Large diameter dolphin piles: The effect of the inner soil on their local buckling resistance

Even though for most hydraulic and geotechnical structures clear guidelines are available, those for flexible dolphins are still missing. Consequently, whether a design is safe is usually subject of debate. Economical dolphin piles are large diameter tubular piles, susceptible to local buckling. After installation the embedded part of these piles remains predominantly filled with soil. The aim of this research is to provide understanding of the effects concerning local buckling and therewith to contribute to the discussion whether a dolphin design is safe. The main research question is: Does the soil in dolphin piles contribute to the local buckling resistance and can this be used in the design of these piles? Gresnigt (1986) prescribes a strain based method to check a pile on local buckling. This method is incorporated in EN 1993-4-3 (2009). The resistance against local buckling depends on the slenderness ratio De/(tε2) . Also, ovalisation influences the resistance. The presence of a soil plug reduces the ovalisation, enhancing the local buckling resistance. An analytical method is proposed in which beam theory is used to determine the dolphin pile deformation and loads on the pile. Literature provides equations to calculate the ovalisation that can be applied in three distinct parts: above the bed level, between the bed level and plug level and below the plug level. The provided equations however, show inconsistencies at the transitions. It is proposed to apply beam theory on the pile wall, modelled as a linear beam with springs providing resistance against ovalisation. Above the plug level, the resistance is provided by the ring behaviour of the shell. Below the plug level, the stiffness is increased with the stiffness of the soil plug. The model overcomes the inconsistencies in ovalisation. The method assumes the stiffness of the plug in the pile can be approximated. However, it is concluded that the suggested stiffness in CUR 211E (2013) is not able to determine the plug stiffness for semi-filled piles. Key parameters that affect the local buckling resistance are identified. These are the plug packing, plug level, slenderness ratio and the soil packing. The influence of these key parameters are studied with the finite element program Abaqus. The parametric study proves that the soil in dolphin piles contributes to the local buckling resistance. A denser packing of the plug improves the capacities of the bending moment and lateral load. Most of all, the dolphin can dissipate more energy. The plug level influences the level at which local buckling is observed, as well as whether an inward or outward buckle occur. The contribution of the inner soil on the local buckling resistance can be compensated by designing a more slender dolphin pile. A more slender cross-section reduces the capacities of the pile. The packing of the outer soil also influences the buckling level and shape. Furthermore, a looser soil packing improves the energy capacity of the dolphin pile. To obtain a dolphin pile that optimises the improvements on the local buckling resistance by the inner and surrounding soil, an installation method is used that does not compact the soil and plug packing and after installation, the plug packing is compacted. With the Abaqus analyses the stiffness of the soil plug is studied. It reveals that the plug is stiffer at deeper levels. For a pile with a diameter comparable to 914mm, when the predicted ovalisation for that pile without a soil plug is less than 8 mm, a plug stiffness of 30,000 kN/m2 can be assumed in the proposed analytical method, regardless of the plug packing. The stiffness is independent of the slenderness ratio. The Port of Rotterdam facilitated a full scale field test with eight piles. The Abaqus model shows reasonable comparison with the field test. The observed local buckling behaviour piles that were not modelled can well be explained with the results of the parametric study.Civil Engineering and GeosciencesStructural EngineeringHydraulic Structure