A system of sub-soil drainage must be constructed to a suitable agreed outfall all to the council’s satisfaction where:
- the winter height of the water table is within 600mm of formation level; or
- the sub-soil is unstable because of being waterlogged; or
- there is a likelihood of water running from or out of adjacent ground; or
- springs, land drains or watercourses are present; or
- the finished road is below existing ground level, regardless of the water table; or
- the sub-grade is likely to be altered due to groundwater.
Backfilling trenches
All drainage (other than filler drains), utility and other trenches in the highway for industrial and commercial premises must be backfilled up to formation level with Granular Sub Base (GSB) type1 granular sub-base material. Backfill on residential sites should be a granular material to the approval of the highway authority (acceptable material will typically include GSB type 1 or material graded to 6F1). Reinstatement standards (such as The National Joint Utilities Group/Highway Authorities and Utilities Committee) do not apply to new development.