Four different projects in Toronto, Ontario, are looked at in brief for their variety of tunneling works in the city and challenges: a small sewer in a residential zone; a large outfall at Ashbridges Bay; a milestone advance for Eglinton Crosstown West rail extension; and, works underway for Scarborough subway extension.
Sewerage investment, large and small, and quite different challenges and experiences in 2022.
At the small scale, a challenge was unexpectedly met during the micro-tunneling work for a new local storm sewer in a residential part of the city. The cutterhead of micro-TBM got tangled up among some of the unmarked tensioned steel tie-backs of the deep foundations for buildings. The drive had been mostly complete but then work stopped for the MTBM to be rescued.
At a bigger scale, and away from residential areas, success was met for a tunneling operation to bore a large outfall tunnel as part of an effluent and environmental improvement program, at Ashbridges Bay.
On transport, Eglinton is more than half way excavated while tunneling is underway on the Scarborough subway.
TORONTO Tangle below
The City of Toronto recently issued an emergency, non-competitive contract for retrieval of the MTBM that got stuck underground for a number of months after it became entangled among tensioned steel tieback wires of long-standing building foundations during construction work for a new storm sewer.
The local authority and project owner said the tiebacks hadn’t shown up on infrastructure research carried out beforehand, such as on as-built drawings, during design or before construction began for the 900mm-diameter (3ft) sewer. The trenchless work was being performed on a 282m-long (308yd) section of sewer between maintenance holes OD5 and OD8.
The new contract – to the contractor, Clearway Construction Ltd – was urged to be issued as a “matter of extreme urgency”, due to “significant health and safety hazard to the public” caused by the unforeseen conditions, according to the City’s Engineering and Construction Dept, in its summary of the advisory Report for Action (RFA), issued mid-February. The FRA was submitted to seek approval for the MTBM rescue contract and storm sewer completion works.
The micro-TBM had been launched in March 2022 to bore the sewer for the Basement Flooding Protection Program, Phase 4 Contract, on Old Mill Drive. However, it encountered the tensioned tiebacks well into the drive and then became ensnared. The steel tie-backs are used for deep foundation shoring of two mid-rise developments in the area.
Wider complications for the local infrastructure arose from the MTBM getting caught up in the unmapped foundation tie-backs. They complications, the RFA said, included: “The integrity of the roadway, nearby subway tunnel and surrounding infrastructure was undermined as a result of ground movement around the microtunnelling boring machine due to excessive ground water and poor soil conditions. This further led to the creation of a sinkhole in the work zone.”
The RFA added that a non-competitive emergency contract was needed to allow the contractor to “review the impacted area and safely proceed with the microtunnelling boring machine rescue operations, including destressing and removing steel tiebacks; and, jet and compaction grouting to stabilise the ground.”
Outfall success
Ashbridges Bay is getting a major new investment in wastewater treatment with a modern plant plus a new 3.5km-long (2.2 miles) outfall tunnel. The capital spend is to improve water quality from treated effluent discharge into Lake Ontario.
Geology in the area of the lake comprises mainly shale with a few other sedimentary rocks, such as limestone, siltstone and sandstone.
The tunnel is entirely within the Georgian Bay Shale Formation, and such shales in southern Ontario have been known to cause time-dependent deformation (TDD). This factor was accounted for in design of the 7m i.d. (23ft) concrete segmental lining.
Planning for the project was undertaken by Hatch, which also performed the design as well as project and construction management. The responsibilities also included preparation of contracts with the geotechnical baseline report (GBR), which included baselining marine conditions to reduce contractor risk.
Contractor on the project is Southland/Astaldi JV, which used a Robbins 7.95m-diameter (26.1ft) single shield TBM, which underwent remote acceptance during the covid pandemic.
Of the remote assembly, in Mexico while suppliers were in the US and some were involved in Canada, Robbins project manager Javier Alcala said in a statement: “It was a challenge for all the people involved due to the pandemic travel restrictions. However, due to good planning and communication we were able to go through the Acceptance Test successfully.”
He added: “I think this might become quite common in the near future.”
The TBM was launched from an 85m-deep (279ft), 14m i.d. (46ft) shaft, in Spring 2021 to bore offshore. A Robbins-made continuous conveyor for muck removal also included vertical transport up the 16m o.d. (53ft) shaft. The crew operated on 2 x 12-hour shifts, five days per week (Monday to Friday).
Daily progress rates up to almost 47m (154ft), or 30 rings, were achieved. This translates to excavation for and installation of a ring every 46 minutes, approximately, says Hatch in a paper with City of Toronto to the Tunnelling Association of Canada conference in 2022.
