Macro- and micro-mechanics of granular soil in asymmetric cyclic loadings encountered by offshore wind turbine foundations

Abstract

Offshore wind turbine foundations are subject to 10 7-10 8 cycles of loadings in their designed service life. Recent research shows that under cyclic loading, most soils change their properties. Discrete Element Modelling of cyclic simple shear tests was performed using PFC2D to analyse the micromechanics underlying the cyclic behaviours of soils. The DEM simulation were first compared with previous experimental results. Then asymmetric one-way and two-way cyclic loading pattern attained from real offshore wind farms were considered in the detailed parametric study. The simulation results show that the shear modulus increases rapidly in the initial loading cycles and then the rate of increase diminishes; the rate of increase depends on the strain amplitude, initial relative density and vertical stress. It shows that the change of soil behaviour is strongly related to the variation of coordination number, rotation of principal stress direction and evolution of degree of fabric anisotropy. Loading asymmetry only affects soil behaviours in the first few hundred of cycles. In the long term, the magnitude of (γ max − γ min) rather than loading asymmetry dominates the soil responses. Cyclic loading history may change the stress-strain behaviour of a soil to an extent dependent on its initial relative density. Keywords Discrete element modelling • Cyclic loading • Soil stiffness • Micro-mechanism Abbreviations a Parameter defining the magnitude of fabric anisotropy a max Fabric anisotropy at maximum shear strain a min Fabric anisotropy at minimum shear strain d 50 Diameter at which 50% of the sample's mass is comprised of particles with a diameter less than this value e Void ratio G Shear modulus M max Maximum moments in a load cycle M min Minimum moments in a load cycle and M ult Ultimate moment capacity N c Coordination number t Deviator stress s′ Mean stress γ max Maximum shear strain (shear strain amplitude) of a cyclic simple shear test γ min Minimum shear strain in a cyclic simple shear test ζ c Ratio of minimum moment to the maximum moment η c Parameter defined to quantify the degree of asymmetry of cyclic loading θ a Direction of the principal fabric σ Vertical stresses on the sample in a cyclic simple shear test τ max Shear stress at maximum shear strain τ min Shear stress at minimum shear strain * Liang Cui

Soil–Structure Interactions for the Stability of Offshore Wind Foundations under Varying Weather Conditions

“…As described by Cui et al [7], the interaction between the monopile and the soil element in front of the monopile is analogic to a cyclic simple shear scenario with constant vertical stress. Cyclic simple shear tests were also performed in the current study.…”

“…Each numerical sample is 50 mm in width and 20 mm in height, containing 8000 disks with sizes ranging from 0.1 mm to 0.3 mm, with a void ratio of 0.227 when vertically consolidated to a vertical stress at 100 kPa, the same as that in experiments. Parameters used in the DEM simulations are listed in Table 2, the same as those adopted and validated in the previous study, where the particle stiffness was calibrated to achieve a similar shear modulus for RedHill 110 sand (1-10 MPa) at 0.5% shear strain under vertical stress of 100 kPa, and the boundary stiffness was set at a higher value to represent the stiffer steel confining rings [7]. The frictional coefficient between sand particles was set to be 0.5, based on the particle characteristics study performed by Cavarretta [31].…”

“…However, long-term cyclic loading can change the soil properties and thus the soilstructure interactions due to various factors, causing either a reduction or increase in soil stiffness, altering the system's natural frequency of vibration, which might set the system into resonance [7][8][9]. The properties of soil, mainly stiffness and damping, are fundamental in this context.…”

Thermal Shock Life Prediction of the SiC Wide Bandgap Power Module Semiconductor Package Considering Creep Behavior of the Ag Sintered Interconnect and Viscoelastic Properties of the Epoxy Molding Compound

“…Since the first cycle starts when both stress and strain are zero, it is noticeably different from the cycles that entered the stable period. 57,58 On the other hand, the plastic deformation is attributed to be the cause of mean stress relaxation. 59 In Figure 8f, the stress relaxation was represented with respect to the strain amplitude from 0.17 to 0.28%.…”

3D Mechanical Characters and Their Fabric Evolutions of Granular Materials by DEM Simulation

“…We can explain the mechanism of the macro-microscopic relations. Under the true-triaxial condition, the granular materials in the stress state of nonhydrostatic pressure lead to stress-induced anisotropy [37][38][39][40], which is helpful in studying the loading influences on contact fabric anisotropy [41][42][43][44]. In practice, granular materials are composed of many irregular particles and voids.…”