Borehole Instability in Decomposed Granite Seabed for Rock-Socketed Monopiles during "Drive-Drill-Drive" Construction Process: A Case Study

Monopiles are commonly used in the construction of offshore wind turbines. However, implementing drive-drill-drive construction techniques in decomposed granite seabed may lead to borehole instability during the window period between drilling and pile driving, resulting in significant project losses. This study provides a comprehensive understanding and approach to address the causes of borehole instability in rock-socketed monopiles in decomposed granite seabed. Using the Pinghai Bay offshore wind farm project in Fujian, China as an example, the details of drive-drill-drive and reverse-circulation drilling techniques employed in monopile construction were introduced. An improved sampling method was utilized to obtain decomposed granite samples, and a series of in situ and laboratory tests were conducted to analyze the physical and mechanical properties of marine-decomposed granite. By examining three cases of monopile construction, the factors contributing to borehole instability during rock-socketed monopile construction in decomposed granite seabed were identified, and corresponding recommendations were proposed. The results indicated that construction technology and unfavorable geological characteristics of decomposed granite are the primary causes of borehole instability. Collapses occurred mainly in highly and moderately decomposed granite layers. Employing smaller boreholes can reduce the likelihood and impact of borehole instability.