The only problem with the above "shuttering jambs" explanation is that the word "jamb" is not a phononym of "jam" but is pronounced to rhyme with "form".

As someone who has worked on shuttering on a large building site I can tell you is is quite easy to get it jammed if your banksman (the person instructing the crane driver) gets it wrong. Although you can get inside and give it a good belt with a sledge hammer before the concrete is poured, once you get to the removal stage that option is no longer available, and you have to separate the shuttering in precisely the right order, or you end up resorting to an awful lot of sawing.

John