Att 3 - Landslide Risk Assessment

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                                       SITE SPECIFIC NOTES

We have classified the site as Class P in accordance with AS2870-2011.
Abnormal Moisture Conditions
Using the guidelines in AS 2870-2011 we have derived a normal ys in excess of 20 mm (refer front
page), which is a measure of the potential of the strata to change volume with changes in soil moisture
(generated by seasonal moisture variations).
During the site visit, we also noted features (see front page) within the zone of influence of the
proposed building footprint, which are specifically mentioned in Clause 1.3.3 of AS 2870-2011 as
contributing to abnormal moisture conditions (AMC). Clause 2.1.2 specifically notes that AMC sites
require a “P” classification and AS 2870-2011 offers the following advice to the footing designer on
the impact of AMC conditions.
Clause 1.3.3 (in part) “Buildings constructed on sites subject to AMC have a higher probability of
damage than that given in Clause 1.3.1”
Clause 1.4.1 General The design conditions specified in Clauses 1.4.2 and 1.4.3 for beams and slabs
supported by the foundation on normal sites shall apply.
For other than normal sites, the design of the footing system shall be by engineering principles to
ensure the footings perform in accordance with Clause 1.3. Design considerations that are particular
to the site shall be considered.
Clause 1.4.3(b) “Past satisfactory performance of similar footings on similar sites”
As the above quoted normal ys does not take into account ground movements generated by the
abnormal conditions, the design engineer must use his/her experience and judgment to ensure that
the design provides acceptable performance. In doing this, the following must be considered:
    •    How the proposed development will change the existing equilibrium of the soil moistures.

    •    The long term impact on the soil moisture equilibrium of existing and future vegetation and
         structures.

    •    Appendix H and/or CH of AS2870-2011.

    •    Effective site drainage.

    •    Past satisfactory performance of similar footings on similar sites.
As the above quoted “normal” ys does not take into account the additional ground movements
generated by the “abnormal” conditions, the design engineer must use his/her experience and
judgment to ensure that the design provides acceptable performance. In doing this, the following
must be considered-
    •    How the proposed development will change the existing equilibrium of the soil moistures.
    •    The long-term impact on the soil moisture equilibrium of existing and future vegetation and
         structures.
Where vegetation exists within the zone of influence of the proposed footing system, the design
engineer shall consult Appendix H and/or Appendix CH of AS2870 in order to provide a suitable
structural design.
Although we are not arborists nor have we been provided with a site feature survey showing the
location, height and tree canopy information, we have provided an initial estimate of the maximum
potential surface movement due to the tree-induced suction change in addition to the normal design
suction change “ytmax” below;
                      Normal ys                 ytmax          Estimated Design Class

                      31 - 40 mm               16 mm                  Class H1

Once a site feature survey is completed and the relevant tree information provided, we can provide
a specific surface movement suction change calculation “yt” to aid the design engineer.




AWT 77775 Site Classification Report
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