geotech report · MCU15/0087

Slope Stability and Geotechnical Assessment
1 Natasha Avenue, Noosa Heads




7.       QUALITATIVE SLOPE STABILITY RISK
         ASSESSMENT
7.1. Introduction to Qualitative Risk Assessment
Risk assessment is a process where potential hazards are observed and/or assessed, and a
judgement made as to the likelihood of that event occurring, and an assessment made of what the
consequences of such an event might be. The ‘risk’ assigned (say high, medium or low) is a means
of explaining the combination of likelihood and consequences in a relatively simple form.

7.2. Risk Assessment Methodology
This qualitative risk assessment has been carried out to assess the geological hazards on the site
and attempt to define the likelihood of that hazard becoming an event, and as a consequence the
resultant risk to property. A qualitative assessment ranks risk on a scale as opposed to a quantitative
risk assessment that defines the risk as a numerical probability.

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 (AGS 2007).

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 a building may be ‘insignificant’ (e.g. the rock is so small that the building damage
might be insignificant). The resultant risk relating to that particular hazard could then be assessed
using the matrix from Appendix C of AGS 2007 to be ‘low’ to ‘moderate’. A copy of Appendix C from
AGS 2007 is attached in Appendix D.

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. In some cases, further investigation work may be required to reduce this
uncertainty.

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.

7.3. 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 C of that document. Generally where risk is assessed
as greater than Low risk, some form of detailed investigation is required and treatment options are
implemented. Where risk is Low or Very Low, the risk is often accepted. Moderate risk or higher
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:



Coffey
GEOTKPAR02068AA/A                                                                                         6
12 August 2015