Updated March 30: Diagrams, photos and the project description have been added.
A report in the March TTC agenda recommends closing the Yonge line late at night for an extended period to speed up work on tunnel liner maintenance north of Eglinton.
From June 2008 to February 2009, the line will be closed from Lawrence to Finch while work proceeds with the tunnel between York Mills and Sheppard.
From March 2009 to July 2010, the line will be closed from Davisville to Sheppard (according to the report) while work proceeds from Eglinton to York Mills. The report is silent on whether a shuttle train would run between Sheppard and Finch.
The hours of early closing would be from Sunday to Friday (Saturday late evening service would not be affected) starting at 12:30 am.
The information here is based on the presentation made at the TTC meeting on March 26, 2008.
The subway system is built in several different ways ranging from open cut running, convention box structures constructed in an excavation, and circular bored tunnels. Some of the tunnel is lined with cast iron rings, and some with pre-cast, reinforced concrete rings.
The pre-cast tunnel liners used on the Yonge Subway between Eglinton and Sheppard were the first pre-cast rings used for subway construction. They are showing their age and design shortcomings.
Although the liner is 6 inches thick, at the bolt pockets it is much thinner at 2.25 inches. Liners used on the Sheppard subway are 8 inches thick and they are also larger so that fewer segments (and their associated joins) are needed.
Among the problems found on the Yonge line are:
- Concrete delaminations above 1 inch
- Corroded reinforcing bars
- Tunnel crown deformation (squat)
The problems had been masked for many years due to the presence of sprayed on acoustic treatment of asbestos and other coatings. As this has been removed, the underlying condition of the tunnel was revealed. Examples of the problems can be seen in photos below. Both sets show the effects of corrosion and leakage between tunnel rings.
“Squat” is caused by pressure of the earth above the tunnel forcing it slightly out of round so that it is now slightly oval.
When tunnels are constructed, there is a deliberate over-safety factor built in to allow for deterioration over the life of the tunnel. However, the effects of pressure and leakage are different depending on the depth of the tunnel. Deep tunnels must withstand greater pressure, and so the safety factor “as built” is lower than shallow tunnels. However, the deep tunnels are below the transition between earth and clay (if you go down far enough, much of Toronto is a pre-glacial lakebed). The clay protects the deep tunnels from the chlorides (salts) that seep down from the road above.
The result of this is that the safety factor for shallow tunnels has dropped much more, proportionately, and tunnels of both depths are now at roughly the same factor. Both are still safe, but the deterioration must be stopped. The planned work includes:
- elimination of leaks
- removal of sound insulation
- filling of the bolt pockets
- application of a thin, spray-on tunnel sealant to prevent further corrosion
- continuous monitoring of liners
The thinnest parts of the liners are the pockets where the bolts holding the liners together.
To reinforce these areas of the liners, they are first cleaned of scaled concrete and other corrosion damage. Holes are drilled into the sides of the pockets, a form is bolted over the pocket, and concrete is pumped into the space below. This concrete pushes into the holes to provide a solid anchor of the additional material with the existing liner. After the concrete has set, the sealant is applied. Core samples are taken to verify that the repaired liner is sound.
As of March 2008, the tunnel leaks have been plugged, although ongoing repairs are needed to stop new leaks as they arise. Removal of the sound insulation is about 1/3 complete, as is the filling of the bolt pockets. However, at the current rate of work, the project target end date of July 31, 2010 will not be met. This date is set to ensure that future deterioration does not compromise the tunnels.
In the current work window, only 145 minutes per night are available for repairs.
With the proposed early closing, the revised work window increases to 235 minutes. Overall, less time is lost to set up and tear down at the work site.
Although these diagrams show an overview of the night’s schedule, several crews are actually working in parallel at different locations on different stages of the project. One crew was recently added, and the TTC feels that this is the appropriate level for the project. There is more to gain from extending the work period than from simply putting more crews on the job.
Late night ridership on the Yonge Subway north of Eglinton is 1,000 per hour or less, and this can be handled by bus shuttles at a small incremental project cost ($200K).