Date of Award

May 2019

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Civil Engineering

Committee Member

Dr. Nadarajah Ravichandran

Committee Member

Dr. Ronald D Andrus

Committee Member

Dr. Kalyan R Piratla

Committee Member

Dr. Laura Redmond

Abstract

This study presents a simplified geotechnical design, design optimization, and finite element modeling of the piled-raft foundation intended for a 130 m tall wind turbine for different site conditions. The sites considered are composed of multilayered soil, clayey soil, and sandy soil. The simplified geotechnical design includes the safety checks (vertical load, horizontal load, and bending moment capacities) and serviceability check (total vertical and differential settlements). The simplified design showed that the final design is controlled by differential settlement requirement. Subsequently, a parametric study was also conducted to investigate the effect of soil strength parameter (undrained cohesion for clay and friction angle for sand) and wind speed on the design. The major drawback of this parametric study is that only one variable is changed at a time. However, more than one variable can change at the same time. Therefore, a reliability-based robust design optimization was conducted using Non-dominated Sorting Genetic Algorithm – II (NSGA-II) coupled with Monte Carlo simulation. In the design optimization, the wind speed and soil strength parameter were considered as random variables, radius of raft, length of pile, and number of piles were considered as the design variables, and the total cost of the foundation and the standard deviation of differential settlement were considered as the two objectives to satisfy. This resulted in a set of acceptable designs forming a Pareto front which showed a trade-off relationship between the total cost and standard deviation of differential settlement which can be used to obtain the design as per the cost and safety requirement. The most optimum design can be obtained using the knee point concept. Further, a three-dimensional finite element model of the piled-raft foundation was developed and analyzed in ABAQUS and the response was compared with the simplified analytical design results. The stress-strain behavior of soil was represented by both linear and nonlinear constitutive models. The soil-structure interfaces were modeled by defining the interaction properties at the interfaces. It was observed that the analytical design resulted in a higher vertical settlement and the horizontal displacement and lower differential settlement and rotation compared to the finite element result. The parametric study conducted subsequently by varying the wind speed and undrained cohesion of soil showed that the difference between the predicted responses from two methods decreases when the load is large and/or soil is soft. Finally, a preliminary study on the development of a new foundation for wind turbine through biomimicry is also presented. Since wind turbine is comparable to a coconut tree, sabal palm tree, and Palmyra tree, the root of these trees is studied to develop simplified configurations with a different number of main roots and sub-roots. The results showed that the performance of the foundation under combined load improved with the increase in the number of main roots while the sub-roots had a negligible contribution to the performance of the foundation.

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