Appendix D - Geotechnical Report

8.           Wastewater Treatment and Disposal
An assessment of the type and configuration of wastewater system that may be suitable for a proposed
development requires:
▪    Assumptions to be made regarding the typical building configuration and probable wastewater flow;
▪    Evaluation of the terrain-soil capacity to accept treated wastewater; and
▪    Consideration of NC wastewater treatment and disposal system construction and siting
     requirements.


8.1          Wastewater Flow Design Allowance
For a typical four-bedroom house (or three bedrooms plus study), we have estimated a peak occupancy
of seven people. This assumption is consistent with current NC development standards and has been
used to demonstrate feasibility of on-site effluent disposal. Should the proposed future residences
exceed this size, it is the owner’s responsibility to demonstrate feasibility of the design, meeting all
relevant regulations.
For each occupant, a typical wastewater flow allowance of 120 L/person/day has been taken from
Appendix H of AS1547:2012. This represents the flow allowance for households with standard water
reduction fixtures (including duel flush toilets, shower flow restrictors, aerator faucets and water
conserving automatic washing machines) supplied by on-site rainwater tanks.
We therefore estimate a typical daily household wastewater flow rate of 840 L/day, based upon an
allowance of 7 x 120 L/person/day.
It should be noted that wastewater flow rates may be reduced by the adoption of full water reduction
facilities (e.g. reduced flush 6/3 L toilets, shower flow restrictors, aerator faucets, front-load washing
machines and flow/pressure control valves on all water-use outlets, etc.). Conversely, the use of extra
wastewater producing fixtures (e.g. spa baths) has the potential to increase the household wastewater
flow rate. Final wastewater flow rates must be calculated at the time of detailed design of the
wastewater system, at building stage once the house design is known.


8.2          Terrain-Soil Assessment
The terrain-soil combination(s) encountered at a site are assessed so that an Expected Available Area
(EAA) can be delineated for suitable methods of wastewater treatment and land application.
Likely land application areas associated with the proposed future lots were up to about 5° (10 %) on Lot
5, however for the remainder of the proposed lots ground slopes of between 6° and 15° (10.5 % to 27 %)
were present. Therefore, sizing of EAA’s will need to consider a slope reduction factor as described in
Table M2 of AS1547:2012 or should be effectively terraced as necessary to reduce slope to < 10%
negating the effect of natural slopes. The addition of a slope reduction rate will result in an increase in
the size of necessary land application areas, which are described in Section 8.4.
It has been assessed from the results of the investigation that the soil profile across the proposed lots
predominantly comprises clayey sands/sandy clays, with sands ranging to coarse throughout, which is
equivalent to Strongly Structured Medium to Heavy Clay (Soil Category 6). This is a categorisation
and has been derived via the method described in Table M1 of AS1547:2012.




Geotechnical Investigation
10 June 2026 | 26073-001-Rev0                                                                                21