The principles of critical state mechanics appear to apply for shales. The isotropic consolidation behavior of four shales have been investigated experimentally at effective pressures of 1to 65 MPa to evaluate some critical state parameters as a function of mineralogy and porosity. Sample volume change as a function of effective pressure was measured to determine the normal consolidation (NCL) and over consolidation (OCL) lines. The slopes of the NCL and OCL (and) are distinct for the four shales. The slopes of the NCL correlate strongly with porosity and with the sum of porosity and swelling clay content, i.e. the water sources. The slopes of the OCL correlate with non-swelling minerals (clays and non-clays), i.e. the elastic components. The results suggest that careful mineralogical and porosity characterizations may be used to estimate both isotropic consolidation and deformation parameters of shales.
Les principes de la mecanique à l'etat critique sont applicables egalement aux schistes. Dans un appareil triaxial on a etudie le comportement de consolidation isotrope de quatre schistes A des pressions effectives de 1 à 65 MPa, afin d'etudier leur caracteristiques en fonction de leur mineralogie et de leur porosite. Le changement de volume de l'echantillon en fonction de la pression effective a permis de determiner les courbes de consolidation normale (NCL) et de sur-consolidation (OCL). Les pentes des courbes NCL et OCL (et) sont distinctes pour les quatre schistes. Le pente de la courbe NCL est en forte correlation avec la porosite, ainsi qu'avec la somme de la porosite et du contenu en argile gonflante, la principale provenance d'eau. La courbe OCL est en correlation avec la teneur en mineraux argileux nongonflants et mineraux non-argileux qui representent les composants elastiques. Π en resulte qu'une caracterisation soigneuse de la mineralogic et de la porosite des schistes permet de connaître leur comportement de consolidation isotrope et de deformation.
Die Grundsatze der Kritischen-Zustands-Mechanik sind auf Tongesteine unterschiedlicher Zusammensetzung anwendbar. In einem Triaxialapparat wurde das isotrope Konsolidationsverhalten von 4 Tongesteinen bei effektiven Spannungen von 1–65 MPa bestimmt. Die kritischen Zustandsparameter wurden dabei in Abhangigkeit von Mineralogie und Porositat ermittelt. Die Messung der Volumenanderung in Abhangigkeit der effektiven Spannungen erlaubte die Bestimmung der Pfade der Erstbelastung (NCL) und der Wiederbelastung (OCL) sowie ihrer Steigungen. Die NCL-Steigung korreliert streng mit der Porositat, sowie der Summe von Porositat und Anteil quellfahiger Tonminerale, welche den Wassereinfluss reprasentieren. Die OCL-Steigung korreliert mit dem Anteil der nicht quellfahigen Tonminerale und Nichttonmineralien, welche die elastischen Komponenten darstellen. Daraus resultiert, dass sie genaue Charakterisierung der Mineralogie und der Porositat der Tongesteine die Beurteilung des Konsolidations- und Deformationsverhaltens ermöglicht.
Shales make up over 75% of sedimentary rocks near the Earth's surface. They are often encountered when excavating foundations and tunnels and are important liners for hazardous waste repositories. Shales are also the most common caprock for oil and gas reservoirs. Clays have been proposed as a possible host medium for the geological disposal of radioactive waste (e.g. Chapman, 1984). Failure of shale formations in repositories. tunnels and wells can increase drilling costs dramatically (e.g. Steiger and Leung, 1988). The prediction of wellbore stability in shale is one of the most important needs in the petroleum industry. Quantitative determination of physical properties of shales are necessary to constrain models of compaction and deformation of these important sedimentary rocks. Isotropic consolidation data are used in models that predict drainage in low permeability shales (e.g. Savage and Braddock, 1991) and models that make risk analyses of ground water movement near hazardous wastes. Using clay compression experiments that related porosity to burial, Skempton (1970) demonstrated that the relation between void ratio and log overpressure is essentially linear for any clay. Moreover, at a given overburden pressure the porosity (void ratio) depends on the amount and type of clay minerals.
