Influence of Cap Rigidity on Pile Group Design

Piles in a group are often connected through a cap which has a relatively small rigidity. In that case, the assumption of fully rigid cap adopted by some pile designers is misleading and can lead to an underestimate of pile deformations and an overestimate of forces and moments in the outer piles of the group, especially for larger groups. This may partially be due to a lack of readily available numerical codes, given that commercial software for pile group design is mainly based on the rigid cap assumption (e.g. Repute, PIGLET, DEFPIG, GROUP). In an attempt to overcome this limitation, this paper presents an extension of the Repute analysis to the fully-flexible cap (i.e. individual piles not connected by cap), a feature which is particularly useful in the design of large pile groups. Validity of the approach is assessed through comparison with alternative numerical analyses and a case history. In addition, a database of real cases is collated showing the relationship between the raft geometry and rigidity. Results provide guidance on the field of applicability of the rigid and flexible cap assumptions, indicating that most large rafts are likely to belong to the perfectly flexible category.