ABSTRACT
This paper describes field monitoring and analysis of an onshore shallow wind turbine foundation. The study has investigated the soil-structure interaction under cyclic and complex vertical-horizontal-moment loading, and correlates this with the observed wind and turbine responses. The results form an important part of the wind-chain, providing information for optimizing the performance of wind turbines and for structural health monitoring systems. The field measurement systems for the foundation, tower and wind fields are described, and the responses for high/low probability wind events are discussed. The implications for soil strength and stiffness degradation due to the associated loading and the general damage accumulation of small load cycles during operation of the wind turbine are examined. The possible consequences for the performance of the wind turbine during extreme loading conditions and over the long-term are also discussed.
R
ÉSUMÉ
Ce document décrit la surveillance sur le terrain et l'analyse d'une fondation d'éolienne sur la côte peu profonde. L'étude a examiné l'interaction sol-structure sous cyclique et complexe vertical-horizontal-charge de moment, et ce en corrélation avec les turbines éoliennes et les réponses observées. Les résultats constituent une partie importante de la liquidation chaîne, fournissant des informations pour optimiser les performances des turbines et des systèmes structuraux de surveillance de la santé. Les systèmes de mesure de champ pour les champs fondation, la tour et le vent sont décrites, et les réponses pour / faibles des événements à haute probabilité de vent sont discutées. Les conséquences pour la résistance du sol et de la dégradation de rigidité due à la charge associée et le préjudice général accumulation de petits cycles de charge pendant le fonctionnement de l'éolienne sont examinés. Les conséquences possibles de la performance de l'éolienne dans des conditions de chargement extrêmes et sur le long terme sont également discutés.
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. However, to achieve this expansion some major technical and policy issues must be addressed by the Canadian wind sector. Some of these issues are associated with the construction and design of foundations for wind turbines. Foundations for onshore wind turbines usually consist of large gravity bases and mono-piles. The geometry and foundation type depends on the wind climate, power regulation philosophy, physical characteristics of the machine, uplift criteria, required foundation stiffness and geotechnical characteristics of the site. The critical analyses for design include bearing capacity and overturning resistance, horizontal/rotational displacements, and dynamic soilstructure interaction.
Extreme wind effects and local storms account for more than 60% of damages to wind farms and $200M annual losses for Canada. Turbines are subjected to millions of load cycles from the wind during their design lives. Some potential performance issues are also related to fatigue experienced in various components from this long-term loading regime. Unfortunately, there is a paucity of appropriate field and laboratory scale datasets, suitable for calibration and validation of current state-of-the-art analytical and numerical design approaches. A multidisciplinary research project is underway to integrate laboratory element testing, scaled physical laboratory testing, full scale field monitoring and numerical modeling of a wind turbine founded on a shallow foundation in carbonate-rich silty clay till in Southern Ontario. A significant component of the project is to provide understanding of the full ‘wind-chain’ from the incoming wind field to the underlying soil and its effects on the performance of the wind turbine.
An important link in the ‘wind-chain’ is the foundation and its response to the cyclic and complex verticalhorizontal-moment loading, placed upon it throughout its lifetime. In order to understand this component of the
‘wind-chain’ a detailed site investigation has been conducted and a full scale long-term monitoring system has been installed on a large onshore turbine. The turbine tower and its foundation have been instrumented with various sensors. The implications for soil strength and stiffness degradation due to the associated loading and the general damage accumulation due to small load

cycles during operation of the wind turbine are examined.
The possible consequences for performance of the wind turbine during extreme loading conditions and over the long-term are also discussed.
2 SITE DESCRIPTION
A large 2.3 MW wind turbine that is located in South-
Western Ontario is the object of this field study. The turbine is one of an 88 turbine commercial wind farm near
Lake Erie. This specific turbine model has a rotor diameter and hub height of 93m and 80m respectively.
The turbine is founded on a shallow octagonal foundation, which can be circumscribed about a circle of diameter (D) of 19m. The foundation is 3m deep at its thickest and tapers out towards the edges. One hundred and fifty meters to the North-West is a meteorological tower instrumented with cup anemometers.
