Energy dissipation can macroscopically synthesize the evolutions in the microstructure of the marine clay during cyclic loading. Hence an energy-based method was employed to investigate the failure criterion and cyclic resistance of marine clay. A series of constant-volume cyclic direct simple shear tests was conducted on undisturbed saturated marine clay from the Yangtze Estuary considering the effects of the plasticity index (IP) and cyclic stress ratio (CSR). The results indicated that a threshold CSR (CSRth) exhibiting a power function relationship with IP exists in marine clay, which divides the cyclic response into non-failure and failure states. For failed specimens, the development of energy dissipation per cycle (Wi) with the number of cycles (N) exhibited an inflection point owing to the onset of serious damage to the soil structure. In this regard, the energy-based failure criterion was proposed by considering the inflection point as the failure point. Consequently, a model was proposed to quantify the relationships between failure energy dissipation per cycle (Wf) [or failure accumulative energy dissipation (Waf)], initial vertical effective stress, IP, and the number of cycles to failure (Nf,E). An evaluation model capturing the correlation among CSR, IP, and Nf,E was then established to predict the cyclic resistance, and its applicability was verified. Compared with the strain-based cyclic failure criterion, the energybased failure criterion provides a more robust and rational approach. Finally, a failure double-amplitude shear strain (gamma DA,f) evaluation method applicable to marine clay in different seas was presented for use in practical geotechnical engineering.
The rock-based sea area has great prospect of development and construction of offshore wind farms (OWFs), and the mainstream construction sites of OWFs in China have shifted from the soil-based seabed to the rock-based seabed area. Previous studies about mechanical properties of seabed materials and bearing characteristics of pile foundation in OWF mainly focus on the submarine soil-based seabed, resulting in lack of direct reference for the construction of offshore wind power in the rock seabed. Therefore, the study concentrates on the investigation of failure criterion of submarine completely weathered granite (CWG) of offshore wind farms in rockbased sea area under cyclic loads. Firstly, dynamic triaxial tests are carried out, and two unique development modes of CWG are revealed under different cyclic loads. The experiments analyze insight stiffness attenuation law and establish the prediction model of stiffness attenuation based on the logarithm formula. More critical, a unique development law of damping ratio of submarine seabed materials is discovered and discussed, and two cyclic failure criteria based on cumulative strain and dissipated energy are put forward to divide the critical CSR under cyclic loads, which gives helpful reference for the construction of offshore wind farms in rock-based sea area.