My understanding of the Legions is that they had specialists for various jobs, and were kind of similar to the U.S./British military in that regard. Except, in this case, they had specialists specifically for things like torture, marching songs, etc.
New troops would be stuck with "dirty jobs" as well. But it doesn't make much sense to me that some barely 20 year old fighter could be so skilled as to create two giant walls around Alesia, bridges, or siege engines - At least, not ones that are as effective as Rome's seemed to be.
I have never heard of a legion of the Roman Republic having "specialized" jobs for torture. It is true that there were guys to carry the eagle/flags (aquilifer), blacksmiths, medics, scouts, etc, but by and large, the soldiers were just general purpose soldiers. And they were capable of incredible engineering feats. Just during the Gallic War, Caesar's legions bridged the Rhine, constructed extensive siegeworks, and constructed ocean-going vessels in the worst conditions of distress and wont. At other times, Roman legions built roads, walls, and even aqueducts. There was an official with the legions called the "prefect of the carpenters/engineers," which presumably directed the construction projects. I talk about this at length over in this older thread, with a lot of good discussion from others and with some good references to check out.