in pseudo-static calculations, where a safety factor against limit conditions is computed. In this paper, semi-empirical relationships are obtained as a result of a parametric integration of an updated version of the Italian strong-motion database, that, in turn, is described and compared to older versions of the database and to well-known ground motion prediction equations. Permanent displacement is expressed as a function of either ground motion parameters, for a given yield seismic coefficient of the slope, or of both ground motion parameters and the seismic coefficient. The first are meant to be used as a tool to develop site-specific displacement hazard curves, while the last can be used to evaluate earthquake-induced slope displacements, as well as to calibrate the seismic coefficient to be used in a pseudo-static analysis. Influence of the vertical component of seismic motion on these semi-empirical relationships is also assessed.</jats:p

Incidence de la microstructure sur la topologie du domaine de bifurcation des matériaux granulaires

International audienceThe detection of bifurcation in granular soils is a major concern in geomechanics. Recent advances in geomechanics have highlighted the key role played by the mesoscopic structures composed of grain loops in the stability of granular media. Based on an elementary grain loop pattern, this paper investigates the influence of the microstructure on the bifurcation domain topology at the mesoscopic scale. This study shows that, at the mesoscopic scale, the bifurcation domain cannot be consistently defined in terms of the stress state only. Expressed in a stress-microstructure space, the bifurcation domain becomes invariant with the contact regime, the initial microstructure, and the loading path. It is also shown that the microstructural anisotropy is as determinant as the stress state with respect to the bifurcation domain topology. Finally, the mechanical and volumetric behaviours of the granular assembly are shown to be correlated through a proper microstructural variable

Détermination de l'angle de frottement entre un géotextile et une géomembrane : effet de la vitesse de cisaillement

International audienceThis study investigates how the shear rate can affect the geomembrane - protective geotextile friction angle. Four types of geomembranes (GMB) were considered (EPDM, HPDE, PP, and PVC) and a single nonwoven needle-punched geotextile (GTXnw) was used to make the interfaces with the geomembrane. Three shear devices were used: a large-scale inclined plane (IP), a shear box (SB), and a small-scale shear device (ssSD). The ssSD allows two shear modes to be compared: one mode involves incrementally increasing the shear stress, and the other involves imposing a constant tangential velocity at the interface. Only the PP GMB- GTXnw was tested with the SB and the ssSD. Inclined plane standardised tests show that for the three interfaces that undergoes gradual sliding (EPDM, PP and PVC GMB- GTXnw), it is shown that a step-by-step experimental procedure gives significantly lower interface friction angle than that given by the procedure from the current international standard, which is explained by the increase of interface shear stress with sliding speed. These observations are confirmed by shear box tests. One major practical result is that, following the nature of geosynthetics, the shear rate applied in large-scale shear box tests should be adapted to assess a safety value of a geosynthetic - geosynthetic interface friction angle

Méthode simplifiée pour estimer la performance sismique de petits barrages en remblai homogènes

International Symposium Qualification of Dynamic Analyses of Dams and their Equipment and of Probabilistic Assessment of Seismic Hazard in Europe, Saint-Malo, FRA, 31-/08/2016 - 02/09/2016International audienceWe propose a new simplified method for assessing the seismic performance of homogeneous small earth dams (height &lt; 20 m) founded on bedrock. Dynamic decoupled calculations were performed on seven accelerograms applied to 33 structural and geotechnical configurations. We studied the influence of embankment geometries and mechanical properties on the prediction of earthquake-induced permanent displacements estimated using Newmark analyses. This paper also discuss the relevance of this model comparing it with existing simplified models and with post-seismic field observations. A regression analysis based on easily accessible parameters characterizing the structure (yield coefficient and initial fundamental period) and the ground motion (peak ground acceleration and Arias intensity) was used to provide a pseudo-empirical predictive equation to carry out rapid preliminary seismic performance assessments

Stabilité sur pente d'une couche de sol au dessus d'une étanchéité par géosynthétiques

