Local amplification of deep mining induced vibrations - Part.2: Simulation of the ground motion in a coal basin

This work investigates the impact of deep coal mining induced vibrations on surface constructions using numerical tools. An experimental study of the geological site amplification and of its influence on mining induced vibrations has already been published in a previous paper (Part 1: Experimental evidence for site effects in a coal basin). Measurements have shown the existence of an amplification area in the southern part of the basin where drilling data have shown the presence of particularly fractured and soft stratigraphic units. The present study, using the Boundary Element Method (BEM) in the frequency domain, first investigates canonical geological structures in order to get general results for various sites. The amplification level at the surface is given as a function of the shape of the basin and of the velocity contrast with the bedrock. Next, the particular coal basin previously studied experimentally (Driad-Lebeau et al., 2009) is modeled numerically by BEM. The amplification phenomena characterized numerically for the induced vibrations are found to be compatible with the experimental findings: amplification level, frequency range, location. Finally, the whole work was necessary to fully assess the propagation and amplification of mine induced vibrations. The numerical results quantifying amplification can also be used to study other coal basins or various types of alluvial sites

Étude en laboratoire par méthodes acoustique : de l’endommagement des roches de craie et corrélation avec des résultats

Les falaises crayeuses qui forment une proportion non négligeable du littoral européen sont le siège d’importants éboulements rocheux. Une meilleure compréhension des mécanismes à l’origine de ces éboulements est fondamentale pour le développement d’une méthodologie basée sur l’écoute microsismique comme moyen de détection des signes précurseurs des grands éboulements de fronts rocheux. Pour mieux comprendre ces mécanismes, une étude de l’endommagement des roches de craie par méthodes acoustiques est effectuée en laboratoire et corrélée avec des résultats in situ. L’étude de l’évolution de l’énergie sismique libérée a montré qu’une augmentation significative de l’énergie sismique des événements prédit la rupture de l’échantillon en laboratoire et l’éboulement sur site. L’étude de l’évolution fréquentielle de signaux a montré, aussi bien en laboratoire que sur le site, que l’initiation de la rupture se caractérisait par un décalage du spectre des signaux vers les hautes fréquences

Evidence of dilatant and non-dilatant damage processes in oolitic iron ore: P-wave velocity and acoustic emission analyses

International audienceUniaxial and triaxial compression experiments were performed on oolitic iron ores to investigate damage processes. Most of these experiments included four indirect measurements of damage evolution, that is, P-wave velocity and maximum amplitude received during pulse transmission experiments, elastic properties (apparent Young´s modulus and apparent Poisson´s ratio) and acoustic emission (AE) monitoring. The mechanical behaviour deduced from strain measurements is dilatant for some samples and non-dilatant for the other samples. However, variations in elastic properties indicate damage processes for all samples. AE source mechanism analysis shows two different microscopic damage processes: (1) for dilatant rock, the development of axial extensive microcracks as well as their interaction and coalescence lead to the formation of shear macroscopic discontinuities; (2) for non-dilatant oolitic iron ore, both compressive and shear micromechanisms take place and interact with macroscopic fractures. A particular consistency between the four types of measurements employed was observed

Technique d’écoute microsismique appliquée au risque d’effondrement dans les mines du bassin ferrifère lorrain

Les bruits émis lors de la rupture des terrains constituent des indices d’initialisation d’un effondrement. La validation de la technique d’écoute microsismique de ces bruits a été effectuée au moyen d’une expérimentation dans une mine de fer en activité. L’expérimentation a consisté à enregistrer, à la surface, les signaux microsismiques correspondant aux ruptures et aux éboulements locaux provoqués au fond de la mine par la destruction volontaire des piliers. Les opérations de dépilage et le foudroyage du toit ont été systématiquement corrélés à une série d’événements microsismiques. L’expérimentation a permis de valider la technique d’écoute microsismique comme moyen de détection des signes précurseurs de la manifestation en surface d’un effondrement, de démontrer l’efficacité de la technique et d’étalonner les paramètres principaux d’un dispositif d’écoute microsismique adapté à la détection et à la surveillance des zones à risque d’effondrement

On the need to improve the seismic risk analysis for nuclear plants safety in France preliminary lessons and recommandations from the research sinaps project.

