Landslide Hazard Report_compressed

                          AUSTRALIAN GEOGUIDE LR7 (LANDSLIDE RISK)
LANDSLIDE RISK                                                    landslide risk assessment for a particular site you
                                                                  should expect to receive a report prepared in
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 defined as "a measure of the probability and
severity of an adverse effect to health, property, or the         Risk to Property
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 any one 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
by special regulations. If you are contemplating                   Possible                    1:1,000
building, or buying an existing house, particularly in a           Unlikely                    1:10,000
hilly area, or near cliffs, go first for information to your       Rare                        1:100,000
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 could impact 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
Risk assessment is a predictive exercise, but since the           be carried out as part of the development, or consent
ground and the processes involved are complex,                    will be withheld.
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, planning 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 of treatment
                       options required to reduce risk to acceptable level. Work would cost a substantial sum in relation to
                       the value 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