I think I'm beginning to understand this at last. Here is a picture from the Northern Programme Twitter feed:
https://pbs.twimg.com/media/CRSdeHCU8AAjOtn.jpg
I take it that Fillie's direction of travel in the picture is from right to left. We can see four of the 20 rams which enable her to advance. They are in varied states of retraction so I assume that an advance of one ring's width has just been completed. The rams have recently been pushing on the completed ring of concrete segments which we see at the right of the picture.
Behind the rams we are looking at the inside of the shell. The completed ring on the right is either entirely or almost entirely beyond the end of the shell.
When the rams started pushing on that ring it was still inside the shell and therefore not yet grouted in.
My problem was that I had been assuming that only a grouted ring could be used for pushing against.
When the rams have finished retracting, the ring assembly mechanism (which must be out of shot to the right) will place the 10 segments of the next ring in the space where the rams now are, inside the shell, starting with the bottom segment and then working up both sides in balance. Then the rams will push on the new ring as the digging at the front permits.
As they push, the tail end of the shell will move forward past the new ring, which will presumably be grouted in (perhaps during the push, working from its right to its left end). By the end of the push, the new ring will be clear of the shell. The rams will then retract and the cycle will repeat.
The all-day lane closures are because of Fillie. The nightly closures are because of resurfacing. That was my point, and it was a new one.
the bus companies eventually went back to parking on the street. Anyone have an update, it's over a year since I was last there so not sure if the problems were sorted - a little check on the ibiza bus website suggests they aren't and are still using the street.
