Study On Cumulative Deformation Of Calcareous Sand Under Cyclic Load

Offshore wind energy had gradually become one of the pollution-free renewable energy with great potential.A large number of wind power facilities were distributed in the offshore of China,among which the offshore wind power pile foundation was the key structure connecting the upper structure and the seabed.The long-term cumulative deformation of soil around the pile was an important factor restricting the safety of wind power pile foundation under cyclic load such as wave load and wind load.Calcareous sand,one of the main soil types interacting with the pile foundations of wind power,was widely distributed in China’s offshore.Aiming at the cumulative deformation of calcareous sand under cyclic load,the cyclic triaxial compression test of saturated calcareous sand was carried out to analyze the effects of confining pressure,initial static deviatoric stress ratio and dynamic deviatoric stress ratio on the characteristics of cumulative deformation.Based on the concept of stress path length,the mechanism of cumulative deformation law of calcareous sand was analyzed.In order to deduce the mathematical expression of relative deviator stress level including initial static deviator stress ratio and dynamic deviator stress ratio,the relative deviator stress level based on stress path was introduced.The calculation model of cyclic cumulative deformation of saturated calcareous sand was fitted by using the test data.The purpose of the thesis was to reveal the long-term cumulative deformation law of calcareous sand,predict the cumulative strain value of calcareous sand under cyclic load,and provide reference for the safety design of wind power pile foundation.Results showed that the double logarithm curve between the cumulative axial strain and the number of cycles showed a linear relationship.Under undrained conditions,except for the specimens with dynamic deflection stress ratio equal to 0.8,the axial strain accumulated in the first cycle of other specimens reached more than 50% of the axial strain accumulated in the 5000 th cycle.When the dynamic deflection stress ratio was equal to 0.2 and 0.4,the percentage reached about 90%.Only when the dynamic deflection stress ratio was equal to 0.8 could the specimen reach the failure condition of 5% axial strain.The larger the confining pressure,the smaller the number of cycles required to reach the failure condition.Under the drainage condition,after the calcareous sand entered the plastic stabilization stage,the cumulative axial strain increased at a very slow rate.With the increase of the number of cycles,the stress-strain hysteresis loop became more and more dense,resulting in the phenomenon of "wave forward" of secant stiffness.The average principal stress,initial deviatoric stress ratio and dynamic deviatoric stress ratio would directly determine the length of stress path.The longer the stress path was,the more obvious the cumulative deformation of calcareous sand was.The volume deformation of calcareous sand mainly came from the rearrangement and distribution of the relative position of calcareous sand particles.Considering the special composition of calcareous sand（extremely fragile marine biological remains）,part of its volume deformation came from the crushing of calcareous sand particles.Theoretical calculation value of the cyclic cumulative deformation calculation model of saturated calcareous sand was in good agreement with the experimental value,which could provide prediction for the long-term cyclic cumulative deformation of calcareous sand.Under the cyclic load of low frequency and low amplitude,the number of cyclic loads must be enough for the calcareous sand to enter the plastic cumulative failure stage,which was meaningful for the design of offshore wind power pile foundation subjected to long-term wave load and wind load.