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Creep rupture of saturated undisturbed clays Snead, David Edward

Abstract

The stress/strain relationship for most engineering materials is known to be time dependent. This is most evident during a creep test in which continual deformations are observed under constant stress conditions. In the laboratory, a specimen of cohesive soil subjected to a constant shear stress may fail after having deformed alt relatively slow rates for a considerable time. This type of failure, termed creep rupture, is also known to occur in the field. Results of drained and undrained triaxial creep rupture tests are presented in this thesis. These tests were performed on a sensitive marine clay from western Canada which was consolidated to various stress histories. Pore pressure measurements were taken during undrained tests using an electrical transducer. In addition to the creep rupture tests, incremental load and constant strain rate triaxial tests were performed for comparative purposes. The strain rate during a creep rupture test was observed to initially decrease as the specimen strained, reach a transient minimum strain rate, and then increase until rupture. Failure was found to be inevitable whenever the strain rate started to increase after having reached a minimum value. Pore pressures measured during the undrained tests did not reflect the onset of creep rupture at the transient minimum strain rate, and therefore, the onset of creep rupture cannot be explained in terms of effective stresses. A relationship was found to exist between the deviator stress, strain and current strain rate during undrained triaxial tests having the same consolidation history. This relationship permitted the prediction of the results of constant strain rate tests based on the results of creep rupture tests. This resulted in an understanding of the interrelation between the transient minimum strain rate of a creep rupture test and the maximum deviator stress of a constant strain rate test. Once the transient minimum strain rate had been reached, the results of creep rupture tests showed that the strain rate was inversely proportional to the time remaining before rupture. This relationship is independent of stress level, consolidation history and drainage conditions. As a result, it is suggested that measurement of deformations in the field can be used to predict the time until a sudden failure would be anticipated. The upper yield strength, defined as the maximum compressive stress which will not cause a creep rupture failure, was evaluated from both creep rupture and constant strain rate tests. It was found that the compressive strength increased as a linear function of the cube root of the strain rate.

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