Lateral Static and Dynamic Response of Single Piles in Non-Homogenous Soils using a Standalone Two-Parameter Foundation Model
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Date
2023-03
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Addis Ababa University
Abstract
In the design and analysis of structures supported on piles, geotechnical engineers
are required to formulate pile responses. Such a formulation involves the
use of subgrade models that relate the interface forces to the displacement.
The primary focus of this thesis is one such relatively new subgrade model,
initially developed for shallow foundations, applied to laterally loaded piles.
The model is a Kerr-equivalent two-parameter Pasternak-type continuum subgrade.
Furthermore, as with most pile applications, the variable nature of the
soil along the pile depth is also taken into consideration. The goal of this
thesis is to investigate the applicability of the aforementioned model for predicting
the static and dynamic inertial response of a single pile embedded
in non-homogenous soils represented by a power function. To this end, the
model in question is initially calibrated by setting equivalences between the
responses predicted and results from finite element analysis. Expressions for
a calibrating factor left open in the model are developed as functions of relevant
parameters. The results from the calibrated model are compared with
the results from the finite element method and found to be comparable. Additionally,
it is demonstrated that the critical slenderness ratio and critical
relative stiffness can be approximated by threshold values form expressions
developed for the calibration factor. Using the calibrated subgrade model, the
static response of the soil is also investigated. Expressions for the pile head
flexibility influence factor and pile head stiffness terms are provided. The
proposed expressions compare well with published results. In addition, utilizing
the unique characteristic of the model being studied, in that the shear
interaction is explicitly accounted for, its effects on static pile response are
also studied. In general, neglecting the shear term results in overestimation
of the pile head displacement. Similarly, the use of a calibrated lower order
Winkler-type model that implicitly accounts for shear interaction is investigated
which by in large provides higher displacement profiles. The calibrated
model is also used in predicting the dynamic inertial response of a laterally
loaded pile in non-homogeneous soil. An approximate energy method is used
to arrive at the dynamic pile head stiffness and damping ratios. With the exception
of fixed-head piles, it is observed that the dynamic pile head stiffness
can reasonably be approximated by the static pile head stiffness for low ranges
of excitation frequencies commonly encountered in seismic events. Curve fit
expressions are also provided for the pile head damping ratio. The effect of
shear interaction is also investigated for the dynamic inertial case. As the
analysis is confined to long flexible piles, very little effect is observed with the
exception of piles in rocking oscillation.