220241.01 - 70 Blanckensee Road, BLACK MOUNTAIN - SECONDARY DWELLING - Slope Stability Report · EXE23/0033

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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.
TABLE 1: AGS ASSESSMENT

                Hazard                         Likelihood                Consequence                    Risk Level
             Global Failure                   Barely Credible             Catastrophic                       Low
         Creep in Shallow Soils                  Unlikely                    Medium                          Low
  Rotational or translational slide down           Rare                       Major                          Low
                 the slope
          Existing batter failure             Almost Certain              Catastrophic                       Low


Based on the results of the AGS assessment, the perceived risk based on the conditions encountered and information
available at the time of the assessment is “Low”.

7         HILLSIDE CONSTRUCTION
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 attached documentation:
 A minimum footing depth of 1.2 m is recommended. Footings should be founded into stiff (or stronger) natural
  clays.
 Fill should be limited to no more than 1 m and should be step keyed into the existing slope. The fill should be
  formed at no steeper than 3H:1V, unless retained by an engineered retention system. All fill should be placed in
  accordance with AS3798.
 Where slopes are not supported by an engineer designed support system, natural slopes within or near movement
  sensitive areas should be no steeper than 2.5H:1V.



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