Prismatic samples of a medium-grained sandstone from Śląsk Colliery (Upper Silesian Coal Basin, Poland) were tested under uniaxial compression, conventional triaxial compression and true triaxial compression conditions. The conventional triaxial compression tests were carried out at confining pressure (p) equal to 12.5, 25.0 and 37.5 MPa. In the true triaxial compression tests the minimum principal stress (σ3) was equal to 25.0 MPa and the intermediate principal stress (σ2) was 1.5, 2 and 2.5 times higher than σ3. Results of the studies show that confining pressure strongly inhibited dilatant behavior of rock samples tested under conventional triaxial compression conditions; the increasing confinement resulted in the growing compaction of the rock material. The effect of dilatancy was also highly suppressed by the intermediate principal stress. While important dilatant, negative volumetric strain corresponded to the peak differential stress at low intermediate principal stress conditions, at high intermediate stresses the rock material was damaged to much lesser extent. As a result, faulting of rock samples in the post-peak region was much more violent and was accompanied by a strong acoustic effect.

Les echantillons parallelepipediques du grès à grains moyens de la mine 'Śląsk' (Bassin Houiller de Haute Silesie, Pologne) ont subi les essais de compression monoaxiale (compression simple), de compression triaxiale conventionnelle (compression triaxale de revolution) et de compression triaxiale vraie. Les essais de la compression triaxiale conventionnelle ont ete effectues à la pression de confinement de 12,5, 25,0 et 37,5 MPa. Dans les essais de compression triaxiale vraie la contrainte principale minimale (σ3) etait de 25,0 MPa et la contrainte principale intermediaire (σ2) etait 1,5, 2 et 2,5 fois superieure de σ3. Il resulte des essais que la pression de confinement inhibe fortement le processus de dilatance dans les echantillons testes dans les conditions de compression triaxiale conventionnelle; l'augmentation de la pression provoquait l'augmentation du compactage du materiau rocheux. Le phenomène de dilatance etait egalement limite de façon significative par la contrainte principale intermediaire. Alors que l'importante deformation volumique negative correspondait à la contrainte differentielle ultime avec les faibles contraintes principales intermediaires, dans les conditions de fortes contraintes intermediaires, le materiau rocheux subissait l'endommagement de façon beaucoup moins importante. En resultat, la rupture des echantillons rocheux au stade post-maximum etait beaucoup plus violente et accompagnee d'un fort effet acoustique.

Prismatische Probekörper des mittelkörnigen Sandsteines aus dem Bergwerk 'Śląsk' (das Oberschlesische Kohlengebiet, Polen) wurden unter der einaxialen Druckbeanspruchung, konventionellen dreiaxialen Druckbeanspruchung und der echten dreiaxialen Druckbeanspruchung geprueft. Konventionelle dreiaxiale Druckversuche wurden bei dem Manteldruck von 12,5 25,0 und 37,5 MPa durchgefuehrt. Bei den echten dreiaxialen Druckversuchen betrug die kleinste Hauptspannung (σ3) 25,0 MPa und mittlere Hauptspannung (σ2) war 1,5-, 2- und 2,5-mal höher als σ3. Aus den durchgefuehrten Versuchen ergibt sich, dass der Manteldruck die Dilatanzentwicklung bei den Probekörpern unter der konventionellen dreiaxialen Druckbeanspruchung stark bremst; die Erhöhung des Druckes rief die ansteigende Kompaktion des Gesteinsmaterials hervor. Die Dilatanz wurde auch durch die mittlere Hauptspannung beschrankt. Wahrend die wesentlichen volumetrischen, dilatanten Verformungen der Grenzdifferenzspannung bei den niedrigen mittleren Hauptspannungen entsprachen, bei den hohen mittleren Hauptspannungen wurde das Gesteinsmaterial viel weniger beschadigt. Im Ergebnis war der Scherbruch der Gesteinsproben im Post-Peak-Bereich viel heftiger und mit kraftigen akustischen Effekten verbunden.


It has long been accepted that dilatancy is a microcracking-related phenomenon that can be treated as an important precursor of the brittle failure of rocks. Dilatancy is interpreted as being due to the development of pervasive microcracking within the rock, with a concomitant increase in void space or voidage. It is understood as the increase of volume relative to elastic changes caused by deformation1 or the inelastic increase in volume during deformation under applied differential stress9.

The phenomenon of dilatancy in rocks is of considerable practical importance since it is associated with possible premonitory signs for earthquakes, mining-induced rockbursts and mine collapses. In general, dilatancy-related precujrsory phenomena (premonitory anomalies) include: crustal deformation (rapid uplift rates, anomalous ground tilts), changes in density of the earth and in the gravity field, changes in the velocity of seismic waves, lowering of the groundwater level, emission of radon and other gases, changes in the magnetic field and electrical resistivity of the earth, changes in pore pressure and flow rate, seismic anisotropy and acoustic emission.

(Figure in full paper)

Although much research has been done to reveal characteristic features of the volume changes in rocks under triaxial stress conditions.

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