Kitsune
Senior Member
Oh wow. I had no idea that the Don Valley was that shallow. The difference in high between the bottom of the valley and the surrounding areas is just 40 metres.
At the Pape terminus of the Relief Line South, the tunnel rests at 90 metres above sea level, while ground level is at 110 metres. At the bottom of the Don Valley, 2.0 km to the north, ground level is at 78 metres above sea level. Lets say they want the tunnel though the valley to be 10 metres below the surface of the valley (same depth as Line 2); that would put the tunnel at 68 metres above sea level. Do the math, and that's a grade of just 1.1%. A subway train can easily handle that grade change.
Of course, the slope between Pape Station and the bottom of the Don Valley isn't constant. Nevertheless, this does indicate that tunnelling under the Don Valley could be plausible.
Its a bit of an optical illusion.... where the end of the wye is, ground level is about 118/119 meters. At the cliff edge of the valley the ground is 128 meters. So it is deep, it just deep compared to the cliff drop. I find Toronto's geology is fascinating. Be interesting if they do tunnel.... that Overlea station could be really really deep. And consider there is 3.48 % grade between Gerrard and Danforth station ... getting under the Don river tunneling would be rather easy on the grade.