An investigation of the driving mechanism of the vibrohammer
The recent development of North Sea oil has seen the construction of massive structures founded in the seabed of the North Sea. Inadequacies in existing site investigation techniques employed for the design of the foundations of these structures have shown that a need exists for a device to permit an accurate and rapid assessment of the soil conditions existing below the seabed. This thesis considers the application of the vibrohammer, a machine capable of driving piles tubes or rods rapidly into the ground under a self-adjusting combination of vibration and impact, as the driving mechanism both for a coring device and a dynamic penetrometer. In so doing it presents the historical development of the vibrohammer from its introduction in the U.S.S.R. circa 1940, together with the parallel development, also in the U.S.S.R., of theoretical treatments of vibro-impact dynamics. An experimental investigation is carried out at both model and full-scale level and confirms that a vibrohammer is capable of self-adjusting the magnitude of the impacts it generates with increasing soil resistance. The capacity of a vibrohammer to produce rapid penetration rates does not arise from a potential to produce large impacts but from the ability to produce an optimum combination of vibration and impact. Optimum machine parameters at which the self-adjustment results in maximum depths of penetration are identified and explained both from the experimental work and a computer simulation of the vibro-hammering process. A possible mechanism by which this self-adjustment occurs is proposed. The application of a vibrohammer as the driving mechanism for a dynamic penetrometer requires the selection of machine parameters different from those which result in maximum penetration depths. In order to maximise the ratio of the measured dynamic soil resistance to the corresponding static soil resistance it is necessary to operate a vibrohammer in a predominantly impact mode. The development of a mechanical adaptor unit by which existing pure vibratory coring devices may be modified to operate as a vibrohammer is presented and discussed in the light of preliminary field test results.