Design and Construction of The Billy Bishop City Airport Pedestrian Tunnel, Toronto, Canada
Design and Construction of The Billy Bishop City Airport Pedestrian Tunnel, Toronto, Canada
Design and Construction of The Billy Bishop City Airport Pedestrian Tunnel, Toronto, Canada
INTRODUCTION
3.1
Introduction
The short term kinematic stability of the backfilled TBMdriven secant bore arch was assessed on the basis of
numerical modeling using the discrete (distinct) element
program UDEC by Itasca Consulting Group, which offers
the advantage of modeling the entire construction
sequence of excavating and backfilling each individual
drift bore, followed by the main tunnel excavation and
support as well as the potential to identify kinematic failure
mechanisms. The UDEC analysis also considers stress
and jointing anisotropy of the rock mass and models the
progressive stress re-distribution following each
construction stage.
UDEC uses a constitutive Mohr-Coulomb failure
criterion model which does not address the time
dependent deformation (TDD) behavior of the Georgian
Bay shale.
However, potential kinematic failure
mechanisms would be expected regardless of TDD
effects. Therefore, the use of UDEC was deemed
appropriate to evaluate these kinematic failure
mechanisms.
3.3
4.1
Tunnel Waterproofing
5.1
Figure 12.
2014)
MPBXs
In an effort to obtain more reliable tunnel sidewall
convergence measurements, two multi-point borehole
extensometers (MPBXs) were installed at station 0+020m
(two tunnel diameters from the mainland portal), one in
each sidewall, in mid-July of 2013. The MPBXs were
installed within 3m long alcoves excavated from the
central cut 1 sidewall to the final main tunnel sidewall (cut
2) profile.
The MPBX data for the eastern wall has previously
been reported by Hurt, et al. (2014), and is updated in
Figure 16. The overall MPBX was 12m in length, with
values of inward deformation (convergence) measured at
the tunnel sidewall and at 2m, 5m, and 8m behind the
wall, all measured relative to a presumed point of fixity
12m behind the wall.
Very little wall movement
(~0.05mm) was recorded until the east sidewall cut 2
excavation work was completed both up-station and
down-station from the 3m long alcove (centered at station
th
th
0+020m) between August 29 and September 6 , 2013,
which resulted in approximately 0.1mm of immediate
elastic sidewall movement followed by a nominal degree
of time-dependent movement. A second discrete jump in
the tunnel sidewall movement (~0.2mm) was observed
when the invert (cut 3) was excavated between October
18-19, 2013, which was again followed by time-dependent
movement. The tunnel sidewall was pushed back slightly
during the concreting of the permanent tunnel arch in midMarch, 2014, but recovered rather quickly;the MPBX
points within the rock mass have since experienced a
nominal residual effect.
After nearly one year of MPBX measurements, the
total inward movement of the eastern wall was only
0.6mm, with about half of this movement being attributed
to elastic movements induced by the sidewall (cut 2) and
invert (cut 3) excavations. The remaining time-dependent
inward movement of the eastern wall (after completion of
the cut 2 sidewall excavation) of approximately 0.25mm is
generally consistent with the typical post-cut2 shortening
SUMMARY