For theory and calculation purposes, you, and I, and the manufacturing industry accept that there is no established standard carried load for the septic tank tops we are refering to. In practice, as evidenced here, tank/top failure rate is extremely low. I believe we have had one reported failure, and it was an enormious piece of equipment, carrying a load as well..... not a riding lawnmower, or a compact tractor.
Is the practice 'endorsed' -obviously not-... It is a "do at your own risk" situation, and as always, each situation is unique, and your mileage may vary.
If it scares you and others, the answer is obviously... don't do it.
Actually not rose collored... I was quoting specs about concrete strength. When item "a" has a determined strength of "x" ( where x is above 0 ), and you arbitrarilly undervalue that strength to 0, then claim that the surface has no load carrying capability, that is introducing significant error into the whole picture. A fudge and safety factor is fine. But that non-reinforced concrete top has positive load carrying capability.. otherwise you could not backfill it. We're splitting hairs anyway. We both agree that the top has limited capacity, and there is a risk associated with driving over it.
"the rose coloured glasses are replaced by just plain clear glass."
Soundguy
"Emperical suits me just fine when driving over a sidewalk but when it comes to a tank top where I could fall into a non friendly environment the rose coloured glasses are replaced by just plain clear glass."