State Departments of Transportation often are requested by contractors to use vibratory drivers rather than the more conventional impact hammers to install piles. Vibratory pile drivers can provide substantial savings by reducing the amount of driving time to final penetration under certain soil conditions. However, the lack of a reliable dynamic method of estimating bearing capacity limits their usefulness. Presently, the most common method to determine capacity is to restrike the pile with an impact hammer, but the validity of this method is unproven and the extra operation reduces the potential savings.
Developing a reliable method for dynamically determining bearing capacity of piles installed with vibratory drivers is a complex problem. Laboratory studies were needed to provide insight into the basic behavior of piles installed with vibratory drivers compared to impact hammers and the influence of various soil parameters on the behavior of piles. Laboratory studies would also assist in the design of future field tests and the analysis of results.
The overall objective of this study was to evaluate the load-deformation behavior of piles installed in the laboratory with vibratory drivers. Specific objectives included: (1) a comparison of load deformation behavior of piles installed with vibratory drivers and impact hammers; (2) the identification of soil parameters that significantly affect load-deformation behavior of piles installed with vibratory drivers; (3) a comparison of load-deformation behavior of piles installed by vibratory drivers with and without restriking using an impact hammer to evaluate the effect of restriking; and (4) the development of a recommended predictive method of determining bearing capacity for further field verification.
Research is complete, and the principal findings have been published in NCHRP Report 316: Laboratory Evaluation of Piles Installed with Vibratory Drivers; only the unedited contractor's final report is currently available. Additional detail on the various experiments is available in volume 2 of the contractor's final report titled, "Supplement to NCHRP Report 316, Laboratory Evaluation of Piles Installed with Vibratory Drivers, Appendices B-Q."