Dynamic shear modulus and damping ratio of deep-sea ultra-soft soil

Seabed surface structures may be subjected to dynamic loads. In order to ensure the long-term stability of them, it is crucial to study the dynamic characteristics of deep-sea ultra-soft soil at seabed surface. Oscillatory shear tests were carried out on deep-sea ultra-soft soil with water content beyond liquid limit by an Anton Paar MCR302 rheometer with strain control mode. The variations of dynamic rheological parameters, dynamic shear modulus G and damping ratio lambda were studied. The test results showed that with the increase of shear strain, the deformation of deep-sea ultra-soft soil was dominated by recoverable elastic deformation and gradually transited to unrecoverable viscous deformation. By establishing the relationship between dynamic rheological parameters and dynamic shear modulus G and damping ratio lambda, the dynamic characteristics of deep-sea ultra-soft soil were discussed. According to the features of dynamic shear modulus G of deep-sea ultra-soft soil, the concept of peak reference strain was proposed, and the relationship between maximum dynamic shear modulus G(max) and normalized water content w/w(p) (w is the moisture content, w(p) is plastic moisture content) was established. Compared with conventional clay, the G/G(max-)gamma (gamma is shear strain)curve of deep-sea ultra-soft soil decreased faster and was less affected by plasticity index. The damping ratio of deep-sea ultra-soft soil was generally high, and increased rapidly with the increase of shear strain. Based on the experimental results, models suitable for describing G/G(max)-gamma and lambda-gamma curves of deep-sea ultra-soft soil were proposed.