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Thesis Format

Monograph

Degree

Doctor of Philosophy

Program

Civil and Environmental Engineering

Supervisor

Dr. Timothy A. Newson

Abstract

Shallow wind turbine foundations are designed based on investigations of the ultimate, serviceability and fatigue limit states. The serviceability limit state design approaches in particular are based on simple isotropic elastic half-space analyses that ignore coupling between loading directions, and soil non-linearity and elastic anisotropy. Many of the wind farms in Ontario are constructed around the Great Lakes basin and a number of these areas are characterized as stiff clayey glacial tills. It is recognized that many of these glacial materials exhibit some degree of strength, stiffness and fabric anisotropy. This research aimed to characterize the anisotropic geotechnical properties of a specific glacial till deposit that underlies a 2.3 MW onshore wind turbine at a commercial wind farm in Port Alma, Ontario, and also assess the anisotropic elastic soil-foundation interaction between the underlying glacial till and the shallow foundation that supports the wind turbine. Additionally, the small-strain and anisotropic stiffness properties of a number of well-known Canadian clays of glacial origin were measured to provide bounds on the range of elastic behaviour that these forms of foundation may be subjected.

High quality laboratory samples of the Port Alma glacial till, as well as the other glacial clays studied, were tested in their intact state using consolidated isotropic undrained (CIU) triaxial tests with locally measured axial strains, CIU tests fitted with orthogonal bender element transducers and resonant column tests on vertically and horizontally trimmed samples. The critical state properties and stress-strain behaviour under monotonic loading of the Port Alma till showed similarities to those exhibited by other glacial clay tills found in the literature. The behaviour of the clay tills was also investigated at very small-strains. Results from bender element and resonant column tests showed that: the small-strain stiffness of the till material increases with increasing confining pressure according to a power law relationship; the stiffness of the material is strain dependent and the elastic stiffness of the material is higher in its horizontal direction than vertical. The effects of the stiffness anisotropy on the soil-foundation interaction for the wind turbine was investigated using a finite element model. The outcomes of the modelling were compared against analytical models found in the literature and measured field responses of the turbine foundation. The results indicate that the vertical and horizontal translations of the working wind turbine foundation are affected by the stiffness anisotropy, while its influence on the rocking behavior can be considered to be smaller.

Summary for Lay Audience

Shallow wind turbine foundations are designed based on investigations of the ultimate, serviceability and fatigue limit states. The serviceability limit state design approaches in particular are based on simple isotropic elastic analyses that ignore coupling between loading directions, soil non-linearity and elastic anisotropy. Many of the wind farms in Ontario are constructed around the Great Lakes basin and a number of these areas are characterized as stiff clayey glacial tills. It is recognized that many of these glacial materials exhibit some degree of strength, stiffness and fabric anisotropy. This research aimed to characterize the anisotropic geotechnical properties of a specific glacial till deposit that underlies a 2.3 MW onshore wind turbine at a commercial wind farm in Port Alma, Ontario, and also assess the anisotropic elastic soil-foundation interaction between the underlying glacial till and the shallow foundation that supports the wind turbine. Additionally, the small-strain and anisotropic stiffness properties of a number of well-known Canadian clays of glacial origin were measured to provide bounds on the range of elastic behaviour that these forms of foundation may be subjected.

High quality laboratory samples of the Port Alma glacial till, and the other glacial clays studied, were tested in their intact state using consolidated isotropic undrained (CIU) triaxial tests with locally measured axial strains, CIU tests fitted with orthogonal bender element transducers and resonant column tests on vertically and horizontally trimmed samples. Results from bender element and resonant column tests showed that the small-strain stiffness of the till increases with increasing confining pressure, the stiffness of the material is strain dependent and the elastic stiffness of the material is higher in its horizontal direction than vertical. The effects of the stiffness anisotropy on the soil-foundation interaction for the wind turbine was investigated using a finite element model. The outcomes of the modelling were compared against analytical models found in the literature and measured field responses of the turbine foundation. The results indicate that the vertical and horizontal translations of the wind turbine foundation are affected by the stiffness anisotropy, while its influence on the rocking behavior can be considered to be smaller.

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