Centrifuge tests to investigate pile run risk in transitional soils

Monopiles, commonly adopted as substructures in wind farms, are typically installed via impact driving. The heavy selfweight of the monopile and the impact hammer required for installation increase the risk of pile runs. Pile runs have been reported in cases of stronger soils overlying weaker layers, as well as in heterogeneous soil deposits (e.g., chalk). However, recent experience from the field showed that none of the reasons above could satisfactorily explain the observed pile run in the presence of silty or fine sandy soils, typically referred to as transitional soils. Conversely, back analysis of the driving data revealed a high dependency of the Soil Resistance to Driving (SRD) on the pile penetration rate. This behaviour is believed to be linked to the drainage response of the transitional soils and pile driving parameters, including impact energy and blow rate. The latter may combine so that Excess Pore Water Pressures (EPWP), without dissipating sufficiently, accumulate to the extent a pile run can be triggered, due to the reduction of the available shear strength of the soil. To investigate this hypothesis, an experimental testing program was conducted using the geotechnical centrifuge. The tests, involving a model monopile driven in a natural silt sample, aimed at demonstrating that the soil conditions believed to contribute to a pile run can be replicated in the centrifuge. Preliminary results of a testing sequence of single blows suggest that the EPWP accumulated around the pile between consecutive blows is responsible for a reduction of the unit shaft resistance.