Axial Winkler springs for vertically loaded piles

Abstract

Axial soil spring stiffness along piles is crucial in the dynamic analysis of offshore jacket foundations. This study proposes a rigorous analytical model for vertically loaded piles embedded in a soil layer based on Green's functions. The stiffness of both end-bearing piles and floating piles can be obtained from this model, which is different from previous studies that have considered end-bearing piles and floating piles by two different models. The study investigates various factors affecting the soil springs. The actual distribution of soil spring stiffness is complicated, even for homogeneous soils. To facilitate the engineering application, the soil spring stiffness is averaged to uniform distribution according to the equivalent work method. The pile with relative pile-soil axial stiffness greater than 1.5 can be treated as a rigid pile in which the soil spring stiffness depends primarily on the pile slenderness ratios. For other cases, referred to as flexible piles, the soil spring stiffness depends on both pile slenderness ratio and pile-soil modulus ratio. Simplified analytical expressions are proposed for the averaged soil spring stiffness of both rigid and flexible piles in different soil profiles. Besides, the soil layer effect is significant for short piles but negligible for long piles.