Single Piles In Liquefiable Ground: Seismic Response And Numerical Analysis Methods

The seismic response of piles in liquefiable ground is an important andchallenging topic in the field of geotechnical earthquake engineering. Through acombination of case analysis, centrifuge shaking table experiments andnumerical simulations, the seismic response patterns of single piles inliquefiable ground were revealed, and the axial pile force and settlement duringpost-earthquake reconsolidation was studied. A complete set of numericalmethod for the analysis of single piles in liquefiable soils was established,consisting of constitutive formulations, numerical algorithms and modellingtechniques. The main novel achievements are as follows:1. A three-dimensional unified plasticity model for large post-liquefactionshear deformation of sand was formulated and implemented for parallelcomputing, based on which a three dimensional dynamic finite element analysismethod for piles in liquefiable ground was developed. The constitutive model isable to achieve unified description of the behaviour of sand at different statesunder monotonic and cyclic loading during both pre-and post-liquefactionregimes. Appropriate stress integration algorithm, three-dimensional stressprojection algorithm and parallel computation techniques were applied in theOpenSees implementation of the model. The potential of the model and itsnumerical implementation were explored via simulations of classical elementand centrifuge experiments. The finite element analysis method was validatedagainst centrifuge shaking table experiments.2. Methods for the analysis of consolidation and reconsolidation inducedpile axial force and settlement with consideration for consolidation process wereproposed. A beam on nonlinear Winkler foundation (BNWF) solution and amodified neutral plane solution were developed and validated against centrifugeexperiments for piles in consolidating and reconsolidating ground.3. The seismic response patterns of single piles in liquefiable ground werestudied, including: basic force-resistance mode, kinematic and inertialinteraction coupling mechanism and major influence factors. Opposite bending direction of piles with and without pile cap under kinematic forces was causedby the difference in rotational constraint at the pile head. Moment caused bykinematic and inertial interaction are opposite for single piles with pile cap,while being of the same direction for single piles without pile cap. The totalmoment caused by dynamic interaction can be viewed as the vector sum ofmoment caused by kinematic interaction and inertial interaction, and is affectedby both the amplitude and phasing of the two types of interaction. Thedominating forces for piles with and without caps are kinematic and inertialforces respectively. Pile residual moment increases with increasing lateralspreading in sloping ground, and could become the peak moment. The existenceof a non-liquefiable layer over the underlying liquefiable layer may cause themaximum moment to occur at the layer interface.4. The axial forces and settlement of piles during post-earthquakereconsolidation was analyzed. The maximum pile axial force caused bypost-earthquake induced negative friction is irrelevant of the reconsolidationprocess, and is only determined by the final state of the ground. However, pilesettlement is dependent on the soil settlement at the neutral plane duringreconsolidation, while the neutral plane position changes during thereconsolidation process.