The sheet pile behaviour for hole weakening

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The evolution of ships requires larger depth or larger terrain loads in ports. Therefore, the existing quay walls require reinforcements. An effective way of reinforcing the structure are underwater anchors, requiring holes in existing sheet piles at the location of significant bending moment. Since this is a new area of expertise, the influence of the hole is still unknown. This report discusses a developed finite element model to study the influence of hole weakening of sheet pile and studies the influence of hole weakening on the sheet pile resistance and behaviour with the discussed model.

The 3DSSI model includes soil modelled by the subgrade reaction model (spring model) the steel sheet pile is modelled with shell elements in the finite element software DIANA FEA. The model includes both physical nonlinear and geometrical nonlinear behaviour and captures the soil-structure interaction. Also oblique bending, which is an existing phenomena of older, existing double U-sheet piles, can be captured with interface elements in the 3DSSI model. Nowadays, sheet piles are also designed using the subgrade reaction model and the sheet pile is modelled as a beam with the appropriate stiffness. The 3DSSI model is able to illustrate the 3D behaviour of the sheet pile, without having to model the soil with solid elements as well. This is beneficial with respect to calculation time.

With the 3DSSI model, the hole weakening of sheet piles can be studied. The influence of different parameters on the sheet pile resistance and behaviour is studied. The parameters studied, based on literature findings, are: hole diameter, hole location (level of the hole in depth), hole spacing (hole centre to centre distance in width), hole in-pan (compression zone) or out-pan (tension zone), sheet pile cross-section class 2 and 3 and oblique bending.

The influence of hole weakening on sheet pile behaviour and resistance is studied by finding in the parameter study for each case the load-displacement curve and from this load-displacement curve, the stiffness is derived. The load-displacement curve and stiffness curve include both steel and soil behaviour.
Resulting from the parameter study, the holes significantly reduce the sheet pile resistance and behaviour. The cases with holes in tension show less reduction in resistance and stiffness reduction. The hole diameter is also influencing the results, but the influence of hole location and hole spacing is relatively small. The failure modes are for holes in compression zone is plasticity around the hole and local buckling in the webs. For holes in tension zone, the failure mode is also plasticity around the hole, but also local buckling in the compression flange. For cases including oblique bending and holes in tension zone do not show local buckling in the compression flange, but in the compression web, due to the inclined neutral axis caused by oblique bending.