RESPONSE OF VERTICAL PILE UNDER LOW FREQUENCY AXIAL VIBRATIONS

Abstract

The vibratory pile driving is a relatively new technique of pile driving which has proved to be very successful under certain conducive conditions. Relatively it is more economical and less time consuming. The pheno-menon of response of a pile under longitudinal vibrations is a complicated one .involving a large number of physical quantities. In the past, both experimental and analytical approaches have been applied to study this complicated phenomenon. Hejazi (1963) proposed an analytical approach for the pile response under vibratory loads. The approach is based on certain idealized conditions and the proposed equations are lengthy and complicated. A unique solution of these equations is not possible. Experimental investi-gations carried out in this field before Ghumman (1985) were mostly confined to small scale model tests, yielding only qualitative results (Kondner and Edwards 1960, Bernhard 1968 and Schmid 1969). After consistent efforts, Ghumman (1985) developed a large size model test set-up to study this phenomenon and studied the performance of vertical piles under low frequency vibrations through limited tests. Later, Singh (1986) utilizing Ghumman's test set up studied the influence of some of the parame-ters e.g. roughness of pile surface, material etc. int1u-encing the pile behaviour. I As previously, the effect of shape of pile on the penetration behaviour of vibro-pile driving has not been studied and tests have been conducted on only circular piles, in the present study 1-Section pile has been utili-sed. The vertical 1-section pile (Sectional area 5.18 cm2 ) is driven under low frequency vibration (f<40 cps) in sand deposit placed at three different relative densi- ties varying from 43.65% to 62.5%. The static force, Fs on the pile is maintained as constant i.e. 50 kg but the dynamic force Fo is varied keeping Fo/Fs ratios of 0.6, 0.8 and 1.0. The penetratin of the pile has been carefully monitored. Analysis of the test data indicates that under low frequency axial vibrations, initially the pile pene- trates suddenly into the sand bed (termed as and then continues to penetrate at a slow rate (termed as Ym) and finally keeps on oscillating without any further penetra- tion. Thus the total depth of pile penetration, Yu is composed of two parts, namely Ys and Ym. The pile pene-tration Yu' Ys and Ym decreases as the relative density of sand increases. The maximum depth of pile penetration is noted at 30 cps which probably is close to the resonant frequency in the present conditions. Ym' the time-depen-dent penetration and the decaying part of its time pene- ii tration curve can be simulated by an equation of exponen-tial form involving a constant, k, which is an index of measure of pile penetration. The data obtained from tests has been analysed utilizing non-dimensional approach and non-dimensional equations have been developed to obtain ultimate depth of pile penetration from different input parameters. iii