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TABLE 3: RETAINING WALL DESIGN PARAMETERS (UNFACTORED)

Material                                      Ɣ (kN/m3)                      Ø (°)                         C’ (kPa)

Very loose to loose sand                           17                          28                                -

Medium dense sand                                  19                          33                                -

Notes: Ɣ = unit weight (moist); Ø = drained friction angle; Ka = active pressure; Ko = at-rest pressure; Kp = passive
pressure; NR = not recommended;


Compaction for retaining wall backfill should be undertaken with due care. Layers should be placed thinner
than for normal earthworks processes (say a maximum layer thickness of 100 mm) and compacted using light
weight equipment such as hand controlled compactive equipment to minimise the stresses on the wall. If hand
controlled equipment is not practical, temporary wall propping will be required.
Walls will need to incorporate full height drainage in accordance with good design practice to minimise
hydrostatic pressure.


5.6 STABILITY AND LANDSLIP MAPPING
Reference to the Noosa Council’s steep land mapping indicates that the site falls within the overlay.
The landslip hazard assessment is based on a number of factors which includes but is not limited to
topography, regional geology and evidence of past instability.
With reference to site features, there was no evidence of slope instability (recent or past) observed during the
site investigation.
A qualitative assessment of the likelihood, consequence and risk to the property in its present condition and
also the proposed development has been carried out in general accordance with the methods suggested in
the Australian Geomechanics Society (AGS) Guidelines (2007).
The prevailing hazard at the site would be downhill soil creep of the upper loose sand; However, as noted
above, there were no signs of instability (including soil creep) observed during this investigation.
Based on the investigation results, and provided that design and construction is undertaken in accordance
with the recommendations given in this report, the risk of slope instability for the proposed development is
considered to be low. AGS suggest that regulators generally accept risk levels of “low” or lower as a level of
acceptable risk.
The following works should be implemented in addition to other items highlighted within this document. These
points are considered to be typical good hillside construction practice as explained in the documentation given
in Appendix A.
•   Any retention system should be designed by a suitably qualified RPEQ engineer and should be designed
    to achieve a safety factor of 1.5 or greater against global stability.
•   All stormwater and surfacer water is to be collected and discharged by an approved stormwater drainage
    system such that overall stability is not adversely impacted. Do not allow areas of the site to become
    saturated from seepage or surface water.
•   Plumbing and services must not be allowed to leak or block, and regular maintenance and inspections
    should be undertaken.
•   Erosion protection of slopes will be required (ie. maintain vegetation on slopes).
•   Overland flow paths will need to be carefully directed away from footings to approved outlets.




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