This depends enormously on where you are: https://www.fhwa.dot.gov/publications/research/infrastructure/pavements/ltpp/08057/07.cfm https://www.fhwa.dot.gov/pub

This depends enormously on where you are: https://www.fhwa.dot.gov/publications/research/infrastructure/pavements/ltpp/08057/07.cfm https://www.fhwa.dot.gov/publications/research/infrastructur... Go farther South and it gets down to eventually nothing. Go far enough North, to permafrost country, and you not only need to dig down to the permafrost but you need to have insulation so the permafrost doesn't melt.