2.1 Soil Description
A detailed site investigation involving laboratory tests and in-situ tests has been performed previously and is reported in detail by Tyldesley et al. (2013). Four boreholes were drilled onsite and soil samples were collected. Various in-situ tests, such as SPT, CPT and cross-hole geophysics were also performed. It was found that the site is underlain by clayey silt tills, rich in carbonates and the deposit can be approximately split into three layers. A heavily weathered upper crust, a partially weathered lower crust and an un-weathered clay till.
Table 1 shows average parameter values for different geotechnical properties for each layer.
Table 1: Average Parameters for Soil Layers
(Gonzalez, 2014)
Property
Upper
Crust
Lower
Crust
Unweathered
Till
Depth (m) 0-1.5 1.5-4.5
Water Content (%) 22-32
Unit Weight(kN/m 3 ) 20.3
Liquid Limit (%)
Plastic Limit (%)
46
21
16-20
21
34
19
Clay (%)
Silt (%)
Sand (%)
40
45
15
29
49
20
4.5-40
16-24
21.6
30
17
31
45
21
Figures 1 and 2 show derived data from one of the seismic CPT borehole soundings, within 10m of the foundation edge. The distinction between layers is noticeable from both the undrained shear strength profile
(S u
) and the small-strain shear modulus (G o
) as shown in the figures. The founding level of the turbine foundation
(3m depth) occurs approximately at the peak strength/stiffness values of the profile and S u
and G o diminish relatively quickly to uniform values at depths of 6-
8m (approximately D/3) as the material transitions into the un-weathered clay till zone.
Figure 1: Undrained Shear Strength (S u
) from CPT
Figure 2: Small Strain Shear (G o
) Modulus from CPT

2.2 IEC Classification and Design Wind Speeds
The wind farm has been classified as an IEC 61400-1
Class IIb site; this categorizes the area as having medium level turbulence intensity and wind speeds (IEC 61400-1).
The expected value of the turbulence intensity at a wind speed of 15 m/s for a class IIb is 14% and the reference
50-year return period 10-minute average wind speed is specified as 42.5 m/s. The project site specific extreme value 50-year 10-minute average wind speeds have been estimated as 33.9 m/s. The annual average wind speed at hub height is 8.5 m/s and the 50-year return period 3 second gust wind speed is 59.5 m/s.
3 FIELD INSTRUMENTATION
3.1 Foundation Monitoring
The foundation has been instrumented with four uniaxial tiltmeters. The tiltmeters use a uniaxial Micro-Electro-
Mechanical Systems accelerometer to determine the angle of inclination from the horizontal with an accuracy of
+/- 5 arc sec up to a full scale reading of +/- 15˚. Readings of the tiltmeters are sampled at 20 Hz. The tiltmeters are installed at each of the cardinal directions, as shown in
Figure 1, to measure the rotation of the foundation in the
N-S and E-W directions. The tiltmeters are denoted TM 1 through 4 and their direction of measurement is listed in
Table 1. It should be noted that while N-S and S-N are in the same plane, the difference comes from the direction taken to be positive.
Figure 3: Diagram of foundation and locations of tiltmeters.
Table 2: Tiltmeter notation and measurement directions
Device
TM1
TM2
TM3
TM4
Measurement Direction
S-N
W-E
N-S
E-W
3.2 Wind Monitoring
In order to compare the response of the turbine to the excitation of the wind, the speed and direction of the wind must be known. Wind speed and direction is collected and averaged at 1 sec intervals from 5 cup anemometers installed on a meteorological (MET) tower located 150 m to the North-West of the turbine. These anemometers are located at 34, 61, 70, 77 and 80 m above ground level.
Wind speed and direction is also recorded at the hub of the turbine (with another cup anemometer) and every turbine on the site measures both electrical output and wind speed simultaneously. For this study the wind speed and direction measured from the MET tower at a hub height, 80 m, will be analyzed.
3.3 Strain Gauges
A Fiber Bragg Grating sensor array of strain gauges as described by Bas et al (2012), has been installed at the four cardinal directions at six elevations up the turbine tower. The array measures longitudinal deformation in the tower and is sampled at 100 Hz.
