Reference Document - RAL25/0020 & OPW25/0194 (As Published)

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To provide an indication of the characteristic ground surface movements (ys) due to changes in
soil moisture, the results of the laboratory testing were input into Douglas’ in-house program
REACTIVE. AS 2870 provides recommended values of change in suction (Δu) and depth of suction
(Hs) for major and regional centres throughout Australia. However, more detailed data published
by Fox (2000), relating climatic conditions to suction was used. A value of 1.2 pF was adopted for
Δu and 1.5 m for Hs in the REACTIVE calculations. This is based on a ‘wet coastal’ climatic zone.

The results of the analysis indicate that ys movements due to normal seasonal moisture content
variation is estimated to be up to 50 mm, consistent with ‘Class H1’ (highly reactive) conditions
based on a full clay profile. A reduced cracking depth has also been considered allowing for fill
placement or excavation (where the cut depth exceeds the lesser of 0.25 H s or 0.5 m).

If ‘abnormal’ soil moisture conditions are experienced at the site, the site classification would
change to ‘Class P’ (problem site) which would require more extensive foundation works or could
result in adverse foundation performance. ‘Abnormal’ soil moisture conditions are defined in
AS 2870 (Clause 1.3.3) and in summary comprise:
•     Recent removal of buildings or structures likely to affect soil moisture conditions;
•     Unusual moisture caused by drains, channels, ponds, dams or tanks;
•     Recent removal of large trees;
•     Growth of trees too close to a structure;
•     Excessive or irregular watering of gardens adjacent to the structure;
•     Lack of maintenance of site drainage;
•     Failure to repair plumbing leaks.

It should be noted that no assessment of the effect of soil moisture change by trees has been
made in estimating the above ys values (either with respect to the removal of established trees
prior to development of building pads, or the proximity of established or new trees to proposed
buildings). Reference to the requirements in AS 2870 should be made by the building designer
in this regard. It should be further noted that the presence or removal of trees can result in
additional surface movement, due to tree-induced suction changes and tree-induced centre
heave. Such tree-induced movement is not included in the ys calculations used to classify the
site.

6.6    Foundations

Provided that site preparation is carried out in accordance with the recommendations in this
report, it is considered that high level strip and/or pad footings founded in either controlled fill or
competent natural soils may generally be adopted. Slabs supported on high level foundations
should be stiffened to suit the expected ground surface movements.




Proposed Subdivision                                                                 237894.00.R.001.Rev0
12-14 Gympie Street, Tewantin                                                               October 2025