Single Piles in Liquefiable Ground : Seismic Response and Numerical Analysis Methods

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Single Piles in Liquefiable Ground : Seismic Response and Numerical Analysis Methods

Abstract

The seismic response of piles in liquefiable ground is an important and challenging
topic in the field of geotechnical earthquake engineering. Through a combination of
case analysis, centrifuge shaking table experiments and numerical simulations, the
seismic response patterns of single piles in liquefiable ground are revealed, and the
axial pile force and settlement during post-earthquake reconsolidation is studied.
A complete set of numerical method for the analysis of single piles in liquefiable
soils is established, consisting of constitutive formulations, numerical algorithms
and modelling techniques. The main achievements are as follows:
1. A three-dimensional unified plasticity model for large post-liquefaction shear
deformation of sand is formulated and implemented for finite element analysis,
based on which a three-dimensional dynamic finite element analysis method for
piles in liquefiable ground is developed. The constitutive model is able to
achieve a unified description of the behaviour of sand at different states under
monotonic and cyclic loading during both pre- and post-liquefaction regimes.
Appropriate stress integration algorithm, three-dimensional stress projection
algorithm and parallel computation techniques are applied in the OpenSees
implementation of the model. The potential of the model and its numerical
implementation are explored via simulations of classical element and centrifuge
experiments. The finite element analysis method is validated against centrifuge
shaking table experiments.
2. Methods for the analysis of consolidation and reconsolidation-induced pile axial
force and settlement with a consideration for consolidation process are proposed.
A beam on nonlinear Winkler foundation (BNWF) solution and a
modified neutral plane solution are developed and validated against centrifuge
experiments for piles in consolidating and reconsolidating ground.
3. The seismic response of single piles in liquefiable ground is studied, including
basic force-resistance mode, kinematic and inertial interaction coupling mechanism
and major influence factors. The roles of kinematic and inertial effects
could differ due to the difference in rotational constraint at the pile head.
Moment caused by kinematic and inertial interaction is opposite for single piles
with pile cap, while being of the same direction for single piles without pile
cap. The total moment caused by dynamic interaction is affected by both the
amplitude and the phasing of the two types of interactions. The dominating
forces for piles with and without caps are kinematic and inertial forces,
respectively. Pile residual moment increases with increasing lateral spreading in
sloping ground. The existence of a non-liquefiable layer over the underlying
liquefiable layer may cause the maximum moment to occur at the layer interface.
4. The axial forces and settlement of piles during post-earthquake reconsolidation
were analysed. The maximum pile axial force caused by post-earthquake
induced negative friction is irrelevant to the reconsolidation process, and is only
determined by the final state of the ground. However, pile settlement is
dependent on the soil settlement at the neutral plane during reconsolidation,
while the neutral plane position changes during the reconsolidation process.

Keywords : Liquefaction Pile foundation Constitutive model Seismic response
pattern Numerical analysis method

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