Here's a link to some interstate standards on pavement. They only show about 10" concrete and 12" gravel. That's zilch for a gravel base. We put more than that into most roads in Winnipeg. Residential maybe around 12", but I'm not a road designer.
https://www.fhwa.dot.gov/publications/research/infrastructure/pavements/ltpp/06121/chapt9.cfm
Edit:
Heres another with 24" gravel depth.
https://www.fhwa.dot.gov/publications/research/infrastructure/pavements/ltpp/06121/appende.cfm
Sorry for polluting the construction thread..
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