Attachment 3 - Revised Slope Stability Assessment

Brett Hage                                                                                      22328-001-Rev1
                                                                                              17 November 2022

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
within the proposed development areas. The nominal 500 m2 land application areas, as shown in Figure 1
would therefore be appropriate, and it is assessed that that there is enough room to site the wastewater
irrigation areas with the necessary setbacks.
The need for a reserve area is generally applied to septic tank systems, producing low quality (Primary
Standard) effluent, due to increased potential for fowling and failure of the trenched land application area to
occur. As the use of an aerated wastewater treatment system is proposed, which are capable of delivering
high quality (Secondary Standard or better) effluent, the risk of fouling and system failure is negligible. As
shallow subsurface drip irrigation is proposed which can be easily repaired in-place as required, the addition of
a separate reserve area is not warranted.
5.3.2        System Positioning
Positioning (‘setback’) of the land application system must be in accordance with the horizontal and vertical
setback distances recommended in Tables T2, T3 and T5 of the Queensland Plumbing and Wastewater
Code 2019, as summarised in Tables 5, 6 and 7 below and on the following page.


Table 1: Setbacks for subsurface land application area for on-site sewage treatment plant
                                                                             Horizontal separation distance
  Site Feature
                                                                             Upslope      Downslope       Level

 Property boundaries, pedestrian paths, walkways, recreation areas,
                                                                               2m             4m           2m
 retaining wall, and footings for buildings and other structures.

 Inground swimming pools.                                                                    6m

 Inground potable water tank.
                                                                                             6m
 (Secondary standard effluent or better)


Note:   Distances measured from the edge of trench/bed excavation or subsurface irrigation distribution
        pipework to the nearest point of the feature.




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