Laying the ground for Ontario’s new line

Toronto’s population is expected to rise from 6M today to at least 10M within the next 20 to 25 years, so its transport network will need to grow fast to keep pace. At 15.6km long and with 15 stations, the Ontario Line is the biggest of four current subway expansion projects for the city – which together make up the largest urban transit programme in Canadian history.

Development of the Ontario Line was announced in 2019 by the provincial government’s procurement agency, Infrastructure Ontario, and regional transport agency Metrolinx. It will link underserved and lower income neighbourhoods with the commercial heart of Toronto, bringing an estimated 47,000 jobs within 45 minutes’ journey time.

4 million anticipated rise in Toronto’s population by 2049

The line is being designed for almost 400,000 journeys a day, with driverless trains running every 90 seconds during rush hour. End-to-end journeys between Exhibition Station in southwest Toronto to the Flemingdon Park neighbourhood in the northeast will take 30 minutes or less.

Exhibition Station in southwest Toronto will be a hub for economic and social regeneration in the surrounding area

Three of the new Ontario Line stations will interconnect with Lines 1 and 2, operated by Toronto Transit Commission (TTC), two with GO Transit rail lines, and one with the Eglinton Crosstown light rail transit line.

There will be two tunnelled sections on the new line, with elevated track to the north and two short sections at grade to the east and west of downtown Toronto.

The new line will include two tunnelled sections and one elevated segment

Preparation for procurement

The line is being delivered under two public-private partnership (P3) procurement contracts and two progressive design-build contracts (PDB – known as early contractor involvement in other parts of the world). There are also several early works contracts for bridge, track and other preparatory activities.

One of the P3s is for rolling stock, systems , operations and maintenance for the entire line; the other is for southern civils, stations and tunnels (see ‘Digging south’).

The PDB contracts are being used for the northern sections of the project: one for tunnels and underground stations (see ‘Digging north’) and the other for elevated guideway and overground stations.

Understanding and mitigating risks has been essential for both procurement routes, to attract the most capable companies to tender for delivery and bid competitively without needing to price for unknown risks.

15 stations

We were appointed in 2020 as part of the technical advisory team that has prepared the overall project for procurement. The technical advisory team is led by HDR, with Mott MacDonald providing specialist input.

Our key inputs have included producing the business case for the line and developing reference design and technical requirements for the alignment, tunnels, stations and track, as well as for a variety of other elements.

These cover utility relocations; ventilation, fire safety and mechanical and electrical systems; traction power supply; signalling, communication and control systems; and the development of transit-oriented communities surrounding stations. We also set out requirements for testing, commissioning and operation.

Across the entire project, we have provided guidance on environmental assessment, stakeholder engagement sustainability and risk management. This has included stretching carbon reduction, climate resilience and social inclusion requirements.

Indicative designs and specifications for transit-oriented communities, with new residential buildings above and adjacent to stations, have taken account of numerous environmental, social and building efficiency factors, including seismic performance. This artist’s impression shows King-Bathurst Station.

De-risking the project

In each discipline, we have developed concept designs to between 10% and 20% detail, depending on the relative complexity and level of risk involved. However, “in a few of the most technically complex areas, designs have been developed much further, to 60% detail, to demonstrate risk mitigation,” says Rob Gould, Mott MacDonald principal project manager for the northern tunnels.

This preparatory work enabled Infrastructure Ontario and Metrolinx to present well developed invitations to tender.

In a few of the most technically complex areas, designs have been developed to 60% detail, to demonstrate risk mitigation.

Getting under way

In November 2022, two P3 concessions were let. The southern civils, tunnels and stations contract was awarded to Ontario Transit Group, led by contractors Ferrovial Construction and Vinci Construction Grands Projets. Work on site began in October 2023.

The contract for rolling stock, systems , operations and maintenance (RSSOM) package went to Connect 6ix, led by infrastructure investment specialist Plenary Americas, supplier Hitachi Rail and contractor Salini Impreglio Canada.

In January 2024, the PDB contract for Pape Tunnel and underground stations was awarded to Pape North Connect – a partnership of Webuild Civil Work and Fomento de Construcciones y Contratas Canada.

