New mention of the 'small rock fall' which I am calling a rockburst. I think the difference between the 'temporary' and 'permanent' lining is another 8 inches of shotcrete, but I don't know. A nice quote:
The client noted in its H1-’09 report that realignment of the tunnel had enabled the machine to improve progress, reducing rock overbreak to minimise excavation in the Queenston shale.
But, surprise!, things are just as bad above the Queenston Shale! They should have stayed just under the Whirlpool Sandstone, and popped out at the last minute. I include a chart for clarity.