Stiffness of granular soils under long-term multiaxial cyclic loading

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.