Dynamic Response Of Piles In Unsaturated Soil And Its Cyclic Loading Testing

Abstract:Pile dynamics is the theoretical basis of both seismic design for building supporting by piles and integrity testing of piles. Owing to the complexity of the problem, the soils were usually assumed to be single-phase or saturated two-phase elastic medium for simplicity in the past studies. As we know, the near-surface soil existing in engineering environment is mostly unsaturated, and consists of three-phase system: solid skeleton and pores, with water and air filled in the pores. Based on the theoretical framework of unsaturated soil, and following the motion equations for multiphase porous media, the soil-pile dynamic interaction, the fundamental solution of internal dynamic source in a semi-infinite unsaturated elastic medium, and the deformation characteristics of pile subjected to cyclic loading are investigated in this paper via analytical and experimental studies. The viscous and inertial couplings between each phase together with the capillary pressure are taken into account during the process of derivation. The main contents of this thesis are listed as follows:(1) The three dimensional lamb’s problem under consideration is to determine the motion of an unsaturated poroelastic half-space subjected to internal excitation. By the application of the techniques of Fourier expansion and Hankel integral transforms with respect to the circumferential and radial coordinates, respectively, the general solution of the governing partial differential equations in the transformed domain is obtained. Furthermore, the closed form Green’s functions of an unsaturated homogeneous elastic half-space to an arbitrary internal harmonic loading is derived, with consideration of the boundary conditions of the surface and the continuity conditions at the depth of loading. The formulas developed are confirmed in comparison with the degenerated solution of saturated soil.(2) Considering the effect of matric suction on the dynamic shear modulus, the interaction model between the unsaturated soil and single pile under vertical harmonic motion is proposed. The resistance factor and vibration modes of soil layer are first obtained from the direct decoupling of the governing equations under axisymmetric conditions. On the basis of the assumptions of perfect contact along the pile-soil and the supporting types at the pile tip (rigid bearing or Winkler model), the motion equation of the pile treated as a bar and described by the conventional1-D structure vibration theory is then solved. The complex stiffness at the top of the pile, the velocity mobility together with the soil impedances in the frequency domain are derived ultimately.(3) By virtue of the Helmholtz decomposition and variable separation method to the decoupling of motion equation, the dynamic characteristics for the horizontal vibration of an end-bearing pile in unsaturated soil are investigated through the three-dimensional continuum model proposed by Novak. The bending, shearing effects of pile are accounted for by modeling the pile as a Timoshenko beam. Moreover, utilizing the dynamic Green’s function for unsaturated half-space under interior horizontal disk loading, the second kind of Fredholm integral equations describing the dynamic response of the laterally loaded single pile are established via the fictitious pile method. The semi-analytical solutions for dynamic stiffness of the pile head are developed in the frequency domain, and the distributions of pile displacement, bending moment and shear force along the depth are examined.(4) With the Novak plain strain model, approximate dynamic impedance of single pile subjected to lateral vibration is first derived within the framework of wave theory of three-phase porous medium. A modified attenuation function of horizontal movements in scattering domain is then obtained, and the response of single pile is further extended to pile groups by employing the principle of superstition based on pile-pile interaction factor. On the basis of the developed solutions, the effects of parameters including the saturation, the excitation frequency and the pile spacing on the dynamic behavior as well as the load sharing are studied. (5) The dispersion equations of Rayleigh waves in a porous medium saturated by two immiscible viscous fluids are deduced, and the variations of wave velocity, displacement distribution of free field and particle motion are discussed with the changes of soil properties. And then, considering the inertia effect of the superstructures, and within the obtained wave field solution for unsaturated soil half space, closed-form expressions for transverse seismic performance are presented through a dynamic Winkler model, for both fully-buried and partially-buried single pile in the unsaturated soil excited by incident Rayleigh waves. In addition, the influences of the saturation, the axial loading and the pile length of extended segment on the seismic response of pile are determined through numerical calculation.(6) The performance of small-scale model piles embedded into red clay and tested under cyclic axial loadings is presented. The influences of cyclic loading levels and rates on long-term dynamic behaviors of pile are observed, and the mechanics of accumulated settlement is analyzed from the views of shear stiffness softening and skin friction degradation. Furthermore, the modified Hardin-Drnevich (H-D) model, being capable of reflecting the fatigue degradation of shear stiffness, is achieved in the FLAC3D, and the behavior of shear resistance degradation during quasi-static cyclic shearing under constant normal stiffness (CNS) condition is numerically investigated.