Testing pile foundations at the ETH Zurich drum centrifuge: recent developments

Motivated by the need to develop rational design methods for the retrofit of existing bridges on pile groups, this paper introduces recent experimental developments at the ETH Zurich drum centrifuge in Switzerland. Four set-ups are developed for vertical, pushover, combined and vibration testing. Their capabilities and limitations are demonstrated using as an example a 2 x 1 pile group on dense saturated sand. Single piles are subjected to vertical loading, exploring the role of installation effects and interface roughness. Pushover loading is employed to measure the moment capacity M-ult of a lightly and a heavily loaded group. In contrast to intuitive expectations, the heavily loaded system mobilises larger M-ult. The developed combined loading apparatus is proof-tested for a shallow foundation. Combined loading under constant vertical load is conducted to derive failure envelopes, revealing significant coupling between lateral and moment loading, and confirming the expansion of the failure envelope with increasing static vertical load. The vibration testing set-up is proof-tested, confirming the possibility of identifying the natural frequency of the system and the small-strain stiffness of the foundation through non-destructive testing. Although the study is fuelled by the ongoing work on pile groups, the developed experimental set-ups are of general applicability for the study of deep and shallow foundation systems.