Slope Stability - lindsays

AUSTRALIAN GEOGUIDE LR7 (LANDSLIDE RISK)

LANDSLIDE RISK

Concept of Risk

Risk is a familiar term, but what does it really mean? It
can be defined as "a measure of the probability and
severity of an adverse effect to health, property, or the
environment." This definition may seem a_ bit
complicated. In relation to landslides, geotechnical
practitioners (GeoGuide LR1) are required to assess
tisk in terms of the likelihood that a particular landslide
will occur and the possible consequences, This is called
landslide risk assessment. The consequences of a
landslide are many and varied, but our concerns
normally focus on loss of, or damage to, property and
loss of life,

Landslide Risk Assessment

Some local councils in Australia are aware of the
potential for landslides within their jurisdiction and have
responded by designating specific “landslide hazard
zones". Development in these areas is often covered
by special regulations, If you are contemplating
building, or buying an existing house, particularly in a
hilly area, or near cliffs, go first for information to your
local council.

Landslide risk assessment must be undertaken by

a_geotechnical practitioner. It may involve visual
inspection, geological © mapping, —_ geotechnical
investigation and monitoring to identify:

¢ potential landslides (there may be more than
‘one that could impact on your site)

the likelihood that they will occur

the damage that could result

the cost of disruption and repairs and

the extent to which lives could be lost.

Risk assessment is a predictive exercise, but since the
ground and the processes involved are complex,
prediction tends to lack precision. If you commission a

landslide risk assessment for a particular site you
should expect to receive a report prepared in
accordance with current professional guidelines and in
a form that is acceptable to your local council, or
planning authority.

Risk to Property

Table 1 indicates the terms used to describe risk to
property. Each risk level depends on an assessment of
how likely a landslide is to occur and its consequences
in dollar terms. "Likelihood" Is the chance of it
happening in any one year, as indicated in Table 2.
"Consequences" are related to the cost of repairs and
temporary loss of use if a landslide occurs. These two
factors are combined by the geotechnical practitioner to
determine the Qualitative Risk,

TABLE 2: LIKELIHOOD

 

 

 

 

 

 

 

 

Likelihood Annual Probability
Almost Certain 1:10

Likely 1:100

Possible 4:1,000

Unlikely 4;40,000

Rare 1;100,000

Barely credible 4:1,000,000

 

 

 

The terms "unacceptable", "may be tolerated", etc. in
Table 1 indicate how most people react to an assessed
risk level. However, some people will always be more
prepared, or better able, to tolerate a higher risk level
than others.

Some local councils and planning authorities stipulate a
maximum tolerable level of risk to property for
developments within their jurisdictions. In these
situations the risk must be assessed by a geotechnical
practitioner. If stabilisation works are needed to meet
the stipulated requirements these will normally have to
be carried out as part of the development, or consent
will be withheld.

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