Well, if you plan on moving rocks, I myself wouldn't want to go and use a nice pack for that at all. If you plan on doing that and don't mind strapping something extra to your pack, go buy a military ruck sakc and frame for that. They aren't quite as comfortable as your average pack, but they can handle the weight and beating.

For me, a normal pack for a 5-7 day trip should only be carrying around 45 lbs. That's what mine usually weighs in around if I'm going out for that long. I'm sure they can carry more weight thatn that, but I have no idea how long their life would be carrying around rocks.
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In all things of nature there is something of the marvelous.-Aristotle