THESIS
2017
xxii, 184 pages : illustrations ; 30 cm
Abstract
Deep excavations inevitably induce stress relief and ground movement, which may affect the
safety and serviceability of nearby existing pile foundations. Although the effects of excavation
on adjacent piles have been studied by some researchers recently, most of the previous studies
have focused on the response of end-bearing piles due to a single excavation.
This research aims to investigate behaviour of a floating pile adjacent to single and double deep
excavations. Three centrifuge tests were conducted to study influence of a single multi-propped
excavation on the response of a floating pile with different restraints. In addition, two centrifuge
tests were performed to examine effects of double excavations with different excavation sequences
on the pile behaviour. Excavation...[
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Deep excavations inevitably induce stress relief and ground movement, which may affect the
safety and serviceability of nearby existing pile foundations. Although the effects of excavation
on adjacent piles have been studied by some researchers recently, most of the previous studies
have focused on the response of end-bearing piles due to a single excavation.
This research aims to investigate behaviour of a floating pile adjacent to single and double deep
excavations. Three centrifuge tests were conducted to study influence of a single multi-propped
excavation on the response of a floating pile with different restraints. In addition, two centrifuge
tests were performed to examine effects of double excavations with different excavation sequences
on the pile behaviour. Excavation process was simulated in-flight in the centrifuge tests. Moreover,
three-dimensional numerical analyses using an advanced hypoplastic model were carried out to
back-analyse the centrifuge tests. Based on the calibrated numerical model parameters, a
systematic parametric study was conducted to evaluate effects of excavation depth, pile location
with respect to excavation, initial working load, soil density and support system stiffness on the
pile-soil interaction.
It is found that a pile-raft settled a similar amount of 0.6% of pile diameter due to excavation as a
free-head pile and an elevated pile group during a single multi-propped excavation. During the
excavation, a downward load transfer mechanism in the pile can be identified. Shaft resistance in
the upper part of a pile decreases due to downward soil movement and a reduction of normal stress
acting on the pile. In order to maintain vertical equilibrium, the pile has to settle to mobilise higher
shaft resistance in the lower part of the pile and toe resistance. Lateral restraints imposed on the
pile head have a significant influence on the induced pile bending moment. Induced bending
moment due to excavation can exceed the pile bending capacity. Results from the parametric study
show downward and upward load transfer mechanisms at different stages of the excavation,
depending on the ratio of the excavation depth to the pile length (H
e/L
p). The pile settlement due
to an adjacent excavation could be underestimated by up to 39% if the designed ultimate shaft and
base resistance before the excavation are used to calculate the change of shaft and base resistance.
The excavation-induced pile settlement is negligible when the pile is located at a distance of more
than 2.5 times the excavation depth from the excavation. In addition, the excavation-induced pile
settlement can be doubled when the design factor of safety is reduced from 3.0 to 1.5. When the
relative density of sand is increased from 30% to 90%, the induced pile settlement is decreased by
up to 75% due to an increase of soil stiffness. Moreover, due to the limited stress relief and soil
movement, the induced pile settlement is insensitive to the support system stiffness when H
e/L
p < 0.4.
During sequential double excavations, the measured pile settlement during the first multi-propped
excavation and the second cantilever excavation are found to be 0.42% and 0.72% of the pile
diameter, respectively. The larger pile settlement in the second excavation is due to a decrease of
soil modulus and a reduction of normal stress acting on the pile. Horizontal soil movements and
pile bending moments are found to reverse during the second excavation. Moreover, the excavation
sequences have limited influence on the induced pile head settlement since the soil experiences
similar stress paths. However, the maximum induced pile bending moment is more than five times
larger if the double excavations are conducted sequentially rather than simultaneously. It is more
beneficial to conduct double excavations simultaneously.
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