Electronic Thesis and Dissertation Repository

Degree

Doctor of Philosophy

Program

Civil and Environmental Engineering

Supervisor

Dr. M. Hesham El-Naggar

Abstract

Laterally loaded structures such as wind turbines, offshore platforms, earth retaining structures, bridge abutments and many other structures impose complex loading regimes on the supporting foundations. These foundations are typically subjected to combinations of vertical, horizontal and moment loadings resulting from the vertical self-weight of the foundation system, superstructure, soil fill and surface surcharge, in addition to significant lateral loads and moments due to soil pressures, wind loads, waves and currents. In the case of shallow foundations, the weight and dimensions must be sufficient to resist tilting and sliding, whilst at the same time preventing failure of the subsoil and satisfying any serviceability conditions. These criteria can be achieved by controlling the width and thickness of the shallow foundation. However, this often requires the use of large and thick footings, which is a problem especially in locations with limited access and in offshore environment, where the presence of gravity bases has a large influence on the movement of the surrounding water and generates significant heave forces.

In this study, an innovative technique is proposed to improve the lateral capacity of shallow footings by providing them with a central short monopile. The interaction between the footing and the monopile in the proposed hybrid foundation system has proven to increase the lateral load resistance and decrease the dependency of the lateral resistance on the vertical load component acting on the system. The lateral load resistance of the hybrid system is generated, even at low vertical load ratios, directly by mobilizing passive lateral pressures on the embedded portion and indirectly through the restoring moment resulting by the bearing stresses underneath the footing. The two phenomena together contribute to the resistance of sliding and rotation of the whole system. An extensive 2D and 3D numerical modeling program, complemented with physical centrifuge testing have been used to study the behavior of the system under various combinations of vertical, horizontal and moment (V-H-M) loadings and generate detailed information about the failure envelopes in three-dimensional (V-H-M) load space for both drained and undrained loading. The numerical study indicates that the interaction between the structural elements of the hybrid system increased the strength and stiffness of the foundation system and mobilized high lateral load and rocking resistances, even at low strains. The resistance was found to rely mainly on the geometry of the system, especially the pile length-to-footing width ratio.


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