Three-dimensional finite element analysis of static compression tests on steel helical piles in clay and sand

This study develops and validates a 3D finite element model to simulate the load-settlement behavior of helical piles. A key aspect of this numerical study is its comprehensive inclusion of various factors influencing helical pile capacity at field tests. In particular, the paper concentrates on the pile geometry (both single and 2 × 2 grouped), soil shear strength reduction brought by the installation-induced softening of the soil in the inter-helix space in both lightly overconsolidated lacustrine clay and medium sand. This numerical study provides a detailed prediction of the concentration of compression-induced soil displacement around the helix plate in terms of the nodal settlement vectors around the shaft. It contrasts with existing helical pile studies by using hyperbolic shear hardening soil (HS) and soft soil (SS) models in 3D finite element (FE) analysis for field tests in stiff consistency clay with an undrained shear strength of 65 kPa, instead of the bilinear elastic-perfectly plastic Mohr-Coulomb (MC) models. The MC, HS and SS models were calibrated using field Cone Penetration Test (CPT) data to analyze full-scale load-settlement field tests. This study compared predicted load-settlement patterns with field data. The numerical analysis revealed that the calculated working load capacity, typically determined at a settlement of 1% of the pile helix diameter, was higher than the actual axial load recorded in the field tests at clay sites. To account for pile installation disturbance and achieve better agreement between the FE model predictions and field observations at clay sites, the key soil stiffness parameters of the FE constitutive models were reduced by 25% to 50%. Furthermore, the results indicate that no adjustments to soil stiffness parameters are necessary when the helical pile is installed in sand. A parametric study varying soil deformation stiffness, helix diameter and axial loading conditions (compression and tension) was conducted to explore how these factors affect the performance of the validated 3D FE model of helical pile in soil.