Abstract
Taiwan lies in the circum-Pacific earthquake zone. The seabed soil of offshore wind farms in Taiwan is mainly composed of loose silty sand and soft, low-plasticity clay. The seismic demand for offshore wind turbines has been given by the local code. Ground-motion analysis is required to consider the site effects of the soil liquefaction potential evaluation and the foundation design of offshore wind turbines. However, the depth of the engineering bedrock for ground motion analysis is not presented in the local code. In this study, we develop a three-dimensional ground model of an offshore wind farm in the Changhua area, through use of collected in situ borehole and PS (P wave (compression) and S (shear) wave velocities) logging test data. The engineering bedrock is the sediment at the depth where the average shear wave velocity of soil within 30 m, Vsd30, is larger than 360 m/s. In this ground model, the shear wave velocity of each type of soil is quantified using the seismic empirical formulation developed in this study. The results indicate that the engineering bedrock lies at least 49.5–83 m beneath the seabed at the Changhua offshore wind farm. Based on these findings, it is recommended that drilling more than 100 m below the seabed be done to obtain shear wave velocity data for a ground response analysis of the seismic force assessment of offshore wind farm foundation designs.
Highlights
Taiwan lies in the circum-Pacific earthquake zone
In order to ensure the stability of the offshore wind turbine foundations, site effects and soil liquefaction must be taken into consideration in its design
The seismic demand for offshore wind turbines is given in the local code for offshore wind farm seismic demand (CNS 15176–1) by the Bureau of Standards, Metrology, and Inspection in the Ministry of Economic Affairs [1]
Summary
Taiwan lies in the circum-Pacific earthquake zone. Offshore wind farms in the western sea area are affected by earthquakes and active faults. To perform a seismic force analysis for the offshore wind farm before a detailed foundation design is done, we need to determine the depth of the engineering bedrock, according to limited soil borehole data. A comparison is provided for the distribution trend of shear wave velocity with depth, calculated by the empirical formula with the experimental data of a resonant column test and the measured values of PS logging. Considering the difference between the application scope of the soil conditions and the analysis results proposed by various scholars to use the SPT-N value to estimate the shear wave velocity, this research compares the measured values of PS logging in an offshore wind farm in the Changhua area with the results of resonant column testing. Shear wave velocity of borehole BH-3 at ##2299 ooffffsshhoorree wwiinndd ffaarrmm ((eeiisstthheevvooiiddrraattiiooaanndd γm iγsmthisetmheomistouisnt iutnwitewigehitg)h. t)
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