Response to Information Request - MCU12/0184

                              AUSTRALIAN GEOGUIDE LR7 (LANDSLIDE RISK)

I LANDSLIDE RISK                                                       landslide risk assessment for
                                                                       should    expect     to    receive
                                                                                                          particular site you
                                                                                                                 a

                                                                                                        report prepared in
                                                                                                             a

Concept of Risk                                                       accordance with current professional guidelines and in
                                                                      a form that is acceptable to your local council, or
Risk is a familiar term, but what does it really mean? It
                                                                      planning authority.
can  be defi ned as "a measure of the probability and
                                                                       Risk to Property
severity of an adverse effect to health, property, or the
                 "
environment.         This   definition   may    seem    a    bit
                                                                      Table 1 indicates the terms used to describe risk to
complicated. In relation to landslides, geotechnical
                                                                       property. Each risk level depends on an assessment of
practitioners (GeoGuide LR1) are required to assess                    how likely a landslide is to occur and its consequences
risk in terms of the likelihood that a particular landslide
                                                                       in   dollar terms.        "Likelihood" is the chance of it
will occur and the possible consequences. This is called
                                                                      happening in anyone year, as indicated in Table 2.
landslide risk assessment. The consequences of a
                                                                      "Consequences" are related to the cost of repairs and
landslide are many and varied, but our concerns
                                                                      temporary loss of use if a landslide occurs. These two
normally focus on loss of, or damage to, property and                 factors are combined by the geotechnical practitioner to
loss of life.
                                                                       determine the Qualitative Risk.
Landslide Risk Assessment
                                                                                       TABLE 2: LIKELIHOOD
Some local councils in Australia          are   aware   of the
                                                                        Likelihood                          Annual Probability
potential for landslides within their jurisdiction and have             Almost Certain                      1:10
responded by designating specific "landslide hazard
                                                                        Likely                              1:100
zones". Development in these areas is often covered
                                                                        Possible                            1:1,000
by special regulations. If you are contemplating
building, or buying an existing house, particularly in a                Unlikely                            1:10,000
                                                                        Rare                                1:100,000
hilly area, or near diffs, go first for information to your
local council.                                                          Barely credible                     1:1,000,000
                                                                       The terms "unacceptable", "may be tolerated", etc. in
Landslide risk assessment must be undertaken by
                                                                       Table 1 indicate how most people react to an assessed
a  geotechnical practitioner. It may involve visual                    risk level.However, some people will always be more
inspection,     geological    mapping,    geotechnical
                                                                       prepared, or better able, to tolerate a higher risk level
investigation and monitoring to identify:                             than others.
      .
          potential   landslides (there may be     more     than
                                                                       Some local councils and planning authorities stipulate a
          one that couldimpact on your site)                          maximum tolerable level of risk to property for
      . the likelihood that they will occur
                                                                      developments within their jurisdictions.       In these
     .  the damage that could result                                  situations the risk must be assessed by a geotechnical
      . the cost of disruption and
                                     repairs and                      practitioner. If stabilisation works are needed to meet
      .   the extent to which lives could be lost.                    the stipulated requirements these will normally have to
                                                                      be carried out as part of the development, or consent
Risk assessment is a predictive exercise, but since the
                                                                      will be withheld.
ground and the processes involved are complex,
prediction tends to lack precision. If you commission a

                                                 TABLE 1. RISK TO PROPERTY

 Qualitative Risk           Significance Geotechnical engineering requirements
                                          -




    Very high        VH     Unacceptable without treatment. Extensive detailed investigation and research, pl~nning and
                            implementation of treatment options essential to reduce risk to Low. May be too expensive and not
                            practical. Work likely to cost more than the value of the property.

      High           H      Unacceptable without treatment. Detailed investigation, planning and implementation ?f treatment
                            options required to reduce risk to acceptable level. Work would cost a substantial sum In relation to
                            the val ue of the property.

    Moderate         M      May be tolerated in certain circumstances (subject to regulator’s approval) but requires
                            investigation, planning and implementation of treatment options to reduce. the risk to Low.
                            Treatment options to reduce to Low risk should be implemented as soon as pOSSible.

       Low            L     Usually acceptable to regulators. Where treatment has been needed to reduce the risk to this
                            level, ongoing maintenance is required.

    Very Low         VL     Acceptable. Manage by normal slope maintenance procedures.




172                             Australian Geomechanics Vol 42 No 1 March 2007