International audienceSINAPS (Earthquake and Nuclear Plants Improving and Sustaining Safety) is a five-year research project starting in 2013 and financially supported by the French government. This project was motivated by the need of characterizing the potential seismic margins of existing nuclear facilities. SINAPS conducts critical analysis at each step of the seismic risk assessment, from seismic hazard, to site effects, interactions between the wave field the soil and structures, structural and nuclear components vulnerabilities current French practices are challenged against site data and innovative methods more adapted to model complex nonlinear processes that occurred during the seismic loadings transfer. The seismic margins are appreciated through all assumptions and the epistemic/aleatory uncertainties treatment during the analysis. The present contribution expose some lessons learned from SINAPS in the area of the seismic hazard assessment. Among them, we present recent published strong motions databases and associated ground motion prediction equations that should be considered as references for SHA in France; SINAPS also investigated Bayesian approach and clearly showed their betterment in the GMPE's selection and weighting process with respect to the classical and less objective expert's advices practice. We also discuss recent progress in the site-specific SHA as the host-to-target and kappa corrections

Analysis of Microseismic Activity Within Unstable Rock Slopes

This chapter illustrates the concept of passive seismics as a method for monitoring the propagation of cracks within a rock mass as a result of load stress or water freezing in view of the use of this technique for rockfall early warning. The methodology is still far from being a standard and consolidated technique. The research is making progress, but just a few real case studies are documented. They are shortly overviewed in the introduction. Then, an interesting field test where crack propagation was artificially triggered up to full rock detachment, while a small sensor network was active, is discussed to show the existence and the characteristics of precursory signals. It follows the illustration of the microseismic monitoring methodology through the description of the Mt. San Martino (Lecco, Italy) sensor network and the discussion of the preliminary results obtained during the initial months of activity. Apparently, the preliminary results show some correlation with rainfalls, but not with temperature. Microseismic spectra are mainly concentrated in the first 100 Hz. This probably means that the hypocentre distances from the sensors are quite longer than 10 m. Electromagnetic interferences are also observed as mentioned by other authors who have analyzed data sets from other microseismic networks installed in mountain regions. They are automatically discriminated from significant signals by a classification software which works on the time/ frequency properties of these events. Hypocenter localization and clustering analysis of the significant events are the planned near- future activities

Analysis of microseismic activity within unstable rock slopes

This chapter illustrates the concept of passive seismics as a method for monitoring the propagation of cracks within a rock mass as a result of load stress or water freezing in view of the use of this technique for rockfall early warning. The methodology is still far from being a standard and consolidated technique. The research is making progress, but just a few real case studies are documented. They are shortly overviewed in the introduction. Then, an interesting field test where crack propagation was artificially triggered up to full rock detachment, while a small sensor network was active, is discussed to show the existence and the characteristics of precursory signals. It follows the illustration of the microseismic monitoring methodology through the description of the Mt. San Martino (Lecco, Italy) sensor network and the discussion of the preliminary results obtained during the initial months of activity. Apparently, the preliminary results show some correlation with rainfalls, but not with temperature. Microseismic spectra are mainly concentrated in the first 100 Hz. This probably means that the hypocentre distances from the sensors are quite longer than 10 m. Electromagnetic interferences are also observed as mentioned by other authors who have analyzed data sets from other microseismic networks installed in mountain regions. They are automatically discriminated from significant signals by a classification software which works on the time/ frequency properties of these events. Hypocenter localization and clustering analysis of the significant events are the planned near- future activities

Impact of climate change on rock slope stability : Monitoring and modelling

The "Rochers de Valabres" is a fractured rock slope located in the French Alps and afected by rockfall. This slope has been chosen as an experimental site for improving knowledge on mechanisms leading to rockfall. Since 2002 scientific investigations (field observations, monitoring and numerical modelling) have been carried out on the site. A seismic network has recorded a significant seismic activity correlated with existing discontinuities. Mechanical measurements using tiltmeters monitor the slope movements in order to improve the knowledge on thermo-hydromechanical process. Different geometrical and mechanical models have been performed. In 2005 the site was chosen to concentrate research on understanding the efffect of climate change on rock slope stability in the framework of a French national program named STABROCK. This paper presents teh results of field measurements and numerical modelling carried out on the experimental site and the complementary investigations will start within the new research. The aim is to study more accurately the impact of environmental factors on rock slope instability mechanisms and to assess the possible effect of the climate change. Technical results will be integrated in a process of risk management