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Technical Note: Strength and stiffness properties of defects within the Hawkesbury Sandstone and Ashfield Shale
The laboratory direct shear test results on defects which were undertaken for three recent tunnelling projects in Sydney β the Lane Cove Tunnel, the CBD Metro (which was not built), and the Northwest Rail Link β provide data to assess the strength and stiffness of bedding planes and joints within the Hawkesbury Sandstone and Ashfield Shale.
The results, which are summarised in Tables 1 and 2, comprise:
- 76 tests from the Hawkesbury Sandstone of 13 bedding planes, ten joints, five shears / clay seams and one saw-cut
- 97 tests from the Ashfield Shale of 13 bedding planes, 11 joints and six shears / clay infill.
The results largely represent planar defects as attested by the descriptions of the individual samples and the displacement plots (vertical versus horizontal) in the result sheets (see Figure 1, for an example). In the case of one dataset, the shear and normal stress values were also corrected for dilatancy as recommended by Hencher and Richards (1989).
Hence, the data can be used to estimate the basic friction (Οb), i.e. βthe frictional component of shear strength for a planar or effectively planar discontinuity i.e. independent of any roughness component causing dilation during shearβ (Hencher and Richards, 2014).
The author has interpreted the results for shear strength and stiffness.
- The peak shear stress (Ο) recorded for the normal stress (Οn) applied during each of the direct shear tests was selected.
- The shear stiffness (ks) was estimated by the author as the gradient of the secant line plotted on the elastic portion of the shear stress versus shear displacement graph (see Figure 1). This is in keeping with the approach taken by Bandis et al. (1981).
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Response to B. G. Look discussion paper
The authors welcome the comments raised by Dr. Look in relation to the paper published in the AGS special edition on expansive soils (Vol. 53 No. 1). The use of AS2870 in subgrade treatment for pavements has been the subject of considerable debate in South Australia and the intention of the paper is to outline an example of its application for the Darlington Upgrade Project and how it compares with other Statesβ approach.
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Discussion on paper by Mackie, Volume 46 Number 4, pp 25-34
This writer accepts the findings of the paper that there is reasonable correlation between core permeability, as measured using gas, with geophysical data from wireline logs. The writer also accepts the statistical methodology used by Mackie in achieving the correlations.
However, the writer is concerned with two inferences in the paper, and one explicit statement. The inferences are as follows.
- Firstly, given that the work was done for a project at Ulan Mine, an inference is that core permeability data may be, or is, relevant to the estimate of groundwater flows and pressures, either before or after longwall mining.
- Secondly, regardless of the relevance of core permeability, an inference is that using gas gives a valid measure of such permeability.
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Cognitive Dissonance in Geotechnical Engineering
2018 AGS Practitioner Award Presentation Tour
Dr Burt Look