January/February 2023 - 75

by the underpinning beams supporting
the now undermined subway boxes.
Any support of excavation in this area
also had to account for small allowable
movements of the soil supporting the
adjacent subway line. The innovative
solution to this tricky challenge was to
use two stacked 12 m (40 ft) long wide
flange waler beam sections installed
between the webs of the shoring piles on
either side of the subway. The north and
south double walers were preloaded
toward each other using a series of
tierods, like a bow and arrow, before
being encased in reinforced concrete.
Once the concrete had reached a
minimum design compressive strength,
the tensile preload on the tierods was
then released transferring the preload to
the reinforced concrete waler beams
and soil behind prior to further excavation.
This system was designed using
the observational method and eliminated
the need for cross site bracing in
the very tight access and allowed
Temporary support of existing subway line
excavation to proceed with enough
headroom to install the lower shotcrete
wall and four rows of soil anchors. The
preloaded waler was monitored
throughout preloading, unloading and
excavation to ensure it met the design
requirements. If movements were to
exceed allowable limits, a strut would be
installed mid-span to limit deflections,
optimistically at a late enough construction
stage to not impede construction.
The overall deflection of the
preloaded waler system after final
excavation was 12 mm (0.5 in) on the
north wall and 0 mm on the south wall,
within the 1/1,000 (12 mm/0.5 in) limit
imposed by the TTC, and much less than
the theoretical non-preloaded deflection
of 30 mm (1.2 in). The overall deflection
of the anchored shotcrete shoring
system was 9 mm (0.3 in), and the
existing subway boxes adjacent to the
excavation saw less than 2 mm (0.08 in)
differential settlement at the joint
locations, within the acceptable range.
Acknowledgments
The overall construction of this station
spans over a decade and two contracts
and involved many companies to make
it a success. Both project teams are
responsible for accomplishing this
challenging construction saga over and
under the existing TTC subway line.
Sarah Speir, P.Eng., is the youngest, and one of
the only female, project managers working on
the Eglinton LRT project for Crosslinx
Transit Solutions. Before starting on the
project in 2015, she got her bachelor's degree
in civil engineering from Queen's University.
Shawna Munn, P.Eng., is a senior
engineer with Isherwood Geostructural
Engineers in Mississauga, Ontario. She has
more than 12 years of experience in the
shoring and specialty foundations field with
a focus on complex excavation retention
systems and subway infrastructure.
DEEP FOUNDATIONS * JAN/FEB 2023 * 75

January/February 2023

Table of Contents for the Digital Edition of January/February 2023

TOC
January/February 2023 - Intro
January/February 2023 - 1
January/February 2023 - 2
January/February 2023 - TOC
January/February 2023 - 4
January/February 2023 - 5
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