Geotechnical Investigation

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                                                                                                         arcosgroup.com.au
                                                                                        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 previous geotechnical investigation was undertaken by Global Testing Pty Ltd (Global) for the purposes of a site
classification. As part of this investigation, Global undertook the drilling of four boreholes around the proposed
location of the dwelling, on the high side of the batter. The encountered material consisted of clay fill overlying natural
stiff clays, increasing to very stiff clay at depth. No groundwater was noted in their borehole logs. Global’s previous
investigation is attached to this report, and has been used to form part of the analysis undertaken by Arcos. An
additional test pit was excavated at a later date, with the soil logged by a Geotechnical Engineer from Arcos. The
test pit log can be found attached, however in summary, the encountered soil profile was consistent with the findings
from Global. Additionally, the material at the base of the batter was assessed by Arcos, where silty sandy clay was
encountered, with a higher moisture content.

5         WALKOVER 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.
A site walkover assessment was undertaken by a Geotechnical Engineer on 27 April 2022. The proposed dwelling
location is positioned towards the northern end of the property in a cleared pad. The house pad is relatively flat with
an approximately 9 m high batter on the northern side sloping down to meet with alluvial plains running alongside the
river. The slope angle of the batter varies from approximately 20° up to approximately 35°. No obvious signs of
disturbed ground, tension cracks or hummocks were noted in the vicinity of the proposed residence during the
walkover assessment. Some surficial soil creep was noted in the steeper parts of the slope where the water flow
paths were present. Based on the nearby river and dam, as well as overly moist soils at the base of the batter, it is
presumed that the standing water table is approximately 1 m below the existing ground level at the base of the batter.



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.”.

Sunshine Coast | Brisbane | Gold Coast