OSPE - The Voice - Winter 2019 - 33

COVER

This article was written by Tony Tomory, BASc., M.Eng., MBA,
Operations Director at WJ Canada

istorically, in Toronto, metro construction has
always been completed by traditional cut and cover
techniques, where a large excavation is dug, the station is
built, the hole is backfilled and the road or other surface
infrastructure reinstated. This works well where roads are
straight, and alignments follow the long grid-like road
pattern of Toronto. Stations have typically been constructed
at a relatively shallow depth of less than 20 metres beneath
the ground surface. The Eglinton Cross Light Rail Transit
Project (ECLRT) forms part of the growing Toronto metro
network requiring deeper lines, up to 40 metres below
ground surface, and several interchange stations within the
existing network. New solutions were required to minimize
the inevitable large-scale road disruptions associated with
cut and cover construction.
The ECLRT is a 19-kilometre-long light rail transit line,
with 10 kilometres of underground twin-tunnels and 14
underground stations. At a cost of $5.3 billion it is the single
largest expansion of the public transport system in Toronto's
history. Several stations are being constructed using topdown techniques and three other stations by what is known
as the New Austrian Tunnelling Method (NATM); Laird,
Avenue and Oakwood stations. Laird and Avenue stations
are each at least 400 metres long, incorporating crossover
and pocket tracks for the train operations, in addition to the
station platforms. These stations are constructed below the
standing ground water level in a broad range of soil types,
including glacio-lacustrine plastic and non-plastic soils (i.e.
clay and sand). The layered nature of the soils within the
aquifer presented significant challenges to the construction
team.

NATM involves short advances, typically one to three
metres, in small narrow headings (forward excavations
into the soil). The exposed soil is immediately supported
using sprayed shotcrete. Full tunnel support requires a
completed ring, top, sides and invert, of shotcrete to have
set to sufficient strength. The headings are only as large as
ground conditions allow. To build anything larger requires
multiple headings which are excavated in sequence to form
the full cavern. When all the headings have been excavated,
the inherently stable circle or 'ring' they form is complete.
The caverns are typically circular or arched shapes to take
advantage of the inherent geological strength in the soil and
to minimize the thickness of shotcrete required to form the
tunnel. The completed station caverns for the ECLRT are
up to 17 metres wide and 19 metres high and required up
to six headings to complete. Pipe arches were necessary to
provide additional support to the roof of these large caverns.
The main contractor for the ECLRT station works is
Crosslinx Transit Solutions, a joint venture of Dragados,
EllisDon, SNC Lavalin, and AECON. Dragados is one of
the world's foremost experts in NATM construction and
is the primary partner in the project's joint venture team
responsible for the mined stations. They brought in expertise

$5.3 BILLION
AT THE COST OF

IT IS THE SINGLE LARGEST EXPANSION OF THE PUBLIC
TRANSPORT SYSTEM IN TORONTO'S HISTORY.

Winter 2020

THE VOICE

OSPE - The Voice - Winter 2019

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