Abstract: As is well-known, the effective vertical stress of surrounding soil element of pile foundation is not identical to the dead weight of overburden soil plus the applied surcharge load. This is because the vertical shearing is developed on pile-soil interface due to the large pile-soil relative stiffness. The analytical solutions for the effective vertical stress, the unit positive (negative) shaft resistance, the total shaft resistance and the axial force (dragload), subject to axial and surcharge loads, are derived by means of equilibrium analyses. Those solutions are calibrated by the reported test result of a single pile in saturated clay. It is shown that those solutions are reasonable and it is necessary to consider the vertical shearing of pile-soil interface. Under the surcharge load, the vertical shearing weakens the effective vertical stress, the unit negative shaft resistance and the dragload. The negative shaft resistances at the pile tip of the cases ? = -0.01, -0.02 and -0.04 are reduced to 91%, 83% and 71% that of the case ? = 0, respectively. Meanwhile, the induced dragload at the pile tip of the cases ? = -0.01, -0.02 and -0.04 are reduced to 93%, 87% and 76% that of the case ? = 0, respectively. The dragload is reduced slight smaller than the negative shaft resistance since it is also controlled by the cross-sectional geometries. This indicates that pile settlement is always overestimated independent of the vertical shearing.

Key words: vertical shearing of pile-soil interface, equilibrium analysis, effective vertical stress, shaft resistance, dragload