Appendix H - Slope Stability Assessment

Brett Hage                                                                                     22328-001-Rev0
                                                                                            14 September 2022

5.0        WASTEWATER TREATMENT AND DISPOSAL
An assessment of the type and configuration of wastewater system(s) that may be suitable for a proposed
development requires:

          Assumptions to be made regarding the typical building/worker configuration and probable
           wastewater flow; and
          Evaluation of the terrain-soil combinations to identify distinct units.

5.1       Wastewater Flow Design Allowance
For a typical four-bedroom house (or three bedrooms plus study), we have estimated a peak occupancy of
six persons. This assumption is consistent with current NC development standards and has been used to
demonstrate feasibility of on-site effluent disposal. Should the proposed residence 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) with reticulated town water supply.
We therefore estimate a typical daily household wastewater flow rate of 720 L/day (6 x 120 L/person/day).
It should be noted that wastewater flow rates may be further 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.

5.2       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.
Several possible areas would be available within proposed Lot 2 for effluent disposal, and it is assumed that
it would be preferable to maintain this on the downslope side of the residence. The terrain below the Lot 2
house site has been categorised as Waxing Convergent in accordance with the slope configurations
described in Figure C2 of AS1547:2012, which also describes the relationship between the shape of the
slope and surface drainage. Such slopes tend to concentrate and accelerate run-off as it moves down slope.

It has been assessed from the results of the investigation that the soil profiles across the likely effluent
disposal area on Lot 2 is equivalent to Light Clays (Soil Category 5), based upon the soil materials
encountered in the boreholes. This soil classification has been derived via the method described in Table
M1 of AS1547:2012. The indicative soil permeability rate (Ksat) for a soil of this category is 0.06 m/d.

5.3       Recommendations
5.3.1          System Type
It is recommended that wastewater from Lot 2 be directed to a suitably sized Household Sewage Treatment
Plant (HSTP) connected to shallow subsurface or surface irrigation systems. The HSTP for proposed Lot 2
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 residential space, it may also be
desirable to use an Advanced Secondary treatment system.
Shallow subsurface drip irrigation is proposed due to the presence of slopes greater than 10 % (6°) away
from the gently sloping crest areas, and certainly within possible land application areas.
Table M1 of AS1547:2012 recommends a Design Irrigation Rate (DIR) of 2.5 mm/day for Category 5 soil.
Therefore, minimum irrigation areas of at least 435 m² for the proposed lot will be required. This includes a
50 % reduction allowance to the DIR, which has been applied due to slopes of around >20 % being present




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