Numerical assessment of negative skin friction on pile groups in soft ground

The mobilization of negative skin friction (NSF) imposes a detrimental load, known as dragload, on piles by reducing their axial bearing capacity and inducing additional settlement. The adoption of sacrificial piles arranged in a group configuration can reduce this dragload. However, the understanding of NSF mobilization in pile groups, particularly in the context of soft soil creep, remains limited. This paper successfully implements an elastic viscoplastic model with a cutting plane algorithm into a finite element package. Numerical investigations, validated through centrifuge tests, are conducted to examine the effect of sacrificial piles on the dragload reduction in the center pile (termed as group effect) at varying pile spacings. A parametric investigation is conducted to quantify the group effect under different variables, including pile spacing, end-bearing soil stiffness and creep coefficient. Results reveal that beyond the spacing of 7 d, where d is the diameter of sacrificial piles, the group effect can be neglected. Furthermore, the group effect is highly dependent on the site-specific creep behavior, becoming less significant under high creep conditions. The reduction in effective stress of soil within a pile group is identified as the primary cause of the NSF pile group effect.