Assessing the Pile Driving Risk Due to the Presence of Boulders

Boulders are known to be present within foundation zone depths at some Atlantic East Coast wind energy development areas, which can make it difficult to level a piled jacket or template and can lead to the progressive collapse of the pile toe, causing premature refusal during pile driving. Although detection and avoidance are preferred over mitigation, numerical analysis methods are available to assess the risk of pile refusal, which allows for informed decisions on whether avoidance is required and what types of mitigation to consider during construction. Detailed numerical evaluation (using one-dimensional wave equation analyses and two- and three-dimensional finite difference and finite element modeling) was performed to develop a better understanding of stresses in the pile during driving. The numerical modeling evaluated the effect of strength, thickness, inclination, shoe length, wall thickness, and lateral continuity on pile stresses. A three-dimensional model of the pile and driving shoe subjected to stress-time histories was used to evaluate the stresses at the pile toe and at the transition from the pile to the driving shoe. Example results are presented to illustrate failure mechanisms of hard layers that include boulders, and high-level guidance is provided on operational sequences and potential contingency measures.