Att 3 - Landslide Risk Assessment

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“Normal” classes also have a minimum allowable bearing               have followed AS 2870-2011 soil descriptions contained in
capacity as outlined in Clause 2.4.5 of AS 2870-2011.                Clause C2.1 rather than AS 1289 because where there is
AS 2870-2011 also has a Class P for problem sites                    a conflict between referenced codes, AS 2870-2011 takes
covering fill, soft or collapsing soils, potential slope stability   precedence.
problems, mining subsidence and abnormal moisture                    Underslab Termiticide Irrigation Systems
conditions. Abnormal Moisture Conditions (AMC) is a                  These are becoming popular and besides serving their
particularly contentious area and Clause 1.3.3 of AS 2870-           obvious purpose, they also inject extra moisture beneath
2011 covers many situations where this clause applies.               the slab at various times (measured in years). This
The most common situations are sites with clay soils                 creates long term “abnormal” moisture conditions that
(normally Class M, H1, H2 or E (ys > 20)) that have either           needs to be addressed at the design stage, therefore if
existing structures or trees or gardens within the zone of           one of these is proposed for this project, the design
influence of the proposed footing. Some of these trees               engineer must be informed prior to preparing the slab. As
may be on adjoining properties. Where this clause is                 a general rule, to cope with these systems, the ys must be
applicable, we have added further explanatory advice. The            increased by about 50%, which will generally result in a
soil shrinkage index (Ips) range quoted in this report was           slab one category higher than would normally be used
assigned after considering the guidelines in Section 2 of            (refer P12, Supplement to AS 2870-2011). Upon request
AS 2870-2011 and from this we have derived a ys, which               we can supply more specific advice.
is the “characteristic surface movement” under NORMAL
                                                                     Acid Sulfate Soils (ASS) & Saline Soils
moisture conditions.
                                                                     Unless specifically stated, we have not considered the
Footings designed in accordance with AS 2870-2011 have               possibility of ASS, which occur around the coastline,
a           long-term            performance              criteria
                                                                     generally below AHD 5.0 and occasionally on broad river
and it should be noted that this does not offer a crack or
                                                                     flood plains at higher levels. Most Councils maintain maps
distress-free performance.          It offers a performance
                                                                     of these areas. In new estates the ASS problem has
criterion that ensures a low probability of foundation
                                                                     normally been assessed and neutralised, but it is
failure, provided abnormal moisture conditions, such as
                                                                     worthwhile confirming this at land sales, if ASS are
over-watering, bad drainage, leaking pipes or nearby trees
                                                                     suspected. In older areas, the council is normally the best
are not allowed to exist or develop.
                                                                     source of advice. ASS, if present, do have the potential to
These performance criteria are outlined in Appendix C of             dramatically shorten the life of footings, slabs,
AS 2870-2011 and under normal conditions a low                       reinforcement and bricks. This advice is also relevant for
incidence of Category 1 damage and an occasional                     saline soils. Unless specifically stated, we have not
incidence of Category 2 damage is expected. This                     considered the possibility of Saline Soils, however we can
appendix is available from our office upon request.                  provide a quotation to complete this testing.
Where Abnormal Moisture Conditions exist and/or are
allowed to continue to develop, then not only will the above
probabilities increase, but the damage will be greater. The
ultimate responsibility falls on the design engineer to
negate the effects of these conditions when they are
known and for the owner/occupier to ensure that they do
not develop. Our responsibility is limited to identifying
these conditions.
If any potential owner is not satisfied with the performance
criteria in AS 2870 (which has been applied Australia wide
since 1986) then prior to footing design, he/she should
consult with the design engineer and have a specially
designed footing more suited to their needs.
Classification Limitations
The content of this report is based on the expertise and
experience of the author representing this company. Our
commission didn’t extend to assessing instability due to
previous or existing sub-surface mining, landslip or
earthquakes, nor did it extend to testing to comply with the
relevant contaminated land act or for acid sulphate soils
(see note below). If, however any of these exclusions was
obvious or where the allotment is within an area where we
are aware of a past history of these exclusions, we have
made comment and given further advice. This report is
based on the assumption that the test results are
representative of the true site conditions. Even under
optimum circumstances, actual conditions may differ from
those reported to exist.          Although our investigation
exceeds the minimum requirements of AS 2870-2011,
economic constraints necessarily limit the practical extent
of any investigation.         We therefore cannot accept
responsibility for conditions encountered on this site
outside the areas tested which are different to those
reported. The positions of these test sites have not been
surveyed and should be regarded as approximate. We

AWT 77775 Site Classification Report
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