Appendix 5. Geotechnical Engineering Report [As Published] · 132007.2399

October 2007                                     -9-                           077682076(AB)


also highlighted. The following observations are made regarding the investigation results as
presented in Table C1:

•     67% of samples show pH after oxidation less than pH3, with 81% of those samples being
      described as grey or dark grey coloured silty/sandy clays or clayey sands of soft or loose
      consistency, respectively.

•     3 out of 6 samples selected for laboratory testing returned SCr results exceeding the
      Action Criteria (0.03%S), with a maximum value of 1.17%S recorded for the ‘marine’
      clay sample from 1.75 m depth in borehole BH8. The other 3 samples returned results
      less than the laboratory detection limit of 0.02%.

•     3 out of 6 samples tested for TAA returned results at or above the Action Criteria (18 mol
      H+/t) ranging between 21 mol H+/t and 66 mol H+/t.

•     1 sample (BH8, 1.75 m) returned a pHKCl value greater than 6.5 and was subsequently
      tested to determine retained acid neutralising capacity (ANC). The ANC was found to be
      262 mol H+/t and is due to the presence of broken shells as noted in the borehole report.
      These broken shells were noted to be mostly coarse, rather than fine grained.

5.5       ASS Extent and Severity

The results from this investigation indicate that ASS having a potential to generate sulfidic
leachate (ie. PASS) exist within the natural grey to dark grey coloured clayey sand and
sandy/silty clay soils in the low-lying south-western part of this site. Dark grey silty ‘marine’
clays in boreholes BH5 (2.4 m to 3.2 m depth) and BH8 (1.4 m to 2.6 m depth) are likely to
be high level PASS.

It should be noted that there is also existing acidity variably distributed throughout the fill
materials and natural soil profile.

Although one of the ‘marine’ clay soil samples tested has an acid neutralising capacity that
could significantly reduce or neutralise the potential acidity, it is our opinion that the ANC
should be excluded from calculations when assessing required liming rates for this site. This
is because the ANC in the soils is derived from broken shells that are generally very coarse,
hence the ANC from those shells is unlikely to be fully available in the field.

The SPP2/02 Guideline requires that the level of treatment for management of ASS is based
on treatment of all existing and potential acidity. The results of the laboratory testing have
been accumulated in an Acid-Base Account to give a Net Acidity Value for each sample in
units of mol H+/tonne as presented in Table C1. These values have been calculated from
sulfur trail potential acidity (SCr or SPOS) plus actual acidity (TAA) plus retained acidity
(SNAS), but does not include ANC.




                                       Golder Associates