geotech report · MCU15/0087

Slope Stability and Geotechnical Assessment
1 Natasha Avenue, Noosa Heads




7.6.2.       Risk Factors
Based on the discussion in Section 7.6.1, the dune escarpment is unstable and has the potential to
slip under adverse conditions. The conditions likely to trigger failure are:

        Increasing the load on the upper sand profile. This could be long term structural or fill loads
         associated with building footings or retained soil. Or short term loads on or close to the batter
         associated with construction equipment such as excavators or piling rigs.

        Undercutting the toe of the batter, such as by scouring during flood flows in the Noosa River,
         could remove the support at the toe of the batter and trigger a longitudinal slide on the face of
         the batter.

        Removing vegetation from the batter face would reduce the vegetative reinforcement in the
         loose sand profile and could trigger a slide.

        Erosion of the escarpment by trafficing the slope, the concentration of flows down the batter
         or the removal of vegetative groundcover could cause localised steepening of the batter and
         trigger a longitudinal landslide.

        Saturation of the upper loose sands by intense rainfall events or localised water
         concentrations such as swimming pool overflow or failure of the stormwater collection system
         would increase the weight of the sands on the batter and reduce their shear resistance thuis
         triggering a slip.

7.6.3.       Landslip Risk – Current Situation
The landslide risk has been assessed in accordance with AGS 2007. A likelihood of Almost Certain
to Likely has been assessed based on consideration of the above risk factors. The consequences
associated with a landslip assuming a structure constructed on a standard footing system as per
AS2870-2011 are assessed as Major. The risk for development without consideration of the stability
risks are thus Very High as per AGS 2007.

Such a risk rating is unacceptable by almost any criteria.

7.7. Slope Stability Mitigation Recommendations
Given the risks assessed for the site, consideration should be given to the results of the risk
assessment when planning the proposed development. With regards to slope stability we
recommend the following mitigation measures to reduce the likelihood of landslip and provide footing
systems to reduce damage should a slip occur.

7.7.1.       Slip Mitigation Measures
The following risk mitigation measures are recommended:

        Vegetation should be established at the toe of the escarpment to reduce the risk of scouring
         at the toe.

        Clearing of vegetation on the escarpment should be minimised and should only be
         undertaken after alternative species with a suitable holding root network has been
         established.

        A suitable groundcover should be established over the escarpment to prevent erosion.



Coffey
GEOTKPAR02068AA/A                                                                                        8
12 August 2015