Load transfer behaviour of super long piles in multi-layer soft soil through field testing and numerical 3D FEM modelling

The load transfer mechanism of pile foundation has received considerable attention over the years, the simultaneous responses that skin friction and base resistances of super-long piles (length L > 60 m) can have in complex soft soil, however, still need greater understanding. This study employs 3D-finite element (FE) analysis incorporating virtual interface elements to simulate the mobilised skin friction and plastic failure (slippage) of pile under ultimate loading. Static pile load tests on 4 different long and large bored piles (1–1.5 m in diameter and 70–80 m in length) embedded in the soft soil region of Mekong Delta are studied in detail through extensive instrumentation along the piles. The results are then used to not only explore load-transfer process, but also validate numerical modelling through a comprehensive process combining multiple-soil layers and −loading stages. The coupled experimental (field) − numerical results reveal the predominant contribution of skin friction exceeding 90 % of the entire bearing capacity before a drop with swift rise in base resistance when reaching a critical condition (displacement s h > 25 mm and load pressure p > 14,000 kPa). The ratio of active skin friction is defined to assess the simultaneous variation of skin friction at different depths, featuring the role of pile length on the mobilisation of skin friction. The study also proposes a novel dynamic method to calculate the strength reduction factor, R i , based on fundamental soil and load parameters, giving a vital means to advancing the use of interface elements when modelling pile foundation in soft soil.