Electronic Thesis and Dissertation Repository

Degree

Master of Engineering Science

Program

Civil and Environmental Engineering

Supervisor

Dr. Timothy Newson

Abstract

With the growing wind industry in Canada, it has become important to optimize wind turbine foundation design. Wind turbine foundations are subjected to combinations of vertical, horizontal and moment loads due to vertical self-weight of the structure and soil surcharge. Additionally, there are significant lateral loads and overturning moments attributed to varying wind forces acting at considerable tower heights above the ground level. In this thesis, the undrained bearing capacity response of circular & octagonal foundations subjected to combined loading is calculated using finite element analyses. Previous works have mostly focused on circular foundations. An octagonal foundation of the style typically used in the wind industry forms the focus of this research. Foundations are either surface based or embedded in homogeneous or heterogeneous soils. The results are expressed in terms of a coherent set of bearing capacity factors and failure envelopes in two dimensional planes (VH, VM and HM). This research also presents a parametric study on the effect of a surficial crust on the bearing capacity of a foundation. Finally, working and design loads for a typical wind turbine foundation are plotted in two dimensional failure planes to investigate if there is a potential `spare' capacity. The finite element study indicates that an increase in soil strength heterogeneity and embedment leads to increases in the uniaxial limit capacities and size of the failure envelopes. For octagonal foundations, the average increase in uniaxial vertical, horizontal and moment capacities due to increases in the embedment is 15%, 52% and 32% respectively. The average increase in uniaxial vertical and moment capacities due to increase in the soil strength heterogeneity is 7.1% and 6.7% respectively, for octagonal foundations. When the shape of a foundation changes from a circle to an octagon, the ultimate uniaxial vertical and moment capacities slightly increase (by 7.7% and 7.2% respectively). Under combined loading, conventional methods are found to underestimate the combinations of horizontal and moment loads that a foundation can resist safely. During eccentric loading, the effective area predicted using the method given by DNV (2002) is also under-predicted.

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