dberg,
Please refer to our typical engineering by downloading the manual. You can find the engineering manual by following this link
http://www.integraspec.com/forum/viewtopic.php?t=21 The engineering is done taking into account different backfill height. But then again it all depends of the building codes in your area.
For example in Ontario, Canada, a six-inch core is use quite commonly used for 3 levels.
There are many options for hanging the floors onto the walls that can also be found in our engineering. Please feel free to contact us for any questions you may have.
Thank you,
Marc Philippe