Seismic Behavior of Pile Embedded in Virgin and Improved Clay: A Numerical Case Study for Kolkata-Paper

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This study examines the seismic lateral behavior of piles embedded in soft clay, with and without ground improvement, using a representative Kolkata soil profile. The profile comprises 10 m of soft clay over 30 m of stiff clay. The authors assess how improvement depth affects pile response under combined axial and lateral loading, considering pile slenderness ratios of 20, 30, and 40 and both free-head and fixed-head conditions.<br /><br />A finite-element model based on the beam-on-nonlinear-Winkler-foundation approach represents soil resistance using cyclic p–y curves, including stiffness degradation linked to earthquake magnitude. Seismic forces are estimated from depth-dependent soil accelerations. The input motion is the 1979 Imperial Valley earthquake record, scaled to Kolkata’s zone factor of 0.16 g. Improved clay is assumed to have twice the Young’s modulus of the original clay.<br /><br />Ground improvement reduces maximum horizontal acceleration near the surface and therefore lowers seismic forces on the pile. Increasing improvement depth raises pile-head stiffness and reduces pile-head deflection, though benefits diminish beyond certain depths. For free-head piles, stiffness gains become negligible at about 6–8 pile diameters (D) of treatment; for fixed-head piles, the corresponding range is about 8–10D. Deflection decreases most rapidly up to approximately 4–5D, then more gradually. The authors identify about 8D as a practical general optimum for the cases studied.<br /><br />The findings are specific to the modeled soil profile and earthquake motion, so they should not be treated as universal design guidance. The study also cautions that increasing pile length or diameter to raise vertical capacity can increase seismic lateral demand and deflection. Ground improvement around the pile may help control this response and maintain acceptable lateral performance.

Keywords

seismic lateral behavior

piles

soft clay

ground improvement

Kolkata soil profile

p-y curves

finite element model

pile-head deflection

axial and lateral loading

earthquake response