Dynamic Characteristics Of Pile With Variable-sections And Its Application In Pile-cap (Beam) Systems

As the increasing application of pile foundations in engineering project,piles with variable-section interface are adopted in more engineering projects to enhance the bearing capcity.Piles with variable-section interface show obvious avantages compared to piles with constant-section interface.However,there are few studies on the dynamic characteristics of piles with constant-section interface using analytical methods.On the other hand,pile foundations are connected with upper structures and then put into use in actual engineering.Piles are often used together with a cap or beam,and these pile foundations are often subjected to dynamic loads caused by wind and waves.The impact of the vehicle load and the ship can easily lead to the section cracking of the pile.At the same time,the pile foundation that has been in service for a long time can easily cause partial damage to the pile body due to the corrosion and become those with variable-section interface.For those pile foundations mentioned above,the traditional testing methods such as static load test,high strain test method,and sonic transmission method do not apply.Above all,this paper studies the dynamic characteristics of tapered pile considering the vertical reaction of surrounding soil and tapered piles with pile cap,the application of low-strain testing on variable section pile-cap and pile-beam systems are investigated.The main work and achievements are as follows:(1)By discretizing the pile-soil system into finite segments,a new relationship between impedance functions of adjacent pile segments is derived,which is called amended impedance function transfer method.A comparison study between the solution proposed in this paper and existing analytical solutions was conducted to give insight to the effects of vertical reaction of the surrounding soil on the complex impedance at pile top and velocity response at pile top subjected to transient excitation.Then a parameters study is conducted to give insight to the coupled effects between the vertical reaction of the surrounding soil and the parameters of pile-soil system on the complex impedance at pile top.(2)Discretizing the pile-soil system into finite segments,the complex shear stiffness on the pile side is derived.Combing the amended impedance function transfer method,the dynamic equilibrium equation is solved to give an analytical solution of the impedance function at pile top.By comparing the solution proposed in this paper with other solutions,the advantage of tapered pile is confirmed further.Then a parameter study is conducted to give insight to the coupled interaction of the vertical reaction of surrounding soil with construction disturbance effect in low-frequency range,which has guiding significance for seismic design of pile foundation.(3)Considering the displacement and stress continuity conditions at the pile-soil interface,ring-soil pile theory(RSPT)are proposed as models of rigid pile-soil interaction that consider the interaction at the variable-section interface of a pile with the surrounding soil.The interactions between adjacent three-dimensional soil layers are simplified as uniformly distributed Voigt models to derive the impedance function at the pile top.The solution simplifies into other previously proposed solutions for specific geometric parameters of the pile shaft.The proposed solution is compared with one that neglects the interaction at the variable-section interface of the pile with the surrounding soil;additionally,the coupled effects of the related pile-soil parameters and the interaction at the variable-section interface with the surrounding soil are analyzed.(4)A variable-section pile-cap model in three-dimensional soil is presented.The corresponding analytical solution in the frequency domain of the pile top and the semi-analysis in the time domain are obtained.The comparison between the solution which considers the interaction of pile variable section with surrounding soil and which does not is conducted,meanwhile the coupling action between related parameters and the interaction of pile variable section with surrounding soil is analyzed.Then,the application of low-strain testing in variable-section pile-cap system is investigated.(5)Based on the research above,the dynamic characteristics of variable-section pile beam system is studied through semi-analytical technique,and the best position for acquiring the signal is given.Some notes are pointed out to guide the application of low-strain testing in necked pile beam system.Above work could developed the foudamental theory of dynamcs of piles with variable-section interface,and have some guiding significance for the application of low-strain testing in necked pile beam system.