Investigation Of Improved Fictitious Soil Pile Method And Its Application In Longitudinal Vibration Problems Of Piles With Nonconstant Cross Sections

Currently, the existing models for the pile-soil interaction problem are mostly non-continuous and lack of extensive investigation on the effect of soil beneath the pile. In order to establish a precise coupling relationship between the pile and soil, a fictitious soil pile method was proposed in this research project to include the effect of stress dispersion at the tip of piles. Based on this method, the influence of soil on the longitudinal vibration behavior of single plies, the impact of welding on the integrity detection of multistage concrete tubular piles, as well as the dynamic characteristics of bored PHC nodular piles under different conditions were thoroughly studied. The main finished work and innovative achievements of this article are listed as following:(1) A fictitious soil pile method was proposed to consider the effect of stress dispersion based on the Mindlin additional stress distribution under the circular vertical loading on the foundation. Further, in combination with the plane strain model, the analytical solutions of longitudinal vibration response of piles in frequency domain and the corresponding semi-analytical solutions in time domain were obtained using Laplace transform and convolution theorem. It is shown that shear stiffness of soil is crucial to determine the complex stiffness of piles according to a parametric study of the influence of different parameters of soil on the dynamic response of piles. The increase of shear stiffness of soil will enhance the stiffness and damping of pile significantly. It was also found that there is a critical value of the thickness of soil between pile tip and bedrock that causing influence on the dynamic response of piles, and this value is about5-7times of the pile radius. Within this range, the stiffness of piles is proportional while the damping of piles is inversely proportional to the thickness of soil.(2) The fictitious pile soil model was further developed from the plane strain model to the axial symmetric continuous soil model, with which the longitudinal vibration model of pile-soil system taking into account of the three-dimensional wave effect of soil around the pile was built considering the radial and longitudinal displacement of soil. Then the displacement potential function and Laplace transform were imported, and the vertical and radial displacement was decoupled by the method of variable separation. Consequently the response of piles in frequency domain was obtained by the impedance transmission and the response of piles in time domain was obtained using Laplace inverse transform. The results obtained by fictitious pile soil method based on the axial symmetric continuous model was compared with that based on the plane strain model, and the drawed conclusion is that the axial symmetric continuous model is better to reflect the natural frequency of pile-soil system.(3) A simplified model of the welding seam of concrete tubular piles was proposed, in which the physical factors that affecting the welding quality are included in the effective stiffness, hence the welding can be theoretically simulated. The fictitious pile soil method was applied to solve the dynamic response at the top of multistage concrete welded tubular piles. The influec of parameters including the effective cross-section and height of welding on the velocity response at the top of piles were analyzed, and it was found that the depth of welding should be less than5mm and the height of welding should be limited to2mm. The fitted results using measured data show that this simplified model can effectively diagnose the influence of welding quality on the reflected wave of tubular piles, thus can provides guidance for the integrity detection of piles.(4) According to the forming mechanism of bored PHC nodular piles, a pile-soil dynamic model was developed regarding to the characteristics of radial nonhomogeneity of soil and the fictitious pile soil method. The dynamic response at the top of bored PHC nodular piles with surrounding nonhomogeneous soil was determined using the transmission method of coefficient matrix. The influence of some factors like the properties of nodular and surrounding cemented soil on the dynamic response of bored PHC nodular piles were also investigated, and the results show that the stiffness of piles is proportional to the density and radius of nodular. It was also found that when the hardening level of surrounding cemented soil is higher, the stiffness of pipe bolck is larger and the damping is smaller, and the cement with mix proportion of40%is most cost efficient. Finally, the hardening rule of surrounding cemented soil of piles was proven through inversion fitting of engineering data.The core achievement of this research is improvement of simulating the pile-soil interaction by proposing the fictitious pile soil method. With the aim of solving problems occurred in engineering practice, the newly proposed method and drawed conclusions in this article will provide reference for the engineering design of pile foundations.