Hi Mike, welcome to the forum.
Its a great picture and maybe a repair job but I dont think its actually spiling for several reasons.
You would never get wood like that to drive in as straight and neat as this and the ends would be all smashed up if you tried it.
Spiling generally goes in at a sharper upward angle than this and with more overlap (ie shorter distance between sets)
The bits of wooden support aren't nearly robust enough for spiling supports.
If you look at the largest size picture, what looks like a second row of boards at the front is actually just two rows of rocks supporting the slats at the front end while there is only one row at the back.
I think it is probably just the form used to construct the arching has been left in, maybe after a repair or just cos that's as far as they got with it.
The stones between the support and the slats can be knocked out when you're done with that section, unlocking the form so you can use it again elsewhere.
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