Somerset Street Reconstruction Part 12: Buliding the O-Train Pathway Tunnel

This is part 12 in the 15-part series on last year's reconstruction of Somerset Street West. In the previous post I described the planning behind the pathway that will go along the O-Train corridor (to be called the Champagne Pathway, I've just learned). Today I'll show the construction of the tunnel that had to be installed under Somerset to make this pathway happen. Eric Darwin of West Side Action blogged frequently during this construction process and I've included links to his relevant posts at the appropriate points.

In May of 2011, when the construction work for the street reconstruction underway, only these spray paint markings on the bridge's sidewalks gave any indication to passersby that a major dig would soon be happening here.

This is the north side of Somerset Street, behind the City Centre building. You can just make out the fence for the O-Train in the distance. The tunnel will cross the view in this photo, coming out right where that group of trees is at the left. Unfortunately, those trees will have to come out to accommodate the pathway.

While this is usually referred to as a "bridge", the bridge part is only the short stretch over the O-Train tracks. The rest of it is a viaduct—a pair of walls containing a bunch of dirt, on which the road surface sits. Much of the Queensway is built this way also.

So in order to excavate the road, the utilities that normally just rest in the ground under the surface needed supporting brackets to hold them in place while the ground was dug out from under them.

The next step was to dig out the significant amount of dirt fill in the stretch of viaduct being excavated. When they were partway down, they cut a horizontal hole through the thick concrete viaduct walls for the top of the tunnel portal (Eric got photos of the actual cutting).

In case it's unclear, this photo shows Somerset, looking toward Hintonburg (You can just make out the top of Takaki Automotive behind the trees). There are two wooden conduits along the edges of Somerset, on the centre-left is the concrete wall of the edge of the O-Train tunnel, and on the right is the concrete wall on the North side of Somerset, behind which is the City Centre complex.

With a transom in place to support the wall above, workers sliced the sides of the tunnel entrance through the concrete. The concrete wall is very thick—at least eight feet thick—and they used a nifty continuous saw to get through it, as Eric captured.

Here's the view from ground level. Only the north side was accessible, but the same stuff was going on on the south side too. You can see the O-Train going under its tunnel, which has very thick concrete embankments, crossing Somerset, that are more or less square with the ground.

The tunnel itself is of rather simple construction, not unlike the one that was installed behind Parliament Hill after rocks started folling down the hill. It's made up of a bunch of precast concrete square 'rings'. I was out of town when the first ones went in, but Eric posted photos of the installation of the first segment and next few tunnel sections.

When they were all in, it looked more or less like this. (It hurts my brain a little to see the top of the tunnel being used as a platform for the work station!)

With the tunnel segments in place, it came time to prepare it for its permanent state. Some tension cables were run through the sections to hold them firmly together (Eric got some good shots of this process), and formwork was prepared for the reinforced concrete collar to fill in the gap between the tunnel and the viaduct walls.

Inside the tunnel, things are quiet. You can see, however, that the ground on the south side of the tunnel is higher than on the north side. This will eventually require some landscaping to balance this out.

Before filling in the tunnel, it received some weatherproofing at the seams (See Eric's photos of the installation of this product)...

...And then they started filling it in. Eric has some excellent photos of the preparations for the filling, including a couple of photos of the backhoe with the backwards bucket.

Eventually, the dump trucks just drove right into the hole (overtop the tunnel) and dumped the fill soil right into its place. This took a lot of loads; consider the size of the hole (already well over half filled) versus the size of the dump truck!

As for the tunnel, it's been sealed off until the pathway is finished by fall of 2012. I'll withhold my cynical comment about the need for this, though I'm sure Eric has made a few.

With the hole filled up, all that was left was to finish the bridge sidewalks (covered in part 9 in the space above it. Oh, and to take down that wooden boardwalk on the far side...

What is that, you ask? The answer to that question is so interesting I thought it deserved its own post, which will be Part 13 of the series!

While you're waiting for that post, or if you didn't get enough about the O-Train pathway in this post or the previous post on its planning, have a look at this great West Side Action post, which makes a number of points about the planning process of this pathway. The design committee for that pathway will be meeting soon to look at some of the issues around it.