Subsoil drains or similar subsoil drainage control systems shall be installed (as necessary) to lead groundwater away from all springs and potential areas of ground water under or adjacent to fills. These drains or control systems shall also be located in the base of gullies and below sidling fills where cut grades exceed 1v:4h.
Temporary subsoil drains shall discharge via flexible jointed pipes to an outlet approved by the Geo-Professional. The drains shall be carefully marked and protected as specified and their locations shown on the As-Built plans. The alignment, size and other details of all other drainage shall also be recorded on the As-Built plans.
NZTA Specification F/2 Pipe Subsoil Drain Construction shall apply except as modified by the following:
- Where subsoil drains are required as shown on the approved drawings or directed by the Engineer, they shall be placed behind the kerb unless shown or directed to be in front of the kerb.
- The subsoil drains shall consist of an approved filter drainpipe 100mmØ to 150mmØ or approved equivalent in a trench backfilled with an approved filter material around the conduit. The conduit shall have a grade of not less than 1 in 200 and discharge into a sump or stormwater manhole as appropriate.
- NZTA Specification F/2 filter material shall not be used in close proximity to HDPE slotted pipe.
- Unless directed elsewhere in these specifications, pea metal or 15mm drainage metal shall be used for backfilling around HDPE slotted pipe.
- Where backfilling a subsoil drain with filter material, the minimum drainage metal cover around the whole pipe shall be 100mm.
- Where a strip-drain is approved, backfill with permeable sand is an acceptable alternative to drainage metal.
- The invert of subsoil conduits at the sump should ideally not be less than 100mm above soffit of the sump outlet. Where this is not practical, the invert of the sump conduit may, with the approval of Council, be lower, but in no case will it be less than 100mm above the invert of the sump outlet.
- Where directed by the Engineer, any permanent wet spot in the subgrade shall be drained to the kerb subsoil drainage system. Where the wet area is below the level of the subsoil drain, it shall be drained using approved filter drainpipes connected to the nearest stormwater system.
Definitions in this section