Deformation and strength characteristics of geomaterials shear loaded for long term

通过实验室剪切试验，研究了多种岩土材料在经受长时间剪切应力加载后，在小应变水平下的变形特征。试验材料选择砂、粘土、砾石、沉积软岩和水泥处理砂。在三轴试验中，不受层理误差影响的轴向应变在局部测量范围小于0.001%。试验结果表明：1)除经水泥处理的土外，在恒定剪应力水平下发生蠕变后，再加载时，在一定应力范围内变形变为弹性。进一步加载后，应力-应变关系回归到原始应力-应变关系中，表现出较明显的屈服。这种行为可以用运动学屈服来建模。在某些情况下，重新加载的应力-应变曲线超过了原始曲线。应变速率对重新加载后屈服前变形特性的影响远小于从各向同性应力状态单调加载时的影响。2)从较高应力水平卸载后，蠕变速率减小。对于相对坚硬的岩土材料，当卸载到较大程度时，可以观察到明显的蠕变恢复。这种行为导致卸载状态下应变速率的影响，即切向刚度随着应变速率的减小而减小。

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.