If you're using the lean mix as essentially soil replacement, as is common, you can use a much lower strength (even 5 MPa would be great soil). Your design parameter will shift from strength to durability. Sulphate resistance and such.
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
Fillcrete as we refer to it is a great product. No compaction, fast, and no waiting for a geotech to test the structural fill. It makes a great solution where the contractor has over-excavated. The downside is the cost. In our region it is only about $40-$60/cu m cheaper than a 25MPa mix, so if you ever need any formwork the cost savings is not always practical if you end up with a two step process. Frugal contractors that do not consider labor costs or testing costs tend to baulk at this option.
