Geotechnical infrastructures may be subjected over their lifetime to long-term loading cycles of varying amplitude, frequencies and direction as a result of the combination of environmental and operational processes. Soil elements surrounding the foundations of these geotechnical systems are in turn subjected to complex six-dimensional stress paths, invariably involving rotation of principal stress axes. Changes of the soil's mechanical properties can lead to changes of the overall structure dynamics, as well as to an accumulation of irreversible deformations. However, the evolution of the soil's response and stiffness under complex long-term cyclic loading scenarios is neither well known nor adequately understood. In contrast to the conditions imposed by standard laboratory tests, this research used a hollow-cylinder torsional apparatus (HCTA) to explore the evolution of the small-strain stiffness of a granular soil under long-term multiaxial drained stress cycles (up to about 6 × 10⁵). Granular soil samples were subjected to stages of regular low-amplitude stress cycles at different anisotropic stress levels interspersed by periodic large-amplitude cyclic loops. A high-resolution local strain measurement system was employed to determine the vertical Young's modulus and shear modulus, both attained in a HCTA at different stages of the testing. It was found that low-amplitude multiaxial stress cycles, involving continuous rotation of principal stress axes, caused a degradation up to about 20% of these elastic soil properties. Within 10⁴ to 2 × 10⁴ cycles, the degraded stiffnesses reached a stable value, which was maintained up to at least 8 × 10⁴ cycles. The stiffness degradation was more pronounced for the shear modulus than the vertical Young's modulus of the soil.

中文翻译：

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长期多轴循环荷载作用下颗粒土的刚度

由于环境和操作过程的结合，岩土基础设施在其生命周期内可能会受到不同幅度、频率和方向的长期负载循环的影响。这些岩土系统基础周围的土壤元素依次受到复杂的六维应力路径的影响，总是涉及主应力轴的旋转。土壤力学性质的变化会导致整体结构动力学的变化，以及不可逆变形的积累。然而，在复杂的长期循环载荷情况下，土壤响应和刚度的演变既不为人所知，也不为人们充分理解。与标准实验室测试所施加的条件相比，⁵）。粒状土壤样品在不同的各向异性应力水平下经受规则的低振幅应力循环阶段，这些应力水平散布着周期性的大振幅循环回路。采用高分辨率局部应变测量系统来确定垂直杨氏模量和剪切模量，这两者都是在 HCTA 测试的不同阶段获得的。研究发现，涉及主应力轴连续旋转的低幅度多轴应力循环导致这些弹性土壤特性的退化高达约 20%。在 10 ⁴到 2 × 10 ⁴周期内，退化的刚度达到稳定值，并保持至少 8 × 10 ⁴循环。剪切模量的刚度退化比土壤的垂直杨氏模量更明显。