Geotechnical Report

                               AUSTRALIAN GEOGUIDE LR7 (LANDSLIDE RISK)

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

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.




February 2019