Model test study on the negative skin friction of piles in unconsolidated soft soil with different initial consolidation degrees

The primary consolidation of unconsolidated soft soil is not yet complete，so it is susceptible to producing excessive settlement，causing drag forces along the pile shaft installed in these soil layers. Few discussions on the bearing capacity of piles(i.e. negative frictional resistance) in underconsolidated soil have been reported，including the influence of the underconsolidation degree. In this paper，a parameter termed as initial consolidation degree，Uz ，was defined to characterize the underconsolidation degree，and three model tests were carried out under different initial consolidation degrees，in which surcharge preloading was considered. The behavior of the negative frictional resistance of the pile under different initial consolidation degrees and surcharge levels was analyzed，and the coefficient of negative frictional resistance and neutral point depth were discussed. The results show that the initial consolidation degree has a significant effect on the negative frictional resistance of pile. With the increase of Uz (i.e.，the decrease of the underconsolidation degree of soil)，the settlement of pile，settlements of soil at different depths，axial force of pile shaft and negative friction resistance of pile all show a decreasing trend，and their behavior is basically similar under different loading levels. The average values of negative friction coefficient under Uz = 0.25，0.50 and 0.75 are 0.545，0.351 and 0.147，correspondingly. Compared to Uz = 0.25，the negative friction coefficients under Uz = 0.50 and 0.75 are reduced by 35.6 % and 73.0 %，respectively. The increase of Uz leads to the neutral point moving downward，but the downward rate gradually decreases. When Uz = 0.25，0.50 and 0.75，the neutral point depths are 0.53，0.75 and 0.76 of the pile depth，correspondingly. In addition，the increase of the surcharge level causes the increase of negative friction coefficient and the decrease of neutral point depth slightly，but the effect is not significant in the holistic view when the initial consolidation degree is identical. The research results can provide technical reference for the analysis and design of negative friction resistance of piles in unconsolidated soft soil. © 2024 Academia Sinica. All rights reserved.