Strain Accumulation in Granular Soils under Cyclic Multiaxial Stress Conditions-Paper

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The study examines how simultaneous cyclic normal and shear stresses affect long-term strain accumulation in granular soil, relevant to offshore wind-turbine monopile foundations exposed to multidirectional wind and wave loading.<br /><br />Researchers tested Hostun RF sand in a Hollow Cylinder Torsional Apparatus, which can independently control axial, radial, circumferential and shear stresses. Three drained tests applied 30,000 cycles at 0.1 Hz after an initial anisotropic stress state. All tests used the same normal-stress cycle; two also included different shear-stress amplitudes, causing the principal stress direction to rotate through ranges of up to 15° and 22.5°.<br /><br />Strain accumulation was nonlinear in every test: it was fastest early in cycling and generally slowed at higher cycle counts. Accumulation increased substantially with greater shear stress and principal-stress rotation. When rotation exceeded 20°, accumulated strain changed by more than an order of magnitude compared with normal cycling alone. In the most severe test, axial strain after the first cycle already exceeded the total accumulation in the other tests after 30,000 cycles, and strain had not reached saturation by the end of testing.<br /><br />Despite differences in cyclic response, the tests showed similar post-cyclic strength characteristics. The results indicate that accounting for multiaxial stress conditions and rotating principal stress directions is essential when predicting long-term soil deformation around offshore foundations. The experimental data can also support the development and calibration of constitutive models for cyclic soil behaviour.

Keywords

granular soil

cyclic loading

strain accumulation

principal stress rotation

shear stress

normal stress

offshore wind turbines

monopile foundations

Hostun RF sand

Hollow Cylinder Torsional Apparatus