Determination of rigidity index for a shallow foundation on a carbonate clay till

Jesus Gonzalez-Hurtado, Tim Newson, Michelle Tyldesley

In the proceedings of: GeoRegina 2014: 67th Canadian Geotechnical Conference

Session: Earth Walls and Foundations

ABSTRACT: ion of rigidity index for a shallow foundation on a carbonate clay till Jesus Gonzalez-Hurtado, Tim Newson Geotechnical Research Centre, Department of Civil Engineering, University of Western Ontario, London, Ontario, Canada. Michelle Tyldesley Golder Associates Ltd., Mississauga & London, Ontario, Canada. ABSTRACT Detailed investigation of the links between operational strain levels and site investigation techniques for foundations of wind turbines have not been previously described in the literature. This paper reports on a study of an operating wind turbine in Southern Ontario. It describes laboratory and field methods used to characterize the strength, stiffness and rigidity index (ratio of shear modulus to undrained shear strength) of the carbonate clay till deposit underlying the large shallow foundation. Various correlations between in-situ tests (geophysics, CPT and SPT) and laboratory results for the stiffness and strength properties are compared. In particular, the operational strain levels for this specific geotechnical problem are investigated and appropriate methods of determining rigidity index and stiffness parameters are discussed. RÉSUMÉ Enquête sur les liens entre les niveaux de souche opérationnelles et techniques d'investigation de site pour les fondations de l'éolienne éoliennes n'ont pas été précédemment décrits dans la littérature en détail. Cet article présente une étude d'une éolienne en fonctionnement dans le sud de l'Ontario. Il décrit des méthodes de laboratoire et de terrain utilisés pour caractériser l'indice de résistance, de rigidité et de rigidité (rapport de module de cisaillement à la résistance au cisaillement non drainé) de l'argile de carbonate jusqu'au dépôt qui sous-tendent la grande fondation superficielle. Différentes corrélations entre essais in-situ (géophysique, CPT et SPT) et on a comparé les résultats de laboratoire pour les propriétés de rigidité et résistance. En particulier, les niveaux de contrainte opérationnelle pour ce problème spécifique géotechniques sont étudiés et on discute des méthodes appropriées pour déterminer les paramètres index et rigidité de rigidité. 1 INTRODUCTION Wind is a major source of renewable energy and is projected to capture 11% of the energy generation capacity for Ontario by 2018. Recent research output for the foundations of offshore wind turbines has been significant. In contrast, the literature for onshore foundation systems is still relatively sparse. Consequently, despite there being similar design issues for turbine foundations across the industry, there is often a diverse interpretation of design codes and understanding of the behavior of these foundations. This can lead to quite different foundation designs on similar wind farms, with the same turbines and comparable geotechnical profiles. This issue is exacerbated in Canada, since there is currently no regional regulatory guidance for site investigation and design specifically for wind turbine. Faced with this situation, it is not surprising that varied approaches to site investigation have developed across the industry. In some cases, despite there being a range of excellent techniques available, generic and relatively crude site investigations can occur, leading to quite conservative designs. Design approaches for serviceability and ultimate limit states for shallow wind turbine foundations are typically based on European codes (e.g. DNV, 2010; IEC 61400-1, 2005). These involve the use of isotropic elastic analyses of half-spaces (Borrowicka, 1943) and empirical modifications of the standard bearing capacity equation for surface founded shallow foundations (Meyerhof, 1953). For clay soils, analyses require accurate and appropriate estimates of strength (su, undrained shear strength) and elastic parameters (shear modulus, G and ). These can be determined from site investigation in a number of ways: empirical correlations, laboratory tests, in-situ tests and geophysical tests. The stress-strain response of soils is known to be complex and non-linear, and is dependent on the mode of loading, fabric anisotropy, rate and time effects, over-consolidation ratio, stress state and strain history. Therefore determining the appropriate test type (or group of complementary tests) that will provide the relevant stiffness and strength properties for specific geotechnical problems can be difficult and is also dependent on the funding available for the site investigation. Researchers have previously developed calibrated correlations of strength and stiffness between certain in-situ and laboratory tests, and data from monitoring of full-scale geotechnical structures (e.g. Mair, 1983). The operational stiffness moduli for structures have been of particular interest and these are usually plotted on shear degradation curves (see Figure 1). The small-strain modulus (Go) is typically determined using shear wave velocity methods and gives a limiting upper value; this is the maximum soil stiffness for a specific void ratio and stress state. Shear modulus (G) is found to decrease non-linearly with shear strain () and shown in a normalized form, with G divided

