Ok, this is getting a bit too freaky for me. The rock erupted 2 km behind the head, in a zone where there had been overbreak, which must have been just before the sandstone. They call it a 'temporary lining', but I don't see much diff. with the final lining.
The rock squeeze is coming faster than I thought. At first I thought they would get stuck in the bad sections because the Squeeze would get them. But then they miraculously Squeaked through, and just left behind this bizarre tunnel profile.
At that point, counting on the Luck of Fools, I pronounced (in my brain) my Main Chance prediction.
They will get away with their funny lining, and will only notice unusual longitudinal cracking before they fill. They will either paste over the cracks and ignore them, and start the water fill.
After 10 years, they will start to notice the tunnel water flow slowing down, and this will set up as a trend. By 15 years they will finally drain the tunnel, and see severe lining damage, much like every other ill-conceived tunnel in this area.
Now, I must amend this prediction to include:
Before they fill, they will have numerous cave-ins. They will dig these out and paste over them.
Now, my real dilemma in my prediction is guessing where exactly they will have the most trouble. I'm not too sure where in the strata this cave-in occurred.