May/June 2023 - 99

A regular column showcasing successful
best practices for sustainable foundations
FOUNDATIONS FOR A SUSTAINABLE FUTURE
Carbon and Cost Savings on Toronto LRT Shoring
The Challenge
The Eglinton Crosstown Light Rail Transit
(LRT) is Toronto's first major new
subway line since the 1990s, and more
than half the line is below grade, with 25
stations and three launch and exit shafts
for tunnel boring machines. For Launch
Shaft 2 (LS2), part of the advance tunnelling
contract that was constructed in
2013-2015, the shaft design was governed
by a prescriptive specification that
included constraints around the
required loading diagram, shoring stiffness
and maximum support spacing.
The LS2 excavation was expanded again
to build Cedarvale Station in 2017-2022.
Specifications for this work and other
underground stations on the LRT line
differed from the advance tunnel contract,
using a performance-based specification
with more design flexibility.
The Solution
By comparing the shoring design at the
interface between both excavations,
which are both 18 m (60 ft) deep cuts
supporting a public park, we can estimate
the added materials needed because
of the prescriptive specification
approach, summarized in the table.
The Results
A strong argument for performancebased
design specifications over
prescriptive specifications is in the
performance of these two shoring walls:
LS2 moved a maximum of 8 mm (0.3 in)
into site and the adjacent station shoring
had a maximum of 7 mm (0.25 in) into
site movement upon completion of
excavation, showing that the additional
reinforcing had a negligible impact on
shoring performance.
AUTHORS
Excavation where the station met the shaft
Launch Shaft 2
Bracing
5 tieback rows
1,400 kN (314 kip) design
load, 7,000 kN (1,574 kip)
total
Pile Size
Pile Toe
Emissions Per Pile
W610x217 at 3 m spacing
(W24x146 at 10 ft spacing)
6m (20 ft)
18 tCO2e
The added shoring materials
increased the LS2 carbon emissions by
80% compared to the station shoring
design, or roughly 530 tCO2e of
emissions. This neglects the difference
between the timber lagging used at
Cedarvale Station and the secant wall
fillers used for LS2, which were required
for the launch shaft's 8-year design life.
Extrapolated over the 13 underground
stations in the new LRT, the
choice to use performance-based design
on the station work resulted in substantial
cost, time, material and environmental
savings. We applaud Metrolinx,
the government of Ontario agency in
charge of the Eglinton LRT, for its transi*Source:
EPA calculator at https://www.epa.gov/energy/greenhouse-gas-equivalencies-calculator
Katie Castelo, P.Eng., and Shawna Munn, P.Eng., of Isherwood Geostructural Engineers, with input data contributions from
Adam Plomske, P.Eng., Green Infrastructure Partners
DEEP FOUNDATIONS * MAY/JUNE 2023 * 99
Cedarvale Station
Deck beam strut at grade
2 tieback rows
1,400 (314 kip) and 1,000 kN
(225 kip) design loads, 2,400 kN
(540 kip) total
W610x125 at 3 m spacing
(W24x84 at 10 ft spacing)
4.5m (15 ft)
10 tCO2e
tion to performance-based shoring
specifications that leverage local knowledge
and experience without affecting
shoring performance and safety.
Additional estimated emissions at LS2 due to
design specifications: 530 tCO2e
The added emissions are equivalent* to:
8,780 tree seedlings
grown for 10 years
67 homes' energy
use for one year
https://www.epa.gov/energy/greenhouse-gas-equivalencies-calculator
May/June 2023

Table of Contents for the Digital Edition of May/June 2023
