In the field of engineering research, the creep behavior of soft clay and the evaluation of its long-term strength are crucial issues in geotechnical engineering. In this paper, the influence of drainage conditions on the creep characteristics and long-term strength of Tianjin coastal clay were analyzed through triaxial creep tests conducted under various stress levels, confining pressures and drainage conditions. The existing methods for determining the long-term strength of soil were discussed and improved, and a novel approach based on stress-strain isochronous curves was proposed to assess the long-term shear strength indicators of the soil. The proposed method was validated through comparative analysis, demonstrating its effectiveness and rationality in practical applications. Results indicate that under the same confining pressure conditions, the creep amount and deformation stability time increase with an increase in the stress level. At the same stress level, the creep amount at each stress level decreases with an increase in the confining pressure. The creep deformation difference is significant under different drainage conditions. During the process of soil creep, the drainage condition is an important factor affecting the shear strength and strength indicators of soil. Under consolidated and undrained creep conditions, the long-term shear strength τ_∞ is about 84% to 93% of the instantaneous shear strength τ₀, among which the cohesion decreases significantly, and the long-term cohesion c_∞ is about 37% of the instantaneous cohesion c₀. Meanwhile, the internal friction angle remains relatively unchanged. Under consolidated and drained creep conditions, the long-term shear strength τ_∞ is about 106% to 116% of the instantaneous shear strength τ₀, with only a minor reduction in cohesion and a significant increase in the internal friction angle. In addition, the long-term internal friction angle φ_∞ is about 110% of the instantaneous internal friction angle φ₀. The research results can provide a reference for the long-term stability analysis and engineering design of soil in coastal soft soil areas.

Offshore wind energy is one of the most important clean energy sources developing in China in recent years. As a new type of offshore wind turbine foundation, the wide-shallow bucket foundation with large diameter has a good ability to withstand moments. It is thus suitable for offshore region in China with soft clay deposits. Moreover, this type of foundation has advantages of onshore prefabrication, foundation-tower-turbine integrated floating transportation and suction installation, resulting in a good benefit for cost reduction. To better understand the bearing capacity of the wide-shallow bucket foundation in clay, numerical analyses are conducted to study the monotonic ultimate bearing behavior. The horizontal and moment ultimate bearing capacities and corresponding failure mechanisms are obtained. By building the velocity field based on the failure mechanisms of the bucket foundations, combined with the virtual work equation and the internal energy loss rate function, the upper bound solutions for the horizontal and moment ultimate bearing capacities of the bucket foundations are established and programmed using mathematical software. The results of the upper bound limit analysis have a good agreement with the calculation of the numerical analyses.