Evergreen Line – Sinkholes and Tunnel Boring

Just returned from the Evergreen Line Project Office where they were hosting an information meeting regarding the tunnel boring machine and the recent sink holes that have happened in College Park.

A few weeks back, the first sink hole happened at Chateau Place, and last week, a second sink hole occurred just off Cecile Drive.

Now, if you’re like me, you might have thought it was some amazing coincidence or stroke of luck that the first sink hole happened in an empty parking lot that was fenced off….   well, obviously, it wasn’t.  The project was aware of the situation that MIGHT occur and had taken pre-emptive action to make sure that no one or property was put at risk as a result of the activities.

So, tonight, we learned what causes the sink holes, what they are doing about it, and many people of the 80 plus in attendance had their questions answered on these and other related questions.

The project engineers started by offering a quick tunnel boring machine 101 education for those of us that aren’t engineers, and it really was quite interesting.  We learned that the boring machine consists of the ‘bit’ at the front, 10 metres high, with many blades and boring surfaces designed to bore into the earth ahead of the machine.  The area immediately behind the blades/bit fills up with dirt to create pressure – I call it back pressure but they did not – that keeps the machine moving forward.  Behind that is the ‘seal’ point to the rest of the machine, with a conveyor at the bottom that takes the dirt out the back and eventually to the trucks waiting below for removal.  While the machine moves ahead it installs the concrete tunnel rings around itself – the balance of the pressure keeps the tunnel from collapsing and minimizes disturbance to only the area being tunneled.  When the machine is moving, everything is in balance and , we are told , it all works very well.  (Inside the tunnel boring machine)

When it changes is when they have to stop the machine for maintenance.  In order to replace the ‘blades’ which get beat up in the boring process, they have to put workers between the bit and the machine, in the space that is normally full of dirt providing ‘back pressure’.   When they remove the dirt, they remove the pressure and the balance the machine has been in with its surroundings is gone (imagine it is like it ‘backed up’ and there was no longer anything supporting the ‘ceiling’ of the tunnel).  Now the surrounding earth from above MAY start to fall into the void created for the maintenance and, depending on the conditions above the machine, this can cause a depression at the surface as the material shifts down.  In the location the machine is in now, this is made worse by the ground water above shifting and causing a change in the ground density.

All very complicated, but honestly when they explained it, it’s understandable, and I think most of the people in the room got it, and felt better knowing that this was a very different dynamic than when the machine would be tunneling under their homes.  Still, disconcerting to know this is going on and the machine is about to go under you, but, I think people, like me, were feeling somewhat better by the time we left !  There are clearly still a lot of people with a lot of concerns, and we need to make sure we keep working with the project team to get the information people are looking for and have the questions answered – a lot of people are concerned about the vibrating and damage being caused to their homes and this needs to be sorted out also.

The project team admitted that that they need to improve their communications, although it was great that they were able to pull this meeting together and provide the information that they did.   But there is a lot of room for improvement on the communications front, and we continue as the City to work with them to make that better.

Also, last night, Council passed a motion calling for a meeting to be held locally for residents that were not able to attend tonight, preferably at Seaview School to share the information.  We look forward to that in the near future, and the project expanding their notification to the general College Park/Glenayre neighbourhoods.

It sounds like another 3-4 weeks before the machine is moving again and the road is re-opened – obviously not great for the residents being impacted, but safety is the highest concern and it sounds like everything that can be done is being done to ensure there is nobody put in harms way.


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Mike Clay

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