Dynamic stability of steel piles

The response of an initially curved cantilever strut with a lumped mass at the tip subject to arbitrary axial impact loads is described, as a model for the dynamic response of piles being driven into the ground. The differential equation of motion defining the lateral movement of impact loaded cantilever struts is derived. This equation includes the effect of inertial resistance to motion provided by both the strut self mass and the mass of a pile hammer. The probable shape of force pulses produced during pile driving were obtained from observations by others and approximated by an equivalent trapezoidal force pulse. The differential equation of motion was solved for this force pulse using the Runge Kutta technique and for physical parameters corresponding to those of a slender steel pile which was observed whilst being driven. The resulting solution was found to agree well with observed pile behaviour during driving in that the magnitude of lateral displacements were very small. It is also shown that the magnitude of lateral displacements reduces as the mass of the pile hammer increases, due to inertial effects. In principle, the theory presented can be used to determine the maximum slenderness ratio for a cantilever pile so as to keep the lateral deflections within a prescribed value for a given pulse shape. (Author/TRRL)