Compiled Reference Documents - EXE250036 (As Published) · EXE25/0036

J002910-001-L-Rev0
28 July 2025


Table 1: Stability Risk Levels
 Risk Level                  Example Implications

                               Unacceptable without treatment. Extensive detailed investigation and research, planning
           VERY HIGH
 VH                            and implementation of treatment options essential to reduce risk to Low; may be too
           RISK
                               expensive and not practical. Work likely to cost more than value of property.

                               Unacceptable without treatment. Detailed investigation, planning and implementation of
 H         HIGH RISK           treatment options required to reduce risk to Low. Work would cost a substantial sum in
                               relation to the value of the property.

                               May be tolerated in certain circumstances (subject to regulators’ approval) but requires
           MODERATE
 M                             investigation, planning and implementation of treatment options to reduce risk to Low.
           RISK
                               Treatment options to reduce to Low risk should be implemented as soon as practicable.

                               Usually acceptable to regulators. Where treatment has been required to reduce the risk to
 L         LOW RISK
                               this level, ongoing maintenance required.

           VERY LOW            Acceptable. Manage by normal slope maintenance procedures.
 VL
           RISK

 Note:

      1)   The implications for a particular situation are to be determined by all parties to the risk assessment and may
           depend on the nature of the property at risk; these are only given as a general guide.


With reference to the qualitative measures of likelihood and consequence given in the attachment Qualitative
Terminology for Use in Assessing Risk to Property, the likelihood of global instability, (i.e., failure through the
underlying inferred extremely weathered sandstone) impacting the proposed pool and additions during the
design life is Rare (i.e., the event is conceivable but only under exceptional circumstances over the design
life), and the consequential property loss could be Minor (i.e., limited damage to part of the structure, and/or
part of the site requiring some reinstatement stabilisation works). A combination of these measures in the
risk matrix results in a property risk classification of Very Low.
Based on the available information and the proposed works, it is our opinion that there is a Very Low Risk of
slope instability for the proposed development assessed in accordance with the AGS 2007, subject to
implementation of the recommendations in this report. This level of risk is generally acceptable to regulators
(i.e., NC).
Site Classification

The site classification derived in accordance with Australian Standard AS 2870-2011 Residential slabs and
footings provides an indication of the likely magnitude of reactive (shrink and swell) movements associated
with normal seasonal soil moisture variations (ys).

The estimated range of ys is calculated in accordance with the methodology presented in AS 2870-2011, by
assessment of the soil suction change, factored for lateral restraint multiplied by the soil layer thickness and
the instability index (estimated from Shrink-Swell index (Iss)).
For the Noosa region, AS 2870-2011 recommends a depth of design suction change Hs of 1.5 m.

Based on our experience in the nearby area within the Tiaro Coal Measures formation, the site is classified
as ‘Class M’ in accordance with AS 2870-2011. The estimated characteristic ground surface movement due
to seasonal moisture variation (ys) is less than 40 mm.

The predicted order of movement is based on the existing soil profile (i.e., no cutting or filling), and moisture
content changes within soils due to normal seasonal moisture variations only (i.e., does not consider soil
settlement).

RECOMMENDATIONS
To obtain a ‘Very Low’ risk classification, the following measures must be implemented for the proposed
construction:




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