Attachment B - Geotechnical Investigation Report AG-220544-RPT-G-01_[Rev A]

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                                                                                        Sunshine Coast | Brisbane | Gold Coast




3         EXISTING STABILITY RISK
Local authorities prepare mapping tools to aid in assessing potential slope stability requirements. The mapping tools
are a “high level” approach where the site specific investigations are specific to a particular site/allotment. It is
important to note that where a site specific investigation and slope stability assessment are undertaken, they take
precedence.
Reference to the available overlays for the area show the site has been initially mapped as “Moderate in terms of
landslide hazard.

4         GEOTECHNICAL INVESTIGATION
A geotechnical investigation was undertaken by Global Testing Pty Ltd (Global) for the purposes of a site
classification. As part of their investigation, 2 bores were drilled to auger refusal at depths of between 1.2 and 1.6 m
below the existing ground surface. The encountered soil profile consisted of high plasticity sandy clays continuing to
the auger refusal depths, where rock was encountered. No groundwater was noted at the time of their investigation.

5         ASSESSMENT
Geotechnical comments are based on the factual findings of the investigation, best practice, local experience,
published correlations; however, are fundamentally founded in opinion and this should be considered.
An assessment of the site was undertaken using available data, local experience and photographs from site. The
site is relatively flat at the front of the block before sloping down towards the east at a relatively uniform angle of
approximately 20°. No signs of previous slope instability were noted in the available data. The mature trees were all
observed to be typically vertical with no misshapen or curved trunks. Misshapen or curved trunks are usually good
indicators for previous slope instability. It is noted that a significant translational slide was observed on a
neighboring allotment to the east, indicating the propensity for slope movements in the area during and after heavy
wet weather.

6         AGS ASSESSMENT
Landslides are caused where a soil or rock mass has a decrease in strength, usually attributed to groundwater,
resulting in the material not being able to maintain its own self-weight resulting in a slope failure.
The Australian Geomechanics Society (AGS) developed a set of tools and procedures for assessing the stability of
a slope. The tools and procedures presented with the AGS documentation form the basis of the assessment
undertaken herein and their documentation is attached for reference.
The guidelines also identify that the regulator typically sets the standard for levels of tolerable risk. AGS defines
tolerable risk as “risks within a range that society can live with so as to secure certain benefits. It is a range of risk
regarded as non-negligible and needing to be kept under review and reduced further if practicable”.
The guides suggest that areas of moderate or greater risk should not be developed unless a rigorous remediation
plan be implemented to reduce the area to a level of Acceptable Risk. AGS defines acceptable risk as “risk which
everyone affected is prepared to accept. Action to further reduce such risk is usually not required unless reasonably
practicable measures are available at low cost in terms of money, time and effort.”.
AGS suggest that regulators generally accept risk levels of “low” or lower as a level of acceptable risk.
AGS prepared a semi-quantitative assessment tool for determining the level of risk for a site. The tool separates the
potential hazards for a site, from a stability context, and rates them with a level of risk frequency. The multiplication
of these individual risk frequency summates as the risk frequency rating which is then measured against the levels
of risk nominated by AGS.
The results of this analysis are shown in Table 1.


Sunshine Coast | Brisbane | Gold Coast