Field measurements of the stiffness of jacked piles and pile groups

Abstract

INTRODUCTION This Technical Note presents a back-analysis of the axial response of some field-scale tubular displacement piles installed by jacking. This study was motivated by the recent development of large pile-jacking rigs. These rigs are sufficiently strong to offer an alternative to bored piling in urban areas where conventional dynamic methods of displacement pile installation are not permitted (White et al., 2002). The most notable feature of the axial response was the very high stiffness. Some pile jacking machines ‘walk’ along the pile wall under construction, so all piles must necessarily be installed at close centres, or indeed touching. This geometry is in contrast to conventional design guidance, which suggests that a separation of typically two or three diameters is required between piles, to prevent harmful interaction effects (BSI, 1986; GEO, 1996). Load test results from jacked piles installed closer than this recommended minimum spacing are presented. The study comprised a series of field tests carried out in Kochi, Japan. Full details are provided by Yetginer et al. (2003), and brief details are repeated below. Test piles were installed singly and in groups forming a ‘cell foundation’, comprising jacked piles at close centres around an enclosed soil block. In this Technical Note, pile group efficiency is defined as the average performance per pile (strength or stiffness) when a group is loaded in unison, divided by the performance of a single pile tested alone. Using this definition, the behaviour of trial piles installed prior to the main group can be linked to the group response. This definition is used by Vesic (1969) and Fleming et al. (1992), and is shown in equation (1) for strength and stiffness. Group strength efficiency, GROUP, is defined in equation (1a), where QSINGLE and QGROUP represent the capacity of a single (isolated) pile and the capacity of a group of n piles respectively. Group stiffness efficiency, GROUP, is defined in equation (1b), where KSINGLE and KGROUP represent the stiffness of a single (isolated) pile and the stiffness of a group of n piles respectively.