4 OBSERVED FIELD DATA
4.1 Wind Monitoring
From monitoring and analyzing the wind environment at the site, an extreme value analysis has been conducted to compare with the site specific values and IEC classification of the site. Peak gust wind speeds for different return periods were calculated by using a superstation approach (Peterka, 1992). This method combines short-term data from multiple stations in a region, into a single superstation, with a large number of station years of data. This relies on statistical independence of yearly extreme wind speeds measured at the different stations. For this study, a single superstation was formed from the hub wind speed measurements of 44 of the wind turbines on the farm, over a two year period (2010-2011). The anemometers on the site give 10-minute average wind speeds and the annual mean 10-minute wind speed across all 44 sites for
2010-2011 was 6.68 m/s.
If each site is treated as being independent and the annual maxima observed at each site, this gives an equivalent of 88 years of data. It should be noted, however, that three of the annual maxima exceed the site design wind speed of 33.9 m/s, and a further two values exceed 25 m/s. An examination of the times at which the

annual maxima occur also reveals that many of them occur at the same time, which means that they cannot be considered to be truly independent. If the unique times are determined and then the largest value found to have occurred at this time, this reduces the annual maxima to a total of 35 values. While a further reduction to 13 values is achieved if only the largest value observed on a particular day is used. These points are plotted in Figure 4, along with a fit of the annual maxima to a Gumbel (Type I) extreme value distribution. The extreme value fit suggests that the 50-year 10-minute mean wind speed at the hub height is 26.9 m/s. Increasing the number of points for the analysis, results in values that are still in the range of 26-
27 m/s. All of these values are less than stated site design value of 33.9 m/s.
The 10-year and 50-year NBCC values for Leamington at a height of 10m are 23 and 27 m/s (shown with the open triangles). When adjusted to the 80m hub height, using the open terrain power law profile specified for use with the NBCC dynamic procedure, this yields a value of
36.1 m/s. The turbine design takes a code design wind speed of 42.5 m/s, from IEC 61400-1. This value is the average annual wind speed multiplied by a factor of 5, which is consistent with the code value of the average annual wind speed of 8.5 m/s from IEC for this site class; this seems to be based on European practice and no justification is provided in IEC 61400-1.
Comparison between the site wind observations and the design wind speeds suggest relatively conservation design values. However, a possible reason for the poor matching of the superstation data to the Gumbel fits and the predicted 50-year wind speeds is the limited range of wind speeds in the original dataset. It should also be noted that these values relate to straight line winds from synoptic weather systems, and localized extreme wind speeds from tornado and downburst events may significantly exceed the code values.
4.2 Tower Response
Previous studies conducted on the tower by Smith et al,
(2014) used the Fiber Bragg Grating arrays along the tower height to determine the bending moment in the tower. The study investigated the maximum base bending moment corresponding to a 10-minute average wind speed at 80m above ground level over a 12 week period.
Figure 5 shows the relation of the maximum bending moment, occurring at the base, with wind speed. The moment magnitudes are seen to increase proportionally up to a maximum of 12 m/s. where rated power production is achieved. Above rated speeds, moment magnitudes gradually decrease with increasing wind speed, while production power remains approximately constant. This is typical for pitch-regulated turbines.
However, at higher wind speeds and in the event of control failures, these moments would be expected to exceed those shown. The highest design overturning moment from IEC 61400-1 (DLC6.1) is 76.2 MNm, which corresponds to a design wind speed of 59.5 m/s.
Figure 4: Gumbel Type I fit of extreme value wind data
Figure 5: Maximum Bending Moment with Wind Speed
4.3 Typical Foundation Data and Analysis
At the completion of this project, a long-term database of the wind environment and turbine system response will be compiled. This will allow the analysis of extreme events, as well as typical operating conditions. It will also provide the basis of a structural health monitoring system for the turbine. For the purposes of brevity, only a typical 2 hour operating data period will be presented herein, with the responses from tilt meters 3 and 4. This will be used to elucidate some of the approaches and findings from the study.
