Behaviour of an over-length pile in layered soils

Piles over 50 m long in layered soils, known as over-length piles, have been widely used in large bridges and high buildings in China, but there has been little investigation into their behaviour. This paper focuses on the behaviour of such piles under vertical loading, investigated using a three-dimensional finite-element method. To obtain reasonable results, it is essential to choose an appropriate element type and stress-strain relationship. It has been suggested that the constitutive models for pile concrete, layered soils and pile-soil interface may be respectively a linear elastic model, a hyperbolic model and a rigid-plastic model. As an example, an over-length pile 75 m long and 2.5m in diameter in layered soils about 300 m deep, used as a field testing pile during construction of the Sutong Bridge in China, was modelled and analysed. The results indicate that the over-length pile is a friction pile. Stability failure may be induced by failure of the soil along its side, and the failure sequence of the soil is from top to bottom with increasing pile load. The effects of pile length and the depth and thickness of the soft soil layer on the pile's behaviour were also investigated.