Response to Information Request - MCU12/0184

Geotechnical and Slope Stability Investigation




A risk is made up of the product of the likelihood of an event, and the consequences of that event

occurring. We have used a methodology guided by the paper ’A National Landslide Risk Management
Framework for Australia’; Volume 42 March 2007 published by the Australian Geomechanics Society in
2007 (AGS2007).

This document was created in an attempt to standardise the terms and methodologies used for risk
assessment for landslide events in Australia, and also to formalise the procedures in conducting risk
assessments. The guidelines use common terms and methods of assessment, combined with a risk
matrix to define risk. The terms used in this document that reference definitions in AGS 2007 are
written in bold.

For example, the likelihood of a small rock falling may be assessed as ’likely’ (e.g.the assessment that

a rock will   likely fall in the next significant rain event) whilst the consequences of that particular rock
impacting building may be ’insignificant’ (e.g. the rock is so small that the building damage might be
              a

insignificant). The resultant risk relating to that particular hazard could then be assessed using the
matrix from Appendix D of AGS 2007 to be ’low’ to ’moderate’.

The risk assessments are based on our assumptions and judgements. The risks assessed using these
methods are generally ranges, and in some cases span two or more categories of risk. The range of
values indicates the uncertainty in the data used to estimate the likelihood and consequences of events.
Further investigation work may be required to reduce this uncertainty.


7.3         Factor of Safety Assessment

Coffey considers that the use of numerical analyses to determine a factor of safety (FOS)and the
subsequent use of such a factor to be unreliable for risk assessment. Numerical methods are not
considered an appropriate means of assessing landslip risks or the stability of structures in natural
environments. However, in order to comply with SCRC Planning Policy 4, an assessment of FOS has
been made and included in this report.

This assessed FOS value has been included in this report to allow SCRC to evaluate the landslip risk
on their terms to their criteria.     Despite this inclusion, Coffey believes that the stability assessment using
the methods set out in AGS 2007 to be the most reliable and representative means of expressing slope

stability risks.

7.4           Implications of Risk Assessment
The implications of the risk levels are discussed in AGS 2007 and a table describing commonly

accepted levels of risk is included in Appendix D. Generally where risk is assessed as greater than
Low risk, some form of detailed investigation and treatment options are implemented. Where risk is
Low or Very Low the risk is often accepted. Moderate risk commonly requires investigation and
treatment to reduce the risk to Low or less.


AGS 2007 provides example implications for risk levels low and moderate risk levels as follows:

      .   Very Low Risk: Acceptable. Manage by normal slope maintenance procedures.

      .   Low Risk: Usually acceptable to regulators. Where treatment has been required to reduce risk
          to this level, ongoing maintenance is required.

      .   Moderate Risk: May be tolerated by regulators (subject to regulator’s approval) but requires

          investigation, planning and implementation of treatment options to reduce the risk to low.



Coffey Geotechnics                                                                                              4
GEOTKPAR01945ANA
23rd April 2013