Physical and Mechanical Characteristics of Bringelly Shale
Department of Civil Engineering, University of Sydney,
In the Sydney metropolitan area the Wianamatta group is a major geological sequence in the Sydney Basin. It is comprised of two shales known as Ashfield shale and Bringelly shale. Both shales are comprised predominantly of claystones and siltstones with occasional sandstone layers. Bringelly shale, which is the top layer in the Wianamatta group, is highly compacted, weakly cemented, and contains significant amounts of swelling clay minerals.
Bringelly shales have very low porosities and in their fresh state high strengths, but it contains swelling clay minerals, swells and disintegrates rapidly on immersion in water and is generally less durable. With conventional drilling techniques core recovery of this shale is low and it is difficult to obtain samples for mechanical testing.
The method of sample preparation involved samples reconstituted to low and high void ratios. Shale with different degree of weathering was tested for durability and strength. Natural water content, Atterberg limits, uniaxial compressive strength, and point load index tests have been investigated. The results were then correlated to mineralogy and internal structure of the rock.
Isotropically consolidated undrained triaxial tests have been performed on specimens for a range of stress levels, over-consolidation ratios and initial porosities. The reconstituted material is seen to exhibit brittle behaviour that is not typical of normally consolidated reconstituted samples sheared from isotropic conditions. The undrained secant stiffness was found to vary with strain level and also to be independent of the consolidation pressure.
Keywords: Sydney, shale, claystone, siltatone, mineralogy
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