土壤液化對單樁基礎動態反應之影響

This study examined the effects of soil saturation and seismic loading on the stability of a monopile structure, a design commonly used for offshore wind turbines. Two centrifuge models prepared, one with fully saturated sand and the other with dry sand, and then subjected each model to the same base input shaking events (1 Hz, 15 cycles, and amplitudes of about 0.12 g and 0.24 g). The natural frequency of the pile, determined inflight by the preshaking method, was about 0.5 Hz. Strain gauges attached on the pile at different elevations were used to measure the bending moment during shaking. In addition, accelerometers, pore water pressure measurements, and laser displacement transducers were used to monitor acceleration, excess pore water pressure generation and dissipation, as well as the lateral displacement of the upper structure. The results showed that, compared to the monopile in dry sand, the pile in saturated sand had a larger bending moment in response to the same amplitude of shaking. For instance, although subjected to the same loadings the bending moment of the pile in saturated sand was at least 1.5 times larger than in dry sand. The displacements of pile head and upper structure and rotation angle observed in dry sand under a seismic loading of 0.24 g was within the allowed range. On the other hand, the monopile in saturated sand under a seismic loading of 0.24 g exhibited a much larger lateral displacement and rotation than the pile in dry sand, 3.6 times in displacement and 4 times in rotation to be exact, both of which in saturated sand were outside the allowed range. The lower stability of the structure in saturated sand can be attributed to the limited support provided by liquefied sand compared to dry sand. The present results thus indicate that soil liquefaction must be carefully assessed when installing offshore monopiles in cohesionless soils.