Evaluation, sampling and testing methods for offshore disturbed sands with plastic fines: A case study

Sandy seabed, prevalent near continental shelves, often contains fines that obviously affect their apparent behaviour. The variation in the physical properties of sands and fines, the difference between seismic and marine wave loads, and the challenges in obtaining accurate geotechnical parameters and undisturbed samples for analysis make it difficult to effectively evaluate the properties of sands with plastic fines in offshore engineering. To address this issue, a method for discriminating and re-sampling sands with plastic fines is suggested, along with a case study to assess the static and dynamic behaviours of such sands from an offshore site in the South China Sea. In this study, evaluation processes for detecting the presence of fines in sands and classifying them as plastic or non-plastic are first recommended to gain a better understanding of such sands. Then, a reconstitution approach of re-sampling for disturbed sand samples from site and compactness assessment guided by the optimum moisture content, is suggested to restore their in-situ characteristics, addressing the difficulty in obtaining undisturbed sand samples. Subsequently, the failure behavior and shear strain development under cyclic loading are analyzed via a cyclic contour diagram to evaluate their mechanical properties under marine environmental loads. The obtained contour diagrams can be further integrated into the constitutive model in subsequent research and used in finite element analysis to simplify the computation for marine foundation design. These results can provide valuable insights for improving understandings and engineering practices related to sandy seabed with plastic fines in offshore environments.