Behavior of Pile Plug in Sandy Soils During and After Driving

ABSTRACT When open-ended piles are driven into the soil, a soil column is created inside. The advance of the soil column with respect to the pile penetration reflects the mode of pile penetration in the soil and influences the pile- soil interaction during and after installation. Formation of the soil co1umn inside open-ended model and large sca1e experimental piles was analysed at four sandy sites. The incremental filling ratio was found to be linked to the impact characteristics. The influence of the soil column on driving results as on static results was evaluated and discussed. In contrast to the driving results, the static loading result were not changed by the removal of the soil column. The plugging effect was explained by an exponential cumulation of stresses inside the piles. INTRODUCTION The majority of offshore platforms are founded on open ended tubular piles. When the piles are driven into the soi1, a soi1 column is created inside the pile. At the start of penetration, the pile cores a soi1 column the height of which is equal to the pile penetration. Then as penetration continues, the filling ratio may vary. At a certain stage, the soil column may act as a perfect plug, preventing any new soil intrusion. If that occurs, the pile penetrates the soil as a closed-ended pile. The advance of the soil column with respect to the pile penetration reflects the mode of pile penetration in the soil which influences the pile-soil interaction during and after installation. In practice the advance of the soil drivability methods use unit skin friction and unit toe resistance independent of the penetration mode of the pile. The contribution of the soil co1umn to the soil resistance to driving and to the static bearing capacity is taken as the lowest resistance of the two fo11owing mechanism. The plugged mechanism requires that the point resistance be calcu1ated over the full pile toe area, and the unplugged mechanism requires that an interna1 skin friction due to the soil co1umn be added to the point resistance of the pile annulus toe area. The same unit skin friction as the external on is used. This may be overly conservative for static design. In this paper, the advance of the soi1 co1umn inside experimental piles is reported -and analysed. The contributions of the soil column to the soil resistance to driving and to the static bearing capacity are evaluated and discussed. Open ended model piles 70 mm in diameter and 2,50m long and large scale piles 324 mm in diameter and 12 and 24 m long were tested at dense sandy sites. The experimental programme was carried out within CLAROM (a research association grouping French oil companies, contractors, one certification body and research institutes working on offshore operations).