minimum thickness of lean concrete

What is the minimum thickness of lean concrete??

in my opinion, i'd say it can be as thick as needed since all you're trying to do is replace soil. if there's a limit, i'd say it would be set by the location of the bottom of the footing (i.e. contractor trying to put mud mat in footing excavation that is "at grade" so that they'd be making the actual footing section thinner). the only reason why i would not use concrete to backfill an over-excavation would be if there is reason to keep the loads as shallow in the soil profile as possible. in this case, you're typically talking about thickness on the order of several feet thick. i suppose you could have a scenario with underlying rock where the other footings are bearing on soil and you would not want one individual footing essentially bearing on the rock. again, thicknesses on the order of several feet.

i am not aware of any specific reasoning why the thickness should be limited if you're simply trying to replace soil. if there are some reasons, i'd definitely be interested to hear them.

The thickness of lean concrete layer shall be 100 mm minimum unless otherwise specified. No form work is necessary for the lean concrete layer. 
For foundation concrete work, side form work shall be used.