The effects of cyclic loading on soft saturated porous rock have been investigated. A typical phenomenon is so called cyclic fatigue in which a material fails at a stress level lower than its static strength. Deformation, strength and behaviour of pore pressure under quasi-static and cyclic loading were studied in undrained test conditions. In addition, creep tests were conducted in order to know the long-term strength. The sample used in the experiments is porous soft tuff whose unconfined compressive strength is about 16 MN/m2. Laboratory experiments have been performed to know the correlation between creep, cyclic loading and conventional strain-rate constant tests. The concept of complete stress strain curve was evaluated.


Les effets de chargement cyclique sur roche poreuse saturee molle ont ete examines. Un phenomène typique est la soidisante fatigue cyclique, dans laquelle un materiel se rompt à un niveau de tension plus bas que sa resistance statique. La deformation, la resistance et le comportement de la pression de l'eau interstitielle sous chargement quasi-statique et cyclique ont ete vises dans des conditions d'essai non draine. De plus, des essais de fluage ont ete faits pour savoir la resistance à long terme. Les echantillons des experiences sont du tuf poreux mou, dont la resistance de compression sans frottement lateral est environ 16 MN/m2. Des essais de laboratoire ont ete faits pour savoir la correlation entre le fluage, le chargement cyclique et les essais conventionels de vitesse de deformation constante.


Die Effekte der zyklischen Belastung auf weichen, gesattigten porösen Felsen sind untersucht worden. Ein typisches Phanomen ist die sogenannte zyklische Ermuedung, wobei ein Material bei einem Druck niedriger als die statische Festigkeit bricht. Verformung, Festigkeit und Betrage des Porenwasserdrucks unter quasi-statischen und zyklischen Belastungen unter undrainierten Versuchsumstanden sind erforscht worden. Weiterhin sind Kriechversuche zur Erforschung der langfristigen Festigkeit gemacht worden. Die in den Experimenten verwendeten Proben sind poröse, weiche vulkanische Tuffe, deren Kompressionsfestigkeit bei verhinderter Seitendrehung ungefahr 16 MN/m2 betragt. Laboratoriumsversuche zur Erforschung der Korrelation zwischen Kriechen, zyklischer Belastung und konventioneller konstanter Verformungsgeschwindigkeit sind angestellt worden.


Foundations of dams, roads and bridges, underground space like tunnels and chambers are subjected to cyclic loading caused by earthquakes, traffics, blasting, etc. The effects of cyclic loading on several different civil engineering materials such as steel, concrete and soil have been investigated. A typical phenomenon is so called cyclic fatigue in which a material fails at a stress level lower than its static strength. However, 'little work in this subject have been done in the area of rock mechanics. The influence of combined stresses and pore Water pressure have not been investigated. It is known that the fatigue curve in cyclic loading is similar to the static creep curve. The reason why is not well documented theoretically or experimentally. Scholz and Koczynski (1979) tried to explain these rock behaviors under cyclic loads with hard crystalline rocks. Their conclusion was that three types of cracking result in dilatancy stress-induced cracking; stress-corrosion cracking and fatigue' cracking. Rock fracture is sensitive to which type is prevalent. In engineering practice, in relation to cyclic loading on rock, an idea of complete stress-strain curve was presented by Haimson (1974). This idea may be useful to explain phenomenologically the similar behaviors between cyclic and creep loading conditions. The purpose of this research is to examine a number of features of rock deformation and fracture that are not well--,observed in more conventional test. Soft saturated porous sedimentary rocks were selected for undrained triaxial tests. Deformation, strength and behavior of pore water pressure under quasi-static and cyclic loading have been investigated. In addition, creep tests were conducted to know the "long-term" strength. The results of these tests are interpreted in view of the complete stress-strain curve.


It is known that the accumulated permanent strain for different upper peak cyclic or static stresses is bounded by the complete stress-strain curve (Haimson, 1974). The envelope of gradually increased repeated (GIR) loading curve is also found to be the complete stress-strain curve (Akai, et. al., 1981) as shown in Fig. l(a). It is recognized that the higher the strain rate is, the stronger is the rock. At the very slow rate of strain test, a rock specimen sometimes does not show even a distinguished peak. The confining pressure in triaxial tests also change the shape of stress-strain curve as depicted in Fig. l(a). Therefore, the infinite number of complete stress-strain curves may be defined with different strain rates and confinements. This means that a surface of complete stress-strain which bounds a state is possibly established in an adequately defined stress, strain and time space. If a complete stress-strain curve is a test path on the complete stress-strain surface, it may be used to predict failure of rock as a result of creep and relaxation.

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