Application of Friction Fatigue Pile Driving Models in GRLWEAP

Abstract

Pile driveability analyses play a crucial part in pile installation planning (hammer sizing, anticipated driving time) and pile design (fatigue concerns). GRLWEAP is one of the predominant pile driving analysis software tools used globally. Traditionally, the offshore community has often wished to use specific SRD prediction methods in driveability analyses. Some of the most popular of these methods (e.g. Alm & Hamre, 2001) employ a ‘friction fatigue’ approach whereby side friction reduces with increasing pile penetration. A past limitation in GRLWEAP has been that friction fatigue methods could not be used in a full driveability analysis, since only a single SRD (valid only for a particular depth of penetration) can be input. This is prohibitive when outputs such as blowcounts are desired at each intermediate penetration depth (e.g. for fatigue analysis), rather than just the final depth. With the introduction of GRLWEAP’s ‘friction fatigue’ module, a way of modelling friction fatigue is now provided, but the fatigue equations are formulated in a specific way that may not be directly applicable to various friction fatigue approaches. Using the Alm & Hamre (2001) friction fatigue method as an example, an approach is presented that shows how friction fatigue models that are not formulated in the same manner as GRLWEAP’s friction fatigue approach can be converted into a suitable form to allow full implementation in a GRLWEAP driveability analysis. This enables driveability analyses to capture the dynamic nature of the SRD as pile penetration increases, producing results that are valid for all penetrations and circumventing the need for alternative workarounds to capture friction fatigue. The general principles presented herein may be modified on similar principles for other friction fatigue models not explicitly captured within GRLWEAP.