Evaluation of time-dependent deformation properties of sedimentary soft rock and their constitutive modeling

A triaxial testing system with local axial strain measurements was developed to accurately evaluate the time-dependent deformation properties of sedimentary soft rock. A series of consolidated triaxial creep tests were conducted on undisturbed samples of sedimentary silt-sandstone and mudstone. Drained creep tests, each lasting for three days, were performed at several deviator stresses in the course of drained triaxial compression. Creep axial strains measured locally at the lateral surface of the specimen were always noticeably smaller than externally measured ones, indicating significant time-dependent effects of bedding error occurring at the specimen ends. In these triaxial tests, the axial strain rate was stepwise changed during monotonic shearing to evaluate the effects of strain rate and its change on the stress-strain behaviour. It is shown that instantaneous irreversible strain and its rate uniquely control the current stress state whether the strain rate is constant or continuously changing. This isotach property is modeled in a simple model to be applied to the limited test condition examined. It is shown that this model can predict adequately the measured stress-strain-time behaviour for various time histories of strain (or stress).

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