The fluctuation of the wind direction over the 2 hour duration is illustrated in Figure 6. The direction is measured in degrees from North, indicating that the wind was predominantly from the South-West. The wind speed had an average value of 12.25 m/s and a standard deviation of 1.62 m/s. The wind speed with time is illustrated in Figure 7 and the turbulence intensity is
13.2% for this period. High-shear, low-turbulence conditions are found to give slightly higher moments between cut-in and rated wind speeds. Low-shear, high-

turbulence conditions give increased levels of shear variance across the tower elevation, potentially exciting higher modes of vibration in the tower.
Figure 6: Fluctuation of Wind Direction a)
Figure 7: Wind Speed Fluctuations
The response of the foundation has been analyzed in a number of ways. First the angle of rotation about the measured axis for each tiltmeter has been calculated and plotted for the 2 hour period, see Figure 8. The mean and standard deviation of the rotation in each direction is also shown. The mean rotations were 0.0330, -0.0318 degrees and .0590, -.0591 degrees with standard deviations of
0.0251, 0.0248 degrees and 0.0471, 0.0467 degrees for tiltmeters 3 and 4 respectively. It can be seen that tiltmeter 3, which measures rotation in the N-S plane, shows smaller deflections than those of tiltmeter 4 in the
E-W direction. It is also evident that the angles of rotation were fluctuating about the neutral point indicating that the foundation is rocking back and forth, and not about a point with positive bias, which will have implications for the instantaneous induced moments. It is worth noting that rocking will be induced not only by the wind acting on the tower pushing it back and forth but also by the rotation of the blades causing rocking perpendicular to the direction of the wind. b)
Figure 8: Foundation Rotation Response
Rainflow counting techniques were also employed to group similar amplitude rotational cycles into bins to categorize levels of strain on the underlying soil. Figure 9, depicts histograms for each of the tiltmeters. A Rayleigh distribution was fitted to each histogram and the statistics are shown. Drawing from the realm of physical oceanography, the equivalent of a ‘significant wave height ’ of the tilt response was also computed; this is defined as the mean response of the highest third of the measured values. Liu and Pinho (2012) presented an empirical relation to find the expected maximum values in a Rayleigh distribution from this quantity for a given number of cycles as shown in equation (1).
θ max
/ θ s
=[ln(N)/2] 1/2 [1]
Where
θ max
/
θ s
is the ratio of the maximum rotation to the significant rotation and N is the number of cycles. For a 2 hour period and approximate cycle frequency based

on the natural frequency of the tower the maximum response rotations were found to be 0.1204 and 0.2246 degrees for tiltmeters 3 and 4 respectively. These values match well with the extreme values shown in the histograms below. a) b)
Figure 9: Rainflow Counting Histograms with Rayleigh Fits
5 DISCUSSION
Standard design approaches for serviceability and ultimate limit states for shallow foundations (e.g. DNV,
2010; IEC 61400-1, 2005), rely on isotropic elastic analyses of half-spaces (Borrowicka, 1943) and empirical modifications of the standard bearing capacity equation for surface founded shallow foundations (Meyerhof,
1953). However, the coupling between vertical, horizontal and moment loads has been shown to change the capacity of the foundation system, particularly for lightly vertically loaded systems (Gouvernec, 2007). Based on pure rotation of the foundation, the ultimate moment capacity is given by, Osman et al. (2007):
M ult
=A*D*S u
*N cm
[2]
Where A is the area of the foundation, D is the diameter and N cm is a bearing capacity factor for moment loading on a rough circular surface foundation (= 0.67).
This gives an ultimate moment capacity for this foundation of 215-290 MNm, depending on the undrained shear strength taken from the soil profile. Hence the ratio of M/M ult
for the maximum operating moment (45 MNm) is in the range of 0.26-0.35.
A different approach that includes the coupled effect of vertical (V) and moment (M) loads suggests that the ultimate moment capacity (Gazetas et al, 2014) is:
M ult
/(V ult
*D)≈ 0.55 * V/V ult
* (1- V/V ult
)
Where V ult is 6.05*S u
* π *D 2 /4
[3]
The design vertical load (V) is 2900 kN, giving a very lightly loaded foundation with V/V ult
= 0.028 to 0.021. This gives much lower moment capacities in the range of 29.5
MNm. Since this analysis is based on a surface foundation, the embedment appears to be an important part of the moment capacity of the turbine stability. Indeed
V/V ult would need to rise to 0.19, to achieve the same rotational stability as the pure moment case.
