Maximum Depth of Liquefaction Based on Fully-Coupled Time Domain Site Response Analysis-Paper

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Empirical liquefaction assessment methods are generally limited to depths of about 20 m, while offshore wind turbine piles may extend much deeper. This study uses fully coupled, time-domain site-response analyses to examine liquefaction in sand deposits up to 100 m deep and assess how constitutive-model choice affects predictions.<br /><br />The analyses model Nevada sand subjected to the east–west component of the 1986 Lotung earthquake. A bounding surface plasticity model (BSPM) represents cyclic soil behaviour and liquefaction; the nonlinear elastic IC.G3S model represents stiffness degradation and wave damping. The researchers compare uniform deposits with profiles combining the two models, and also compare results with an empirical assessment based on drained analyses.<br /><br />The empirical method predicts liquefaction to roughly 11 m in a 15 m deposit under the original motion, and to the full 15 m under a motion scaled to twice its amplitude. For deeper deposits, it indicates reduced liquefaction susceptibility. Fully coupled analyses using BSPM alone generally predict less liquefaction than the empirical method: the original motion does not fully liquefy the 15 m profile, and the 30 m and 100 m profiles do not fully liquefy at any depth. However, when IC.G3S is used in deeper layers, less wave energy is dissipated, increasing motion in the upper soil. This produces greater pore-pressure and effective-stress reductions; the upper 10 m of the 100 m profile fully liquefies.<br /><br />The results suggest that BSPM calibration focused on liquefaction resistance may overestimate damping. This overdamping can suppress wave motion and underestimate liquefied-zone depth, potentially leading to non-conservative designs for deep foundations. The authors recommend further investigation and incorporating damping checks against laboratory data during model calibration.

Keywords

liquefaction

offshore wind turbine

site-response analysis

deep foundations

cyclic soil behavior

bounding surface plasticity model

wave damping

effective stress

pore pressure

Nevada sand