A screw pile is a special-shaped pile with several advantages, including good bearing capacity, economy, and rapid construction. The calculation of the screw piles' ultimate bearing capacity in the individual bearing failure state remains controversial. To address the problems of an unclear failure mechanism and the pile–soil contact relationship in screw piles, we conducted large-scale direct shear tests using a partial amplification method. The variation law for soil stress and the failure

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... tern of soil around the screw teeth were analyzed. The bearing capacity of the screw shear plate with screw teeth was found to be significantly higher than that of the plane shear plate, while that of the screw pile first increased and then decreased with an increase in the screw pitch. The optimal screw pitch allowed the determination of the maximum bearing capacity. Furthermore, the optimal screw pitch was generally equal to the critical screw pitch, which distinguished the individual bearing failure from the cylindrical shearing failure. A new calculation method for the critical screw pitch and ultimate bearing capacity in the individual bearing failure state was presented, and its rationality was proved using the direct shear test results. The calculation of the critical screw pitch considers the shear strength of soil and the geometric parameters of the screw teeth, making it more widely applicable. These results can provide a theoretical basis for the subsequent design of screw piles.