Dynamic soil–structure interaction of monopile supported wind turbines in cohesive soil.

Offshore wind turbines supported on monopile foundations are dynamically sensitive because the overall natural frequencies of these structures are close to the different forcing frequencies imposed upon them. The structures are designed for an intended life of 25 to 30 years, but little is known about their long term behaviour. To study their long term behaviour, a series of laboratory tests were conducted in which a scaled model wind turbine supported on a monopile in kaolin clay was subjected to between 32,000 and 172,000 cycles of horizontal loading and the changes in natural frequency and damping of the model were monitored. The experimental results are presented using a non-dimensional framework based on an interpretation of the governing mechanics. The change in natural frequency was found to be strongly dependent on the shear strain level in the soil next to the pile. Practical guidance for choosing the diameter of monopile is suggested based on element test results using the concept of volumetric threshold shear strain.

Winkler springs (p-y curves) for liquefied soil from element tests.

In practice, piles are most often modelled as "Beams on Non-Linear Winkler Foundation" (also known as “p-y spring” approach) where the soil is idealised as p-y springs. These p-y springs are obtained through semi-empirical approach using element test results of the soil. For liquefied soil, a reduction factor (often termed as p-multiplier approach) is applied on a standard p-y curve for the non-liquefied condition to obtain the p-y curve liquefied soil condition. This paper presents a methodology to obtain p-y curves for liquefied soil based on element testing of liquefied soil considering physically plausible mechanisms. Validation of the proposed p-y curves is carried out through the back analysis of physical model tests.

Effect of particle loss on soil behaviour.

Soil particle loss can result in strength and volume reductions which are difficult to predict. This paper investigates the influence of the removal of fractions of selected particle sizes under different confining pressures. The mass loss process was reproduced by the dissolution of selected salt particle sizes and fractions from uniform Leighton Buzzard sand. The dissolution tests were performed in a triaxial cell customised to allow circulation of pore-fluid thereby allowing the dissolution/removal of the salt fraction. Test results from previously conducted oedometric dissolution tests and subsequent triaxial dissolution tests all show increases in void ratio. From the triaxial tests, a reduction in shear strength with increasing ductility was observed. Volumetric and strength behaviour were found to be related to the particle size and fraction material removed while shear-wave measurements obtained pre- and post-particle removal indicate significant changes in small-strain stiffness.