CANADA | PROJECTS
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.
Left:
Toronto’s Ashbridges Bay has Canada’s largest outfall tunnel
PHOTO CREDIT: ROBBINS
Spring 2023 | 17
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