Osman et al. (2007) presented the mobilisable strength design approach, where plastic solutions are used to scale stress-strain relations into loaddisplacement. Following this method, a foundation response ratio of the maximum design moment to the operational moment is computed. The design moment corresponding to the 50-year gust design wind speeds is
76.2 MNm. This yields a foundation response ratio, M/M ult of 0.3. Using the relations provided by Osman et al, 2007 the shear modulus ratio G/G o
has been estimated at 0.25.
The small strain stiffness G o
at a depth of D below the foundation from CPT data is 60-80 MPa, as shown in
Figure 2. Applying the shear modulus reduction for nonlinearity and dynamic loading, an estimate of the shear modulus is 15-20 MPa. It should be noted that the shear modulus reduction ratio is typically suggested to be 0.25 for the presumed typical strain levels of 10 -3 for wind turbines by the DNV standard (DNV-OS-J101, 2010).
The applied overturning moment (M) of the wind turbine for the average wind speed of 12.25m/s for the 2 hour period is 45 MNm, (see Figure 3). The mean absolute value of rotation θ, for this 2 hour period is
0.0325 and 0.0602 degrees for tiltmeters 3 and 4

respectively. Using the Borrowicka (1943) equation (4), and the mean rotation from tiltmeter 4 (the larger of the two means) the operational shear modulus is found to be
10.3 MPa.
K
R
= 8GR 3 /(3*(1ν)) [4]
Where G is the shear modulus, v is the Poisson’s ratio;
R is the radius of the foundation.
Interestingly the measured operational rocking stiffness K r
appears to be 40 GNm/rad, which is a little lower than the manufacturer
’s specified minimum rocking stiffness of 50 GNm/rad. This may be attributed to inaccuracies in the measurements of the tiltmeters at
20Hz, which will be confirmed by complementary accelerometer measurements on the foundation. Or may suggest that the mechanism of rocking used in the analysis may not be wholly appropriate; Gazetus et al.
(2014) suggest that rotation and localized plastic deformation around the foundation toe is more likely for lightly (vertically) loaded systems subjected to moments.
This is similar to the leaning instability mechanism identified by Potts, (2003) for the Leaning Tower of Pisa. If this is found to be the case then the support of the soil at the sides of the embedded foundation, above the foundation level may be more important that currently assumed.
Assuming proportional responses of the moments and rotations to the applied wind speeds, the 50-year gust wind speed would give average rotations of 0.1 degrees and soil strains in the range of 10 -2 to 10 -3 , with reductions of G/G o
to 0.2. This is generally in the same strain range as assumed by design codes, such as DNV OS-J101
(2010). For fatigue calculations, IEC 61400-1 (2005) assumes up to 1000 strain cycles of this magnitude in a
20 year design life of a turbine. Further cyclic stress-strain laboratory testing is being conducted on the site soils to determine the accumulated strain damage due to applied strain cycles over a typical turbine life-cycle. Additional work is also being conducted to investigate the effects of operating activities such as emergency stops and turbine restarts.
6 CONCLUSIONS
There is currently a lack of full scale field monitoring data sets of operating wind turbine structures available in the public domain. This leads designers to use traditional and potentially conservative methods when designing foundations for wind turbine structures. As demonstrated herein, the loading and response relationship of these structures is complex and the application of more innovative analyses can match field data. It is anticipated that upon completion of this study a better understanding of the foundation response to cyclic and complex verticalhorizontal-moment loading will be established. This will help with validation and calibration of state of the art design approaches, leading to more robust and economical designs, potentially extending the life-cycles and investments made by wind turbine owners and operators.
ACKNOWLEDGEMENTS
The authors would like to acknowledge the help and support of JJ Davis, Paul Dawson, as well as the comments and suggestions of Jamie Smith. The financial support of NSERC for the first author is also acknowledged.
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