Offshore Clays - Parameter Calibration of a New Hybrid Hyperbolic Model-Paper

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The paper develops a calibrated hybrid model for predicting shear-modulus reduction and damping in offshore clays under cyclic loading. It draws on 105 resonant-column test programs using undisturbed samples from 12 offshore sites worldwide, covering a broad range of plasticity indices (PI), overconsolidation ratios (OCR), depths, and soil consistencies.<br /><br />The model combines a hyperbolic normalized shear-modulus relationship with a damping correction method. It uses four parameters: reference shear strain (the strain at which the secant modulus falls to 70% of its maximum), a curve-shape factor, a damping adjustment constant, and minimum small-strain damping. The reference strain is calibrated as a function of PI and OCR; the other parameters are assigned recommended average values because clear relationships with these soil properties were not found.<br /><br />PI is the main control on both stiffness degradation and damping behavior. Increasing PI generally shifts the modulus-reduction and damping curves toward larger strain levels. OCR effects on modulus degradation depend on PI: they are negligible for low- and medium-plasticity clays, but become more evident for high- and very-high-plasticity clays. The observed OCR-related modulus trend differs from some previous studies, and its cause warrants further investigation. Damping results are highly variable, with no clear overall OCR relationship; the available data also do not establish reliable trends for minimum damping.<br /><br />The proposed curves are intended for offshore geotechnical applications, including nonlinear seismic site-response analysis and soil–structure interaction assessments for fixed structures such as wind turbines. The authors caution that variability may reflect factors not addressed in the study, including mineralogy, particle-size distribution, confining pressure, and sample disturbance. For large or critical projects, they recommend direct laboratory measurement of dynamic soil properties.

Keywords

offshore clays

cyclic loading

shear-modulus reduction

damping behavior

resonant-column tests

plasticity index

overconsolidation ratio

hybrid model

dynamic soil properties

seismic site response