NATURAL FREQUENCY OF PILES IN COHESIONLESS SOIL

Abstract

The lateral load capacity of piles has, ,bring the past decade, d unin ated studies relating to pile behaviour both isolated and in groups, a major portion of the work, however, being confined to static studies. Dynamic studies on piles and soil-pile systems are important because of the wide range of applications of pile systans which are subjected to dyn nic loads due to ocean waves, machine imbalance and a horde of loads which could be directly or indirectly placed in this category. The principal objective of dynamic studies being the detennin ati on of the natural frequency of isolated piles and pile groups. Embodied herein are results obtained from the dynamic studies conducted with the inclination of determination of the isolated piles and pile groups. The variation of the natural frequency with change in parameters such as the mass of superstructure, lateral load, spacing and the number of piles has also been investigated. The natural frequency of piles, both when isolated and grouped together, is found to decrease with the increase in magnitude of the vertically sustained load and the lateral load. In pile groups, however, an increase in the natural frequency can be achieved with increase in spacing provid& it is less than where 6d is the diameter of the pile' VII beyond which no appreciable increase can be envisaged. Similar affects on the natural frequency of a pile group can also be imposed by increasing the number of piles upto a maximum of six piles in any group independent of the spacing in that particular group, any increase beyond the above limit produces no marked change on the natural frequency. The natural frequency of a pile group can be predicted on the basis of the dynamic tests conducted cn a single pile as well as the load deflection characteristics of a single pile. These predictions have tj high degree of correlation with the values obtained from actual experiments conducted on pile groups.