Somerset rises up and over the O-Train on a viaduct. A viaduct is rather like a bridge, but it is not hollow underneath; instead two side walls hold up a long berm of dirt with the road on top. The only bridge portions are over the O-Train itself and over a bit of City Centre Avenue.
Another viaduct is the portion of the transitway from the O-Train overpass west, over the top of Baysview Avenue, and then descending into the cut that enters Tunney’s Pasture.
An underpass is required for the new bicycle path that parallels the east side of the O-Train corridor. A large hole will be dug out of Somerset Street, and the new underpass installed in precast pieces of concrete. Cut and cover. No shoring or supports will be required for the sides of the hole, as the back sides of the viaduct walls will form the north and south shoring, and the O-Train tunnel wall acts as the west side shoring. On the east, it will be a slope during construction to access the hole.
I noticed today that a large amount of timberwork is going up on the south side of Somerset, just beyond the edge of the bridge. It looks like it will be a temporary boardwalk sidewalk so that peds and cyclists can continue by the site during the 4 to 6 weeks the road will closed to vehicular traffic.
It would otherwise be very long detour indeed for peds and cyclists who use this as a major east-west route, for people accessing the schools on the west side (Devonshire, St Francois) and Plant Rec Centre on the east side.
I have a hard time believing that the wooden contraption is a detour boardwalk; that would require two cuts in the railings, and the engineers are very unkeen to touch the railing. While the only thing I have to go on is your photo, I’d suspect it to be a platform for the construction crew. I would guess that the tunnel was going to be done in two or more sections, allowing peds and bikes to traverse the section not underway at any given time. Time will tell.