Modelling of Cyclic Soil Degradation

This thesis entitled "Modelling of cyclic degradation - Development of a cyclic accumulation model and the application to a gravity based foundation" presents a new soil model to analyse the behaviour of undrained soils subjected to cyclic loading. Cyclic loading on offshore support structures for wind-turbines results in complex design challenges, and current codes and standards offer limited guidance on the assessment of undrained cyclic soil behaviour. To assess the undrained cyclic behaviour, a new procedure is being developed within the finite element software Plaxis, the UDCAM-S procedure. This new procedure is designed to asses the undrained cyclic behaviour of clays in the design of offshore support structures. This study focuses on the development of this procedure and the application to a gravity based foundation for a large wind-turbine placed on clay. Large cyclic wind and water loads acting on the turbine are transferred through the structure to the foundation, resulting in cyclic shear stresses in the clay. These shear stresses lead to an increase in pore pressure due to the restriction of pore pressure dissipation, and thereby a decrease in effective stress. With each cycle, shear strains are developed due to the decrease in effective stress: the average, cyclic and permanent shear strains. Permanent shear strains accumulate as a result of accumulated pore pressures due to cyclic shearing of the soil, the irreversible deformations of the soil. With the accumulation of pore pressures and plastic shear strains the soil degrades in terms of the undrained shear
strength and the stiffness of the soil.

The degradation of the soil as a result of a design storm is determined with the UDCAM-S procedure. The design storm is simplified by analysing the cyclic loads in terms of load parcels, to use the load data in the strain accumulation procedure (Andersen, 1976). With the result of the strain accumulation procedure, the degraded behaviour of the soil can be determined by means of scaled contour diagrams for the behaviour in direct simple shear, triaxial compression, and triaxial extension. Parameters of the NGI-ADP soil model in Plaxis are fitted onto the degraded stress-strain curves, resulting in a parameter set accounting for the degradation of the soil by the given design storm. The results of the UDCAM-S procedure have been used to model the influence of cyclic degradation on the stability of gravity based foundations. The investigated cases gave insight in the influence of the different steps in the UDCAM-S procedure on the outcome of a finite element analysis. It has been found that VHM capacity of gravity based foundations can be reduced significantly by large equivalent number of cycles for a design storm. As the soil strength decreases with increasing equivalent number of cycles, the maximum moment resistance is mobilized at lower vertical loads. This results in a large reduction of the HM capacity at higher vertical loads. The same effect has been found on the stability of the foundation for different widths of the foundation. The equivalent number of cycles is the dominant factor influencing the stability of the foundation. This is due to the accumulation of large strains due to a design storm, resulting in a softer clay.

Details

Title	Modelling of Cyclic Soil Degradation
Pages	110
Language	English
Format	PDF
Size	6 MB
Download Method	Direct Download
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