Failure model of soil around enlarged base of deep uplift piles

Abstract

This paper presents an investigation into the failure behaviours of low-cohesive (c ≤ 10 kPa) or non-cohesive (c = 0) soils around the enlarged bases of deep uplift piles. An axisymmetric elasto-plastic finite-element method is used for analysing displacements and plastic strains in the soil–pile system induced by uplifting. Numerical results are examined in detail to reveal the relationships of the accumulated equivalent plastic strain contours in soils and the uplift displacements of piles with the diameters of enlarged bases. The investigation finds that for the pile to reach ultimate uplift failure, the pile has to experience a large upward displacement. A serviceability failure limit of pile upward displacement equal to 10% of pile shaft diameter is adopted for determining pile uplift resistance capacity. At serviceability failure limit, examination of the accumulated equivalent plastic strain contours in the soils leads to the establishment of two logarithmic spiral functions for the estimation of the plastic zone envelope and the slip surface around the enlarged base. The plastic zone envelopes and slip surfaces are axisymmetrical, peach-shaped, closed and curved surfaces, completely beneath the ground surface. The models and functions have been used to formulate analytical solutions for estimating uplift resistance capacity for deep piles with enlarged bases.