Several methods, including X-ray radiography, have been developed for the investigation of the characteristics of water-saturated quasi-brittle materials. Here, the water content is one of the most important factors influencing their strength and fracture properties, in particular, as regards to porous building materials. However, the research concentrated on the three-dimensional fracture propagation characteristics is still significantly limited due to the problems encountered with the instrumentation requirements and the size effect. In this paper, we study the influence of the water content in a natural quasi-brittle material on its mechanical characteristics and fracture development during in-situ four-point bending by employing high-resolution X-ray differential micro-tomography. The cylindrical samples with a chevron notch were loaded using an in-house designed four-point bending loading device with the vertical orientation of the sample. The in-house designed modular micro-CT scanner was used for the visualisation of the specimen’s behaviour during the loading experiments. Several tomographic scans were performed throughout the force-displacement diagrams of the samples. The reconstructed 3D images were processed using an in-house developed differential tomography and digital volume correlation algorithms. The apparent reduction in the ultimate strength was observed due to the moisture content. The crack growth process in the water-saturated specimens was identified to be different in comparison with the dry specimens.
A comparison of rock quality designation (RQD) parameters obtained by drill core analysis and the RQD determined using alternative methods is presented using metamorphic rocks such as migmatized gneisses, migmatites, and amphibolites. Methods of borehole-wall imaging using high-resolution acoustic logging, optical televiewer, and simple video inspection as well as the structural analysis of exploration drift walls oriented subparallel to the analysed boreholes are used for alternatively evaluated RQD values. Results show that, in most cases, the RQD index measured directly on a drill core section reaches lower values (about 10%-30%) than the RQD evaluated by alternative methods. Significant differences in determination of the rock mass failure degree using classical and alternative methods may occur, especially in sections with higher frequency of the metamorphic foliation planes created by biotite-rich bands or by dark-coloured melanosome layers. The nature of biotite itself is favourable for the creation of pre-disposed jointing planes, in which the increased drill core disintegration caused by drilling technology or core handling can occur. Results show it is highly appropriate to make these comparisons on a larger scale and thus verify whether the observed differences affect the qualitative classification of the rock mass.Résumé : Le présent article présente la comparaison entre les valeurs de RQD (rock quality designation) obtenues par analyse de carottes de sondage et celles déterminées à l'aide d'autres méthodes, dans le cas de gneiss migmatisés, de migmatites et d'amphibolites. On obtient des images des parois de puits de forage au moyen de la diagraphie acoustique à haute résolution, de sondes optiques ou d'une simple inspection vidéo et on réalise une analyse structurale des parois de la galerie d'exploration subparallèles aux puits étudiés, et ce afin de déterminer d'une autre manière les valeurs de RQD. Les résultats ont montré que, dans la plupart des cas, l'indice RQD mesuré directement sur une section d'une carotte de sondage atteint de faibles inférieures (d'environ 10-30 %) à celles estimées à l'aide d'autres méthodes. On peut constater d'importantes différences entre les valeurs obtenues lors de l'estimation du degré de rupture de la masse rocheuse, selon que l'on utilise des méthodes classiques ou alternatives, en particulier dans le cas des sections comportant un nombre plus élevé de plans de foliation métamorphiques formés de bandes riches en biotites ou de couches de mélanosomes de couleur sombre. De par sa nature, la biotite favorise la formation de plans de diaclase préexistants, dans lesquels la désintégration de la carotte de sondage causée par sa manipulation ou par les techniques de forage utilisées peut être plus importante. Les résultats montrent qu'il est essentiel d'effectuer à plus grande échelle les comparaisons décrites ci-dessus afin de vérifier si les différences observées influent sur la classification qualitative des masses rocheuses. [Traduit par la Rédaction] Mots-clés : i...
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