First tunnel segments go under Somerset

Recall that there is a pedestrian-cyclist tunnel under construction. The tunnel will take users under Somerset Street, parallel to the O-Train.

Segments of the pre-cast tunnel were delivered today. Each giant precast block of concrete has a female and male “end”. There are holes cast in the tunnel segments to tie them all together and some holes in the sides which will have pins that hold them to the concrete reataining walls of the viaduct.

The first piece installed is the north portal. It got its inside leading edge a bit crumbled in the process (visible on the far left of the pic below). There is only 5cm clearance on the sides and top of the hole, so it has to be very precisely placed:

Note the steel beam bolted into the concrete pad that is the stopper for the first piece.

I didn’t see them drag the big tunnel piece with the bulldozer. Instead, they used it as a heavy object, attaching chains between it and the tunnel segment and using “come alongs” (winches, pulleys, and a ratchet lever) they manually moved it centimeter by centimeter. Getting the first one perfectly placed was important.

In the picture below you can see the tunnel segment on the far left, with the leading edge slightly damaged: (most of the visible concrete in the picture is the bridge abutment itself, chipped away by jackhamers to widen the opening a bit after the saw cut hole proved a bit too tight)

Eventually, the second piece of tunnel was dropped into the hole onto the foundation, and winched into place. A type of grit was put on the foundation first, to help the tunnel piece slide or glide a bit (tombstone makers and sculptors use ordinary white sugar for the same purpose; put between two slabs of granite the sugar makes them slide around easily).

Then the overhead cables were disconnected: (note the lubricating grit spread on the conrete to the left of the tunnel piece)

It was then winched into place with the “come along” block and tackle gear. But before that could happen, they moved the first piece of tunnel over about 4″ to the left:

 This was a slow process; eventually a large power shovel was called in to help push the pieces into place: (pic below is taken from above and “inside” the hole)

As of Monday night, there were two tunnel pieces put in place; and two more were awaiting their crane ride on Tuesday. I expect the main section of the tunnel will go quicker, since they are not having to insert it into a portal. But after most of the tunnel pieces are put in, there will be the portal to cope with on the south side.

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