International audienceGeosynthetics are widely used on slopes in several civil engineering structures such as waste facilities and hydraulic works, generally named geostructures, in particular to assure the water-proofness of these works. Generally, several geosynthetics are associated to meet the various expected functions (waterproofness, drainage, anti-puncturing...), so the stability of materials implemented over these geosynthetics systems is widely dependent on characteristics of the geosynthetics interfaces between them or in the contact with grounds surrounding the system and on the geosynthetics internal sta-bility especially in the case of geocomposites. On the basis of these characteristics, different methods of calculation are used to justify the stability of these works. The present communication approaches the various aspects of the justification of the stability of these works. A first part is dedicated to the calculation of stability for the thin layers on slope on a geomembrane lining system of dam or waste disposal which can be realized on the basis of the standard NF G 38-067 (AFNOR 2017). But in numerous situations, this approach presents limits and questions remain open on the hypotheses and the consideration of the behaviour of materials. In a second part, we propose answers, on one hand, we have therefore developed a numerical approach based on the use of the FLAC software which enables to realistically model the behaviour of geosynthetics taking into account the mechanical nonlinearities of the materials and interface constitutive laws; on the other hand we lead works for the determination of interfaces and internal shear resistance of geosynthetics ; new proposals of operating procedures are made to overcome the limits of the current standards

Mécanique des mésostructures dans les matériaux granulaires bidimensionnels

International audienceMacroscopic behaviours of granular materials derive from the fabric distribution and evolution. Two kinds of fabric in the meso-scale can be distinguished: column-like clusters, called force-chains, which carry a majority of the external loading; and the loop-like clusters, called meso-loops, cells enclosed inside by contact branches. Some significant features in themeso-scale and how they influence themacroscopic behaviour of granular materials are investigated under the drained biaxial loading path. Results show that: (1) the macroscopic mechanical performance of the granular material can be associated to the evolution of the proportions of L3 (meso-loops with 3 sides) and L6+ (meso-loops of number of side no less than 6); (2) there exists a single mesoscopic topology as the signature of the critical state of granular materials; (3) the force-chains adjacent area is the main source and contributor of the global dilatancy of the granular material.Moreover, the instability of the idealised hexagonal loop, as the representation of L6+, is analysed. It is shown that under a loading path with a constant confinement, it is possible to proceed an elastic buckling, if there is a sufficient high confining stress or a sufficient low α0 (the initial opening angle of the hexagon, initially controlling the elongation of it along the axial direction). α0 and the stiffness ratio kt /kn (the tangential stiffness over the normal stiffness) are found to be greatly influential to the instability of the hexagon, through influencing the Mohr–Coulomb line of the hexagon

Dam and levee failures: an overview of flood wave propagation modeling

Dams and levees are hydraulic structures that are designed to resist to water pressure, but they occasionally break because of constructional defaults or unexpected events. When such a failure occurs, water can be suddenly released and create a flood wave more dangerous than natural events. Hydrodynamic modelling of such waves includes both estimating the flow hydrograph at the structure site and propagating the latter hydrograph downstream the structure. For engineering purposes, simplified erosion models are used to obtain the breach hydrograph for earthen embankments while instantaneous failure is often assumed for concrete structures. For flood wave propagation, 1-D or 2-D shallow water equations are solved using numerical schemes that can deal with supercritical and subcritical flow regimes. Uncertainty is likely to be high because it is difficult to calibrate the numerical model for flows much stronger than actual observations. Particularly, because of high velocities, sediment transport is likely to occur and to modify the risk parameters; numerical modelling can help defining the range of uncertainty due to this sediment transport but calibration is even more difficult than for hydrodynamic modelling. The description of these latter models is illustrated on various events, the Malpasset dam failure that occurred in 1959 during the first filling, the 100 year flood in Agly coastal plain that resulted in levee breaching in 1999 and the Ha! Ha! dam failure that caused huge geomorphological changes along the downstream valley

Multiscale failure modeling in granular soils

International audienceSolving boundary value problems requires sufficiently robust constitutive models to be implemented. In this context, this presentation focuses on slope stability. For this purpose, an advanced micromechanical model for granular soils is presented.This model introduces an intermediate scale made up of elementary hexagonal patterns of adjoining particles. This is a breakthrough with respect to current micromechanical models that generally describe the material by a single distribution of contacts. It is shown how specific degradation laws influence the existence of a bifurcation domain, detected by the vanishing of the determinant of the symmetric part of the constitutive tensor. The bifurcation domain, located inside the plastic limit surface, gathers all mechanical states at which a failure may occur, depending on the incremental loading applied. This basic notion is proved to be fundamental in order to predict the occurrence of large, catastrophic landslides