2029 is when testing and commissioning are due to start

And in February the PDB contract for the elevated guideway and stations was let to Trillium Guideway Partners, led by Acciona Infrastructure Canada and Amico Major Projects.

Following bid reviews, our role progresses to checking the compliance of detailed design and construction plans, and then resolving any technical issues or disputes, and assuring quality, as work advances.

From 2029 we expect the project will be in testing and commissioning phase, to enable the line to enter operation in 2031.

Tunnelling challenges

“The Ontario Line will connect with existing TTC subway lines at three stations. That involves sinking very large access shafts for lifts, escalators, stairs, ventilation and services either side of TTC’s stations and mining large diameter caverns for our platforms beneath them,” says Mott MacDonald tunnelling practice leader Mark Ramsay.

20mm permitted track settlement per 10m

Toronto Transit Commission set exceptionally stringent limits for tunnelling related ground movement – a maximum of 20mm over 10m length for track settlement, and 10mm to 15mm per 10m for vertical deflection in the access shafts’ primary walls.

Over half the line is underground. Along the southern segment, tunnel and station structures will be excavated in shale bedrock overlain with consolidated glacial soils. To the north, the route alignment is in looser glacial soils with high groundwater.

Earth pressure balance (EPB) tunnel boring machines (TBMs) were specified for the twin bore tunnels to minimise risk of settlement. However, Mark notes that the TBM drives are fairly straightforward. “It’s the caverns where things get interesting,” he says.

Digging south

Past glaciation and ongoing tectonic activity have put the shale under significant stress, which is released during excavation, causing fragmentation. “The rock heaves and groans for two to three months when you create a large cavern,” says David Taylor, Mott MacDonald principal project manager for the southern segment.

We mobilised a specialist modelling team from our office in Madrid, Spain, to develop detailed computational models of rock jointing. This analysis defined the rock’s propensity to relax and deform when mined, as well as the extent to which it may fracture and collapse.

This modelling was critical for the southern segment, which has six stations and a crossover that allows trains to cross between ‘up’ and ‘down’ lines within the shale.

The caverns for the platforms and the crossover will be built using the sequential excavation method (SEM). This method will also be used for several sections of running tunnel and to connect platforms to the cut and cover station access and to the ventilation and emergency egress shafts.

At Osgoode station, the SEM platform tunnel crosses under the cut and cover box carrying TTC’s existing services

Modelling was carried out for each of the caverns to determine the ground characteristics, design feasibility and construction requirements, paying close attention to settlement control.

We used INSAR, a satellite surveying technique, to generate millimetre-accurate surface level baseline data against which settlement can be monitored as construction advances.

The SEM caverns must be completed in advance of the bored tunnels, which will be driven by the TBMs from west to east.

SEM excavation of large diameter caverns

The SEM method breaks down the excavation of wide caverns into a carefully controlled sequence, in order to maintain equilibrium in the surrounding ground.

A small ‘top heading’ is excavated first, then the material underlying that – the ‘bench’ – is removed, and finally the ‘invert’ or lowest part. The length of each heading and depth of each bench are carefully designed.

Rock bolts will be installed to support the crown (or roof) and walls. Sprayed concrete linings will provide temporary ground support, with in situ poured reinforced concrete used to create the permanent lining. We designed the linings to cope with horizontal and vertical pressures as the ground adjusts around the tunnel.

Sequence of SEM excavation in a large diameter cavern

Digging north

There will be another two interchange stations on the northern segment, involving the excavation of deep shafts next to TTC Line 2, and large caverns underneath it. Ground conditions formed by water bearing glacial soils at one of those stations – Pape Station – make it the northern segment’s standout technical challenge.

Track and platforms for the existing Line 2 at Pape Station were built in the 1960s and formed in a cut and cover concrete box. The structure is highly sensitive to differential settlement from the Ontario Line construction as the box itself has no shear or moment connections between its structural panels. At the time, its designers expected the surrounding ground to hold all the elements in place and saw no need for structurally strong connection between them.

As a result, stringent settlement control is required. That will start with consolidation of the loose glacial soils by injecting cement grout, effectively creating weak in situ concrete.

Then, a tunnel for the Ontario Line will be carefully mined beneath (see ‘Pipe jacking to minimise settlement’).

