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EN
Water content may have a crucial effect on the petrophysical and mechanical properties of high porosity rocks such as chalk. Consequently, the water fluctuations may significantly influence the behaviour of engineering structures that are constructed in chalks. It is the aim of this study to illustrate how the elastic and strength properties of chalk can evolve considering water content variation using P-wave velocity and analytical analyses. To study the influence of water on the ultrasonic compressional wave velocity (VP), measurements of travel time of three samples subjected to different water content were performed. The Uniaxial Compressive Strength (UCS) in dry and saturated chalk specimens was also estimated. To clarify the possible influence of lithology on the physicomechanical properties of rocks, three samples of phosphatic chalk were studied in thin sections. Automated mineral analysis and textural imaging of the samples were performed using an FEI QEMSCAN®. The analyses of the Ultrasonic tests reveal that changes in the water content are associated with variations in the P-wave velocity. Based on P-wave velocity changes as a function of water content, analytical models have been used to predict the elastic and strength properties. The data indicate that the presence of water significantly reduced the elastic and strength parameters of the chalk, a result that is in agreement with the UCS laboratory tests. The derived equations can be used for predicting the elastic and strength properties of high-porous chalk from the P-wave velocity as a function of water content. This approach may avoid the necessity for time-consuming laboratory testing.
2
Content available Effect of Confinement Stress on Rock Mass Stability
EN
A huge number of factors controls rock mass failure, but it is mainly influenced by the state of stress and in particular on the bearing capacity and failure mechanism of the massif. The evaluation of rock mass strength in confined and unconfined compression, as well as its tension strength, are key issues to understand rock mass behaviour prior to failure. A connection between the laboratory analyses of the rock mass and the practical use of the obtained data is presented in the current work. The strength properties, confinement effect and failure mechanisms are successfully studied in volcanic rock specimens from an underground mine. In order to estimate the confinement effect on rock mass strength properties, different confined compression stresses on rock specimens are applied. In addition, the crack initiation and propagation in rock samples are observed and rock mass failure mechanisms are studied. The obtained data is used for stability analyses of an underground openings through determination of the safety factor. The obtained results of the safety factors underlined the influence of the confining stress on the rock mass. The tendency of increasing values of the shear safety factor and decreasing values of the tensile safety factor as confinement increases is found. This is an important observation that would allowed more accurate predictions of the stable and unstable zones of the underground openings to be carried out, and thus the stability of the rock mass to be improved.
EN
A sequence- and cyclostratigraphic interpretation of early Anisian (Aegean) shallow-marine carbonate ramp deposits, exposed in outcrop sections west of Tserovo village, NW Bulgaria, is presented. The hierarchical pattern identified can be interpreted in terms of Milankovitch cyclicity with elementary sequences representing the precession (20-kyr) cycle, small-scale sequences the short eccentricity (100-kyr), and medium-scale sequences the long eccentricity (400-kyr) cycle. Palynology provides a robust stratigraphic framework. The study of sedimentary organic matter, revealing variations of terrestrial input, sorting and fragmentation of phytoclasts, and prominent acritarch peaks, allows the interpretation of environmental changes and contributes to the cyclostratigraphic and sequence-stratigraphic framework. The detailed documentation of syndepositional soft-sediment deformation structures confirms their laterally traceable distribution within the depositional sequences and makes them good palaeoenvironmental indicators. Anisian ramp systems of the western Tethyan realm thus were subjected to highly dynamic regimes, recording the interplay between sea-level changes in tune with orbital cycles and ramp morphology.
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