Preliminary Geotechnical Investigation · OPW21/0315

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Generally, the action criterion from the chromium suite of tests, which triggers a requirement for ASS
disturbance to be managed, derived from the Soil Management Guidelines is as follows:
•    Net Acidity (TAA + SCR + SNAS) of greater than or equal to 0.03 %S for disturbances of greater
     than 1000 tonnes

Based on the laboratory test results, both the existing sand fill and natural silty sand are considered to
be acidic. However, only the natural sands are considered to contain PASS, having the potential to
generate sulfuric acid when disturbed and exposed to oxygen. The measured acidity in the sand fill is
not likely attributed to sulfides (based on the SKCl results), but rather naturally occurring acidity
commonly found in non-ASS soils.

       7.5.3 Management and Treatment of ASS

As the disturbance of the acidic soils cannot be avoided, these disturbed soils will require some
treatment.

While the non-ASS acidic sand fill material does not require specific treatment, appropriate
management of the natural acidity should however be carried out to reduce the risks of environmental
impact in order to comply with the general environmental duty of care where appropriate (ie.
neutralisation of any acidic leachate).

It is recommended that a guard layer of neutralising agent (eg. aglime) be spread under and also over
the top of stockpiles of acidic soils to limit the generation of acidity from the surface of the stockpile
and any leachate emerging near the base. A nominal guard layer rate of 2.5 kg/m2 is recommended.
Reapplication of the guard layer will be necessary under areas of repeated temporary stockpiling.

However, the natural sands will require neutralisation treatment in order to reduce the risk of oxidation
and consequent reduction of pH to produce acidic conditions potentially damaging to the environment
and to buried steel and concrete.

‘Ag lime’ should be used as the preferred neutralisation material for the management of ASS as it is
usually the cheapest and most readily available product for soil neutralisation. This material is mildly
alkaline (pH of 8.5 to 9), of low solubility, and does not present any handling problems. The ‘ag lime’
comprises calcium carbonate typically made from limestone that has been finely grounded and sieved
to a fine powder.

Table 4 provides indicative liming rates for neutralisation of the different ASS material likely to be
disturbed.

Table 4: Indicative Liming Rates
                        Material                                       Lime Application Rate for Treatment (kg/m3)
                   Existing Sand Fill                                                 Not required
                     Natural Sands                                                         4.0
                                                                   3
Notes: Assumes a bulking up factor of approximately 1.5 tonnes/m




Proposed Residence                                                                                   95120.00.R.001.docx
33 Wyuna Drive, Noosaville                                                                                  October 2020