CS-8 SUBSOIL DRAINAGE FOR EARTHWORKS AND ROADS ...

CS-8 SUBSOIL DRAINAGE FOR EARTHWORKS AND ROADS
TABLE OF CONTENTS
CS-8.0 Subsoil Drainage for Earthworks and Roads .......................................................... 1

CS-8 SUBSOIL DRAINAGE FOR EARTHWORKS AND ROADS
CS-8.0 Subsoil Drainage for Earthworks and Roads
a) Earthworks
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.
b) Roads
NZTA Specification F/2 Pipe Subsoil Drain Construction shall apply except as modified by
the following:
i) 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.
ii) 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.
iii) NZTA Specification F/2 filter material shall not be used in close proximity to
HDPE slotted pipe.
iv) Unless directed elsewhere in these specifications, pea metal or 15mm
drainage metal shall be used for backfilling around HDPE slotted pipe.
v) Where backfilling a subsoil drain with filter material, the minimum drainage
metal cover around the whole pipe shall be 100mm.
vi) Where a strip-drain is approved, backfill with permeable sand is an acceptable
alternative to drainage metal.
vii) 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.
viii)
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.
Updated 01/07/2013 Page 1