RÉSUMÉ: mination of rigidity index for a shallow foundation on a carbonate clay till Jesus Gonzalez-Hurtado, Tim Newson Geotechnical Research Centre, Department of Civil Engineering, University of Western Ontario, London, Ontario, Canada. Michelle Tyldesley Golder Associates Ltd., Mississauga & London, Ontario, Canada. ABSTRACT Detailed investigation of the links between operational strain levels and site investigation techniques for foundations of wind turbines have not been previously described in the literature. This paper reports on a study of an operating wind turbine in Southern Ontario. It describes laboratory and field methods used to characterize the strength, stiffness and rigidity index (ratio of shear modulus to undrained shear strength) of the carbonate clay till deposit underlying the large shallow foundation. Various correlations between in-situ tests (geophysics, CPT and SPT) and laboratory results for the stiffness and strength properties are compared. In particular, the operational strain levels for this specific geotechnical problem are investigated and appropriate methods of determining rigidity index and stiffness parameters are discussed. RÉSUMÉ Enquête sur les liens entre les niveaux de souche opérationnelles et techniques d'investigation de site pour les fondations de l'éolienne éoliennes n'ont pas été précédemment décrits dans la littérature en détail. Cet article présente une étude d'une éolienne en fonctionnement dans le sud de l'Ontario. Il décrit des méthodes de laboratoire et de terrain utilisés pour caractériser l'indice de résistance, de rigidité et de rigidité (rapport de module de cisaillement à la résistance au cisaillement non drainé) de l'argile de carbonate jusqu'au dépôt qui sous-tendent la grande fondation superficielle. Différentes corrélations entre essais in-situ (géophysique, CPT et SPT) et on a comparé les résultats de laboratoire pour les propriétés de rigidité et résistance. En particulier, les niveaux de contrainte opérationnelle pour ce problème spécifique géotechniques sont étudiés et on discute des méthodes appropriées pour déterminer les paramètres index et rigidité de rigidité. 1 INTRODUCTION Wind is a major source of renewable energy and is projected to capture 11% of the energy generation capacity for Ontario by 2018. Recent research output for the foundations of offshore wind turbines has been significant. In contrast, the literature for onshore foundation systems is still relatively sparse. Consequently, despite there being similar design issues for turbine foundations across the industry, there is often a diverse interpretation of design codes and understanding of the behavior of these foundations. This can lead to quite different foundation designs on similar wind farms, with the same turbines and comparable geotechnical profiles. This issue is exacerbated in Canada, since there is currently no regional regulatory guidance for site investigation and design specifically for wind turbine. Faced with this situation, it is not surprising that varied approaches to site investigation have developed across the industry. In some cases, despite there being a range of excellent techniques available, generic and relatively crude site investigations can occur, leading to quite conservative designs. Design approaches for serviceability and ultimate limit states for shallow wind turbine foundations are typically based on European codes (e.g. DNV, 2010; IEC 61400-1, 2005). These involve the use of isotropic elastic analyses of half-spaces (Borrowicka, 1943) and empirical modifications of the standard bearing capacity equation for surface founded shallow foundations (Meyerhof, 1953). For clay soils, analyses require accurate and appropriate estimates of strength (su, undrained shear strength) and elastic parameters (shear modulus, G and

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Jesus Gonzalez-Hurtado; Tim Newson; Michelle Tyldesley (2014) Determination of rigidity index for a shallow foundation on a carbonate clay till in GEO2014. Ottawa, Ontario: Canadian Geotechnical Society.

@article{GeoRegina14Paper480,author = Jesus Gonzalez-Hurtado; Tim Newson; Michelle Tyldesley,title = Determination of rigidity index for a shallow foundation on a carbonate clay till,year = 2014}