Field tests on negative skin friction of steel pipe piles in high backfilling soils

For the piles in large-area high fill soft soils, rubble mound may cause negative skin friction on piles because of the settlement of backfilling soils. To grasp the influence of the negative skin friction, field tests are carried out on a steel pipe pile of Yangshan Port of Shanghai. The knowledge on key parameters such as distribution, magnitude and temporal variation of the negative skin friction of the pile, location of neutral point and downdrag force is acquired. The results indicate that: (1) the time effect of the negative skin friction on the pile is obvious. The time for the negative skin friction of soft soils to reach its peak is shorter, while that for the good soils with large embedded depth is longer; (2) the measured negative skin friction coefficient of rubble mound is up to 0.8, while the coefficient of silty soft clay is only 0.04, the coefficients of other soils are in good agreement with the technical code of China; (3) in nearly nine months after rubble mound construction is completed, the down drag force of the pile continues to increase. The maximum downdrag force is up to 7740.4 kN, depth of the neutral point is between 0.62L ~ 0.68L, and the negative skin friction generated by the rubble mound is up to 184.4 kPa. Attention should be paid to the design of similar projects in the future, and appropriate measures should be taken to reduce the negative skin friction.