I'm not convinced by a lot of this. A lot of words are used in different ways on construction sites, and the use by labourers may not be technically accurate.

What I am sure of, celtic-lyrics.com notwithstanding, is that the phrase is "shuttering jams". 40 years ago, anything you cast concrete in was called shuttering - the terms 'Formwork' and 'Falsework' came into more general use in the 1970's. If there's any shuddering in dam formwork, I'd run for the hills PDQ. And when a dam is under construction, there's no water to cause a blockage.

Brian Behan's song Building up and tearing England down includes:

I was on the shuttering jam on the day that Jack McCann
Got the better of his stammer in a week
He fell from the shuttering jam
And that poor old stuttering man
He was never ever more inclined to speak

So I go for either the shuttering jam(b) as the vertical supports to the dam shutters, or the general use of 'jam' in the sense of wedging - the shutters for dams are fixed by bolts and wedges from the part already concreted. (gnu suggested this in the other thread).

Phil