Numerical analysis of the bearing characteristics of inclined helical anchors in clay under vertical uplift loading

Abstract

This study examines how inclination angles affect the pullout resistance of helical anchors embedded in clay. By validating a finite element approach with in-situ test data, a semi-cylindrical numerical model featuring a three-helix anchor was developed. We evaluated the structural response and failure mechanisms across inclination angles ranging from 0° to 30°. Given that the load–displacement profiles demonstrate a strain-hardening trend lacking a distinct peak, the ultimate pullout capacity was defined using a 50-mm settlement threshold in accordance with relevant national design codes. Findings indicate that the pullout resistance stays relatively stable for inclination angles up to 10°, whereas it drops sharply at higher angles. Specifically, installing the anchor at a 30° tilt results in a 63.63% reduction in capacity compared to a plumb installation. Furthermore, the load-transfer mechanism transitions from a combined action of helix-plate end bearing and shaft friction to a localized failure governed by the soil shear strength above the tilted shaft. Quantitative analysis reveals that the maximum equivalent plastic strain (PEEQmax) abruptly increases by a factor of 3 to 4 once the tilt reaches 15°, indicating a shift from a broad global shear zone to a highly concentrated local shear band. Additional assessments on softer clay confirm that the 10° critical threshold and the degradation pattern remain highly consistent, proving the universal applicability of this mechanism across varying soil stiffness levels. This research offers a robust theoretical basis for the engineering design of inclined screw piles.