A sinkhole opened up on Yonge next to this site (30 inches deep and 15 meters long). You can actually see it starting to form in the road cracks in Marcanadian's photo above. I suspect activity on this site may have altered a local accumulation of ground water. If enough water was removed, the soil would shrink in volume.
I take it you are referring to the section beside the ramp? I'm not sure why it's wood there instead of concrete (what you refers to as struts are known as lagging, while the vertical members are piles), but I am pretty certain that there are no underground connections planned here, whether for a contemplated PATH extension or any other reason.
Nah it's definitely structural. The tarps hold up most of the wall.
Good question, I've never really thought about it but the only thing that I can imagine is that some pieces of dirt probable come loose in high winds and could be a hazard to workers and equipment. If anyone knows the real answer I'd love to know.
Yes, as they're cutting into the bedrock below small pieces of shale and limestone often break off overhead due to the vibration. You can see that the tarps have trapped quite a lot of falling debris if you look at the lowest point of the wrap.