Date of Award
Master of Science (MS)
Ronald D Andrus
Kalyan R Piratla
The generation of sustainable energy from wind has received global recognition in recent years. Large-scale wind farms with tall towers are required to meet the renewable energy demands. Taller towers produce higher power due to steady wind with higher speeds at higher altitudes. The site for building a wind farm is primarily selected based on wind conditions, accessibility to the site, and subsurface conditions. In cases where available land consists of soil with poor geotechnical properties, the construction of foundation can become expensive primarily when the foundation must sustain a substantial horizontal and moment loads induced by a tall wind turbine. In such a circumstance, the soil near the ground surface may be improved to enhance the strength and deformation properties of the soil to achieve substantial economic benefit. The study conducted shows the analytical design, 3D finite element analysis, and cost analysis for a piled-raft foundation for a tall wind turbine on in-situ and improved clays.
Initially, the analytical design of the piled-raft foundation for 80 m tall wind turbine on the in-situ soil was completed using the contemporary geotechnical design methods. The final design of the piled-raft foundation in the unimproved ground for design mean wind speed of 80 mph, consisted of 24 auger cast piles each 48.4 m long and 0.457 m in diameter. The raft was designed to be a circular raft, 8 m in diameter and 1 m in thickness. Then, five depths of ground improvement using cement soil mixing (CSM) around the piled-raft foundation were considered, and analytical design was performed for each case. The five successive depths of ground improvement correspond to 0.25, 0.3, 0.35, 0.4, and 0.45 times the diameter of the raft. Two design approaches were used: the first one was to determine the effectiveness of the ground improvement and the second to evaluate the performance. For the first design approach, the length of the piles was adjusted while keeping the number of piles, the diameter of the raft, and the cross-section of pile constant to meet the safety and serviceability requirements. The length of the pile decreased by 79.64 % for the highest depth of ground improvement in comparison with the unimproved case. On the other hand, the differential settlement increased by 73.91 %, and lateral deflection increased by 57.57 % due to the shortening of piles, but these deformations were within the design requirements. For the second approach, the length of the pile was kept constant at 48.4 m, and the deformation behavior of the piled-raft was studied. The differential settlement decreased by 12.9 %, and lateral deflection decreased by 33.05 %. The factor of safety against axial load increased by 104.9 %, and the factor of safety against the moment increased by 126.4 %.
To gain further insights into the performance of the piled-raft foundation, three-dimensional finite element models for the piled-raft foundations and supporting soil were created and analyzed using ABAQUS. The FE model created adopting the design outcome from the first approach from analytical design (length of pile varies with ground improvement) lead to a 16.37 % increase in horizontal deflection and 56.67 % increase in differential settlement for the highest level of ground improvement. The FE model created adopting the design outcome from the second approach from analytical design (length of the pile remains constant with ground improvement) leads to a 29.38 % decrease in horizontal deflection and a 1.1 % decrease in the differential settlement.
A parametric study was performed by varying the undrained shear strength of soil by ±1standard deviation (σ). The length of the pile increased by 24.38 % with positive variation in undrained shear strength and decreased by 17.36 % with negative variation in undrained shear strength soil. Cost analysis performed by adopting the length of the pile for various cases of ground improvement led to the conclusion that ground improvement reduced the total cost of the foundation.
Phuyal, Saphal, "Effectiveness of Ground Improvement Around Piled-Raft for Tall Wind Turbines in Weak Soil: Analytical and Finite Element Analyses" (2020). All Theses. 3312.