FEATURE ARTICLE Comparison of Impact Versus Vibratory Driven Piles in a variety of ground conditions. However, despite the evolution in hardware, the design codes used to predict the behaviour of vibratory piles have not advanced at the same pace. Existing design guidelines used to predict pile capacities are largely based on impact-driven pile research, with limited or no modification to account for variation in the installation process. Vibratory hammers are commonly used to install sheet piles Pipe pile installation for the New NY Bridge (courtesy of New York State Thruway Authority) Every year, DFI’s Committee Project Fund supports innovative research in areas critical to the deep foundations industry. The project “Comparison of Impact versus Vibratory Piles” was selected in March 2014. This project was supported by the Marine Foundations and Driven Piles Committees. Dr. Paul Doherty, with GDG, geotechnical engineering consultants, based in Ireland, coordinated the research. Current engineering standards have been based on impact driven piles and do not explicitly consider the effect of vibratory installation. The research project will assess the impact of installation procedures on long-term pile performance. The research examines whether installation using vibratory or traditional impact hammers affects the axial and lateral resistances of driven piles. The project consisted of compiling the published literature and completing case studies to identify any trends in pile performance with different hammer types. Here, we describe the project background, the potential benefits for industry, and offer some preliminary results. We hope to expand the database of relevant case studies, and encourage all practitioners in the deep foundations industry to submit details of projects where the relative merits of impact and vibratory piles can be quantified. This is crucial to ensure the project findings are robust. Background Vibratory hammer technology has been used to facilitate deep foundation construction for 80 years. During that time, the equipment has evolved to a high standard; vibratory hammers are now capable of installing piles more than 70 ft (21 m) in diameter due to the high rate of penetration, the reduced ground vibrations and the possibility of extraction. Sheet piles are often used for temporary or nonstructural applications, so the axial, lateral and serviceability performance is of lesser concern compared to driven pipe piles. The latter are commonly used to support bridges, buildings and offshore structures. The deep foundations industry is particularly interested in vibratory installed piles for marine applications. There are two advantages to these piles: lower noise contributes to an improved environmental profile, and faster installation can lead to huge economic savings. However, to ensure accurate and robust design of such load-supporting foundations, it is critical to understand the soil-structure behaviour of vibratory driven piles and how the pile performance may be influenced by the installation process. RWE vibro pile testing project Despite extensive international research completed on pile behaviour over the past 50 years, there is no well-defined database of vibratory driven piles or research that compares the axial or lateral performance of impact and vibratory driven piles. This lack of published data means that offshore piles are most commonly installed using impact driven hammers, and where vibratory hammers are used, a conservative pile design is normally adopted. In European offshore wind projects, a vibratory hammer is often used for the initial penetration, but the installation process reverts AUTHOR Paul Doherty, B.E., C.Eng MIEI, MICE, Ph.D., Managing Director, Gavin and Doherty Geosolutions (GDG) DEEP FOUNDATIONS • JAN/FEB 2015 • 63
