Premature refusal can pose a threat to driven pile installation. One contingency measure that has been applied is pre-drilling ahead of the pile tip to ease driving. While some practitioners may assume that carefully conducted drilling will not give any adverse effect, little evidence is currently available to assess the possible impact on long-term axial capacity. This paper describes a standard gravity (1g) small-scale model laboratory study in which 18 small closed-ended piles were jacked into constrained 300mm cubes of stiff high overconsolidation ratio (OCR) natural London clay, with and without pre-drilling. Four control piles were also installed without any boring. Pile loading tests performed after appropriate equalisation periods showed that pile capacity reduced systematically as the ratio of the drill hole-to-pile solid area ratio increased. The model experiments showed comparable reductions in capacity to those in earlier model and field studies on soft Mexico City clays. An effective stress analysis is suggested to help the application of the model tests to other soils types and pile geometries. Capacity reductions are concluded to be likely when pre-drilling is performed in highly overconsolidated offshore clays. The results have implications for both new piling operations in exceptionally hard clays and the re-analysis of existing installations in a broader spread of soil types, where refusal may have resulted from pile hammer capacity limitations.

1. Introduction
1.1 Use of pre-drilling to ease pile installation

Driving refusal has been encountered frequently during offshore foundation pile installation. One field contingency measure applied to achieve target penetration with tubular piles has been pre-drilling ahead of the tip to ease driving. The development of more powerful hammers and improved driveability predictions has reduced the incidence of refusal events, but consideration still has to be given to this possibility when dealing with unusually hard or dense materials.

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