Centrifuge modeling and numerical analysis of the behaviour of suction caissons in clay

Cao, Jianchun (2003) Centrifuge modeling and numerical analysis of the behaviour of suction caissons in clay. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Although suction caissons have been used as mooring systems for offshore structures since 1980s, the working principles of caissons installed in clay and subjected to uplift loading have not been fully understood. Those phenomena include (1) installation resistance during both self-weight and suction penetration; (2) distribution of excess pore pressures, EPPs induced by installation in the soil and their dissipation after installation; (3) setup development; (4) failure mechanism and corresponding appropriate parameters for reasonable prediction of pullout capacity; and (5) distribution of EPPs in soil during pullout of a suction caisson. This study was to investigate these phenomena using both centrifuge modeling and finite element analysis (FEA). -- The first part of this study focuses on centrifuge modeling on the behavior of suction caissons in normally consolidated (NC) or slightly overconsolidated (SOC) clay. All caissons were installed in-flight by both self-weight and active suction. The undrained shear strength profile, the penetration resistance profile, the distribution and dissipation of the EPPs in clay during both installation and pullout phases, the passive suction, and the pullout capacity were measured. -- Centrifuge test results indicated that the penetration resistance depended not only on the soil properties but also on the effective stress in soil which was influenced by the EPPs. The initial EPPs in the soil induced by installation of a suction caisson can be described using cylindrical cavity expansion theory. The consolidation time of EPPs can be reasonably predicted using two methods: (1) radial consolidation theory (initially developed for driven piles) through adjusting the radius by the annular base of the caisson wall; and (2) modified Bogard and Matlock method (originally derived for driven piles) by adjusting the diameter or the wall thickness of the caisson to keep the D/Δ ratio (diameter/wall thickness) to a maximum value of 48. -- A confined general shear (CGS) failure mechanism is appropriate to describe the soil failure for sealed suction caissons in clay under fast upward loading. A displacement of 4 to 10% of caisson's diameter is required to mobilize maximum reverse end bearing (REB). The REB is in range 40 to 60% of the total pullout capacity, resulting in a REB factor in the range of 6.5 to 10.8. Moreover, a setup curve used to predict the wall skin friction resistance of the caisson at different times after installation is also proposed. -- The second part of this study focuses on the numerical investigation of the behavior of suction caissons subjected to vertical loading. A finite element model based on centrifuge model test SAT06 was developed. The caisson-soil interaction was simulated using interface elements. A new method, in which the water inside the caisson is simulated by a very soft porous-elastic material, was introduced to simulate the development of passive suction. This numerical model was validated using the centrifuge experimental results. -- Finite element analysis (FEA) results confirmed the failure mechanism of suction caisson in clay observed in the centrifuge tests. Although the passive suction versus pullout displacement curve obtained using the FEA was slightly different from that obtained from the centrifuge tests, the maximum suction was almost the same, and the pullout force versus displacement curve obtained using FEA was close to that obtained from the centrifuge tests. FEA results also indicated that the distribution of the EPPs in the soil inside the caisson is different from that in the outside soil during pullout of a suction caisson. -- In summary, this study has combined centrifuge modeling and finite element analysis to investigate the performance of suction caissons in clay. The study presented in this thesis clarifies several working principles of suction caissons to provide better understanding. They include the friction coefficients for predicting the installation resistance during both self-weight and suction penetration, the distribution of EPPs in clay induced by installation and their dissipation with time, the setup development, the failure mechanism with corresponding parameters, the EPPs in clay during pullout of the caisson, and the simulation of passive suction using FEA.

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