Rat wall questions

Well I have forms up and 3 yards of sand coming tomorrow. The person I talked to at ready-mix plant told me to put down a minimum 3" of sand. so now my I am hearing that I need a rat wall......my brother says to dig a trench down 18" The ready-mix guy said 10" 18" sure seems like overkill to me....The building inspector did not mention anything about a rat wall and he is away this week so can not ask him... If I do dig out this rat wall should I do it before sand or after sand? and does it really have to go down 18"? TIA
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