Modelling Anvil Dynamics to Improve Pile Driveability Analyses

It is common practice to perform wave equation driveability analyses, in order to assess the adequacy and performance of the driving hammer selected for the installation of foundation piles, as well as the driving stress induced fatigue damage in the pile. In commercial software for driveability analyses, the anvil, transferring the hammer load from the ram towards the pile wall, is assumed rigid. However, the larger the pile, the larger the anvil and the less it will behave like a rigid body. This paper describes the findings of modelling the dynamic behaviour of the anvil during pile driving, in order to come to more realistic pile driving predictions, especially in large diameter piles. A semi-analytical model capturing the dynamic deflection of the anvil was developed, based on the Lagrange equation. This flexible anvil model was validated against finite element results, as well as against field data. The response of the combined model for ram, flexible anvil and pile reflects both finite element and field data very well, at least if contact between the three bodies is established via a penalty stiffness approach. An impulse-based contact mechanics approach did not bring the expected numerical robustness. The use of the flexible anvil model in driveability analyses results in a significant increase in both calculated energy transfer to the pile and calculated peak force, thus likely affects pile driveability predictions and estimation of fatigue damage, too. Taking into consideration a) the cost impact of a realistic hammer selection, and b) the possible benefits of a more precise evaluation of the fatigue damage as a result from stress estimation during driving, it is recommended to consider this flexible anvil behaviour for more accurate drivability predictions, especially for large diameter piles. Future validation of the suggested anvil modelling with more field data is considered as necessary to further improve its accuracy.