Transient kinematic pile bending in two-layer soil

The dynamic response of piles to seismic loading is explored by means of an extensive parametric study based on a properly calibrated Beam-on-Dynamic-Winkler-Foundation (BDWF) model. The investigated problem consists of a single vertical cylindrical pile, modelled as an Euler–Bernoulli beam, embedded in a subsoil consisting of two homogeneous viscoelastic layers of sharply different stiffness resting on a rigid stratum. The system is subjected to vertically propagating seismic S waves, in the form of a transient motion imposed on rock outcrop. Several accelerograms recorded in Italy are employed as input motions in the numerical analyses. The paper highlights the severity of kinematic pile bending in the vicinity of the interface separating the two soil layers. In addition to factors already investigated such as layer stiffness contrast, relative soil–pile stiffness, interface depth and intensity of ground excitation, the paper focuses on additional important factors, notably soil material damping, stiffness of Winkler springs and frequency content of earthquake excitation. Existing predictive equations for assessing kinematic pile bending at soil layer interfaces are revisited and new regression analyses are performed. A synthesis of findings in terms of a set of simple equations is provided. The use of these equations is discussed through examples.

Research highlights► Stiffness contrast between upper and lower layer, soil damping, depth of upper layer, waveform of input motion were investigated. ► Simple equations were provided to account for spring stiffness k adopted in Winkler models and damping ratios D employed in free field response analyses. ► New regression analyses were carried out for computing transient pile bending moments at the soil layer interface. ► Existing predictive equations for assessing pile kinematic bending at soil layer interfaces were revisited. ► Three alternative procedures were provided to solve the problem in the realm of routine engineering calculations.