Offshore wind turbines (OWTs) are subjected to long-term cyclic loading, such as wind and wave loading, leading to axial-torsional bidirectional cyclic loading in the pile surrounding soil. This bidirectional cyclic loading results in the cumulative deformation of the pile foundation. The larger cumulative deformation endangers the operational safety of the wind turbine system. In China, offshore wind farms mainly consist of clay layers and their cyclic mechanical characteristics need to be examined. Therefore, a series of bidirectional cyclic loading tests are carried out by using a hollow cylindrical torsional shear apparatus to investigate the effect of different cyclic stress levels on the cumulative strain. The results revealed that cumulative strains stabilize as the cycles increase when the cyclic stress level is less than the critical cyclic stress level. An empirical model based on the test results is developed to reflect the cumulative deformation characteristic. In the bounding surface constitutive model, the plastic modulus interpolation function is improved by using the established empirical model,the effect of loading and unloading routes on the variation of plastic modulus is investigated. Furthermore, a modified constitutive model is used to predict the cumulative deformation of soil under the effect of bidirectional cyclic loading. The predicted results agree well with the findings achieved from the tests.

This paper investigates shear banding as a possible failure mode for silt-clay transition soils under general three-dimensional stress conditions. Drained and undrained true triaxial tests with constant b values were performed on tall prismatic specimens of such soils with systematically varying silt contents. Based on the values of critical plastic hardening modulus, shear banding does not govern the strength characteristics of the soils for b values less than 0.2. For larger b values, shear band formation is essentially critical as it takes place in the hardening regime of the stress-strain curves prior to the smooth peak failure points. An increase in silt content appears to move the onset of shear banding to lower levels of shear in the stress-strain relations of the silt-clay transition soils. It is also demonstrated that a non- associated constitutive model with a single hardening law is capable of accurately predicting the onset of shear banding in normally consolidated silt-clay transition soils based on bifurcation theory. (c) 2024 Production and hosting by Elsevier B.V. on behalf of The Japanese Geotechnical Society. This is an open access article under the CC BY- NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).