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Abstract
The Patonga Claystone, a red bed facies in the Narrabeen Group of the Sydney Basin, is one of the most unfavorable rock units in the basin from a geotechnical
point of view. This rock unit is composed of sandstone, siltstone, mudstone and claystone. One of the unfavorable characteristics is the low shear strength, which causes instability of cut slopes; another is its slaking-prone
behaviour. Numerous measurements of geotechnical properties, along with extensive mineralogical and geochemical determinations, were carried out to identify cause of this slaking behaviour. Key techniques were the use of
quantitative X-ray diffractometry for mineralogical analysis, and the determination of slake durability index and related properties to evaluate the slaking behaviour
under both standard and more extended conditions. Standard (two cycle) slake durability test results indicate a range from low to high slake durability index
values, with some mudstone samples having very low durability and some sandstones having very high slake durability indices. Jar slake test results indicate that the rock samples break rapidly and/or develop several fractures (Ij = 4) in an as-received state, but degrade to a pile of flakes or mud (Ij = 1) if the samples are oven dried before testing. The results for jar slake testing of
oven-dried material are comparable, for individual samples, to those obtained from the more comprehensive slake durability tests. The mineralogy of the samples was evaluated by quantitative X-ray diffraction techniques using the
Rietveld-based Siroquant processing system. Comparison to independent chemical data show a generally good level of agreement, suggesting that the mineralogical analysis results are consistent with the chemical composition of the
individual rock samples. Good correlations were also obtained between clay mineralogy determined from orientedaggregate XRD analysis of the <2 micron fraction and the results from powder diffractometry and Siroquant analysis
of the whole-rock samples. Evaluation of the slake durability characteristics and
other geotechnical properties in relation to the quantitative mineralogy suggests that quartz and feldspar form a rigid framework in the rocks that resists the
disruptive pressures that cause slaking. Expansion of the clay minerals by various processes, including the incorporation of water into the interlayer spaces of illite/smectite as well as changes in pore pressures associated with entry of water into micro-fractures in the clay matrix, are thought to produce the disruptions that cause slaking and degradation. An abundant clay matrix also reduces the strength of the rock materials, probably because of the less rigid nature of the clay minerals relative to the quartz and feldspar particles.
As well as the mineralogy, the loss on ignition (LOI) and water absorption percentage were found to provide good indicators of longer-term slaking behaviour. Both properties are also related to the overall clay content. Rock samples with water absorption values of <10, 10-15 and
>15% behave as highly durable, intermediate and less
durable materials respectively. Rocks with LOI values of
greater than 5% by weight behave as less durable rock
materials, at least for the strata encompassed by the
present study. The water absorption and LOI values were
also used to develop a predictive model of slake durability
characteristics for the different rock materials in the
Patonga Claystone, providing a relatively simple basis for
predicting longer-term stability in a range of geotechnical
studies.