Constructing the crossing starts with the excavation of two 40m deep access shafts, one to either side of TTC’s Line 2 station. Each shaft will measure 67m by 38m in plan. The shafts will be formed with diaphragm wall panels that are 1.5m thick and 3m wide to create a watertight structure. Panels will have T-section, adding stiffness and minimising the risk of movement.

As shaft excavation advances, props will be installed and pre-loaded to prevent the diaphragm walls from moving. In the lower half of the shaft, anchors will provide additional resistance to ground and water pressure.

Pipe jacking to minimise settlement

The new Ontario Line platforms at Pape Station will be formed by connecting the two deep shafts using pipe jacking, to keep settlement below the 15mm limit.

A total of 44 steel tubes, each 1.8m in diameter, will be forced through the ground from one shaft, under Line 2, to the second shaft – a distance of 30m. The pipes will form a box, with 15 pipes each top and bottom, and seven either side. (The pipes’ large diameter will allow for easy access to the leading end, to clear any obstructions encountered as jacking takes place.)

The thick walled, large diameter pipes are already stiff, and will be further stiffened by inserting a steel I-beam into each and backfilling with grout.

Before excavation of this box begins, compensation grouting will be used to lift Line 2 by approximately 6mm. Steel frames will be installed within the jacked pipe box to provide support as excavation progresses.

Thick steel tubes will be driven from one shaft to the other, to form a box for the new platforms – this minimises ground disturbance that could affect the existing line above.

Four dimensional communication

Talk to any member of our diverse team on the project, and they will highlight the importance of stakeholder engagement and communication in achieving this rapid progress. Frequent, open and information-rich dialogue with stakeholders has been essential to move the project through planning, definition and approval.

Engagement and communication have been hugely assisted by 4D modelling – detailed 3D models of the project’s spatial configuration, and how it will change over time, show how the project will be developed step by step. This has been used to make technically complex information accessible to non-engineers, from city hall to community groups.

Modelling was initially embraced to help design teams visualise the interface between new Ontario Line assets and existing buildings and infrastructure.

“Running 4D scenarios promoted open discussion about how to address issues and enabled rapid optioneering, with side-by-side comparison of different solutions,” says Scott Kerr, Mott MacDonald lead for 4D construction modelling.

Eliminating conflicts and working out interfaces and sequences before issuing invitations to tender substantially de-risked the project for bidders.

Construction planning involved 4D modelling, showing how work would progress over time and allowing sequences to be checked (images are of Pape Station)

Construction planning involved 4D modelling, showing how work would progress over time and allowing sequences to be checked (images are of Pape Station)

Construction planning involved 4D modelling, showing how work would progress over time and allowing sequences to be checked (images are of Pape Station)

Construction planning involved 4D modelling, showing how work would progress over time and allowing sequences to be checked (images are of Pape Station)

Leading on sustainability

We had provided sustainability consultancy to Metrolinx under an earlier framework agreement. From this work, we knew Metrolinx was interested in the Institute for Sustainable Infrastructure’s Envision standard, which uses 64 indicators to guide projects towards better social, environmental, climate and economic outcomes.

William takes huge pride in the fact that the Ontario Line is the first P3 project in Canada to use and achieve the Envision standard.

1st P3 project in Canada to meet the Envision standard

Sustainability has been embedded into the procurement specifications, covering construction and operation. Key requirements included all buildings – including stations – achieving LEED Gold standard; embodied or operational carbon in infrastructure must be 20% lower than the baseline assessment and procurement must incentivise energy efficiency.

Building carbon reduction and climate resilience requirements into the project also enabled Infrastructure Ontario and Metrolinx to secure a CA$500M grant from the Canadian government, through its Climate Lens project funding initiative.

A 70-year track record in Toronto

Mott, Hay & Anderson, one half of the 1989 merger that created Mott MacDonald, designed Toronto’s first subway route, the Yonge Subway, in the 1950s. We’ve worked on every metro project in the city this century, with major roles on:

• Toronto-York Spadina Subway Extension (2006-17): 8.6km long, with six stations
• Eglinton Crosstown LRT (2010-22): 19km long, with 25 stations
• Scarborough Subway Extension (2023-30): 7.8km long, with three stations