there is always the "if it is done right" clause... 
but if your soil is good, it shouldn't be a problem. we get in our 10 ft circular monitor cages and crawl around in there on our hands and knees, and the soil is fine. of course, if you try and walk, with all of your weight on your two small feet, then sometimes you will collapse through.
so on a MUCH smaller scale, in a small setup, with a small Uro, and a couple rocks that weigh a couple of pounds, i wouldn't worry about that collapse.
sure, sometimes burrows come too close to the surface, but under weight they crumble more than collapse. it isn't like some 7.0 earthquake, and everything disintegrates and the Uros get crushed before they can get out.
of course, if you are using straight sand, or a poor soil like potting soil, especially really dry, then that is altogether different, but i don't think the animals could really "burrow" under that heavy stuff (like rocks) so much as excavate a bit under it and just slide underneath it.
the soil is already heavy, it should "pack" well enough under its own weight (assuming good water content) that collapsing burrows just really isn't much of an issue...
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robyn@proexotics.com
Pro Exotics Reptiles