Characterization of Pile Driving Induced Ground Motions

Abstract

Pile installation is a complicated, energy intensive process where codes and regulatory standards provide some guidance, but little is understood about coupling and transmission of pile driving energy into and through the ground in the form of vibrations. These vibrations can cause direct structural damage and damage due to settlement of granular soils. This thesis presents results that give insight to concepts that are still in question concerning pile driving induced vibrations using impact hammers. These results are the outcome of an innovative research comprised of three components: (1) full-scale ground monitoring during impact driving of H-piles in the field, (2) small scale pile driving testing in a controlled laboratory environment and (3) numerical analysis of the impact pile driving process using 3D finite element analysis. Field pile driving vibration data were collected from five project sites. The mechanisms of energy propagation during impact pile driving were evaluated by installing sensors in the ground, starting very close from the pile (0.5 ft) and moving away at different radial distances and depths, generating the first data set of its kind. Analysis of the data reinforces the hypothesis of the wave propagation field generated by impact driven piles. Body waves radiate from the pile tip in a spherical wave front. Shear waves propagate outwards from the pile shaft in a cylindrical wave front. The shaft transfer starts only after the pile tip passes below and observation point (sensor). The Rayleigh wave development reported by various researchers is not verified. Attenuation of the peak particle velocity and increase of the shear wave velocity at increasing distances from the pile is also confirmed. A widely used attenuation formula (Bornitz equation) was fitted to the recorded measurements and was found to be a good model to describe the energy degradation through the soil when impact driven piles are used as a source. The attenuation coefficients are in agreement with earlier documented findings. A process to evaluate the potential for a granular soil to undergo shakedown settlement is presented based on the field measurements from the tested sites. This concept can serve as a first guide for identifying potentially troublesome sites with similar site conditions. A decrease of particle velocity and an increase of shear wave velocity with increasing distances from the pile is also verified. Reduced-scale physical experiments of pile driving were conducted in the laboratory. The controlled environment of a homogeneous and properly characterized soil profile allowed for investigation of the mechanisms of energy transfer from the pile to soil without the complexities encountered in the field. The generated wave field follows the pattern found for field testing measurements. The contribution from shear waves is not “seen” by the installed sensors until the pile tip reaches their elevation. These trends substantiate the existence of two wave fields, spherical and cylindrical, generated from a linear source as the pile, a behavior also observed in the field. The pile driving induced vibration field was modeled using a 3D finite element dynamic analysis. It is clearly shown that a cylindrical wave front emanates from the shaft and a spherical wave front radiates from the pile tip. Preliminary results of calculated ground motions in the very close proximity of the pile showed good agreement with recorded ground motions in the laboratory.