The paper adds that, excluding slight delays due to local issues such as period of groundwater inflows and some washout of two-component grout and a few instances of resulting concrete lining flexure damage, the average advance rate over the entire tunnel drive was 17.7m (58.1ft) per working day.
Further, the best weekly TBM advance rate was 167m (548ft) when 1111 rings were placed in total, and equates to sustained daily progress of 33.4m (109.6ft).
Following the drives, in a statement, Alfredo Garrido of Robbins Field Service, said: “This is a wonderful type of geology for our machines. During the entire excavation, a total of seven cutters were changed. The wear behavior is incredible, between 2mm and 5mm.”
He added: “Every 25 machine cycles, it was necessary to stop the excavation to probe drill hole in front of the cutterhead to check for possible water. The drilling was done basically every day, stopping the machine for a few hours, but it was very necessary.”
The outfall tunnel will discharge the treated effluent up through 50 vertical risers and ports at its end for diffusion into the waters of Lake Ontario. The risers are each 1m-diameter (3.2ft) stainless steel pipes that extend 50m up from the tunnel crown and through the lakebed, and fitted with FRP ports to disperse the treated effluent into the lake waters.
Tunneling for the last stretch of the outfall, boring on the section below the location of the series of risers under the lake, was a challenge.
In the statement, Southland project manager, Joe Savage, said: “The team really worked together to overcome some tough ground conditions and high water inflows in the tunnel.”
The existing wastewater plant and discharge outfall are around 70-years old – making then both one of Toronto’s as well as the country’s oldest such effluent treatment and outflow facilities. But the infrastructure does not satisfy the latest environmental standards for effluent discharge and also it is under-capacity for increased, modern demands.
Additionally, the assets are old and coming to the end of their useful service life and so capital investment of some description would be required. The city’s investment in a new, replacement plant as well as the gravity-fed outfall tunnel (design flow capacity of the outfall is 3923 million litres/day) are to improve the water quality of Lake Ontario as well as the shoreline close the Ashbridges Bay area. With such benefits, the investment is also expected to bring a notable boost to the regional economy.
In its project information, Hatch says the available hydraulic head between the liquid level in the outfall shaft and the lake level during peak flow conditions is approximately 1m (3.2ft) and “cannot be exceeded.” It added that given the tight conditions, comprehensive analysis using both computational fluid dynamics (CFD) and spreadsheet hydraulic models were employed to achieve the parameters. The approach, it adds, helped to develop “the most economically feasible design that minimizes the head loss” through the shaft and outfall tunnel.
TORONTO TRANSPORT TUNNELS MAKE PROGRESS
Tunneling on Toronto’s Eglinton Crosstown West Extension reached the halfway mark in late February.
‘Renny’ – the first TBM launched on the project – dug more than 3km (1.86 miles) of the approximately 6km-long (3.73-mile) tunnel, less than a year after tunnel boring began.
The second TBM, known as ‘Rexy’, is not far behind. Since July 2022 it has driven 2.45km (1.5 miles) and reached the first headwall location at Martin Grove Road.
The 9.2km-long (5.7-mile) extension of the Eglinton Crosstown LRT will run from the future Mount Dennis station to Renforth Drive and will operate mainly underground. Once complete, it will create a continuous rapid transit line that stretches from Scarborough, through midtown Toronto, and into Mississauga.
Tunneling started from the tunnel launch site near Renforth Drive, with the TBMs driving to west of Scarlett Road, where they will be retrieved.
Also in the city, tunnelling is also underway on Scarborough Subway Extension.
The Herrenknecht TBM, named ‘Diggy Scardust’, will dig about 6.9km (4.2 miles) of the 7.8km-long (4.85-mile) tunnel, with daily advance rates of approximately 10m-15m (32ft- 50ft) anticipated.
The remaining portion of the tunnel will be built by the stations, rail and systems contractor, Scarborough Transit Connect, which is a consortium of three companies – Aecon Infrastructure Management, FCC Canada and Mott MacDonald Canada.
At 10.7m-wide (35ft), the three-station Scarborough extension will be the first subway tunnel in Toronto to house two tracks operating in both directions. As tunneling commenced, crews also started work at the project’s extraction shaft site.
Strabag was awarded the design, build and finance contract for the tunnel in September 2021.
Work has also started on early upgrades at Finch Station that will set the groundwork for future construction on the Yonge North Subway Extension.
The Eglinton Crosstown West Extension, Scarborough Subway Extension, Yonge North Subway Extension and Ontario Line are four priority subway projects in the Greater Toronto area, representing the largest subway expansion program in Canadian history.
The four projects of the Greater Toronto area will add more than 40km (25 miles) of new subway service to the rapid transit network.
