Time-dependent dragload and neutral plane of a pile beneath an approach embankment on stratified clay: unified method versus coupled consolidation analysis

Abstract Purpose Piles built before an approach embankment on compressible clay are dragged down as the clay consolidates. The unified method gives the long-term dragload and neutral plane from an assumed toe stiffness and fully mobilised shaft shear, and says nothing about time. This study measures what those assumptions cost for a pile through high-plasticity clay over low-plasticity clay. Methods A 0.6 m by 18 m pile carrying 600 kN beneath a 3 m embankment (54 kPa) was analysed with the unified method and with a coupled finite-element consolidation model with a frictional interface. The clay was modelled as equivalent elastic and as Modified Cam Clay, with consolidation followed for 600 years. The model was checked against an independent one-dimensional solution and four meshes; twelve variations were run. Results Final dragload was 444 kN with Modified Cam Clay and 459 kN with elastic clay, against 801 kN from the unified method; the neutral plane lay at 12.6 and 13.5 m instead of 14.25 m. About 166 kN of the gap came from toe stiffness and 180 kN from partial mobilisation over a 9 m transition zone. Dragload lagged settlement: at 34% consolidation the pile carried 19% of its final dragload. With a draining bearing stratum, 61% arrived within the first year. Conclusion With inputs consistent with the model, the rigid-plastic method located the neutral plane within about 0.2 m but overestimated the dragload by 32 to 41%. Toe response and drainage of the bearing stratum controlled magnitude and timing more than interface strength did.