Date of Award

12-2015

Document Type

Thesis

Degree Name

Master of Science (MS)

Legacy Department

Civil Engineering

Advisor

Ravichandran, Nadarajah

Committee Member

Andrus, Ronald D.

Committee Member

Chen, Qiushi

Abstract

Wind turbines play a vital role in producing sustainable and clean energy to fulfill the growing energy needs. Energy generated from wind turbines being more sustainable and cost effective, many countries have taken a major step to develop wind turbines with large capacity. There has been a significant reduction in the levelized cost of energy (LCOE) since the 1980’s from the development of larger wind farms and from consistent performance improvements of wind turbine components. The power generation capacity of wind turbines has increased significantly over the years with the use of taller towers. When the tower height increases, the loads on the foundation increase and the foundation becomes significantly larger. The geotechnical design of foundations for taller wind turbines has also become complicated presenting unique challenges for each location. In this research, the economic advantages of using raft foundations, pile group foundations and piled-raft foundations are discussed based on the geotechnical design results and parametric studies using both analytical calculations and numerical simulations using GROUP and PLAXIS 3D software. For the analytical design, the axial load, lateral load and bending moment at the base of the tower were computed assuming a tower height of 130 m and design wind speed of 160 mph. All the geotechnical parameters required for the design of the foundation were obtained from a geotechnical engineering report for a location in North Charleston, SC. The final design of the raft resulted in the circular raft with a radius of 18 m, thickness of 1 m and depth of foundation of 1.5 m. The final design of the pile group with a pile cap resulted in 40 pre-stressed concrete piles (PCPs) of width 0.61 m (24 in.) and varying length from 25 m to 40 m. Out of 40 PCPs, 18 were arranged in the circle of radius 5.3 m and rest in the radius of 6.7 m. Based the minimum requirement of the pile cap, a circular pile cap of radius 7.43 m and thickness 1.0 m was selected. Similarly, the final design of the piled-raft foundation resulted in a raft of radius of 7.5 m and thickness of 1.2 m and 32 pre-stressed concrete piles of 0.457 m (18 in.) width and 10 m length. The piles were arranged in two radii: 16 inner piles in the circle of radius 5.3 m radius and 16 outer piles at 6.7 m radius. Based on the stiffness of the raft and the pile along with pile-soil-pile interaction factor the load shared by the piles was calculated to be 85 % of the applied load. The comparison of the volume of material required for each foundation type showed that the piled-raft is the most economical option for the given site conditions and loading. The performance of the pile group foundation with a pile cap was then investigated numerically in a coupled manner using an industry standard finite difference software called GROUP and the performance of the piled-raft foundation was investigated using a finite element software PLAXIS 3D. A parametric study was performed to develop a relationship between the volume of material, which can be directly related to the cost of the foundation, versus input and design parameters such as the tower height, wind speed and pile size. For each case of parametric study, the minimum safety and settlement requirements were always maintained by adjusting the number and length of the piles. It was found that an increase in wind speed, tower height and pile size resulted in a nonlinear increase in volume of material required while an increase in the number of piles decreased the volume of material up to a certain number of piles and then it started to increase. Another study on the effect of the amount of piles with regards to settlement revealed that the increase in number of piles while keeping the size and length constant, resulted in significant decrease in settlement up to certain number of piles. Thereafter, increasing the number of piles had negligible contribution in reducing the settlement. This finding is consistent with the results obtained from the previous analysis, where the optimum number and length of piles were determined.

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