FOUNDATION SOIL RESPONSE TO WIND TURBINE GENERATOR LOADING
Executive Summary Dynamically loaded wind turbine generator (WTG) foundation design requires a specialized design process due to abnormal loading conditions over a large bearing area. Multiple foundation options exist to support WTGs. A large octagonal mass of reinforced concrete is the most commonly used foundation type. Two high-capacity (? 1.5 MW) WTGs were instrumented in the upper Midwest of the US. The instrumentation was oriented to take advantage of the predominant wind direction in each site. This thesis focuses on the interpretation and analysis of data from these two instrumented WTG foundations. Ten soil deformation gauges were installed at each site. The main purpose of the soil strain gauges (SG) was to measure the deformation of the underlying bearing soil at different locations and depths. Pressure gauges (PG) were installed to monitor the dynamic pressure distribution underlying the octagonal WTG foundations. At one of the sites (Site A), thermal dissipation sensors and micro-electro-mechanical system accelerometers (MEMS) were installed to monitor volumetric water content change and foundation block rotation, respectively. Turbine towers were also instrumented with strain gauges to estimate moment transfer from the tower to the foundation. Improvement of design approaches for dynamically loaded foundations, such as WTG foundations, requires knowledge of stress-strain transfer mechanisms. Data analysis from field- instrumented WTG foundation systems can be used to validate present-day design assumptions and to provide new and mechanically accurate approaches. Observations of contact pressure distribution, foundation soil deformation, and transferred moment from tower to foundation assist in understanding the mechanistic and dynamic behavior of WTG foundations and soil bearing response. Based on this research effort, changes in soil pressure and strain were highly related to wind direction and speed. At normal operating conditions, the most variation in pressure distribution was observed towards the edge of the foundation. Normalized pressure amplitude was around 0.35 (pressure/pressure average of analyzed data set) for the pressure cells locate at the edge of the foundation. Interior pressure cells (e.g., PG-2, PG-3, and PG-4), on the other hand, exhibited lower amplitudes (? 0.10). This indicates that the outer portion of the foundation is more susceptible to stress changes. Although pressure was distributed across entire the foundation footprint, pressure response was not uniform. Pressure shifts were observed in the cases of startup and shutdown conditions. During the shutdown sequences, greater pressure fluctuations were observed (e.g., 17% in PG-3, 48% in PG-4). Pressure data analysis indicated that maximum and minimum pressures occur during turbine shutdown. Pressure spikes were observed during shutdown varying from 2 kPa to 10 kPa depending on the location of the pressure cell. Pressure cell - soil stiffness interaction is required for analysis of this type of field data. Under-representation was observed due to pressure drop in pressure cells. These decreases which under-represents the calculated static dead load of 78 kPa are attributed cell-soil stiffness difference and ?bridging? phenomenon. Soil strain was also non-uniform in distribution, both horizontally and vertically. The highest elastic soil deformation (0.02 mm over the gage length of 300 mm) occurred at the leeward site of the predominant wind direction. Moreover, soil deformation decreased systematically with depth. Strain level at full power production was computed as 0.006% immediately beneath the foundation and approximately 80% of this strain dissipates within 1.7 m. A commonly assumed cyclic strain level of 0.1% for design purposes (Det Norske Veritas)may significantly over-estimate strain levels experienced in the field for sites with stiff clay, such as these two instrumented sites in the mid-west. The observed displacement and pressure trends were symmetric depth dependent and highly correlated to wind direction and speed and location. Thermal dissipation sensors indicated that gravimetric water content does not significantly change over time as the foundation soil is shield from most environmental changes by the concrete block. The observed changes are most likely related to large seasonal changes (23% � 2%). According to the MEMS accelerometer analysis, tilts were computed as 0.38o at S30E, 0.16o at 90W, and 0.18o at N30E. These tilts create approximately 0.5 m sway (in amplitude) at the top of the WTG.