Appendix D - Geotechnical Report

Good quality topsoil obtained from excavations could be used to supplement the soil profile across the
proposed irrigation areas, forming narrow mounds/swales along the contours.


8.3          Existing Wastewater Infrastructure
Existing sewerage treatment and disposal infrastructure servicing the existing residence on Lot 3 was not
observed during the walk-over.
While no indications of inadequate operation or failure (e.g. odours, wet/soft/boggy ground, leaking
effluent, or irregular grass growth) of any system or application areas was observed at the time of our
site visit.
Should the owner wish to retain the existing system to service the existing residence, the system
(including disposal field(s)) must be inspected, and serviced as required, by a licensed plumber to ensure
that it remains fit for purpose and conforms with NC requirements. If the existing system is deemed
unsuitable, it is noted that sufficient space is available within proposed Lot 3 to site a new wastewater
treatment system as described in Section 8.4.
All wastewater installations must be completed by a suitably qualified and licensed plumber, experienced
with on-site wastewater systems. Structures associated with any disused or discontinued wastewater
treatment systems and trench areas should be fully removed (grubbed out) as part of operational works
and replaced with controlled fill so as to not conflict with future site use.


8.4          Recommendations
8.4.1        System Type
It is recommended that wastewater from each of the future residences be directed to a suitably sized
household Sewage Treatment Plant (STP). The STPs must be licensed to produce at least Secondary
standard effluent, with Chief Executive approval from the Department of Housing and Public Works.
However, to maximise available space and to reduce setback requirements (e.g. to the gully invert on
Lot 2) it may be necessary to use an Advanced Secondary treatment system (Ref. Section 8.3.2 below).
The STPs should be connected to either a surface or shallow subsurface drip irrigation system. To
demonstrate feasibility of the lot layout, the use of shallow subsurface irrigation has been assumed for all
lots as this provides a more conservative design. The use of surface irrigation is not supported on sites
with slopes greater than 10 %.
Table M1 of AS1547:2012 recommends a Design Irrigation Rate (DIR) of 2 mm/day for Category 6 soil.
Therefore, a minimum irrigation area of about 420 m² (840 L/day / 2 mm/day) would be theoretically
required. However, an additional 20 % reduction in DIR shall be applied for LAAs with slopes of 10 % to
20 %, and 50 % for slopes between 20 % and 30 % inclination (as described in Table M2 of AS1547:2012).
The calculated EAAs for respective lots based on encountered slopes are set out in Table 5 overleaf
below. This may be reduced at final design stage depending on the final location of the building and
land application area and associated slope configuration.




Geotechnical Investigation
10 June 2026 | 26073-001-Rev0                                                                             22