Attachment 5 - Geotechnical Assessment · MCU18/0015

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RTI_11/055 Folio B392

November 2003 -8- 03632248-001

 

initiating instability. Local erosion control measures (e.g. gabion baskets) to protect the creek
banks both here and elsewhere should be considered in detailed design. ,

The presence of this possible slip indicates that the steeper parts of the sand dune are close to
equilibrium (ie. factor of safety ~ 1). Our stability analysis indicates that when the
groundwater level rises 0.7 m above the lower interface of the very loose sand, the minimum
factor of safety reduces less than one for a 5 m long slip circle located on the lower part of the
slope (ie. in the bank of the creek). For a fully specified slip surface through the very loose
sand and extending up slope for a distance of about 20 m, the factor of safety is about 1.3,
being slightly less than acceptable for long-term stability.

From the above observations and stability analyses, it is considered likely that surface
slumping of the sand can occur within about 20 m of the creek after periods of heavy or
prolonged rainfall. This needs to be considered in the detailed design of the.footings for
structures (Section 7.3).

Subject to the implementation of the geotechnical advice in Section 7,0, it is our opinion that
there will be a Low to Moderate Risk of slope instability affecting the proposed development.
Such a level of risk is manageable provided that the proposed treatment options are adopted.

This risk assessment has been carried ott following the AGS Sub-Committee Guidelines on
Landslide Risk Management (March:2000)» Relative levels of risk and their implications are
given in the table below.

Table 4: Relative Risk Leyels & Implications for Development

 

 
 

 

VH VERY HIGH RISK | Extensive detailed investigation and research, planning and
implementation of treatment options essential to reduce to
acceptable levels; may be too expensive and not practical.

 

H HIGH RISK Detailed investigation, planning and implementation of

treatment options required to reduce risk to acceptable levels.

 

 

 

 

 

M MODERATE RISK | Tolerable provided treatment plan is implemented to maintain or
reduce risks, May be accepted, May require investigation and
planning of treatment options. ,

L LOW RISK Usually accepted. Treatinent requirements and responsibility to
be defined to maintain or reduce risk.

VL VERY LOW RISK | Acceptable. Manage by normal slope maintenance procedures

 

 

 

Note:(1)The implications for a particular situation are to be determined by all parties to the risk assessment; these are only given as a general guide.

Golder Associates