Geotechnical Investigation Report · OPW26/0149

                                                                                             PTG/04982 – 0001 – Rev0 – Rev0



    9.2.3      Site Formation Recommendations
The site formation works to be carried out should include appropriate controls to protect batter slopes and exposed
work areas from concentrated stormwater flows. Bunding or diversion channels should be employed to ensure any
water from rainfall events during construction can be directed to appropriate locations and managed accordingly.


    9.2.4      Filling
The fill for the dam should be placed and compacted under Level 1 inspection and testing in accordance with site
specific design drawings and specifications. Minimum earthwork requirements should be in accordance with
‘AS3798–2007 Guidelines on Earthworks for Commercial and Residential Developments’. In summary, AS3798
guidelines recommend the following:
»   Strip vegetation and uncontrolled fill (if any);
»   Test rolling using the following plant and load conditions:
  o A static smooth steel wheeled roller with a mass not less than 12 tonne and a load intensity under a wheel load
     not less than 6 tonnes;
  o A pneumatic tyred plant with a mass not less than 20 tonnes and a ground pressure not less than 450 kPa per
     tyre; or,
  o A highway truck with rear axle loaded not less than 8 tonnes with tyres inflated to 550 kPa;
» Subject to site specific conditions, any soft areas identified might need to be removed and replaced with select
   material;
» Prior to placing any fill, the existing ground surface should be cut into near horizontal benches, with each bench
   having a slight ‘back-fall’ into the slope. The material won from the benching may be set-aside, and re-used during
   the bulk filling;
»   Structural fill should be laid and compacted in near horizontal layers, commonly termed ‘lifts’, with each lift
    generally not to exceed 300 mm uncompacted thickness;
»   Structural fill for the dam should be compacted to a 100% minimum dry density ratio (DDR), with a moisture
    content variation not exceeding either -2% or + 5% of the optimum moisture content (OMC). This requirement
    should result in the majority of the fill being compacted slightly wet of optimum; and,
»   All fill to be utilised for the dam must comprise fine-grained, cohesive material only; granular soils with a sand
    content greater than 30% and/or a gravel content greater than 20% are not appropriate and can lead to increased
    permeability of the dam structure.


Other earthwork considerations include:
»   Over-compacted clay soils can be prone to future softening if subject to seepage, potentially compromising the
    long-term integrity of the embankment, and should be avoided;
»   The most suitable plant for compaction is a pad-foot roller for cohesive soils with varying amounts of silt and clay;
    and,
»   Any overly wet soils would need to be dried out before reuse or continuation of earthworks. Drying of in situ wet
    soils is accelerated by tyning the upper 0.1 m to 0.3 m depth and baking-out (to within -2% to +5% of the OMC
    value) during sunny weather.


PTG can provide Level 1 supervision and testing following site specific design drawings and specifications and AS
3798–2007 if required.


    9.2.5      General Dam Construction Advice
In addition to the pond outlet that will help maintain the pond at the target level, it is recommended that the dam
construction include a secondary, emergency spillway. The emergency spillway is recommended to help minimise
uncontrolled over-topping of the dam should the primary outlet become blocked, or when the influx of water into the
pond exceeds the flow-rate of the outlet. If the emergency spillway were to be activated, the volume and rate of water
discharge has the potential to be significant. As such the emergency spillway is recommended to be specifically
armoured to prevent scour of the dam structure and the land below in the event that it ever becomes activated.
Armouring may include, but should not be considered limited to:
»   Geotextile surface matting to help stabilise surficial soils;
»   Specific erosion matting where discharge velocities and volumes are anticipated to be significant during over-
    topping events;

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