Change in elastic properties of sands under very large number of low amplitude multiaxial cyclic loading

Abstract

Geotechnical structures built in the offshore environment, such as offshore wind turbines (OWTs), are invariably subjected to millions of complex and irregular loading cycles during the design life. OWTs are subjected to continuous cyclic loading originating from both environmental (wind and wave) and operational sources (rotors dynamics), having variable frequencies, amplitudes and loading directions. Such loads are transferred to and resisted by the soil surrounding the foundation, but the eventual changes of mechanical properties of the soil during the application of such high-cyclic loading is still not understood. Changes in soil stiffness properties, possibly related to soil fatigue effects, may compromise the serviceability of the OWTs (progressive accumulation of deformation or tilting). OWTS are also dynamically sensitive structures and changes in soil stiffness may shift the system excitations towards the structural resonance frequencies leading to a dangerous amplification of the dynamic loads. Therefore, for the first time, the present research aims to shed some light on the evolution of quasi-elastic soil properties under a very large number (up to 105) complex multiaxial loading cycles of small amplitude, typically experienced by the soil surrounding monopile foundations of OWTs. Through three-dimensional finite element analysis of a monopile foundation subjected to horizontal load, typical stress paths experienced by soil element around the OWT foundation have been studied. The complex multiaxial nature of the stress paths has been revealed and it has informed and directed the high-cyclic, multiaxial loading soil element experimental program. Due to the multiaxial nature of the stress paths, the Hollow Cylinder Torsional Apparatus (HCTA), offering four degrees of freedom in stress control and capable to reproduce generalised multi-axial stress conditions, has been employed. The HCTA was equipped with a complex local strain measurement system, composed of six non-contact displacement transducers (0.1 μm resolution) which obtained an accurate estimation of Young’s and Shear moduli at very small strain levels and captured their evolution during the application of a large number of cyclic loading. The experimental tests have shown that the long-term application of multiaxial loading cycles in a stress direction different from the preloading (thus implying rotation of principal stress axis during cyclic loading) can lead to a decrease of quasi-elastic properties of the material. The amount and rate of degradation was found to be dependent on the amplitude and direction of cyclic loading, and on the average stress level. A stable value of quasielastic properties was found to be after reach around 1.5-2·104 cycles although this may be also affected by the relationship between the pre-cycle and cycle loading directions. Unavoidable and unplanned interruptions of the hydraulic supply for the loading system periodically imposed large unloading-reloading cycles on the sample. A tendency to temporarily recover the value initial stiffness (or even a slightly higher value of stiffness) before the application of the loading cycles has been recorded. However, further application of multiaxial stress cycles resulted in a systematic repeated stiffness drop.

Date of Award	6 Nov 2018
Original language	English
Awarding Institution	The University of Bristol
Supervisor	Andrea Diambra (Supervisor) & Erdin Ibraim (Supervisor)