Long-term evolution of negative skin friction on piles in coastal soft ground: Effects of vacuum-preloading depth and surcharge

Pile foundations for coastal infrastructure often penetrate mucky soft soil layers. Continued consolidation settlement under self-weight and subsequent surcharge induces significant negative skin friction (NSF) and causes neutral plane (NP) migration. Vacuum preloading accelerates consolidation and reduces post-construction settlement, but deeper untreated soil may continue settling under surcharge if treatment depth is insufficient. Currently, long-term NSF development and NP migration under different treatment depths and surcharge conditions remain unclear. This study conducted centrifuge model tests using shallow and deep treatments, each with and without surcharge, over a five-year prototype service period. Test results show that deep treatment significantly reduces long-term surrounding-soil settlement, shortens the axial-force accumulation range, and lowers the peak axial force, which stabilises earlier. Although its effect on the NSF peak is limited, deep treatment shortens the NSF-affected zone and causes the NP to move upward and stabilise. Surcharge increases pile and soil settlements, pile axial force, and NSF, but affects NP differently with treatment depth: migration is pronounced under shallow treatment but limited under deep treatment. Compared with shallow treatment, deep treatment reduces the average NSF coefficient by 18.9% with surcharge and 20.0% without surcharge, supporting vacuum-preloading depth selection and long-term pile-performance assessment in coastal soft ground.