Technical Note: Strength and stiffness properties of defects within the Hawkesbury Sandstone and Ashfield Shale

Robert Bertuzzi


The laboratory direct shear test results on defects which were undertaken for three recent tunnelling projects in Sydney – the Lane Cove Tunnel, the CBD Metro (which was not built), and the Northwest Rail Link – 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 (ϕb), i.e. “the frictional component of shear strength for a planar or effectively planar discontinuity i.e. independent of any roughness component causing dilation during shear” (Hencher and Richards, 2014).

The author has interpreted the results for shear strength and stiffness.

  • The peak shear stress (τ) recorded for the normal stress (σn) 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).