One is not allowed to do a post tension slab *without* engineering. Period.

But was that the question you were asking?

I think this answers the question *I* thought you were asking.

From the IRC (underlining and bold are mine)
- R401.4.2 Compressible or shifting soil. Instead of a complete geotechnical evaluation, when top or subsoils are compressible or shifting, they shall be removed to a depth and width sufficient to assure stable moisture content in each active zone and shall not be used as fill or stabilized within each active zone by chemical, dewatering or presaturation.

In other words, one *can* have the geotechnical testing and following engineering done to design a slab sufficient to withstand movement caused by the soil, but, *IF* one *is not* going to do any soil testing and engineering, one SHALL remove that soil to a depth and width sufficient to assure ... blah-blah-blah.