Historically, 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 20m beneath the ground surface. The Eglinton Cross LRT Project (ECLRT) forms part of the growing Toronto metro network requiring deeper lines, up to 40m below ground surface, and several interchange stations with 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 19km-long light rail transit line, with 10km of underground twin-tunnels and 14 underground stations. At a cost of CAD 5.3bn (USD 3.8bn) it is the single largest expansion of the public transport system in Toronto’s history.
Several stations are being constructed using top-down techniques and three other stations by the New Austrian Tunnelling Method (NATM); Laird, Avenue and Oakwood Stations. Laird and Avenue Stations are each at least 400m 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 glaciolacustrine 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 1 up to 3m, in small narrow headings.
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 17m wide and 19m 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 (CTS), a joint venture of Dragados, EllisDon, SNC Lavalin, and Aecon. JV partner 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 from around the word to work on the project, including from Spain and the UK. WJ Groundwater has used many novel dewatering techniques in tunnel construction, beginning with the Dockland Light Rail Project in East London, the London Underground Jubilee Line and, most recently for several stations and cross passages for the Crossrail project which runs east-west under the city of London (Soon to be opened as the renamed Elizabeth Line).
The stakes are high when mining below the standing ground water level—any shortcomings in the dewatering systems can lead to significant delays, and far worse, instability of the mining face can result in loss of material, subsurface instability and settlement. Dewatering needs to be done right. This involves a combination of engineering design, know-how and a can-do attitude in the field under any conditions the subsurface presents. CTS needed a partner that would work together with them through to the end, and invited WJ to bid on their project. WJ was selected as the dewatering contractor for the three mined stations and would go on to win six other stations along the alignment after opening their first Canadian office on Eglinton Avenue.
WJ began in earnest with 3-D finite element analysis of the groundwater drawdown. From the beginning, the groundwater drawdown models highlighted that the Oakwood Main Entrance underground access, also called the cross-cut, was the most challenging aspect of the project. This was the deepest excavation and the ground conditions featured a low-permeability saturated silty sand, with a soil matrix that fines downwards. The base of the crosscut was founded just above a sand/ clay interface. Sand/clay interfaces are especially challenging – to halve the depth water above the interface requires having the well spacing so doubling the quantity of wells. The aquifer held 18m of water above the clay interface, 17m of which needed to be drained by the dewatering system. As the excavation depth approaches the clay interface, the well count risks growing exponentially. Proposing ejector wells and deep wells from the road surface, WJ designed the dewatering system to maximise the groundwater drawdown to as close to the sand/clay interface as practically feasible, while highlighting that construction would remain demanding.
The ground held many challenges. Excavation unearthed three thin clay horizons within the rarely excavated lower part of the aquifer, ranging between a few inches and a few feet in thickness. Critically, regardless of the thickness, a continuous clay layer inhibits the downward drainage of water, significantly reducing the effectiveness of the surface dewatering system in achieving the drawdown to the base of the excavation. A contingency plan was implemented comprising arrays of wellpoints targeting the clay interfaces, installed from various locations within the machine-bored tunnels and from within the partially completed mined cavern. WJ implemented 24-hour operations within the tunnels to make sure that the ground was always dry ahead of the excavation. WJ was able to complete this feat, without any delay to the mining schedule. This is significant, as NATM construction costs run to tens of thousands of dollars a day, and as everyone who knows anything about NATM works: the mining can never stop.
Even with more than half the caverns complete, the dewatering along Eglinton remains a day-to-day challenge. Tight timelines and access constraints require a collaborative and flexible team able to adapt to the new demands that this exciting project brings.
