Model test on extraction behavior of suction caisson in layered soil under axial loading

After 20-25 years of service, suction caisson for offshore wind turbines needs to be extracted. Model tests were carried out investigating the suction caisson extraction behavior in layered soils under axial loading. When the suction caisson is extracted from sand, clay-over-sand, sandwich soil, or sand-over-clay, there is a quick increase in the extraction resistance to the peak value, followed by a slow decrease. On the contrary, the extraction resistance increases rapidly then remains almost constant in clay. The variation of passive suction during extraction is consistent with that of extraction resistance. When the pullout rate increases from 0.1 mm /s to 10.0 mm /s, the extraction resistance increases 2.9, 3.0, 2.6, 1.9 and 4.2 times in sand, clay-over-sand, sandwich soil, sand-over-clay, clay respectively. The peak extraction resistance with the drainage holes open is smaller than that with the drainage holes closed in the same soil layer type. To improve extraction efficiency, a variable-rates extraction method is proposed. The soil plug is intact when the extraction rate is high. On the contrary, when the extraction rate is relatively low, the formation of cracks and a water layer inside the soil plug may cause a reduction in passive suction and extraction resistance.