THESIS
2013
xvii leaves, 218 pages : illustrations ; 30 cm
Abstract
Tunnel construction inevitably causes soil stress changes in the ground and hence induces
ground movements. Uncontrolled ground movements induced by tunnelling may cause
cracking in buildings and gas mains, or induce additional loads on piles of nearby structures.
In urban cities, it is not uncommon to encounter existing piles located close to tunnel
construction. Estimation of the effects of tunnelling on existing pile foundations of buildings
poses a major challenge to designers and contractors.
The objectives of this research are to investigate the fundamental mechanisms of twin tunnel-soil-pile interaction three-dimensionally and to quantify effects of twin tunneling on pile
capacity. Two major research methodologies, namely centrifuge modelling and finite element
analysis,...[
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Tunnel construction inevitably causes soil stress changes in the ground and hence induces
ground movements. Uncontrolled ground movements induced by tunnelling may cause
cracking in buildings and gas mains, or induce additional loads on piles of nearby structures.
In urban cities, it is not uncommon to encounter existing piles located close to tunnel
construction. Estimation of the effects of tunnelling on existing pile foundations of buildings
poses a major challenge to designers and contractors.
The objectives of this research are to investigate the fundamental mechanisms of twin tunnel-soil-pile interaction three-dimensionally and to quantify effects of twin tunneling on pile
capacity. Two major research methodologies, namely centrifuge modelling and finite element
analysis, were adopted. The advancement of tunnel excavation process was simulated in-flight
by controlling volume loss (V
1) at 1.0% at each stage of tunnel excavation. In addition to
measurements of ground surface settlement and pile settlement, bending moment and axial
force induced in pile by tunnelling in different stages of construction were recorded. In the
centrifuge model tests reported in this thesis, the effects of cover-to-diameter (C/D) ratios of
twin tunnels on a single pile of 0.8 m diameter and 19.2 m long were investigated. In addition,
the effects of different construction sequences of twin tunnelling on pile capacity at different
depths were simulated and the behaviour of a 2×2 piled raft due to the construction of twin
tunnels were also studied. In order to enhance our fundamental understanding of the three-dimensional
twin tunnel-soil-pile interaction, systematic three-dimensional numerical back-analyses
of centrifuge tests and parametric study were carried out. Three key dimensionless
groups, i.e., relative location of twin tunnels with respect to pile depth (Z/L), the normalized
distance between each tunnel and pile with diameter of tunnel (H/D), and volume loss due to
tunnel construction are considered in numerical parametric analyses.
It is found that the settlement of a pile induced by twin tunnelling is closely related to the
depth of each tunnel relative to the pile. With a horizontal distance of 0.25D from each tunnel,
the maximum cumulative pile settlement occurs when the twin tunnels are located at the pile
toe (i.e., Test TT), owing to the significant reduction of confining stress induced by the twin
tunnel excavated near pile toe. The excavation of the first tunnel results in a pile settlement of
about 1.9% of the pile diameter. Similar magnitude of pile settlement is also induced by the
construction of the second tunnel. Based on the displacement-failure load criterion proposed
by Ng et al. (2001a), the apparent loss of pile capacity (ALPC) is about 21% after the
construction of the first tunnel construction, and increases to about 36% (cumulative) after the
construction of the second tunnel. The cumulative pile settlement due to twin tunnelling near
the toe is about 2.2 times larger than of that caused by twin tunnelling near the mid-depth of
the pile. The ALPCs for twin tunnelling near mid-depth of pile are about 14% and 20% after
the excavation of the first and second tunnels, respectively. It is revealed that the construction
sequences of twin tunnelling at different depths have a significant effect on the cumulative
pile settlement and hence pile capacity. The pile settlement induced by twin tunnelling with a
tunnelling sequence ST (first tunnelling near the mid-depth of the pile shaft and then second
tunnelling near the pile toe) was 33% larger than that with a tunnelling sequence TS (the first
tunnelling near the pile toe and then second tunnelling near the mid-depth of the pile shaft).
The measured transverse tilting of piled raft due to twin tunnelling near pile toe is 0.18%,
which is close to the limit (i.e., 0.20 %) for tall buildings proposed by Eurocode 7 (2001). Due
to twin tunnelling near mid-depth of pile, the measured transverse tilting of piled raft is only
23% of that induced by twin tunnelling near pile toe. For all the cases studied, the induced
bending moment due to twin tunnelling is insignificant.
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