Experimental Investigation of the Horizontal Bearing Capacity of Offshore Wind-Turbine Monopiles with Grouting Protection Against Scouring

Seabed scouring can uncover the buried component of wind-turbine monopiles and this reduces their horizontal bearing capacity. Because rigid piles with large diameters and low slenderness ratios have been widely used in offshore wind farms, there is an urgent need to address this issue. In this study, grouting protection, as a new countermeasure against scouring, was examined, with scouring tests under constant-current, regular-wave, and combined wave–current conditions being conducted, along with static and cyclic load tests on the monopile. The scouring characteristics and horizontal bearing behaviours of a pile with grouting protection (Pile a) and a pile without grouting protection (Pile b) were investigated. We found that the ultimate scouring depths of piles with grouting were much lower than those without under constant-current scouring, regular-wave scouring, and combined wave–current scouring. The HL2Dγ′−y0D and p-y curves of the piles were obtained, which showed that grouting protection can significantly strengthen the ultimate horizontal bearing capacity under a static loading. The pile with grouting was more stable under cyclic load compared to the pile without grouting. There are two explanations for the strengthening of the horizontal bearing behaviours of piles with grouting protection. First, grouting lessens the loss of pile burial depth brought about by scouring. Second, grouted seabed has a much higher sediment stiffness than ungrouted seabed, thus the grout dramatically increases the strength of the pile– sediment structure.