Building a Subway Station In a Crowded City

Toronto's Eglinton Crosstown LRT line has been under construction for what seems like forever. It might open next year. A large part of the line is underground, which has created massive disruption for several areas in north Toronto, especially for station construction. 

There has been an effort to minimize disruption and several stations have been built using construction techniques that are new to the Toronto area. Urban Toronto has a report about how some of the stations were constructed using "top down" or "mining" techniques that I found quite interesting.  

"At certain locations, it's sometimes advantageous to look at top-down construction. What do we mean by that? We put in shoring along the perimeter and we dig down just enough to put in the roof slab of the structure. And we build the permanent roof slab to sit on the shoring system along the perimeter. And, when that's done, we can reinstate much of the surface—the road and the sidewalks—while we're continuing to dig down to the bottom level of the eventual permanent station structure. What that allows us to do is reinstate the road surface faster, so there's less overall duration of the disruption to the community while we're building the station, so that's the primary advantage of that top-down construction.

"Basically, you dig down just enough to put the roof in, support the roof on the shoring system, and then you backfill up and reinstate the road surface while you continue to dig down below… and that construction happens underneath the road system that is now reinstated, with less disruption."

If you are interested in transit or engineering, have a look at the documentary, The 15 Billion Pound Railway, about the construction of London's Elizabeth line, parts of which should open next year.