On the Triaxial Testing of Sand for Offshore Wind Turbine Foundation Design-Paper

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The paper examines how laboratory triaxial testing choices affect the characterization of sand for offshore wind turbine foundation design. As turbine foundations grow, assuming sand always behaves under fully drained conditions may be unreliable; foundation loading can impede drainage, and compression tests alone may not represent the varied stress paths around foundations.<br /><br />Researchers tested three North Sea sands at different densities and stress states using drained and undrained triaxial compression and extension tests, alongside constant-volume direct simple shear tests. They also compared strain- and stress-controlled shearing and assessed the effects of different consolidation creep periods.<br /><br />Results showed that drained tests generally produced higher peak friction angles than undrained tests, particularly for cleaner, denser, more dilative sands. Compression and extension responses differed: compression tests mobilised greater deviatoric stresses, while extension tests could indicate higher friction angles for dilative sands. Direct simple shear stresses were generally lower than compression-test stresses. Extension specimens were prone to necking, limiting measurable strains; stress-controlled shearing could delay necking and extend the useful strain range, but rate effects must be considered. Longer creep periods had little effect on friction angle but affected extension-test stiffness and peak response. Dilation estimates also varied with drainage, stress path, and test type.<br /><br />The authors recommend selecting test conditions and derived parameters to match the foundation’s expected loading and stress paths, rather than relying exclusively on drained compression tests. They also advise careful control of specimen preparation, saturation, consolidation, creep, shearing mode, and dilation calculations. These choices are essential for obtaining representative parameters and avoiding overestimation of foundation strength.

Keywords

offshore wind turbine foundations

triaxial testing

sand characterization

drained and undrained conditions

peak friction angle

direct simple shear

stress path

dilation

compression and extension tests

foundation design