Experimental investigation of perforation drilling in stiff-over-soft clay

The installation of independent-leg jack-up rigs in seabed sediments where a strong layer overlays weaker soil can lead to a catastrophic ‘punch-through’ event, with potential leg buckling or toppling of the unit. In an attempt to mitigate this risk in layered clays, an industry practice known as ‘perforation drilling’ is sometimes used. This process is also colloquially referred to as Swiss cheese drilling. The rationale is to reduce the bearing resistance of the upper stiff layer by perforating it with drill holes. Although drilling directly underneath the spudcan is reported to ease punch-through, little guidance to how many or where drill holes should be located is available. This paper reports two series of model tests of a 40 mm diameter (DD) spudcan footing vertically installed in a stiff-over-soft clay deposit. In this preliminary study, the effectiveness of perforation drilling was investigated by drilling holes of different spacing, depth and distribution both underneath and outside the immediate perimeter of the penetrating spudcan. Drilling patterns were chosen to not just follow the conventional method of holes directly below the spudcan, but to coincide with failure mechanisms previously observed during spudcan penetrations in stiff-over-soft clay soils. The method of producing the holes, whether through drilling, coring or water jetting, was also investigated. Results show that punch-through can be mitigated when the layered deposit is punctured in a zone immediately outside the spudcan periphery.