Floating Piles

Abstract

The construction of an artificial island is one of the solutions to be able to drill for oil safely in the Caspian Sea. On this island the required facilities are installed. Construction of an artificial island however, results in loading of the subsoil and will trigger settlements. Consolidation is one of the processes which cause the settlement. Differential settlements can damage the shallow foundations on the island. In order to prevent this, an alternative foundation on piles needs to be considered. Piles placed in a thick, soft soil layer will have low end bearing capacity, and thus need to activate skin friction. Since the surrounding soft soil is consolidating, these piles will have their own challenges. The relative movement between pile and soil determines the zones of negative and positive skin friction. Positive skin friction will need to be mobilized to support pile loading. Applying load on the head creates additional pile settlements, thus expanding the zone of positive skin friction. The consequence of this is that the bearing capacity of the pile is increased. This research looks into the two dominant parts of these friction piles in a consolidating soil profile. The driving mechanism, consolidation and subsequent settlement of the subsoil, is studied. Besides that the behavior of the interface between the soil and the installed piled is considered. Site investigations performed at the reference project are interpreted and a ‘Caspian’ soil profile can be determined. Besides this general profile a ‘Centrifuge’ soil profile is determined, which is revised to fit the specifications of the centrifuge. A one layer ‘benchmark’ situation is created to check the different calculation methods. The reaction of the ‘centrifuge’ and ‘benchmark’ soil profiles to the new loading situation is calculated and can be checked with centrifuge measurements. Recreating a two layer soil profile, containing stiff and soft clay, is done in the centrifuge. Msettle, a Hand Calculation and the Plaxis Soft Soil model give an accurate representation of the settlement in the physical model. The settlement in time seems to be described accurately as well. Calculations are done with a constant permeability. The “settlement in time” lines are calculated with Msettle and give a good image of the decrease in permeability during the performed centrifuge tests. The measured compression parameters for Kaolin Clay agree well with the values found in the literature. These compression parameters are tested in an oedometer test, and are given in a one dimensional stress space. Recalculation of the compression parameters into isotropic stress dependent parameters does not give the same settlement predictions. The used relationships given in the Plaxis 2D Material Models manual do not seem to give an accurate relation. The performed direct shear box tests show that the peak friction on the pile-soil interface is mobilized at displacements of about one millimeter. Further shearing led to a decrease in maximum shear stress, the so called residual value.