Difficult to say. Sure, 4 inches may be the max design value, but you may also have a reasonable cost (for the performance) by modifying soils to meet lower criteria. This has to be weighed with the desired performance and the available alternative methods.
I had a hotel that insisted they could not have the potential movement of a SOG, but not the cost of a structurally supported floor., PT slab is what they wanted, but it required overexcavation to get movements into acceptable amounts.
Over excavation and PT Slab are not all that uncommon in CO. The PTI manuals are geared a bit more to the practice in areas such as TX. CO can easily see soils/conditions that exceed the numbers provided in the manuals.
ym can vary a lot. But to help you out I generally see
ym center between .5 to 1.75"
ym edge between .9 to 2.5"
I design PT slabs in CO and TX and yes there is a difference between the two areas. Even a major difference it seems between the geotechnical engineers and the way they prefer to obtain the PT design criteria.
Cost effectiveness as TDAA mentioned becomes a balance of over-excavation and native materials. Beware, don't over-excavate and replace with structural fill if no major precautions are used to drain the site. If not you might have a huge bath tub.
