Reference Document - MCU24/0023

GJ Gardner Homes Gympie                                                                            23236-001-Rev0
C/- Covey Associates Pty Ltd                                                                         7 August 2023

4.5       Drainage
During construction, temporary open swales and diversion drainage may be required to divert seepage
and/or runoff away from the construction area.
Permanent site drainage must be installed to ensure that stormwater runoff and residence overflow is
directed away from structures and footings (e.g. positive gradients and/or channel drains), and also not
allowed to flow over adjacent fill areas. Uncontrolled flow of stormwater over fill batters (e.g. from residence
overflow during high intensity storms) has the potential to saturate and weaken the fill, potentially leading to
erosion and slumping failure. Any erosion affected areas should be reinstated as soon as possible.

Specific drainage recommendations are as follows:

          Capture any stormwater from level areas and direct it into controlled drainage to prevent saturation
           of foundation materials around the structures.
          Create diversion drains (i.e. contour berms) above the gully heads as shown in Text Figure 12.
          Remove any stockpiles of vegetation or debris which might create poor drainage on the slopes
           below the residence and also re-profile uneven ground around removed tree root balls to promote
           run off of surface water and prevent ponding.
          Apply surface and erosion protection to any exposed slopes including as a minimum revegetation
           with surface protection matting and scour protection in areas where required.
Subsurface drainage must be installed behind any retaining walls (e.g. slotted ‘aggi’ pipe wrapped in filter
‘sock’ placed in gravel backfill). Depending on matters such as the extent of drainage installed and site-
specific groundwater conditions, the designer must make an assessment of hydrostatic pressure to be
accounted for in the design of the retaining structure.

All excess stormwater collected around the proposed development should be directed by pipes or surface
channels (e.g. rock lined ‘dry creek’ beds) to discharge at least 10 m downslope of any structure,
embankment or effluent disposal area. Stormwater outlets should be positioned on natural spurs. A surface
dissipation device such as rock weir or rock lined contour drain must be installed at the stormwater outlet(s).

Subsurface disposal of stormwater must not be undertaken (including infiltration pits).

4.6       Driveway
Ongoing instability in the form of batter scours or slumps is likely to occur on both the cut and fill
embankments along the upper, north-western part of the driveway, thus requiring maintenance and repairs
on an ongoing basis.

To minimise maintenance requirements, the following mitigation and drainage control measures are
recommended for the driveway:

          Any over-steep batters should be reprofiled to batter slopes of 1V:2.5H (or 1V:1H in low strength or
           stronger rock). Where space and slopes prevent re-profiling retention using engineer designed
           retaining structures could be considered.
          Revegetate any bare soil exposed. The use of native species appropriate to steep slopes and this
           region are recommended. Hydromulch may be used on soils exposed in batters.
          Create diversion drains above the crest of the cut and fill embankments and direct flows to chute
           drains at preferable locations to outlet below in a controlled fashion.
          Install water bars at regular intervals to direct surface water into a swale drain along the up slope
           side of the driveway.
          Install appropriately sized culverts at regular intervals to transfer stormwater flows from the swale
           drain to the down slope side of the driveway.
          Protect the swale and culvert inlets/outlets from erosion by placement of rock fill, or similar.
          Regularly inspect and clean out culvert inlets to prevent blockage by leaf litter, soil, vegetation, etc.




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