Deformation and strength characteristics of geomaterials shear loaded for long term

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The deformation characteristics at small strain levels of a wide variety of geomaterials when loaded after having been subjected to shear stress for a long duration of time in the field were investigated by means of laboratory shear tests. As the geomaterials to be tested, sand, clay, gravel, sedimentary softrock and cement-treated sand was selected. In the triaxial tests, axial strains, which were free from effects of bedding error, were measured locally from less than 0.001%.The following was found from the test results :1) Except for cement-treated soils, the deformation become elastic for some stress range when loaded again after creep deformation has occurred at a constant shear stress level. As loaded further, the stress-strain relation rejoins the primary stress-strain relation exhibiting a relatively clear yielding. This behavior can be modeled as the kinematic yielding. In some cases, the reloaded stress-strain curve overshoot exceeding the primary curve. The effects of strain rate on the deformation characteristics before the yielding upon the restart of loading is much smaller than that observed when loaded monotonously from the isotropic stress state.2) The creep rate decrease after unloaded from a higher stress level. For relatively stiff geomaterials, when unloaded to larger extents, noticeable creep recovery can be observed. This behavior leads to the effects of strain rate at unloading states in such that the tangential stiffness decreases with the decrease in the strain rate.