Study And Application On Vibration Characteristics Of Tapered Pile Based On Gradient Section Model

The tapered pile was firstly used in in the Soviet Union in the 20th century.Due to the special geometric characteristics indicating the gradual change of the pile diameter,the pile body has better bearing performance.Compared to the traditional equal diameter pile consuming the same material,the tapered pile can bear more.Therefore,this pile takes the advantages of engineering applications.In the last decades,experts have studied the static characteristics of tapered piles through theoretical calculations,finite element analysis,indoor models,field tests,etc.In the other hand,studies of dynamic theory of tapered piles are more based on the simplified model of traditional equal section pile,which have defects.This paper proposes to establish a non-equal section pile model,and based on this,the following main work is done.1.The dynamic equation is solved through finite difference method and mpedance transfer method.The non-equal section pile model is proved true by theoretical comparisons and model test.Compared with the traditional equal section pile model,calculation results are similar on case of the low soil shear wave velocity.With the increase of soil shear wave velocity,there is a significant difference between the results.The longitudinal vibration velocity of pile top calculated by non-equal section pile model is lower than that of the simplified model.2.Three parameters including pile length L,pile diameter in the half pile Rm and tapered angle a are used to describe the geometric characteristics of tapered piles.The calculation shows that the longer the pile length,the weaker the reflection signal from bottom of pile,and the smaller the amplitude in the frequency response curve.Also,As Rm increases,? decreases,the reflected signal from bottom of the pile will rise slightly,the C region curve will sink significantly.At the same time,the frequency response curve in the low frequency region will increase,while the frequency response curve in the high frequency region keep still.3.Longitudinal wave velocity of pile and shear wave velocity of soil can influence the dynamic response of pile.The larger the longitudinal wave velocity of pile,the higher the reflected signal and the amplitude in the frequency response curve.In the homogeneous soil,as the shear wave velocity of the soil increases,the time domain curve sinks,and the reflection signal is weaker.As for the frequency domain curve,the amplitude is smaller.In heterogeneous soil,when the difference in the shear wave velocity of the soil layer becomes larger,the fluctuation of the time domain curve is also greater,while longitudinal translation of reflection signal occurs.The Soil inhomogeneity also influence the amplitude of frequency domain curve.4.The parameter ?=d/D,indicates the ratio between diameter of pile top and diameter of pile bottom.There is a critical ?0 where the reflected signal from bottom of pile disappears,which makes it hard to figure the length,integrity,etc.The calculation shows that as pile length increases,elastic modulus reduces,shear wave velocity of soil increases,the critical ratio ?0 becomes larger.However,Average pile diameter has little effect on the critical ratio ?0?5.The defect length of tapered pile has less influence on the bottom reflection signal,while it influence the location and height of defect signal.The deeper the position of defect,the lower the signal.As the defect of tapered pile increases,signal of defect becomes greater and signal of pile bottom reduces.On the aspect of frequency domain,defect of tapered pile reduces the number of formants.Also oscillation amplitude and same-order resonance frequency of frequency response curve change as the defect changes.6.The parameter ?=d/D,indicates the ratio between diameter of pile top and diameter of pile bottom.The parameter ?s indicates ratio between impedance of defect section and impedance of normal pile section.As for tapered piles with defect,the signal of pile bottom reduces as the parameter ? decreases.And there is a critical diameter ratio ?0 where the signal of pile bottom disappearsd,which increases with pile length.In the other hand,the larger the cross-sectional defect ratio ?s,the weaker the reflected signal at the bottom of the pile.And there is a critical parameter ?s0 where the signal of pile bottom disappears,which increases with pile length.Therefore,it's necessary to reasonablly design of pile length and taper angle in case of dissappeared pile bottom signal...