Experimental Evaluation of Backfill in Scour Holes around Offshore Monopiles

When a monopile is installed in sandy or silty soil a scour hole will form around the pile due to erosion. A scour protection consisting of rock infill can be employed to protect against the forming of a scour hole. As scour protection is highly expensive the most economic solution might be to allow the forming of a scour hole and hereby design the monopile with a larger penetration depth.

The depth of the scour hole will change over time as the scour depth will increasewhen currents are dominating and backfilling of the scour hole will take placewhenwaves are dominating. Several researchers have investigated the time scale of erosion in small scale experiments and found a dependency on Shields parameter and the Keulegan-Carpenter number. Further knowledge is however needed in larger scale. Regarding the rate of backfill, further knowledge is needed for both small and large scale. Currently there is no knowledge concerning the relative density, and hereby also the strength and deformation properties of the backfilled soil material. The strength of the backfilled soil and the time scale of respective erosion and backfilling is of high importance when designing the steel structure of the foundation for fatigue.

A backfill test has been performed in the LargeWave Channel (GWK) of the Coastal Research Centre (FZK) in Hannover.The relative density of the backfilled soil material has based on soil samples and CPT measurements been determined to be in the range of 60–80%. The normalized time scale of backfilling is found to be small in comparison with existing small scale experiments.