Investigation of the collapse mechanism of open ended piles during installation

The open-ended large diameter piles, used by Woodside Offshore Petroleum Ltd. as foundation for the offshore platform in Goodwyn-A, Australia, experienced deformations during their installation. The cause of this deformation was not clear as they were never observed before. Pile tips seemed to have obtained a sort of peanut shape deformation. The Geotechnical Laboratory was asked to do experimental research to see i f the observed peanut shape was reproducible and to find out which parameters were of influence on the deformation process. Next to this investigation, a proposed theoretical model had to be validated which could explain the pile's collapse mechanism. The influence o f t h e so-called D/t-ratio, which shows the pile diameter divided by the wall thickness, was given a major role as parameter o f influence and thus needed to be investigated thoroughly. A parameter study was started. Scaled open-ended piles were driven in a soil sample which was placed within a pressure vessel. Various cell pressures could be imposed on the soil sample. Piles could thus be driven in the soil under varying soil stresses. Tt appeared that an initial damage always had to be given to the pile tip to obtain deformation during pile driving i f a certain amount of cell pressure was placed on the soil sample. It seemed that a guiding system, used to ease the entrance o f a pile in the pile sleeve, could be the cause o f this initial damage. The pile sleeves were used to batter the piles in the soil by around 1.5 degrees to avoid piles would touch at greater depth. Much time was spent on the development of a usable geometrical model that could reproduce the shape o f the pile i f certain measured values were used as input. Next to this, a new model had to be developed which could explain the pile-soil interaction during pile driving. A model was made in which soil and pile behaviour is represented by springs having certain stiffnesses. Formulas with which these rigidities could be calculated were made with the aid o f the existing formulas for 'Stresses in Tunnel Linings' by A. Verruijt. A final test series was done in which piles having various D/t-ratios were given certain initial damages. The cell pressure needed to obtain plastic pile deformation was looked at. It seemed that the gradient within the initial deformation of the pile tip had a major influence. Piles having larger gradients required less cell pressure to deform. Finally, a comparison was made between the theoretical calculated values for the maximum radial deformation and the experimental results. Piles for which the calculated radial deformation exceeded a certain value always seemed to be deformed further in the experiments. It can thus be concluded that the proposed model can determine very roughly whether a pile will deform during driving or not.Hydraulic EngineeringCivil Engineering and Geoscience