Likes it moist and will dominate planting, suppress other plants in time if successful.

Given your description of the planting site, it's possible that the site would act as a large perched water reservoir rather than draining quickly, or even normally. Some people believe drainage through a large pore medium is good, but that is not the case, and when a water retentive medium is placed above a large pored medium, water can be held in the upper medium rather than drain normally.

Thanks,
@ bbrandon: now that you mention it, I remember that issue from a lecture and that I tried to get my head around the soil physics.

I discussed this with a tree expert whom I know from work:

his comment was:
- go for a draught resistant species (thanks, bboy)
-if in doubt, choose smaller species, as ones with larger
canopies will suffer in the long run
-mix in claish soil underneath the tree stuff, mix it all avoid strict/ seperated layers

lets see whether I can talk some sense into those project folks,

thank a lot, bye, Lin

and yes, I pointed out that in the long run, the perennials will perish

Will any of the material settle over time and make the tree root area have air pockets in a year or so? I guess that could be corrected. I would wonder that if it was my yard, so I mentioned it.