The minimum and maximum depth of cover depend on the pipe characteristics and the laying conditions.
By convention, manufacturers make a distinction between:
- the backfill area. ;
- the careful backfill area comprised of:
- the trench bottom and pipe surround backfill up to at least 0.10 m above the assembly crown for flexible pipes;
- the trench bottom and surround up to the horizontal diameter for rigid pipes
- the extant soil.
The backfill zone conditions the pipe stability and/or protection. Its execution must meet variable demands depending on:
- the pipe characteristics (rigid, semi-rigid, flexible);
- the external loadings (depth of cover, wheel loads);
- the more or less rocky or heterogeneous nature of the ground.
The backfill zone varies according to the area involved (rural, semi-urban or urban), and has to take road stability into account.
Other constraints also affect the laying conditions:
- keeping the pipe frost-free (minimum depth of cover),
- passing through critical safety areas (railways, motorways, etc.), which require special arrangements,
- current regulations and local requirements applicable to roadways.