QUANTIFY THE PERFORMANCE OF SUCTION CAISSONS FOR THE EXTENSION OF WIND TURBINE NO. 1

A programme of centrifuge tests has been performed at the National Geotechnical Centrifuge Facility (NGCF) at the University of Western Australia (UWA) to provide insight on the performance of suction caissons installed and loaded in a clay-over-sand soil profile. The primary purpose of the centrifuge testing was to assess the response of specified suction caisson geometries to controlled loading scenarios. It is understood that the centrifuge testing campaign will help to determine the viability of using suction caissons as foundations to support a 15 MW offshore wind turbine at an offshore location where the seabed is known to comprise a marine clay and clayey silt overlying a marine sand.
Three model caissons were considered in the centrifuge testing campaign – with a total of six caisson tests completed. Two of the models were 1:100 reduced scale replicas of field scale suction caissons with outer diameters of 7 m, skirt lengths of either 12.5 or 16.4 m, and skirt thicknesses of 50 mm. The third model was a 1:200 reduced scale replica of a suction caisson with an outer diameter of 17 m, a skirt length of 16.4 m, and skirt thicknesses of 90 mm. The model caissons were instrumented with various pressure sensors to allow for measurement of differential pressure across the bucket lid. The soil model considered in the centrifuge tests comprised a 10.7 m thick kaolin clay layer overlying a sand. In one soil model the sand layer was saturated with water while in the other the sand was saturated with a viscous pore fluid to help achieve an undrained response during cyclic loading.
A self-weight installation phase was modelled by penetrating the caisson to a prescribed load before (typically) commencing a suction assisted installation phase. The suction installation was achieved by continuing to hold the prescribed self-weight load while extracting fluid from the interior of the caisson at a constant flow rate using an actuated syringe pump. Two caisson tests in the water-saturated sand sample did not include a suction installation phase and were instead jacked to prescribed embedment depths before reducing the vertical load to the caisson self-weight. This jacking was followed by the application of suction pressure whilst ensuring that there was no further caisson embedment. Four caisson tests in the viscous fluid-saturated sand sample involved both self-weight and suction-assisted installation stages.
The tests involved both ‘grouted’ and ‘ungrouted’ installations, with the latter included to investigate the scenario in which there would be no grouting between the underside of the caisson lid and the soil surface (as was the case for the meteorological mast caisson). The ‘ungrouted’ installations were conducted on tests using the 7 m diameter caisson, whereas the ‘grouted’ installations were conducted on tests using the 17 m diameter caisson. For the ‘ungrouted’ installations, caisson penetrations were 11.9 m for the caisson with the 12.5 m skirt length and 15.4 m for the caisson with the 16.4 m long skirt, which corresponds with fluid-filled gaps of 0.6 m and 1.0 m respectively. For the ‘grouted’ installations the embedment depth was between 14.6 m and 15.0 m, with the grout infill modelled by including a 1.0 m thick acrylic disc at the caisson lid invert such that the maximum achievable skirt embedment was 15.4 m.
After installation the caissons were (typically) subjected to either monotonic or cyclic loading sequences. The monotonic loading sequences resulted in no more than 65 mm additional (permanent) vertical settlement of the caisson at prototype scale while the cyclic loading sequences resulted in no more than 8 mm additional (permanent) settlement.
The ‘post-testing’ vented drained tensile capacity of the 7 m diameter caisson was approximately 130 kPa for the caisson with the 12.5 m skirt length and 220 kPa for the caisson with the 16.4 m skirt length. Despite the different installation approaches adopted (suction-assisted vs jacked) in the two tests with the 12.5 m skirts, the resulting tensile capacities were almost identical.
The ‘post-testing’ vented drained tensile capacity of the 17 m diameter caisson was 86 kPa and 91 kPa in two separate tests involving different loading sequences, with one of these tests including application of a high compressive stress after the loading sequence and immediately before extraction. The sealed undrained capacity of the same caisson was 463 and 483 kPa in two separate tests – i.e., greater than five times that of the vented capacity – despite the application of a high compressive stress immediately before extraction in one of these tests.