In situ and laboratory tests in dense sand investigating the helix-to-shaft ratio of helical piles as a novel offshore foundation system

Abstract

Whilst small diameter helical (or screw) piles are used extensively for near-shore and onshore projects, the potential to use these foundations in the offshore environment is receiving increasing attention. Potential advantages of helical piles over conventional foundation systems include their high capacity, ease of installation (avoiding pile driving), ease of removal and lowcost for mass deployment. In situ testswere performed in Germany with a drill rig Bauer BG42. A series of field tests, finite element analyses and laboratory tests on helical piles are being conducted as part of a joint research project between BAUER Maschinen GmbH and University College Dublin (UCD). This paper shows the in situ tests conducted under real conditions. Besides, the results of small scale 1 g laboratory tests on 1/10th scale piles which were performed in a large sand box at UCD are extensively described. A series of experiments were performed which investigated the effect of the ratio of helix (or helical plates) to shaft diameter, and helix diameter on the pull-out capacity of helical piles. The results suggest that the optimum pile design has a helix-to-shaft diameter (defined as wing ratio) between 1.5 and 2.0. Larger helical plates were significantly harder to install, whereas smaller helical plates provided little resistance to vertical loads.